So here's a glioma cell transfected
with a leucine rich protein. It has all
those amide protons on the left. This is
a control plasma. We turn it on and we
see only our tumor lighting up.
What is the beauty of the
system? It's that we can design a family
of reporter genes with all different resonance frequency and we can saturate
specific frequencies so we can do multicolor imaging, sort of analog to
fluorescence where you have all these different wave lengths. I think it will be really important in the
future to be able, just like immunocytohistochemistry to look at multiple cell
populations simultaneously specifically to look at these interactions, also
when we co-graft multiple cells.
So this is a summary of the
techniques that are currently available.
It's a very crowded slide. I am
sure they will be posted online so you can maybe look at it later. I think the three that are most closely to
becoming clinically important when we start to use human embryonic stem cell
therapies in humans are listed here: positron emission tomography; SPECT or
gamma camera imaging for systemic injection; and also MRI as you note that it's
primarily for correct delivery and local engraftment, less for systemic. The resolution are here. The number of cells, it's possible to see
single cells at the right resolution. I
should also say that CT and ultrasound and there are things happening right now
with contrast agent that we'll hear more about.
If you're interested, some of this
work is covered in deeper context in this review article that came out last
month, if you want to look that up, with a lot of references.
So, in conclusion, the iron oxide
label cells and compatible catheters, and, again, the few centers set up as of
yet because the special catheters allows MR-guided realtime targeted delivery
and, essentially, accurate cell delivery and it can have various
application. And then after we do that,
then reporter genes are needed to serve as a beacon for cell survival,
uncontrolled cell proliferation, as well as cell differentiation.
And I think that at the end it's
probably going to be a combination of techniques, so why not put in a PET
reporter gene in a cell and at the same time label it with iron oxide so we can
do both delivery and follow it. I think
there's an unrecognized potential for interventional radiologists who will do
this in their practice or in their academic setting. That it is something that imaging people
always talk about they want to track cells to see where they go, to
follow. Yes, but more important is to
deliver them in realtime using specific interventional instruments and
catheters and devices.
So I do think that needs to be an
integrated part also with discussions with the stem cell people is how they
want to deliver these cells and where because of those issue.
I should acknowledge a lot of
people, also people I borrowed slides, and NIH funding. We have worked with a company in Baltimore
with mesenchymal stem cells. We have
islet cell protocol, got a lot of islets from Justin Diabetes Center.
And I leave it here. Thank you.
(Applause.)
CHAIR URBA: So we have time for a couple of quick
questions.
DR. SNYDER: Jeff, which of the markers that you used to
mark cells are already being used in patients and are approved for patients?
DR. BULTE: Yes.
So the only one that approved is indium-oxine and they are the ones that
have been used in patients just recently the thymidine kinase, the reporter
gene from the herpes simplex virus and the Theradex label.
DR. SNYDER: And those could be applied to stem cells in
your opinion?
DR. BULTE: Yes.
There's no difference. Well, you
get a practical issue is that embryonic stem cells may be harder to transfect. You know, there are other issues there. They grow in embryo bodies. Do we get the label. But, principally, yes, there's no difference,
right.
DR. TAYLOR: So a number of these MRI studies with
Theradex we and others have done and shown that as cells don't survive the
label is taken up by macrophages and other cells and actually doesn't
accurately reflect the cells you transplanted.
So I'd like your comments on that in terms of clinical relevance.
And then, two, that's clearly not a
quantitative measurement at this point.
So can you talk about what the limitations might be clinically then in
terms of how we over or under estimate effects if cell number is really
critical to some of the deleterious side effects?
DR. BULTE: Yes.
I've heard the question many times before. There's a limitation by labeling cells with a
fluorescent dye, a lipophilic dye, Theradex indium-oxine it leaches when a cell
dye is taken up. In case of the
Theradex, the contrast itself disappears quickly since the macrophage is
biodegraded. But the Theradex
application is really the correct delivery in realtime, initially.
After that, we don't know if the
cells are dead or alive because the contrast is going to stay there. Another problem is the label disappears
quickly if cells start to proliferate uncontrollably. So the holy grail is an MRI reporter
gene. It's the holy grail. And several centers have been working on
that, including our artificial approach.
People use ferreting. We are not
there yet, so that's one way. Yes, so
that's a potential artifact.
Your second question was about --
what was your second question?
DR. TAYLOR: How to do with the lack of quantitation?
DR. BULTE: Yes, yes.
So the MRI is not quantitative.
Reason being it's very simple, we do not a priori if our cells are
clustered in groups or if they homogeneously disperse. That affects the MRI contrast differently and
that's the problem of that. So the way
to do this is indium-oxine or PET bioluminescent imaging, you know
semi-quantitative.
What does that mean? We do not really know the depth where the
cells are, so we have to correct for the attenuation of the signal. We have to make an estimate so it's
semi-quantitative.
Also, by the way, indium oxine
leaches out, binds transferring. You get
a lot of artifacts, liver uptake, spots that the cells actually are not
there. Each technique has it's
problem. I'm just hoping in ten years we
have a good MRI reporter gene that's safe.
The reporter genes we are using are artificial, so we're creating things
that don't exist. It's a whole other issue
we have looked at and everything looks fine, but I don't know.
CHAIR URBA: Dr. Chien?
DR. CHIEN: I was going to ask you, this is a two-part
question. One is, what's the minimum
resolution that you can get? I mean how
many cells can you pick up? I know it's
obviously not a single cell level detection.
So what is the minimum mass of cells that you can actually detect,
100,000?
DR. BULTE: Can you put it on four again, sir, on the
visual? Real quick I can also tell
you. Actually I should point it out
because it's a very important question.
It's in this column.
The sensitivity, the number of
cells, for the reporter genes, bioluminescent imaging, and our artificial , and
you go five to ten thousand cells, which you have not talked about them. PET, actually, I have asked the experts. I've not gotten an e-mail yet, but I think
it's a low number, perhaps 100 or so.
The MRI part with the side, that's a tricky one. And, again, it's not an easy answer. It depends on the field strength. The higher the field strength the
better. These particles use
contrast. It depends on the voxel size,
the resolution.
I would say clinically we can see as
much as a 1,000 cells in the lymph nodes in animal systems. We can use bigger magnetic particles, but we
can see single cells in vivo at high field in an animal, but this will be
clinically translated.
DR. CHIEN: Yes, that's what I thought. And the other question I had is, don't you
think you could, like many other surgical protocols, because a lot of this is
kind of surgery interventional, are you going to optimize the entire delivery
protocol in animals without having to have all these sophisticated imaging
things because then you can use, you know, sort of realtime, single-
cell-level-almost analysis with lacZ reporters and optimize that and then just
go into humans with an optimized protocol that you don't have to demand
realtime feedback of the delivery of the cells while the operator is delivering
it on the table?
DR. BULTE: Am I allowed to disagree with you?
DR. CHIEN: I didn't say that was the case. I just asked what you thought.
DR. BULTE: Yes.
Reporter genes are not realtimed because you get the substrate and it
takes a little while, you know, to accumulate in that case. I think in the case of myocardial infarct, if
somebody is poking around at the area of the infarct, at the moment within
seconds at least it will be known in realtime and the patient is dying, you
know, and is claustrophobic and is in the system, I think it is very important
to do that fast, not to see afterwards if the cells were injected at the right
place.
Same with the lymph nodes, you're
suggesting it's not important to do in realtime, but --
DR. CHIEN: I'm saying is is that once you work out the
protocol, okay, so for example, for direct injection in a specific location in
the heart for example, that I don't know that you necessarily -- you need
readout to know where you're at in the heart, but you may not necessarily have
to get realtime disposition of the cells in the heart because you've already
optimized the protocol for delivery.
DR. BULTE: But if you don't know where you've put the
cells, you want to find --
DR. CHIEN: Well, no.
You can figure out where you put the cells by electrophysiological feedback,
like NOGA and things like that. You
don't need to see it, right?
DR. BULTE: Okay.
DR. CHIEN: Okay.
Anyway, I don't want to argue with it.
CHAIR URBA: Dr. Firpo?
DR. FIRPO: You know you mentioned that a suicide gene
in, again, acyclovir selection and there's a couple of papers out on that now,
more than just the one you talked about.
But are you aware of any studies where people have done selection to
kill the tumor and then allowed the mouse to live after that to see if it comes
back?
DR. BULTE: Following treatments? Okay.
No, I'm not aware. I don't know
how long the -- you know, if it kills all the cells. From the signal it looks like they're all
dead, but if a few cells remain. I think
it's a matter of dose. I've heard if you
inject small numbers of these cells that they may not form tumors. It's just like injecting a subcutaneous
tumor. If you inject 100 cells, you
don't get a tumor in a nude mouse, but you have to give maybe 100,000
cells. I don't know.
It just depends on how many cells
survive. I guess you could do
experiments by dosing 10, 50, 100, 200 and see if they form tumors that the
same scenario will apply, that the number of surviving cells could again form
tumors. So it depends on the specific
setting.
DR.
GOLDMAN: Jeff, to just follow up on Dr.
Chien's question. So the MR resolution
is going to depend upon the cellular density, of course per voxel. So just as a base level of maximal
resolution, how many cells are required per technique, whether by proton or by
polyamide, cells per voxel can be detected let's say in a clinical 3T magnet?
DR. BULTE: Yes.
So for clinical 3T magnets I think it's fairly safe to say for MRI
within a voxel somewhere between 500 and 1,000 cells, and we're talking here 500
micrometer resolution in each direction, like a cube. The MI probe is less sensitive. Currently we can only really do it very well
in animal magnets of higher fields. The
sensitivity is about somewhere around 10,000 cells I think at this point.
The most sensitive tracer are these
iron oxides. They're the most, so that's
going to be the limit.
CHAIR URBA: We'll take two more questions. Dr. Woo and then Dr. Weir.
DR. WOO: Yes.
We heard from previous presentations that the formation of teratomas is
those dependent of the undifferentiated embryonic stem cells, and the threshold
may be around 100,000 plus/minus. And
yet in one of your slides, I think it's the first slide of section four, you
show a teratoma forming in a mouse and you indicated you only need one cell for
causing trouble. So I'm kind of confused
as to what is it that is really needed to form a teratoma.
DR. BULTE: No.
There's a misunderstanding and I understand your misunderstanding. That slide was with one bone marrow stem cell
that reconstitutes the entire bone marrow to see the power that when you have
one cell that it starts proliferating.
Eventually you can see that. So
that one cell was the bone marrow reconstitution experiment.
The other one that I followed up was
the teratoma slide. So I understand the
confusion, but they are separate studies.
DR. WEIR: You've given us a wonderful look into the
future. But I wanted to ask about just
conventional radiology techniques as far as monitoring an inject site, for
example, for teratoma formation. Just
how sensitive do you think it could be if you were looking at the spinal cord
or if you were looking at some other site as far as getting a clue that there
was a teratoma being formed?
DR. BULTE: Yes.
So currently the way it will be done right now is purely
anatomical. Right? We get a soft tissue mass or perhaps a skin
or keratonin, whatever. So they'll
anatomical at that time. It's just like,
in general, in tumor formations, the whole issue. By the time a tumor is detected anatomically
with MRI, it's already too big. So you
want to have more sensitive methods to detect it in its very early stage.
So how many cells you need in order
for it to detect it? I think it's the
same issue as the sensitivity of these cells perhaps. So I think the bottom line is, is the
sensitivity equal or higher than the number of cells that are needed to form a
teratoma. So if you can detect fewer
cells than are needed to form a teratoma, I think that's a good thing. So at that point you can maybe see it earlier
than the cells are able to form a tumor, something like that.
CHAIR URBA: Thank you.
It's time to move onto the public
hearing part of the meeting. I'd like to
share this announcement before we start.
Both the Food and Drug
Administration and the public believe in a transparent process for information
gathering and decision making. To ensure
such transparency at the open public hearing session of the advisory committee
meeting, FDA believes that it is important to understand the context of an
individual's presentation.
For this reason FDA encourages you,
the open public hearing speaker, at the beginning of your written or oral
statement to advise the committee of any financial relationship that may have
with any company or any group that is likely to be impacted by the topic of
this meeting.
For example, the financial
information may include the company's or a group's payment of your travel,
lodging, or other expenses in connection with your attendance at the meeting. Likewise, FDA encourages you at the beginning
of your statement to advise the committee if you do not have any such financial
relationships. If you choose not to
address this issue of financial relationships at the beginning of your
statement, it will not preclude you from speaking.
Our first speaker is Ms. Amy
Comstock Rick from Parkinson's Action Network.
MS. RICK: Thank you and good afternoon. My name is Amy Rick and I am actually here in
my capacity as president of the Coalition for the Advancement of Medical
Research, which is a non-paying position by the way. But I also serve as CEO of the Parkinson's
Action Network. Aside from that I cannot
think of any conflict of interest that I have.
I drove my own car from my home this morning.
The Coalition for the Advancement of
Medical Research is a coalition that was formed in 2001 as a direct response to
the President's policy restricting federal funding for embryonic stem cell
research, as you all know, for lines that were derived after August 9th,
2001. The coalition is comprised of
patient groups, individual research institutions, some which are represented
here, as well as associations of
researchers. We have over 100 members.
Our mission remains fairly
consistent, which is to promote regenerative medicine with a prime focus on the
lifting of the President's policy on the restrictions for federal funding.
As I'm sure you all know, human
embryonic stem cell research has been quite a focus for patient groups over the
years. With the legislation that has
gone through Congress, as well as the scientific breakthroughs that do get a
fair amount of media attention, disease groups, Parkinson's, spinal cord
injury, diabetes, many cancer, many, many others, the patient advocacy
community as been quite focused on the progress. In fact, to the point that a few years ago
when the legislation was a hot topic in Congress, we were fearful that the
patient community was at a place where, if the legislation passed, that they
would be expecting FDA approval and treatments immediately.
I think we are not at that place any
more. I find that you have a very
educated patient or affliction spinal cord injury population who understand in
a very sophisticated way the issues surrounding human embryonic stem cell
research, both potential as well as risk.
It is quite enlightening to the
patient community that, in spite of the President's restrictions laid down in
2001, that the science has moved forward, not as quickly as it would have but
for the restrictions, but both with private funding as well as the emergence of
-- merging a fair amount of state funding the science has moved forward and it
is inspiring to patients and we commend the FDA for actually having the
advisory committee meeting to address some of the issue that we hope you will
be facing in the coming months and years as you begin to see applications for
approval for clinical testing using human embryonic stem cells.
As you all know, we actually
anticipate even hopefully more eligibility for federal funding in the coming
months and years as the three main presidential candidates are all from the
U.S. Senate. So we happen to have on
record that all three of them voted twice in favor of the stem cell research
enhancement act which would have allowed federal funding for human embryonic
stem cell research on lines derived, would have lifted the President's
restriction if they were left over embryos from IVF clinics that otherwise
would have been discarded and a few other limitations. So given the fact that we have this record,
we do anticipate that in 2009 more research will, in fact, be funded using
federal funds.
It is in that context of hopeful
anticipation for this field that I want to raise two cautions to this
committee. One would be our request on
behalf of CAMR and the patient communities that in spite of the high visibility
and great amount of controversy that there has been around human embryonic stem
cell research that you not put an extra layer of risk averseness or safety
requirements simply because the nature of the visibility or the controversy on
the issue.
It is the risk benefit analysis, if
you will, which I understand is extremely complicated on all these issue, not
unique to stem cell research, and is not formulaic in any way, but the fact
that there's external controversy I would plead that you do not, as scientists,
allow external controversy in any way to interfere with your analysis.
And the second caution that I would
raise is, in fact, not as directly related to stem cell research. But in an article in Bloomberg News this
week, actually about this meeting, Dr. Robert Lanza was quoted as saying, in
this field there can be no risk whatsoever.
Now, I know that all of us know you can't take that literally because
there's risk in everything in life. But
I ask that you -- for the diseases that we're talking about in this room this
morning, Parkinson's, spinal cord injury, juvenile diabetes, cancer, if you
could, these decisions cannot be made in a vacuum. We are talking about risks, but you're
talking about risks as you know balanced against the life of living with a
chronic, progressive disease like Parkinson's.
You're talking about cancer, juvenile diabetes. There is the ever present, horrible risk of
living and dying a miserable death with one of these diseases or with this
injury, and if I ask, as you always do, to keep that in mind as you're
assessing the risk of some of the very serious questions that we heard about
this morning, scientific questions.
Thank you.
CHAIR URBA: Thank you very much.
Next we'll hear from Dr. Chris
Airriess, California Stem Cell, Incorporated.
DR. AIRRIESS: Dr. Urba, committee, thank you for the
opportunity to speak today.
First off, I am speaking on behalf
of a private company, California Stem Cell, and I'm an employee of that
company. We are actively developing
therapies and are engaged in preclinical development currently in spinal
muscular atrophy and ALS, as well as spinal cord injury, so some of the
diseases of the previous speaker has just brought up.
The stem cell research field is
currently at a turning point. Research
findings enabling the scalable, current, good manufacturing practice production
of human cell populations at extremely high purity move the therapeutic potential
of stem cell derived treatments from the real of hope to that of practical
application.
At California Stem Cell we have
conducted extensive safety testing along the lines that have been discussed
here this morning on our human embryonic stem cell lines, as well as the high
purity, differentiated human cell products of these lines. Studies such as these help to minimize the
risks of potential therapies to prospective patients.
Until the technology for safe and
scalable generation of patient specifics outlines has proven, compassion
compels us now to use existing technologies to develop therapy as addressing
devastating and currently untreatable human disorders.
With appropriate safety testing and
careful administration of safe and effective immunosuppressive regimes,
emerging therapeutics based on current human embryonic stem cell technologies
are an immediate and viable solution for treatment of the widest variety of
such conditions.
We've been highly impressed thus far
with the dedication and insights of the team of Mercedes Serabian and her
colleagues here at CBER. Two items in
particular that we feel will be conducive to the efficient development of stem
cell-based therapies are continued opportunities for early interaction with the
FDA through the pre pre-IND process.
This has been very beneficial to us so far.
We've got a lot of valuable feedback
and we encourage the continuation of this process. And we would like to see clarity, which I'm
sure is coming, on the FDA's requirements for preclinical efficacy in safety
for stem cell therapies in the form of a formal guidance document.
Again, I thank you all again for the
opportunity to speak today, and we also thank our key partners, the ALS Therapy
Development Institute, Families With Spinal Muscular Atrophy, Johns Hopkins
University, the University of California, Irvine.
CHAIR URBA: Thank you.
Now, notice of this meeting was made
available to the public and anyone wishing to speak was asked to register prior
to the meeting. However, we have a few
moments of additional time if anyone else in the audience wishes to address the
committee at this time.
If not, we'll adjourn for lunch
until 2:05. Thank you.
(Whereupon, the foregoing matter
went off the record at 1:02 p.m.
and went back on the record at
2:07 p.m.)
A-F-T-E-R-N-O-O-N
S-E-S-S-I-O-N
2:07 p.m.
CHAIR URBA: If everyone could please take their seats,
we'll begin the afternoon session.
Okay. So we'll get started with the afternoon
session, which, if you remember Dr. Bauer's presentation this morning, was to
address three rather broad questions.
And the first question up for discussion is on the slide that's before
us. And I guess just to set the stage, I
will read it.
Inappropriate Differentiation and
Tumorigenicity, and what we're being asked to consider and discuss are:
Criteria for selection of clinically
relevant animal species or models that support engraftment of the administered
human embryonic stem cells, for example, optimal strategies for evaluating
potential host rejection of administered stem cell-derived products?
What may be the optimal site of
implantation in the animals in order to obtain meaningful test results?
What sorts of study durations are
required?
And what is the most appropriate
dosing method, that is, absolute undifferentiated human embryonic stem cell
number versus percentage of undifferentiated stem cells present in the product
to extrapolate cell doses tested in animals to plan the clinical dose?
So that's where we'll start. And Dr. Goldman, if you'd like to kick off
the discussion?
DR. GOLDMAN: Sure.
So trying to break that down a bit more operationally, both
tumorigenicity and inappropriate differentiation can be looked at as functions
of model and disease environment, of course of site, site of implantation
especially for nervous system targets, function of the survival and the study
duration, the cell dose, of course how that cell dose is calculated, whether
before or after transplantation or as a function of both. And then, of course, both tumorigenicity and
differentiation state have to be looked at as a function of the immune state as
well whether we're dealing with immunocompetent patients, I mean suppressed
patients, or as far as disease models are concerned, immunocompetent,
suppressed or deficients.
So essentially we're looking at a
combinatorial function of all those variables and we need to establish an
algorithm for being able to apply these as uniform criteria across cell types,
and at least in some reasonable fashion across disease models.
So I'll start just discussing at
least the issue of tumorigenicity from the standpoint of just presenting a
couple of questions for the committee, and then of course looking at
differentiation or inappropriate differentiation from the same standpoint.
Tumorigenicity strikes me with the
most important issue, at least in my own mind from what we heard this morning,
is what constitutes a tumor, how to define it?
Of course there was already some debate, if you will, in terms of
whether a tumor could be benign, whether a histologic benigness connoted
physiologic outcome benign nature. The
issue of whether infiltration and to what degree was a measure of tumorigenicity
and to what extent that precluded the use of ES or ES derivatives.
And then really in a more
fundamental level, how looking at histologic tissues can we define a
tumor? Should we look at the division
rate, the survival rates over time, the expansion rates of the population over
time? Do we need to assess that from the
standpoint of the proliferation or turnover rates of need of cells in the organ
into which the cells are being transplanted?
Or as we looking rather for an absolute absence, as the case may be, of
proliferation or undifferentiated expansion?
What kind of markers can be used to define anaplasia? What kind of markers can be used to define
undifferentiated expansion?
Now, of course, we heard the ES
markers used as indices of the persistence of undifferentiated cells in grafts. But worrying about more from the standpoint
also of the things not discussed. What
happens as ES derivatives are implanted?
And if those derivatives are still at the progenitor state and
undergoing expansion themselves, what allows us to define whether the
undifferentiated expansion of already committed progenitors, when does that
become a tumor?
There are instances, some mentioned
earlier, some others in literature, of undifferentiated neuroepithelial
expansion of ES-derived neuroepithelial cells.
This may be a problem in a variety of organ systems. So it's not just a question of
undifferentiated ES persisting in a graft, but also of their mitotically
competent derivatives. At what level do
we need to exercise control over their expansion?
At what level do we permit the
implantation or introduction of any persistent, undifferentiated ES cells, or,
as the case may be, still mitotically competent derivatives thereof? And what kind of markers can we use to define
the existence of these cells? The cancer
literature, of course, has a number of markers defined phospho-Akt survive in a
variety of proteins that can be used for identification purposes, but they tend
to correspond to markers of anaplasia or uncontrolled expansion.
Cells that are no longer controlled
by normal cell cycle checkpoints. The
issue is going to be, I think, for this field as progenitors are transplanted
and then undergo persistent expansion that will not necessarily express markers
of anaplastic transformation. At what point
can we define that as normal expansion versus uncontrollable?
So those are all questions, but they
really come down to the point of, what can we tolerate in terms of implantation
of potentially undifferentiated or partially differentiated cells that are
still capable of expansion? So I would
broaden the issue behind just ES cells
and behind just
ES-derived
teratomas or teratocarcinomas.
So that segues into the issue of
differentiation and to my mind it's very much a parallel question. What constitutes inappropriate
differentiation?
And so it's essentially by
definition. Ectopic differentiation of a
functional, mature phenotype in an area in which that phenotype would not
normally be present or in numbers in which that phenotype would normally not be
represented, would represent inappropriate differentiation.
But we don't yet have criteria by
which to establish whether an inappropriately differentiated pool is
dangerous. Under what circumstances
inappropriate differentiation may be relatively harmless? In what cases it may be beneficial? There are no general rules where this is
concerned and these outcomes may depend very much on the disease state and the
disease target.
In my own field, if we put glial
progenitors into say a demyelinating lesion and we're looking oligodendrocytic
differentiation and we see astrocytic, well that would be inappropriate. If we go into let's say a stoke bed and pick
up astrocytic differentiation, well, that would be beneficial. So this is very much a function of the
disease target as well as the cell type that's being implanted.
Then even appropriate
differentiation into a set of phenotypes that is correct, if you will, for the
organ may still not yield an appropriate functional outcome, and so the necessity
becomes to match, essentially, the quantitative representation of phenotypes
generated from progenitors that may be capable of giving rise to multiple
phenotypes.
For islets, for islet progenitors
derived from ES, it's one thing to look for beta cells as an appropriate
cellular target, but one may expect these progenitors to potentially give rise
let's say to alpha or delta cells, but potentially antagonizing the effects of
the beta cells. For example, in the
examples mentioned earlier of dopanergic production, well those same
progenitors as derived from ES can also give rise to serotonergic and gabergic
cells which in vivo/in vitro can potentially antagonize within the steroid of
some of the effects of the dopanergic neurons.
So even when we have ES that are
giving rise to the cell types of interests and even the precise representation
of cells that would normally be derived from those progenitors, unless the
proportions are correct that we're going to need, we may see if not dangerous,
then at least counterproductive effects that would potentially diminish
ultimately efficacy, as well as in some cases potentially presenting safety
issues as well.
And so we need to define and
characterize the state of differentiation that we want cells to be implanted
at, what types of lineage potential they have at that point, and it becomes a
big of a ying yang in that by the time we have cells that are sufficiently
mature to yield the cell type of interest with the highest possible fidelity,
in other words the purest possible population of the cell type of interest,
well by that point we're far enough down the lineage and the cells may be post
mitotic, they may not tolerate the engraftment well, and so we may not have a
viable, engraftable cell population.
On the other hand, the still
mitotically competent cells that may have much greater efficacy, as well as
survivability upon transplantation, may be those that potentially may give rise
still to undesired phenotypes that may still be capable of uncontrolled expansion. And so this is the, essentially, dilemma I
think that we all face is establishing what is the appropriate stage of
differentiation for transplantation and how essentially enriched or purified do
those populations have to be at the time of transplantation? So I'll leave those as essentially entry
points for discussion.
CHAIR URBA: Do you have a couple of comments on what you
would do preclinically to identify and answer those questions?
DR. GOLDMAN: Well, then it's a question of disease target
and, specifically, the cell type of interest.
I think these answer are going to be, to the extent that answers can be
derived for any, but I think that conceptions are going to be driven by exactly
what disease target and exactly what cell types are being used.
So, for example, my own target of
interest, the hypermyelinating disorders.
This is a set of disorders where glial progenitors, as Jane was
discussing before for example, can be productively used. However, here's little ability to control the
oligodendrocytic versus astrocytic differentiation of these cells. We're still learning what the rules are.
But we know that by the time the
cells are oligodendrocytes, oligodendrocytes
at least primate and human, oligodendrocytes are every bit as post mitotic as neurons, and
so they become very difficult to engraft.
And someone in order to have a working strategy has to be able to give
the cells are progenitors.
And so then the issues of persistent
expansion and potentially unregulatable differentiation come to the fore. And the imperative then becomes matching up
the disease target to the phenotypic potential and likely in vivo activity and
behavior of those cells. That can be
productively done with some disease targets, not so much with others. So I don't know that there's a one size fits
all answer to these.
CHAIR URBA: But if you were going to take the oligo cells
to trial, you would take the earlier stage of differentiation, which is more
susceptible to some of the problems we talked about. What preclinical things would you want to see
have done before you actually went and did that clinical trial?
DR. GOLDMAN: So I would want to see a definition of the
expansion kinetics of that cell population over time in vivo in animal models,
in animals models that severally replicate the disease target of interest. The net population expansion from the
standpoint of how many cells do you have at given points of time after the
initial transplantation, taking out for very long periods of time essentially
for the experimental models we typically use, mice and rats, the life of the
animals.
I would want to see what the mitotic
rate, the fraction of cells in cell cycle was as a function of time. And what point the mitotic index of the
implanted cell population fell to that of the background, essentially the
native cell population of the same phenotype in vivo, in other words of the
host cycle kinetics, at what point do they actually match up.
At that point I would want to make
sure that there was no heterotrophic migration of the cells into in this case
non-white matter areas, in other words that we weren't seeing heterotrophic
foci oligodendrocytes in areas of gray matter for example. I would want to see that there was no overt
anaplastic transformation of any of the cells at any point. And that there was no evidence of
heterotrophic differentiation to non-glial phenotypes, much less non-neural.
I think all of those are critical
safety end points, and that in the final analysis that the overall cell number
was in no way perturbed by virtue of the graft, and that the final
representation of phenotypes at least was analogous to that of the native
tissue that one was trying to either repair or replace. So that, in effect, we're looking for the
establishment over long periods of time of histologic and, therefore,
physiologic normalcy.
CHAIR URBA: Two other questions I think that tie in with
all the other questions and then we'll let other people comment.
Is the mouse and rat model, is that
as far as you go, or do you think you've got to establish it --
DR. GOLDMAN: Depends on the disease target. So, for example, the congenital
hypomyelinating disorders, the only models that exist, by and large, are mice. And yet there's no reason in terms of the
known biology, of course we only know what we know and we don't know what we
don't, but in terms of the known biology, there's no reason why those models
wouldn't be reflective of the congential hymyelinations of humans, of children.
On the other hand, if one is looking
at, for example, models of adult either traumatic injury or stroke, the issue
then becomes what the kinds of sizes involved, what the kinds of unique
features of the primate and human vascular supply to regions that are being
challenged, that one, I believe, does need larger animal, and preferably
primate models because there are primate and human specific features of the
anatomy, and in some cases of the cell biology, that require large animal, or
as the case may be, primate modeling.
So, again, it depends on the disease target that one is approaching I
believe.
CHAIR URBA: Is there an acceptable rate of tumor
formation or does it have to be zero?
DR. GOLDMAN: I would say zero, and just getting back to
the point that we raised earlier, at least in the spinal cord and brain, there
is absolutely no such thing as a benign tumor.
It doesn't matter what they look like histologically.
CHAIR URBA: Would the acceptable rate of tumor formation
be target specific too, or would you generalize that to all the things you
heard about today?
DR. GOLDMAN: This, of course, is all personal
opinion. But with few exceptions, the
disease targets of regenerative medicine are not ones that, to my mind, should
permit the genesis of tumors, of cancers from implanted cells. When one is talking about biologic
therapeutics or any therapeutics in the setting of diseases with short life
spans, of course the bar becomes smaller.
So, for example, one may tolerate
the potential development of lymphoma or leukemia decades out in the setting of
chemotherapy, radiation therapy of say a child with leukemia, because you're
looking at extending the life span tremendously and you essentially are faced
with an all or none situation. Most of
the disease targets that I think we're all looking at in regenerative medicine
are a bit different in terms of these are disease that often are chronic, they
often are involving tissue loss over long periods of time, and so looking at
that from the standpoint of playing out essentially the cost benefit over time,
from my own standpoint I don't see the risk of tumorigenesis as being really
justified.
Again, I'm speaking with very broad
strokes, but in general terms I don't think it's justified. There are examples, such as Huntington's
disease where one may think in terms of inducing endogenous progenitor cells,
where the patient may have a relatively short life span and will potentially be
using a strategy that may increase the risk of brain tumorigenesis over time. But then just as the example before with
childhood leukemias, if we're looking at potentially extending the life span of
that individual considerably beyond what it would normally be, then the risk of
late stage tumor formation becomes I think justified.
But it's for disease targets where
the life span is not significantly curtailed or is curtailed but not
potentially to the point where the appearance of tumorigenesis would
necessarily be where it's need to be, that's where I don't think it's
justifiable.
CHAIR URBA: Thank you.
Dr. Taylor?
DR. TAYLOR: Thank you.
In thinking about optimal
preclinical study design I think you made an excellent point. I think the first thing is that the
preclinical studies have to be clinically relevant. So they have to be done in animal models that
reflect the disease state at which we're looking and they should reflect the
questions that we're likely to see clinically with regard to migration, whether
route of administration matters more than site of delivery, and I think we're
really asking the question, what it really comes down to, what can cells do
versus what they actually do, and so we need to design preclinical, in vitro
studies to examine both the potential of the cells as well as in vivo studies
that show not only the potential, but what actually happens given the
population that you ultimately are going to use.
I wonder if, perhaps, given the
preclinical data in mice at least suggest site of undifferentiated cell
administration really has an impact on tumorigenicity, and given that we heard
this morning that vulnerable sites exist, perhaps site localization studies
should be recommended to include direct administration of the final cell
product to some of those vulnerable sites, most likely vulnerable sites to see
if, in fact, tumorigenesis or some other adverse effect is going to be an
issue.
We've also heard that dosing
matters, cell number matters, and I think back to some of the angiogenesis
assays and tumorigenesis assays that we all did when we were looking at
angiogenic cell types, and there's a standard subcutaneous tumorigenesis assay
that we all do to determine if cells are angiogenic. It seems to me that we can develop similar
sorts of assays for the uncontrolled cell proliferation potential of these
cells.
And then, finally, I wonder if -- I
think we need to be careful not to add an extra burden on embryonic stem cells,
but at the same time recognize that given the fact that we don't yet know to
what degree minimally differentiated cells have adverse events, we need to
develop strict definitions of what potency and release criteria are going to
be, and I'll speak to that more when we get to the next question.
I guess there is one other thing
that I would like to say and that is patient criteria have to fit in here
somewhere. It may be that an age of a
patient matters, just like disease matters.
Age may matter, sex may matter, co-morbidities may matter, and so I
think as we begin to ask some of these questions about the both positive and
negative potentials of these cells, we have to consider the context in which
the cells are going to be delivered.
DR. SNYDER: I just wanted to just briefly reiterate a
point that Steve started to make and Doris began to make, too, and that's that
in answer to your question as to what should our cell population be that we put
in there and how do we know. It really
so much depends on having really faithful animal models that reflect the real
disease. And that in turn goes hand in
hand with a better understanding of what the pathophysiology of the diseases
are we want to treat, and that, in and of itself, is turning out to be a moving
target.
You know five or six years ago, for
example, for ALS, the slam dunk answer would have been, well, we just need
motor neurons. Now as our sophistication
about the nature of ALS is growing, we're starting to realize, well, sure,
motor neurons are important, but maybe we actually do need astrocytes, which
means that perhaps we need to somehow figure out how to put in a mixed population
of cells that'll be necessary.
This kind of thing I think even
extends to our view of embryonic stem cells, and this is a point that Willie
and I started talking about. On the fact
of it, to say I'm going to use embryonic stem cells in a neurologic disease, it
would be a deal breaker to say, oh, and by the way, I'm also getting some
mesoderm, I'm getting some vasculoendothelium, and I'm getting some smooth
muscle. Ordinarily, you would say, well,
I guess you can't use your cells.
Well, with a broader view of how
we're starting to appreciate the injury niche, we realize the injury niche is
neural elements, and also vascular elements, and some extracellular
matrix. It may be exactly what we want
to be able to reconstruct a niche that you have a cell that safety gives you
some elements and then some support elements, and Willie talked a little bit
about the cross talk.
So I guess the point comes back to,
even as cell biologists, we still have to go back to really good, faithful,
animal models. And that's independent of
whether it's a larger animal or a small animal.
We just need models that at least model the path of physiology and then
be prepared to revise our notion of what pathophysiology is and what you really
want to do to fix something that's broken.
CHAIR URBA: Yes. I
think your summary was brilliant. I
think you've really summed it up. The
difficulty, of course, there are so many questions simultaneously. It's trying to solve a complex puzzle. So let me just suggest parsing this out into
the cell types because each cell, when you examine it, interrogate it, there
are different questions. So ES cell, the
intermediate, which would be the progenitor committed but can still have
renewal capability in the differentiated cell.
And so in the ES cell, what's
interesting about this is is that your starting material, which would be kind
of like let's think about this like a monoclonal antibody, and then you want to
purify the soup and eventually get a purified humanized antibody, in this case
the definition of your optimal study material is actually a cell that will form
a tumor and so that's very different. So
you have to have criteria to ensure that the cell is what you want to begin
with.
So, obviously, first thing, there
has to be criteria that it is an ES cell that meets criteria of interest. That can be done by transcriptional
profiling. It can be done by -- and
other issues. I'm not sure. I was concerned about the karyotypic
abnormalities, but I think that can be sorted out.
The other thing that I was quite
impressed with is how little, how few assays there are for monitoring for
teratoma. I mean if we, and I say we
meaning the whole field, we're all interested in our diseases, heart disease,
neurologic disease, et cetera. But what
we're not doing is focusing on a core barrier where, if we don't get over it,
we're not going to be able to do anything.
And that is having better assays for teratoma. There's just no question.
I look at this and I say, the idea
that you're going to take an ES cell, up it into a nude rat, and have any kind
of confidence that this or a derivative, however you're purifying that, be it
ibolometric or by FAC sorting, okay, choose anything. There's a kind of a gradient there. I think it's not stringent enough and there
are things that you can do. So here's a
couple of things and I'm surprised to hear that people haven't done this.
First of all, you can do
xenotransplantation without rejection.
Take a human cell, put it into a pig, you just have to do it at a window
where tolerance is not fully established.
And so you could do it in a very late fetal stage in pigs or in
non-human
primates.
You could give it systemically. You could put it into the fetal
circulation, and if those cells, if they
were tagged and did not produce tumors anywhere after about three months or
four months, and they were put in very large quantities, I would have
confidence that that cell preparation was as good as we could tell
non-tumorigenic.
The difficulty of using a rat or a
mouse is, okay, the good news, it's not being rejected; the bad news is all the
competence factors, receptors, growth factors, et cetera, won't even tickle
appropriately the human ES cell. Many
times they are variant enough that they won't even activate the downstream
pathway. And all of those effects,
including the niche itself, are not going to be operative and so you wouldn't
have that confidence. So I think that's
one issue.
The second issue is we need better
surrogates. We need, if you will, a
cholesterol for teratoma. This sounds
very cardiological. Sorry. But tumors secrete factors. There could be signatures. They could be unique. If nothing else, in a pig you could detect a
human antigen for sure if you did this correctly. I say it as a group. I'm not saying --
We just have not been aggressive
enough in developing these assays that we need.
What I don't think we should do is just settle. So I am a very strong advocate for embryonic
stem cell research.
On the other hand, I think we have
to let science take us there and we have not be aggressive enough in developing
these assays. That's very clear to
me. And until we do and have confidence
in them, and so what we need to do is we need to take ES cells and then differentiate
them and see when do they lose their tumorigenic capability and what level.
So you can say, well, we won't
tolerate any tumors. Okay. Well, I can understand that. But on the other hand we expose ourselves to
mutagenic stimuli every day. We get
X-rays. Okay. We know there's a defined risk, but we accept
it. Okay. You manage it. I think that's what this is.
We have to figure out what is a
tolerable risk for whatever dose of whatever the number of cells, or
whatever. But until we get quantitative
with that, we are just guessing and I don't think guessing is good enough.
The other point is is the niche,
which I'm sensitized to, is that, of course, you know I'm a cardiologist and
we've -- not me, but our field has been putting cells into the heart for some
time with quite mixed and ambiguous results.
But one of the things I think is going to be clear depending upon the
organ system and the disease is that there's going to be a different mix of
benefit ratio. It has to be very
carefully examined so that the benefit's always -- and I think you were saying,
that the benefit's always got to be in favor of the patient. Okay, that's point one.
Point two, there are going to be
certain areas where getting the cells back out is not going to be trivial and
there has to be some thought as to what you're going to do if things go
wrong. And that's one of the things I
think that I share your view on this CNS as being difficult there because, if
things go wrong, it's hard to undo them.
And in other cases it might be a little bit easier. So that would be one issue.
Now, the other thought was
progenitors. So one thing to think about
is, I didn't hear this at all today, is that we do know that, and these factors
and pathways are being flushed out every day, that ES cells, including human ES
cells, can be directed to go in a certain direction in response to specific
queues. Some of these are well defined
factors. Some of them are used every
day. They don't have to be a gene. They don't have to be gene therapy.
But if you can get a cell, an ES
cell, to move in a certain direction, it may move away from being a
teratoma. So you might be able to design
protocols to move it so it has zero potential to be teratoma and still has the
potential to be your cell. Maybe not the
cell that your neighbor wants, but maybe the cell that you want. And I think figuring those first steps out
and standardizing, so you want mesoderm, you hit everything with this; you want
ectoderm, you hit everything with this; you want endoderm, you hit everything
with this, and I think that you can flush this out to take those steps.
Now, this is not a product, but
somebody has to take those steps because that could eliminate from a finite to
perhaps darn near zero because you've moved away from the gold standard of ES
cells, which is the ability to make a tumor.
So I think was one step.
The other things is is there's clear
evidence in humans that you can implant cells that are progenitors into an
organ, into a living human being and it won't cause cancer and that is myoblast
therapy for heart failure. Well, it
didn't work in the Magic trial. I know
it's still a thought. I know you're
aware of some of this work. They didn't
get tumors because the cell has a limited capability of replication.
So I think just the fact that it
divides doesn't necessarily mean it's going to be a tumor. It might be good. You might get an amplification signal. So I think each cell has to be looked at
very, very carefully and I think putting some attention on getting the
intermediate and characterizing it rather than the ES cell itself, because if
you can catch the intermediate and purify it and characterize it, you've got
it. You know what you have and you know
what you don't have.
It's not an ES cell, and you can
reproducibly say these are the criteria that we are going to establish that you
have that intermediate. It has to
fulfill these criteria then you go forward with it.
The last one with the differentiated
cell type, what I was amazed at is is that the criteria for what people would
accept for a cell that they would be thinking about, maybe not yet, but think
about putting in a human being, it's clear that the technology for isolating
this cell has not purified it to a level of homogeneity where you could be sure
that it was a cell, even the cell you wanted.
It doesn't mean it didn't have a few cells in there that you want, but
that 90 percent or 95 percent of the cells were the cells you want. So the one criteria is you need to figure out
is the cell you want.
The other is is that it could be a
variant of the cell you want. So, for
example, there are many different types of heart cells. You have a pacemaker cell. You don't want to put a pacemaker cell in
your ventricle if you want to contract.
It's not going to be a good thing, you know, you have electrical
confusion.
I think these sorts of issues, it
looked like a heart cell, expressed myocin, but it had spontaneous pacemaking
capability. We didn't know that. There may be an advantage of having a more
homogeneous starting material that will go in a certain direction rather than
thinking that all the cells that express these four markers are all the same
cell. I think that could be dangerous.
And that's it. That's all I have to say.
CHAIR URBA: Take your niche argument one step
further. The mouse and rat don't provide the other factors, but the pig
does? Would a non-human primate provide
more and better and are you arguing --
DR. CHIEN: No. I
think value and primate would be better.
I was just concerned that people would be upset that I was advocating
giving human ES cells in utero to non-human primates. But I think if that's what it takes to have
complete confidence that you're ready to do something as potentially
revolutionary and as exciting and as valuable as this, I think it's worth
it. And you do it in very high dose.
It would be kind of like the Ames
Assay, you know, you just really put a huge dose of cells systemically into the
fetal circulation. They would be
genetically tagged lacZ, something else you could see very quickly, and then
you see did any tumors form in the progeny over three months.
CHAIR URBA: Dr. Weir?
DR. WEIR: Ken's brought up all kinds of points that
need further discussion. But I did want
to get back to specificity of the animal models and how valuable they are, and
Evan and you talked about that, and particularly in relation to diabetes. Because as far as dosing and characterization
of what you want to put in, it seems to me that diabetes is actually rather
unique in that regard because we have so much experience with just experimental
islet transplantation.
We know what the beta cell mass
should be. We have very defined markers
as to what a differentiated beta cell is.
So you can put the human embryonic stem cell precursors in and see
exactly what they turn into. And you can
put them in various transplant sites, as in fact De Novo Company has published
recently, and get very good characterization which should allow you to make
very good guesses as far as what you should implant into humans. In other words, what beta cell mass do you
need?
And, in addition, the whole
relationship between the beta cells and the non-beta cells I think can be
defined in animal graft sites in a way that it'll have a lot of predictive
value. The issue of malignancy is really
a different issue, which I think you're never off the hook with that in terms
of safety. But, at any rate, I just want
to make that specific point about diabetes.
CHAIR URBA: Dr. Allen?
DR. ALLEN: I'd just like to address the issue of
animals. I think I'm a little
surprised. I mean we are worrying about
niches and the exact, you know, replicating the disease. And I guess my bigger concern with all this
discussion so far is the ultimate in almost all of the experiments we're
talking is xeno experiments, the xeno transplants. I've heard nothing today that explains to me
why we would not have on the insistence a strong suggestion towards doing
allogeneic experiments within species.
So, as a veterinarian, I mean I have a potential interest in this
because, obviously, my patients could potentially benefit.
But there is at least
technologically no particular reason why one couldn't, for example, do a dog or
a pig allogeneic experiment. I find it
strange that we not consider, if we were looking at
cell-based
therapies, we would not wish to assess, we make decisions on the efficacy,
potential efficacy of treatments by looking not at what a human cell would do
in a goat, but what a goat cell would do in a goat. And it seems strange that we would suddenly
abandon that simply, as I said, for expediency.
So much as I don't want to see
unnecessary animal experiments being done, I think if a model exists in a large
animal that can be tested, for example, a spinal cord injury model, if it
exists and it can recapitulate the phenotype of the disease in humans or the
injury in humans, that it would be appropriate to at least explore that.
If it doesn't exist, then I think doing
a mouse xeno, potentially, experiment is appropriate, but I find it sort of
strange that the concept of the two animal rule, not only are we not requiring
the necessary two animals, but we also are accepting xeno data and then we're
talking about tumors.
DR. CHIEN: No. I
don't necessarily disagree with you. But
there's one important point here, and that is that, as we've already heard, human
ES cells and mouse ES cells are quite different. And I would suspect that pig ES cells and all
creatures great and small, if you were fortunate enough to get their ES cells,
are going to be slightly different. And
the markers are sure as heck going to be different. There will be some that will be conserved.
So you can do all of this and still
not answer the safety issue because you didn't do it on the actual
material. So my concern was really
safety, not efficacy. I'm not arguing
you with efficacy. If you want to get
there, that's certainly important. But I
think for safety, you have to have a way in an animal to test the human
material that's going to go into the human being and you're going to have to
have that nailed as well as humanly possible.
DR. SALOMON: So there's a lot going on here. The comment I'd make first would be on the
animal models.
I think if we're trying to give
advice to the FDA on how they might put together a guidance document for
sponsors, I think that's why we're here, I think we're going to have to be a
little bit more specific about these animal model recommendations.
I think that we first have to ask
ourselves, and I think you've picked up on that theme, in your comments is,
what is the animal model supposed to be telling us? First of all, sure, it's easy to say that we
want to have these animal models that have integrity with respect to human
diseases. But the fact is is that most
animal models are poor approximations of human diseases, diabetes, the NOD
mouse, inflammatory bowel disease, multiple sclerosis with myelin immunizations,
and I could go on.
Secondly, immunosuppression, we're
going to mouse models and test immunosuppression? I can give seven days of cyclosporine to a
kidney transplant and get long-term tolerance.
Now, you want me to try that in a human patient? Doesn't work.
Xenotransplantation, talk about
complicating life dramatically. I've sat
on this committee and others talking about xenotransplantation for years. So I just want to point out the fact that
what a sponsor should do with the animal model is answer specific questions in
incremental fashion that relates to what they want to do in a human
patient. And the idea of expecting
sponsors to concoct these really complicated, multifactorial animal models
before we'll let them do a human clinical trial, that part I think is wrong.
DR. CHAPPELL: I wanted to bring up two points when
discussing phase one trials in question three.
But Dr. Chien already started along that road, but maybe we can keep
these in the backs of our minds.
The first one is the issue of what
incidents of toxicities we are going to tolerate. And I warn you against saying you'll tolerate
zero toxicities only, even the most severe ones, because you can do sample site
calculations and realize you need -- no finite sample size will demonstrate
that. And by saying you want zero
toxicities in early phase, what you're doing is you'll maybe see zero
toxicities by luck and then in a randomized phase, big clinical trial later on
you'll probably find some of those if they're at all common and then wonder why
you weren't warned in advance.
Cancer research has gone down that
road to great confusion. They define
acceptable toxicities in terms of one toxicity out of a certain number in a
clinical trial, but those numbers varied and so nobody really knows what the
percentages are in cancer vaguely under a three, but it's very vague and
there's tremendous confusion.
And I urge you, even though it may
seem artificial, to say, to tell us statisticians and others that design
clinical trials what percentage would be acceptable, arbitrary as it may be, five percent
teratomas, three percent teratomas, et cetera.
I mean things work out much more neatly and are much more broadly
interpretable if you actually have a number even though you may not want to
give us one.
The second issue is surrogate end
points. We're placing tremendous demands
on these phase one trials, first of all in terms of toxicities. Right?
We're not going to tolerate a fifth of people having grade three or toxicities as in cancer. And so we need some surrogate for toxicities
as Dr. Chien pointed out.
But, also, the comments so far seem
to indicate that we want some evidence of efficacy in a phase one clinical
trial, and usually I think of surrogates as the enemy for phase three
trial. But for phase one we're going to
need surrogates of efficacy and toxicity I think because we won't have the
sample size to determine rates of clinically important relevant efficacy and
toxicity events.
So you'll hear from me again in the
third question on those.
CHAIR URBA: Dr. Gunter.
DR. GUNTER: Well, I think the discussion has been very
good and I have to say I agree with most of the points that have been
made. I just wanted to pitch in
regarding the suggesting about allogeneic
animal model, and I understood those to be a model where you would be
using embryonic stem cells or derivatives thereof not from the product you're
going to be giving to people. And I can
understand theoretically why that's important.
But, in the end, I'm afraid
concerned sponsors are going to spend a lot of time developing a product that
they never intend to give to people and end up trying to justify comparability
between that product and what they actually want to give to people. And based on what I've seen about abilities
to characterize these cellular products, I think it's going to be hard to
establish that comparability, and so data you get from an allogeneic animal
model might be very difficult to interpret.
So I'm just concerned about that approach as useful. That's my main point.
I would also suggest that maybe we
shouldn't expect our animal models to solve -- one animal model to solve every
problem. I understand the need for
animal models to establish efficacy, and I think in this field we do need to
show some evidence of efficacy prior to phase one. But an animal model for efficacy doesn't
necessarily have to be the same one you use for safety.
I think the FDA put together a very
good background package for us and there I saw pretty good evidence that
immunocompromised rodents were actually quite a sensitive vehicle for testing
tumorigenicity, and there's some numbers that I have in some of these
references. But it's fairly impressive
how these models can detect very low numbers of tumorigenic cells.
I agree with the comment that we
need to define what a tumor is. There's
certainly a need for standardization in the field and that would help all
sponsors if they knew what kinds of assays the FDA was looking for.
And, finally, just a suggestion and
this may be way off the wall. But in an
earlier meeting we had a very impressive group from the National Toxicology
Program who discussed a proposal for testing the safety of retroviruses. It may be reasonable to consider their
involvement in this problem given the importance of the field in trying to
develop some standardization of tumorigenicity
assays.
Thank you.
CHAIR URBA: Do you have a specific response?
DR. ALLEN: Yes.
Just one. I think just to be
clear, my interest in allogeneic models is, I mean apart from anything else,
we're looking at data now that have been done with essentially dosing
experiments, spiking experiments, dosing experiments where you increase the
number of these potentially less differentiated cells and say there's an
increased tumor risk. What I'm saying is
that some of that science needs to be done in an allogeneic model rather than
xenogeneic.
For example, it may be that if you
gave a pig a pig embryonic stem cell line that was actually very, very low or
highly manageable risk of tumor, and that it wasn't in fact that dose
dependent, unlike when you do a xeno model, so I think it's sometimes -- I'm
not suggesting the burden all falls to industry. What industry has to do goes in parallel with
what science has to do. But within that
framework I think there's an enormous amount of information.
I just find the concept of ignoring
a xeno environment strange. I mean I
think we have to get information on an allo environment as well and use that to
our best. And I think, say, just
confirming the fact this risk of teratoma and these other things really exists
in other species when it's not a xeno environment I think is really important.
CHAIR URBA: Dr. Taylor?
DR. TAYLOR: I think it's important that we remember,
although we're talking about embryonic stem cells here, there are other stem
cells that have already been down many of these pathways and that a number of
these questions have been answered for some of those in terms of whether or not
it's appropriate to do allo or xeno testing.
And so I think we need to learn from what's already been done and only
make different recommendations if we feel the risk benefit ratio is
significantly different.
And in the situation where
tumorigenicity comes up, I think that that's going to directly relate to the
product involved and how well the product involved has been characterized both
in vitro and in vivo. We know that
differentiation directly relates to tumorigenicity. That the more differentiated a cell type is
and the purer a cell type is with regard to differentiation, the less likely it
is to be tumorigenic.
So I certainly think that this can't
be dissociated from the cell product and the initial composition. That maybe, in fact, we set more rigorous
guidelines for the composition and understanding the product and the potential
benefit of that product, then define a few safety assays, tumorigenicity assays
that have to be employed, just like they do now for other cell types, and then
go forward from there.
DR. SNYDER: Yes. I
just had a quick followup on the animal model comment. That certainly when we're working out the
biology, at least in neurobiology, we always stay within the species to work
out some of these things. And the only
time we tend to move into using human cells is, as Kurt and Ken said, when
you're actually dealing with a product that might go into humans.
The one part about having good,
predictive animal models comes down to the realization that the road of
clinical trials is littered with failures that were based on jumping from an
unfaithful animal model to humans. Brain
tumor therapies come to mind as one glaring example of where the clinical
trials simply failed because they were based on poor models of brain tumors and
we're only now getting to derive better animal models.
So I think that even though we're
all -- certainly animal models for safety is one thing. Animal models for efficacy I think have to
better reflect what we think we're going to anticipate seeing in the humans. Otherwise, we'll get into clinical trials
that'll simply fail and give the stem cell field just a black eye, I fear.
DR. FRIEDLANDER: I think a lot of very important points have
been discussed quite eloquently by most my colleagues. I'd like to just emphasize and go over a few
that came up.
First of all, I couldn't agree more
that I think it needs to be indication specific. That depending upon the disease you're
looking at and the cell source you're looking at, what you're expecting those
cells to do, the bar is going to be very different.
So, for example, we might have a
much higher tolerance for some side effects in someone who is going to be dead
within two months or a year in spite of the treatment than something which is a
more chronic disease and might threaten some vision for example. I think the micro environments are very
different.
For example, if we take cells from
the progenitor cells we work, we put into the eye, they don't proliferate, they
do certain things. You put them into a
model of brain tumors, they proliferate as he says by Ki-67 staining. So, again, depending upon what you're looking
at, I think you can have different expectations and different results.
In terms of the animal models, I
agree. I think it's very important you
have something that's physiologically relevant, that's disease
appropriate. However, there are many
instances in which you can best perhaps expect proof of concept. So, for example, rodent eyes, for example,
are intrinsically resistant to ischemic damage.
Yet this is the vast majority of diseases that blind you do so as a
result of abnormal angiogenesis and reflective ischemic conditions. So I think you have to be careful about what
kind of a bar you set there in terms of how rigorous that's going to be.
I think in the consideration we
discussed a little bit about the size of the animal eye and we can get by by
doing rodents and is it really necessary to expand to larger eyes. So let me give you another example. When you inject tiny volumes into tiny
eyes, much of it comes back out
again. You get reflux. So if you're looking at dosing or you're
trying to get a sense about how effective the population is, if three-quarters
of the stuff refluxes out, you really need to think about another model. So, again, I think depending upon the disease
and the indication you're looking at, larger eye models might be more
appropriate.
I think depending where those cells
wind up, again, the micro environment, they will spread and proliferate and do
things very differently than if they're localized in another area. And, finally, again, I think the eyes is
where most of our experience is. If
you're proposing putting something
sub-retinally
into an eye that's diseased, that already has extensive degeneration as opposed
to something which is normal, you're going to see very different results also.
So, again, I think the
appropriateness of the model you're using become very, very important.
CHAIR URBA: Dr. Gerson?
DR. GERSON: I'd like to make an observation comment and
then a suggestion, specifically in the area of the issue of benign and
malignant teratomas.
First of all, in the presentations
this morning, I was struck with the interest in sort of understanding the
tumorigenicity potential of the ES cells and the differentiated cells. I did not come away from that with the
impression that there was a desire to instill tumorigenic ES cells in any
setting regardless of the disease characterization.
So I think we would -- I would
suggest that we sort of reinforce that.
I think that there are ways to inch into this field therapeutically
managing risk benefit with an emphasis at the start on safety and require, as
best we can, that we not encourage therapeutic trials with any intention of
tumorigenic cell infiltration. And there
are ways to manage that.
One is to make sure there is no ES
phenotype by BCR of gene expression. I
think we have some pretty clear data on that.
I would implore the field to develop a single, unified approach to a
tumorigenicity assay, should be reasonably straightforward. I don't really care what it is, where it is,
whether it's right or not because it never will be right. But if there was a standardized assay, so on
a relative basis one could assess the risk of tumorigenicity, I think that
would be wise.
And I think one can revise downward
that stringent approach as there's experience with differentiated ES cells over
time, but to start with a less stringent approach in my mind doesn't make any
sense.
CHAIR URBA: Dr. Goldman.
DR. GOLDMAN: This is just a followup to Dr. Gerson's point
really, and it really picks up from Dr. Chappell's before.
I think we need to distinguish
between toxicity accruing to inappropriate differentiation and then
disease-dependent toxicities, a la the interaction of those inappropriately
differentiated phenotypes in the disease setting versus toxicity accruing to
tumorigenicity. I think we can
reasonably look at inappropriate differentiation toxicity in the same way we
look at any treatment associated risk and simply assess the cost benefit by
virtue of that patient's condition or that group of patients' conditions.
But the issue of tumorigenesis from
ES I think is more fraught with really fundamental dangers. We don't work in a vacuum and having any
malignant cancers generated from inadvertently introduced, still
undifferentiated ES in patients would be to my mind disastrous for the field
and it would take very few patients suffering cancers from ES transplants to
potentially put a really significant damper on the field potentially for
years. And so I'm really looking at it
from that standpoint as much as the precise balancing of cost benefit in
suggesting that we have a higher threshold for potentially accepting toxicity
due to tumorigenesis from ES rather than the toxicities where we're
vis-a-vis
inappropriate differentiation, which essentially is just what Dr. Gerson just
said.
CHAIR URBA: Would you identify yourself?
DR. MCDONALD: Yes.
Dr. John McDonald, Johns Hopkins.
I really wanted to clarify two issues.
One is embryonic stem cells being similar, as other points have
mentioned and our approach needs to be similar as well and I think regulating
this. And the second one being a comment
on primate, on modeling in the use of primates.
So first of all, I mean obviously
for political reasons embryonic stem cells are different. But, biologically, every cell that we're
starting came from an early embryonic stem cells and progenitor, and the same
regulatory issues that would apply to any cell being transplanted, whether it
be derived from a late-stage progenitor or a late-stage progenitor derived from
ES cells, should also be similar.
There's not a great need to be treated dramatically different and that
would be a mistake, in my opinion.
But we do need to be aware of the
risks in the field, and Dr. Goldman's comment I believe strongly in, too. For example, it would be relatively easy to
get approval to do a single patient with a drug, or stem cells, if you're a
doctor a or surgeon at a hospital. And I think there we need to be very
stringent, because all it takes is one negative. Even whether that patient was going to die
from the disease in two months anyways, a negative there, something going awry
would cost the field a tremendous amount.
So I think we just need to be
careful, as Dr. Goldman mentioned in the cost benefit analysis, just we may --
keep the same stringency across, and the stringency and regulation of ES cells
in medical usage should really not be much different than the regulatory approaches
that we have with other cellular forms of delivery that have been commonly used
over the last 20 years.
My second comment is on
modeling. And as many of the speakers
have brought up, modeling is an issue, because, it's inherent in the a name;
it's just a model. But we've seen over
history also the use of primates, even to appropriate, you know, approximate
the size of what you're trying to treat, like your comment about the eye. Oftentimes, you know, it's really just a
band-aid, and it's not getting us the data we want, and you could figure out
from the beginning that it didn't give us the data we want.
So we don't want to impose barriers
to progress forward just simply because they've been put there before. But I think, traditionally, a lot of the
requirements to go to larger animals hasn't given us the data we want, and
predictably wouldn't give us the data that we want, and it sets the bar higher,
but it also can set a false barrier.
And the great example of this is the
Parkinson's disease transplantation trials, where adverse events were seen with
transplantation with dyskinesias and so forth that weren't observed in
primates. Well, it turns out that the
dosage that they gave in primates was so substantially lower, you know, because
they couldn't get enough cells to do the transplantation. Examples like that, you know, I think we can
look back and see many of them in our field.
Now, it made everyone feel better to
do those primate trials, but it didn't help at all in terms of predicting. In fact, it gave a false sense of security.
CHAIR URBA: Can you go to the microphone, please?
DR. ISACSON: So you're actually right in point, but wrong
in fact. So the primate model would not
give an L-dopa, so that didn't have any prior dyskinesia. That was the reason they didn't give the
appropriate answer.
CHAIR URBA: Did you know that?
DR. ISACSON: Cell dosage was not an issue. They were given a very large transplant, but
they didn't have any prior L-dopa exposure, which is what's the difference
between the real clinical situation.
DR. MCDONALD: So what was the difference in the dosage --
what was the dosage range used in primates versus the dosage range used in
humans?
DR. ISACSON: Similar.
DR. MCDONALD: What were they?
DR. ISACSON: They were three to four transplant per
case. But the primate may not --
DR. MCDONALD: How many cells, it's the cells, number of
cells? This is kind of getting at the
issue of dosage. Is dosage number of
transplant sites, is dosage number of cells, is dosage number of surviving
cells?
DR. ISACSON: Obviously, all of the above, but the
difference was really that -- and we have actually used the primate model with
L-dopa, which is now giving us answers.
So it's not actually an absolute version of what you said. With L-dopa you get dyskinesia in the primate
model, and then you can test -- it's the same side effect of transplants that
you see in the patient. So just a point of clarification.
DR. MCDONALD: Well, thanks for
clarifying that fact. My point had
nothing to do with the dyskinesia; that was just an example. But I think you bringing that up pointed out
the issues of just simply dosing across species and size is substantial --
DR. SNYDER: Well it actually brings
up more about the modeling. So what Ole
was mentioning then, the primate model from which the human trials was launched
did not accurately reflect the human population. The human population typically is started on
pharmacology first, then gets the transplants.
Now, the monkeys were also started
on pharmacology, then got the transplants.
Now they do show the dyskinesias, which would have predicted what we
would have been seeing in the human population.
DR. MCDONALD: All right.
Thanks.
CHAIR URBA: Specific response for that?
DR. CHIEN: Yes. I
just wanted to come back, because I just
wanted clarify one point, and that is is that we would not be asking, if we were
to ask for non-human primate data for human ES cell safety issues about, are
the cell preparations you're going in with, do they have any residual cells, or
any residual potential in a primate situation in normal, okay, to form a
teratoma? To me, that seems not
unreasonable. It's not that much
different from what you have to do if you're going to have a humanized
antibody, and go into the clinic.
Okay? I mean, what's more
dangerous: monoclonal antibody, or human ES cell-derived products? It's a no-brainer, okay? You should at least do what you're going to
do with monoclonal antibodies therapy.
I don't think it's asking -- now to
ask to create a whole humanized disease model in the eye, or whatever else it
is that you do, and then to get the ES cells from humans, and then to show that
they would have efficacy, that's another issue.
We're talking about a standardized tox assay. I think this is not asking too much.
CHAIR URBA: Dr. Woo?
DR. WOO: Yes. I
thought this morning's presentations were very informative, and this
afternoon's discussion is very, very stimulating.
My comments really has to do with
the product and the potential tox. From
what we hear this morning is that, in some of these trials that are going
forward, we're hearing, 95 to 99 percent purity sounds pretty good, and we can
go ahead and do the transplant, and so on, and hopefully the other one to five
percent of cells is going to be just bystanders, and even if they're
inappropriate cells, they may not cause any particular pathology.
And yet, on the other hand, I'm
hearing that the cells, these undifferentiated human ES cells, will have some
problems in chromosomal instability, and there has been observation of
aneuploidy when you do the chromosome spreads, and so on. Well, to me, aneuploidy, that's a hallmark
for tumorigenicity. And then we hear
there's intrachromosomal deletions, and this chromosome and that. Well, if you can see intrachromosomal
deletions in the finite number of cells that you look at in the metaphase
spread, and we are talking about transplanting millions and millions and
millions of cells out of which one to five percent, we don't know what they
are. That, to me, seems like a very --
it's a condition that is conducive to the development of teratomas and
teratocarcinomas in the recipients.
So I would really urge the field to
spend much more time trying to purify these cells before really going into
"translation of medicine" in too fast and too big of a leap. To minimize the formation, like Richard says,
you know, you can never minimize everything to zero, but the best we can do
within the technical capabilities is certainly something that we should try
before we go into transplants.
And then, of course, how do we know
that we have minimized the risk? Then I
think Ken Chien made a point that I totally support, which is, we need to
develop assays to know what we're dealing with, whether it's gene profiling, or
whether it's secreted products, or for cholesterol, whatever it is.
(Laughter.)
DR. WOO: And so, finally, it seems to me that every
sponsor can potentially be dealing with a human ES cell line that is
proprietary, and that means each company is dealing with a product that is
completely different from one another.
So for every one of those products, it is the responsibility of the
sponsor to demonstrate to the FDA that they have purified the products, and
they have developed assays to determine that this is the threshold of cells
that will not form tumors in the animal model, whatever animal model that is
appropriate, and then the maximal dose of the transplant is not going to exceed
the number of cells that will form tumors in the animal models. I think this kind of criteria will have to be
set before we just jump into human patients and say, let's do it, and hope for
the best.
Thank you.
DR. GERSON: I just have a quick concern. There was discussion earlier about animal
models in the context of toxicity and efficacy, and I think that's very
appropriate. I'm a little bit concerned
about the results of a negative animal model that's designed to look at the
tumorigenicity of these cells in a specific disease entity because you're
much more likely to have negative data
without any standardization. So I would
argue that there should be, if anything, a standardized model for
tumorigenicity so that, on a relative basis across the field, we can assess
what's going on.
CHAIR URBA: Dr. Chen, from the FDA, you've been patiently
waiting.
DR. CHEN: Okay.
I heard there are many good suggestions there, but there are still some
points that the panel haven't covered, which is the study duration. I mean, you have animal that you suggest is
rodent, or a large animal. Does the
study need to be carried out to the extent of the life span of the animal, and
how long are you supposed to follow them?
DR. MCDONALD: Okay.
So I'll address that issue. But
first, I want to make a comment.
You know, so we spend a lot of time
talking about tumors, as an example. But
just to stimulate some thought related to this question, let's talk about graft
versus host disease, and host versus graft disease, which is, in my mind, a
much more likely, huge negative outcome than tumorigenicity. And, you know, there are standards for
dealing with this.
If you want to look at a cost
benefit analysis, anybody who's gone to medical school, and has been on one of
those units where they do bone marrow ablation to get rid of a solid tumor, and
then replace with other things, just making it through that is an unbelievable
negative. And then to only have to
succumb with, actually, a fair percentage of people to graft versus host, or
host versus graft disease, which can come in many forms, shapes and sizes in
terms of severity and onset both in terms of time, you know, being subacute,
subchronic, or very long term, and yes, so I think some discussion on how long
should begin with looking at how long do we already do.
What is the standard in the FDA now
for cell-based therapies? Let's take a
look at bone marrow replacement. How did
they have to demonstrate in an animal model that things didn't go a
negative? And then we can