**Food and Drug Administration**

**Center for Biologics Evaluation and Research**

**DRAFT Report of the Biological Response Modifiers Advisory**

####### Subcommittee on Xenotransplantation

**Meeting of June 3 and 4, 1999**

Holiday Inn, Bethesda, MD

## 

### Subcommittee Participants Consultants

#### Hugh Auchincloss, Jr., M.D., Chair\* Antonio Benedi\*^++^

#### Jonathan S. Allan, D.V.M.\* William G. Lawrence, J.D.\*^++^ 

#### John M. Coffin, Ph.D. \* E. Steve Woodle, M.D.\* 

#### Martin S. Hirsch, M.D.\* 

#### Richard Kaslow, M.D., M.P.H.\* [Guest Participants]{.underline}

#### Nicholas W. Lerche, D.V.M.\* 

#### Abbey S. Meyers\*^+^ John Conte, M.D. 

#### Claudia A. Mickelson, Ph.D.\* Marian Michaels, M.D., M.P.H

#### David Onions, Ph.D. Robert E. Michler, M.D. 

#### Prem S. Paul, D.V.M., Ph.D.\* Ralf R. Toenjes, Ph.D. 

#### David Sachs, M.D.\* 

#### Daniel Salomon, M.D.\* 

#### Harold Y. Vanderpool, Ph.D., Th.M.\* [Guest Speakers]{.underline}

#### LeRoy Walters, Ph.D.\* 

####  David Cooper, M.D., Ph.D. 

#### [NIH Participant]{.underline} Rima F. Khabbaz, M.D.

####  Taylor Wang, Ph.D. 

#### Mary Groesch, Ph.D. 

#### 

### CDC Participants FDA Participants

#### 

#### Walid Heneine, Ph.D. Eda Bloom, Ph.D. 

#### Louise E. Chapman, M.D. Louis Marzella, M.D., Ph.D. 

####  Philip Noguchi, M.D. 

####  Jay P. Siegel, M.D. 

#### \*Temporary Voting Members Karen Weiss, M.D. 

#### ^+^ Consumer Representative Carolyn Wilson, Ph.D. 

#### ^++^Patient Representative 

**DRAFT**

This summary report for the June 3-4, 1999 meeting of the
Xenotransplantation Subcommittee of the Biological Response Modifiers
Advisory Committee was approved on
[\_\_\_\_\_\_\_\_\_\_\_\_.]{.underline}

I certify that I attended the June 3-4, 1999 meeting of the
Xenotransplantation Subcommittee and that this report accurately
reflects what transpired.

[\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_]{.underline}
[\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_]{.underline}

Gail Dapolito Hugh Auchincloss, Jr., M.D.

Executive Secretary Chairman

###### Introduction

The meeting of the Xenotransplantation Subcommittee was called to order
at 8:30 a.m. on June 3, 1999. With Hugh Auchincloss, Jr., M.D. presiding
as Chairman, the subcommittee discussed three main topics: (1) recent
data from pre-clinical and clinical studies searching for evidence of
PERV infection of transplant recipients, (2) issues regarding the
development of FDA regulatory policy for xenotransplantation, and (3)
the nature and timing of future clinical trials of solid organ
xenotransplantation. The meeting was attended by approximately 125
people.

A conflict of interest statement was read into the public record which
stated that members with the appearance of a conflict of interest based
on their work with products which could be affected in the future were
given waivers to participate. Copies of the waivers are available from
the FDA Freedom of Information Office.

A statement was read to the subcommittee by Dr. Michael Schmoeckel,
M.D., Munich Xenotransplantation Research Group, Munich, Germany during
the open public hearing in the afternoon of June 3.

The scientific presentations of June 3-4 are referenced in the attached
agenda and roster.

**CHAIRMAN'S SUMMARY REPORT - DRAFT**

**Topic I: Data From Pre-Clinical and Clinical Studies Seeking Evidence
of PERV Infection of Transplant Recipients.**

Representatives from the FDA, the CDC, and several private companies
described their studies of both non-human primate and human recipients
of transplants from pigs. These studies are on-going and not all of the
data, derived from several hundred animal and human recipients, are
currently available. However, the information that was presented was
judged by subcommittee members to be the product of well-performed,
high-quality assays. Together, the data at this time support the
conclusion that there is no significant evidence that PERV infection of
a transplant recipient has occurred as a result of xenotransplantation.

Several important caveats were mentioned regarding this conclusion: (1)
the negative data from some of the studies in non-human primates are
somewhat less reassuring because it has never been demonstrated that
baboon cells can be infected with porcine endogenous retroviruses, (2)
the patients who have been studied have not received transplants under
conditions that represent the highest risk of PERV transmission (i.e., a
combination of (a) removal of anti-gal antibodies from the recipient or
of expression of the gal determinant by the donor, (b) inactivation of
the recipient's complement system, (c) long-term survival of donor
tissues, and (d) on-going immunosuppression), (3) the assays that have
been used, while of high-quality, continue to be improved and new assays
are being developed, and (4) the data that have been gathered have not
yet been fully reported.

The essence of the subcommittee's further discussion of this topic
involved the effort to validate a large body of negative data when there
is no true positive control. Subcommittee members described a number of
steps to enhance our confidence in the data:

1)  **Species selection for further testing:** Non-human primate studies
    of potential PERV transmission are most reassuring when cells of the
    recipient species have been shown capable of PERV infection. For
    this reason, rhesus monkey studies are better than baboon studies at
    this point, when they are performed for the sole purpose of
    examining the risks of viral infection. On the other hand, on-going
    analysis of baboon and other non-human primate recipients should
    continue to be performed when these species are used for
    xenotransplantation studies designed for other purposes, especially
    as future data may demonstrate the ability of baboon cells to be
    infected.

2)  **Microchimerism:** The persistent survival of donor cells in
    transplant recipients -- "microchimerism" - makes it difficult to
    determine with certainty that detection of PERV in transplant
    recipients represents viral infection of the recipient rather than
    the presence of viral DNA sequences in the donor cells. Currently,
    investigators quantitatively assess by PCR the amount of PERV DNA
    sequences relative to repetitive porcine DNA sequences. The relative
    amounts determine whether the PERV signal indicates microchimerism
    or true infection. This is a good approach, but it leaves an
    inherent inability to detect low levels of PERV infection when
    microchimerism is present. It would be useful to determine the
    actual sensitivity for true PERV infection using this technique. It
    might also be possible to enhance the sensitivity of this technique
    by cell sorting techniques to remove contaminating donor cells prior
    to viral analysis. However, some members of the subcommittee
    suggested that viral infection that remains at such a low level as
    to avoid detection in the setting of microchimerism might also
    represent a level that is not a major public health concern.

3)  **Tissue Samples:** While human PBMC have been infected with PERV in
    vitro under certain circumstances, they have not always been found
    to be susceptible in other assays. Nonetheless, the subcommittee
    continued to favor sampling of PBMC from patients as the primary
    cell source, until such time as there is a sound rationale for
    seeking other cell types for analysis. This recommendation reflects
    partly the ease of obtaining these cells, but also their importance
    in potential disease transmission should PERV infection actually
    occur. Other tissues should be examined as they become available in
    a recipient's natural clinical course.

4)  **Plasma Samples:** In addition to PBMC, the subcommittee strongly
    encouraged the analysis of plasma for the presence of virus using
    RT-PCR techniques. While there are important caveats to the
    interpretation of any potential positive findings using this
    approach, careful documentation of PERV expression in plasma of
    xenograft recipients would be of considerable importance.

5)  **Development of an Antibody Assay:** In keeping with its previous
    recommendations, the subcommittee urged that assays to detect
    antibody responses in transplant recipients as an indicator of PERV
    infection should be developed further. In contrast to the situation
    when the subcommittee last met, several assays of this type now
    exist and have been used in some of the studies of transplant
    recipients. However, these assays have not yet been developed to a
    level that is comparable to standard antibody assays for many other
    viral infections. Nonetheless, a significant shift in the
    subcommittee's recommendations to the FDA is that all sponsors of
    xenotransplantation trials should be required to be actively
    involved in the analysis of their patients' sera for evidence of
    antibody production. The subcommittee urged the FDA to use its
    influence to promote the sharing of reagents and samples among
    companies to promote the most effective development of better assays
    of this type. The subcommittee continued to recommend that sponsors
    bank serum samples for further analysis using improved assays that
    may become available in the future.

6)  **Quality Control**: The subcommittee expressed the view on several
    occasions that the quality of the work being performed by the
    different private companies and public agencies would appear to
    represent state-of-the-art application of modern technology.
    Nonetheless, to the extent possible, it would be desirable to have
    different laboratories exchanging reagents and samples in order to
    achieve the highest possible level of quality control for their
    assays.

7)  **A "Gold Standard":** The goal in this evolving analysis of the
    risks of PERV transmission is to develop the best possible assays in
    three areas: (1) assays to detect PERV transmission to recipient
    cells using DNA PCR, (2) assays to detect evidence of viral
    infection by recipient antibody responses, and (3) assays to detect
    viral expression in plasma by enhanced RT-PCR techniques. The FDA
    should be seeking the achievement of all three types of assays by
    all company sponsors, although there is no clear standard yet
    available in any of these categories. Therefore, further basic
    science research is important in this field to define, for example,
    the cell types and species most likely to be infected by porcine
    endogenous retroviruses. In addition, the particular standards set
    by the FDA will continue to evolve over time.

8)  **What If A "Positive" Occurs:** While it is far from clear that a
    positive result in any patient using these types of assays would
    necessarily represent a public health danger, the subcommittee
    suggested that the presence of clear evidence indicating that
    infection of human patients with a porcine endogenous retrovirus had
    occurred would almost certainly require a temporary suspension of
    further clinical activity as more complete data were obtained and
    reviewed by an appropriate group of experts. The more difficult
    problem will be to determine the significance of marginal test
    results. Undoubtedly, there will be instances where probable false
    positives or weak signals will warrant further investigation without
    the need to halt all other clinical activity. One suggestion was
    that a Data and Safety Monitoring Board, or its equivalent, might
    help to evaluate the significance of such suggestive but
    indeterminate findings.

**Topic II: The Concept of Relative Risk**

The FDA presented its definition of xenotransplantation: "the use of
live cells, tissues, or organs from a non-human animal source
transplanted or implanted into a human, or used for ex vivo contact with
human body fluids, cells, tissues, or organs that are subsequently given
to a human recipient". The subcommittee accepted this inclusive
definition as appropriate for the description of what the FDA should
regulate. It was recognized, however, that it might be possible for
humans to be exposed to some of the infectious risks associated with
xenotransplantation in ways that fall outside of this definition. For
example, Porcine Factor 8, used for the treatment of hemophilia, could
expose, and may conceivably already have, exposed human patients to
PERV. These products are currently regulated appropriately through other
branches of the FDA and would not fall under the proposed definition of
xenotransplantation.

Even this definition, however, generates a potentially enormous range of
products that might require FDA regulation. Therefore, the FDA asked the
subcommittee if they could identify particular types of
xenotransplantation that might fall under the definition they had
generated but still not necessarily require the same degree of intense
scrutiny. Some of the characteristics they suggested for consideration
included: (1) species of source animal, (2) cell lines vs. fresh tissue,
(3) dose of implant, (4) encapsulation devices, (5) duration of
exposure, (6) degree of immunosuppression, and (7) recipient behavioral
factors.

The subcommittee considered this concept of "relative risk" carefully,
but in general the members did not find that there were many instances
in which it would be useful at this time. While it is almost certainly
true that some forms of xenotransplantation are more risky than others,
the degree of difference was felt to be insufficient in most cases to
make a difference in a setting where the estimated public health risk of
xenotransplantation is already judged to be extremely small. There were
some exceptions to this conclusion. For example, the use of cell lines,
as opposed to whole organs from living animals, would not require the
extensive documentation of parental breeding sources that has been
required by the FDA thus far. In addition, the co-culture of human cells
with cell lines derived from invertebrate animals was felt by
subcommittee members to represent an exceptionally low risk for
transmitting certain types of infections to human recipients. On the
other hand, some of the factors identified by the FDA were felt to
increase the potential risks of xenotransplantation. For example,
recipients with behavioral patterns that make them unlikely to comply
with careful monitoring in the future would be more risky to the public
health. Members of the subcommittee pointed out, however, that the
possibility of excluding participants from trials of xenotransplantation
for behavioral reasons poses an ethical issue that merits more extensive
discussion and clarification. Finally, the subcommittee noted that short
term contact with "bridge" xenotransplants might actually increase the
public health risk because it would be less obvious that xenogeneic
exposure had occurred.

This discussion of "relative risk" took place at a time when the
subcommittee was only just beginning to grapple with the implications of
the broad definition of xenotransplantation proposed by the FDA.
Therefore, this topic may well be reviewed by the subcommittee again in
the future as product regulation by the FDA presents new examples of
exposure to living animal tissues.

**Topic III: Preclinical Studies and the Initiation of Clinical Trials
of Solid Organ Xenotransplantation.**

The third topic presented to the subcommittee was the potential
initiation of clinical trials of solid organ xenotransplantation.
Several key elements were stressed in this discussion: (1) the
subcommittee was not asked to vote approval or disapproval of any
specific clinical protocol, (2) the trials being considered involved
solid organ transplants and not cellular transplants (examples of which
have already been considered by the FDA and are in active clinical
investigation), (3) given the FDA's announcement that it would not
currently consider clinical trials of xenotransplantation from non-human
primate donors, the topic for discussion focused exclusively on
potential pig-to-human clinical trials, and (4) the primary purpose of
this discussion was to provide both potential sponsors and the FDA with
a sense of the types of questions subcommittee members would be asking
and the sorts of answers they would find satisfactory during future
considerations of specific clinical trials that might be proposed.
Against this background, the subcommittee considered two broad issues:
(1) what types of results in non-human primate trials would the
subcommittee expect to see before the initiation of different types of
clinical trials and to what degree would the limitations of working with
non-human primates allow the initiation of clinical trials when survival
in the pre-clinical trials was less than would be satisfactory in
clinical practice, and (2) which sorts of clinical trials would the
subcommittee find most satisfactory for the initiation of solid organ
xenotransplantation.

The subcommittee heard presentations from several independent
investigators in the field of xenotransplantation, from two potential
sponsors of clinical trials (Nextran and Imutran), and from the FDA.
There were some differences in the message conveyed, but substantial
agreement was expressed on a number of key points: (1) that
extraordinary progress has been made in recent years in overcoming
hyperacute rejection, especially by the generation of transgenic pigs
expressing human complement regulatory proteins, (2) that subsequent
rejection of pig organs by primates is mediated by a process known as
acute vascular rejection, involving a humoral component and probably
other elements of the innate immune system and the complement cascade,
and (3) that strategies to overcome this mechanism of rejection have not
generally been successful. The actual survival of pig-to-primate
xenografts has varied in different experiments, but in general, the
available data suggest that 50% survival of functioning, life-sustaining
pig organs in non-human primates would be less than one month using
currently available techniques.

One of the questions the subcommittee considered was how to assess
survival data in non-human primates and whether alternative in vivo
experiments might provide better pre-clinical information. There was
agreement that the management of non-human primates in complex protocols
of organ transplantation presents extraordinary difficulties that exceed
those in clinical practice. In addition, some of the immunologic
reagents available for clinical trials may not react with the molecules
of non-human primates making it impossible to test some potential
clinical protocols in a non-clinical setting. Finally, there is no
guarantee that results in non-human primates, even under the best of
circumstances, will necessarily predict human outcomes. Despite these
clear limitations, the subcommittee indicated strong agreement that
pig-to-non-human-primate studies of functioning orthotopic xenografts
are important and probably the best source of pre-clinical data that can
be obtained at this time. In addition, the subcommittee noted that
allogeneic transplants have survived for many years in non-human
primates in several different laboratories, indicating that it is not
inherently impossible to manage these animals over prolonged periods of
time. Finally, it is important to recognize that the failure of the
pig-to-non-human primate xenografts in recent experiments appears to be
occurring because of a biologic process that is incompletely understood
at this time and for which there is no clear therapeutic solution. Thus,
the short survival of xenografts in non-human primates does not seem to
represent a limitation of the model, but rather a limitation of our
current therapeutic strategies for xenogeneic transplantation.

For these reasons, the subcommittee generally suggested that solid organ
xenotransplantation should not enter clinical trials until there was a
"reasonable expectation for success" and that survival in non-human
primates approaching that which would be felt necessary to justify
clinical trials would be required as an adequate justification for
initiating these trials. One possible standard that was suggested to the
subcommittee was that before initiating trials of clinical cardiac
bridge transplantation non-human primates experiments should show 90%
survival of orthotopic heart transplants at 60 days, and 50% survival at
90 days. For trials of definitive xenotransplants of either hearts or of
kidneys, the requisite survival in non-human primates might perhaps be
much longer, up to a year in some cases. There was no formal effort to
define exact standards of this sort by the subcommittee as a whole, and
many individuals expressed a range of alternative views (sometimes
suggesting that much less stringent standards might be appropriate for
individuals facing imminent death with no effective alternative
therapy). There was agreement, however, that the effect of
xenotransplantation on the survival of subsequent allogeneic transplants
should be assessed prior to the initiation of clinical trials of bridge
xenotransplants. In addition, the subcommittee appeared to accept the
view expressed by the FDA that pre-clinical studies should also be used
to optimize whatever clinical trial was being proposed as much as
possible.

The subcommittee also considered the types of clinical trials that would
be most reasonable to initiate solid organ xenotransplantation. Four
potential trials were considered: (1) cardiac bridge transplantation for
patients who face imminent death, who are not candidates for ventricular
assist devices, but who are candidates for an allotransplant. The number
of such candidates would not be large and would often include children
or very small adults too small for assist devices; (2) definitive
cardiac transplantation for a small number of patients who are not
candidates for allotransplantation; (3) definitive kidney transplants
for a small subset of dialysis patients whose high level of
sensitization essentially precludes them from allotransplantation and
for whom dialysis had become either unacceptable or effectively
impossible. This would again represent a small, but identifiable group
of patients; or (4) the use of heterotopic cardiac xenotransplants.
However, the number of heterotopic procedures being performed with
allogeneic organs was considered to be too small to warrant initiating
trials of xenotransplantation in this manner. The subcommittee
acknowledged the serious need for more organs in several other areas
such as lung transplantation. However, some subcommittee members noted
that at this time lung transplantation raises additional questions about
the control of infectious disease risks, and liver transplantation
involves a host of complex concerns over physiologic function.
Therefore, the subcommittee did not feel that it would be appropriate at
this time to consider the use of xenogeneic organs, other than hearts or
kidneys.

The concept of bridge transplants is attractive because it would not
require the achievement of the same long-term survival that is currently
obtained with allotransplantation and because it could potentially
improve the chances for survival for individual patients. However, the
use of bridge transplants would not improve the overall survival for
society resulting from organ transplantation (because there would
ultimately remain the same number of allogeneic organs for the pool of
recipients) and indeed bridge transplantation might be expected to
decrease the overall benefit of allotransplantation by allowing an even
sicker group of patients to receive the scarce human organs. For these
reasons, the subcommittee did not express enthusiasm for bridge
transplantation as a way to initiate solid organ xenotransplantation. If
this approach were to be used, the subcommittee thought it would be
reasonable to expect that the technique had first shown promise for
increasing the well-being of potential recipients such that they would
become more, not less likely to survive and benefit from subsequent
allotransplantation. Specifically, in the case of heart transplant
candidates, it was suggested that at least two months of healthy
survival would be required to allow recovery and correction of end-organ
damage plus an additional period of survival to allow a reasonable
expectation of finding a suitable human organ. On the other hand, the
subcommittee suggested that initial clinical trials of bridge
transplantation should necessarily proceed on to allotransplantation if
possible, even if the xenotransplant were functioning extremely well at
the time of this option.

The use of a xenotransplant as definitive therapy for cardiac or renal
disease would present the opposite balance of factors: trials of this
type would require much better evidence that long-term survival can be
achieved, but the benefits of success would be much clearer. Given the
FDA's important role in the initiation of clinical trials, subcommittee
members urged the agency to give due consideration to the potential
benefits of xenotransplantation as well as to the potential risks.
However, by a very substantial majority, the subcommittee indicated that
the pre-clinical data available at this time do not warrant the
initiation of clinical trials of definitive solid organ
xenotransplantation.

An additional feature regarding the initiation of clinical experiments
is that decisions to proceed with particular trials of solid organ
xenotransplantation might seem reasonable when considered on their own,
but might have severely negative effects on the progress of the field in
general and on the public's perception and response if they generate
dramatically negative results. The sentiment was expressed that the FDA
should keep this consideration in mind as they review potential trials
of solid organ transplantation in the future.

**DRAFT**

RESPONSE TO THE CHAIRMAN'S SUMMARY REPORT

FDA Xenotransplantation Subcommittee, Meeting #2, June 3-4, 1999

The report provides an excellent summary of two days of discussion. I
would like to highlight one procedural suggestion from the Chairman's
report and add a procedural suggestion on a topic that was not
explicitly discussed at the meeting.

Under Topic I, Point 8, the Chairman's Summary Report introduces the
notion of appointing a body similar to a Data and Safety Monitoring
Board (DSMB) to assist the FDA in determining whether a bona fide
infection occurs in a patient as a result of a xenotransplant. This
procedural approach would allow a single body of outside experts to have
access to the data from multiple clinical trials on a confidential basis
and to provide their advice to FDA much more rapidly than is possible
with a regular committee or subcommittee meeting. The DSMB of the AIDS
Clinical Trials Group, NIAID, NIH, could provide a model for such a
multi-trial review body. This DSMB may also provide guidance on how one
can fulfill (or otherwise deal with) the requirements of the Federal
Advisory Committee Act in establishing such a group.

Under Topic III, the Subcommittee discussed the future initiation of
clinical trials with solid organs from non-human organ sources,
especially the pig. I would suggest in this case that the FDA consider
having the Subcommittee function as a public review body for the early
solid-organ transplants - and perhaps even for the already-initiated
trials that involve introducing non-human cells into the brains of
patients or using sequestered non-human cells as filters for patients
with diminished organ function. The first transplants of solid organs
into patients, whether on a bridge or a longer-term basis, will be major
events, and the public and policymakers should be adequately prepared
for these events by public advisory committee meetings that thoroughly
examine both the biomedical context and the ethical, legal, and social
implications of these events.

As a corollary to this second suggestion, FDA could and perhaps should
encourage sponsors who are moving toward clinical trials with solid
non-human organs to submit "preclinical protocols" outlining how they
plan to carry out such clinical studies. This approach has served the
NIH Recombinant DNA Advisory Committee well, both in 1987 as the first
gene-therapy studies were being planned and in 1998-1999 when the issues
surrounding in utero gene transfer have been considered. Detailed
discussion of future clinical protocols may have the beneficial result
of expediting the progress of xenotransplantation because important
questions are anticipated and resolved well before the clinical trials
begin.

LeRoy Walters

June 29, 1999