BIOLOGICALS RESPONSE MODIFIERS ADVISORY COMMITTEE
MEETING #33, OCTOBER 10, 2002
Retroviral Gene Therapies for the Treatment of
Patients with
Severe Combined Immunodeficiency – Safety Issues
CBER
is convening this meeting in response to the recent notification of an adverse
event in a clinical trial in France that uses retroviral vector-mediated gene
therapy in children with X-linked Severe Combined Immunodeficiency. In particular, we are seeking the advice of
the committee on how to proceed with similar clinical trials in the US
(Question for the Committee is found on the last page of this document).
Severe Combined Immunodeficiency (SCID) is a group of
inherited disorders that all share a defect in T cell differentiation giving
rise to deficiencies in immune cell function (5). Current
therapeutic options include bone marrow transplantation. In those cases where a HLA-identical donor
(meaning that the donor marrow is a perfect match for the recipient) is used,
survival is 100%, as reported in a long-term study of infants with SCID (3). Survival
is reduced to 78% in those children who receive HLA-haploidentical donor marrow
(the donor marrow is 50% identical to the recipient) (3). Although
bone marrow transplantation seems to result in normal T cell function, most
children who receive the HLA-haploidentical marrow still have abnormal B cell
function, resulting in the need to treat with intravenous immune globulin in
over 60% of the cases (3). In
contrast, a study in neonates comparing data on bone marrow transplantation
performed in 21 SCID infants who received the transplants in the neonatal
period found that the survival rate was 95%, even in those cases where the
transplant was from a haploidentical donor (9).
The genetic lesions underlying many of the clinical forms
of SCID have been elucidated (5). One type
of SCID caused by a genetic defect in the gene encoding adenosine deaminase
(ADA) can be successfully treated in 90% of the patients by weekly
administration of PEG-ADA (ADA coupled to polyethylene glycol) (5). Defects in
the gene encoding the common gamma chain (gc) have also been shown to cause X-linked SCID. Other genetic defects resulting in SCID
include mutations in the gene encoding Jak-3, interleukin-7 receptor alpha
chain, Rag-1 and Rag-2, or CD45 (reviewed in (5)). The
inheritance pattern is either X-linked or autosomal recessive for all these
known genetic mutations. The facts that
SCID is caused by a genetic defect and that the genetic defect underlying the
disease is known, in most cases, make SCID an attractive target for gene therapy
approaches, whereby one could potentially correct the genetic defect by
providing a normal copy of the gene.
Initial
clinical trials using a gene transfer approach were performed in children with
SCID-ADA by treating their T cells with a retroviral vector encoding the ADA
protein. While T cells carrying the
retroviral vector sequences have been detected long-term, the levels have been
very low, and the continued use of PEG-ADA rendered the studies difficult to
interpret with regard to clinical benefit of the gene transfer (2) (1) . Several
subsequent studies have been performed in children with SCID-ADA using
retroviral vectors to deliver the ADA gene to hematopoietic stem cells (reviewed
in (6)). Again,
patients were maintained on PEG-ADA and the levels of T cells carrying the
retroviral vector sequences were maintained for years after treatment, but
always at low levels. The success of
the gene transfer itself was again difficult to assess because of the
concomitant administration of PEG-ADA.
More recently, gene therapy clinical studies have been
initiated in children with X-SCID, and for the first time, retroviral vectors
have been used to treat hematopoietic stem cells has resulted in not only
laboratory evidence for gene transfer, but also laboratory and clinical
evidence of immune function suggesting there may be clinical benefit (4) (7). Evidence
of successful engraftment was reported in 4/5 infants treated with CD34+
hematopoietic stem cells that were exposed to a retroviral vector encoding gc. In
addition, longer-term follow-up data on these four patients, varying from 1.6
to 2.5 years at the time of the report, indicated almost normal numbers of T
cells and natural killer (NK) cells as well as normal responses to antigen
proliferation in vitro or after immunization.
In addition, unlike those patients who receive haploidentical bone
marrow transplants, the levels of antibody production were sufficient to
obviate the need for intravenous immunoglobulin adminstration. Importantly, the children who were treated
in this study were showing evidence of normal growth and ability to lead normal
lifestyles (7).
Retrovirus vectors most commonly used in clinical trials of
gene therapy are based on a murine gammaretrovirus. The vector sequences are deleted compared to the wildtype virus
so that cells exposed to retrovirus vectors express only the therapeutic gene
product, but do not make new viral particles.
This is a critical safety feature of all retroviral vectors used in
clinical trials of gene transfer.
However, because the parental murine gammaretrovirus can, under some
circumstances, cause tumors in mice via insertion of retroviral DNA into the
host cell genome, retroviral vectors have always been perceived to carry the
potential risk of tumorigenesis. While
most integration events of the vector DNA are not expected to cause harm to the
cell or to the patient, there is an unknown (but thought to be low) risk that
in some cases the integration event may result in activation of neighboring
genes which could result in uncontrolled cell division or a tumor (an event
called “insertional mutagenesis”). Since
tumorigenesis is thought to be a multi-step phenomenon, it would be likely that
an additional event would be required before a vector insertion at a given
locus would necessarily result in tumor formation. In all cases, the potential risk of tumorigenesis from a
retroviral vector has been included in informed consent documents used in
retroviral vector-based clinical trials in the US.
Recently, these assumed risks were demonstrated to be real
when scientists reported that acute myeloid leukemia developed in mice
receiving hematopoietic stem cells transduced with a retroviral vector (8). In all
cases the leukemic cells had the same site of insertion of the retroviral
vector, causing inappropriate expression of the gene at the insertion site
(Evi1). However, it was postulated that
in addition to the dysregulated expression of Evi1 that additional factors,
such as the transgene used in the retroviral vector and the target cell
population, likely contributed to the occurrence of leukemia (8).
The long-recognized risks of tumorigenesis from retroviral
vectors were initially addressed by FDA/CBER initially nearly 10 years ago when
a letter was issued to all sponsors of gene therapy clinical trials using
retroviral vectors requesting life-long follow-up of all subjects who
participated in these clinical trials.
The policy was later published (10/18/2000) in a guidance document: Guidance for Industry: Supplemental Guidance on Testing for
Replication Competent Retrovirus in Retroviral Vector Based Gene Therapy
Products and During Follow-up of Patients in Clinical Trials Using Retroviral
Vectors (available at http://www.fda.gov/cber/genetherapy/gtpubs.htm). The guidance document recommends that all
subjects should be followed life-long on an annual basis. In addition, the topic of long-term
follow-up was also discussed at several previous meetings of the FDA
Biologicals Response Modifiers Advisory Committee (November, 2000; April, 2001;
and October, 2001 – transcripts are available at http://www.fda.gov/cber/advisory/brm/brmmain.htm).
Adverse Event in Retroviral Vector Gene Therapy Clinical
Trial in X-SCID in France
One child in the gene therapy clinical trial in X-SCID
children in France (4) (7) has had a serious adverse event related to the
retroviral vector gene therapy.
Although the clinical trial is not under US IND, the clinical
investigator has been very cooperative and has shared many of the data with
CBER. The child was treated three years
ago and had positive clinical and laboratory evidence of immune function. He had a mild lymphocytosis in April, 2002,
preceding a varicella zoster virus (VZV) infection (chicken pox). He was able to clear his infection, but
maintained a somewhat elevated, but stable, T cell count, until August, 2002,
when the T cells began to increase an additional 10-fold and the child
presented with hepatosplenomegaly. At
that point he was treated with steroids and vincristine, to reduce his T cell
counts, and subsequently also received Daunorubicine. His T cell counts have been reduced to 500, and the patient is in
good condition.
The expanded T cells are gamma delta T cells, and are
monoclonal with respect to both the form of the T cell receptor expressed and
the site of retroviral vector insertion into the genome. Using a PCR-based method, the investigators
have shown that the retroviral vector has inserted into the first intron of the
LMO-2 gene on chromosome 11. There is
over-expression of LMO-2 in these cells, suggesting that the vector insertion
may have caused dysregulation of the LMO-2 gene expression. LMO-2 (the second member of the LIM-only
family of genes) is normally expressed during early stages of hematopoietic
differentiation and its expression appears to be critical for development of
lymphoid and myeloid cell lineages (reviewed in (10)). In
addition, the chromosomal translocation t(11;14)(p13;q11) in T-ALL (acute
lymphocytic leukemia) results in joining of the T cell receptor D or J segments
to the LMO-2 locus. This translocation
is thought to be the result of aberrant RAG-mediated V(D)J recombination,
highlighting the multi-step nature of the leukemogenic process (10).
It is important to consider that there are likely several
factors that may have played a role in the T cell expansion in this
patient. The retroviral vector
insertion and activation of LM0-2 may have been a necessary step in these
events, but the insertion alone may not have been sufficient. Additional factors that should be considered
are the role of the VZV infection in stimulating T cell proliferation and
possible genetic predisposition, since there are two childhood cancers in the
family, including a cancer in the patient’s sister.
Upon notification of the adverse event in the gene therapy
clinical trial in France , FDA/CBER reviewed the currently active gene therapy
clinical protocols under IND in the US.
We identified three clinical trials that were most similar to the one
ongoing in France in terms of the clinical indication, target cell, retroviral
vector, and route of administration.
While the serious adverse event in France was being evaluated, we placed
each of the INDs in SCID subjects using retroviral vector-mediated ex vivo
transduction of CD34+ hematopoietic stem cells on clinical hold, pending
further analyses of this event. In
addition, we notified sponsors of similar clinical trials that are in active or
inactive status (i.e., no longer actively treating patients) of this event and
requested that they contact their patients’ families to discuss the event and
its implications. We now seek the
advice of the committee and its experts to determine what future regulatory
actions should be taken.