Regulatory officials and researchers are saying that insertion of a retrovirus-based vector near a particular host gene appears responsible for the development of leukemia in two children participating in a gene therapy clinical trial in France. These findings complicate this gene therapy clinical trial, which is widely regarded as perhaps the most promising in terms of achieving efficacy among many dozens of others in the United States and Europe. Indeed, few of these trials have yielded signs of health benefits and, in one case, the gene transfer procedure is blamed for the death of a young participant in 1999 at the University of Pennsylvania, prompting intensive reviews of this field and widespread efforts to improve gene delivery systems (ASM News, February 2000, p. 67; December 2001, p. 625).

The French clinical trial, led by Alain Fischer and Marina Cavazzana-Calvo of Hopital Necker-Enfants Malades in Paris, involves almost a dozen very young patients with X-linked, severe combined immunodeficiency syndrome (X-SCID), which often is fatal because it leaves such children highly susceptible to infections.

The gene therapy strategy in this trial depends on introducing a gene encoding the [gg]c subunit of a transmembrane interleukin receptor protein, which is missing or mutated in children with X-SCID and is required for normal development of key lymphocyte cells needed for a functional immune system. Subsequent clinical procedures included extracting hematopoietic stem cells from each of the patients, treating those cells with a retrovirus vector containing the ãc subunit-encoding gene, selecting cells that were transduced by (incorporated into) that gene, and then reinfusing them into the individual patients.

This procedure proved effective for 9 of 10 infants who were treated since 1997, enabling them to develop functional immune systems. However, last October, about 30 months after the genetically treated cells had been infused into him, one of those children developed leukemia. By the end of last year, the French scientists disclosed that another of the successfully treated children, also at about 30 months after receiving such an infusion, also developed leukemia.

In both cases, according to subsequent investigations, the retrovirus containing the ãc subunit-encoding gene had inserted into or near the LMO-2 locus within each of the two children's lymphocytes. LMO-2 is considered a proto-oncogene, and disruption of its usual control leading to its activation seems, in part at least, to explain the proliferative expansion of lymphocytes and leukemia that occurred in these two cases. Both children with leukemia appear to be responding well to treatments to control their cancers, and the immune system of the one who first received such treatments remains intact (it is too early to determine how the other child's immune system will respond), according to Fischer. Meanwhile, other children in the trial are being monitored for signs of leukemia.

These two leukemia cases led French investigators to stop taking new patients into this clinical trial.

While the first case of leukemia in France led RAC members and FDA officials to recommend placing many gene transfer experiments on clinical hold, the second case led FDA officials to ``enlarge the scope of the retroviral vector hold without regard to disease'' but restricting that hold order to protocols involving retroviral vectors targeting hematopoietic stem cells, according to Philip Noguchi, who directs the Office of Cell, Tissue and Gene Therapies in the FDA Center for Biologics Evaluation and Research. Of those on hold, the original three protocols involving several forms of SCID will remain suspended while many of the other 27 that involve life-threatening diseases are being considered for restarting as new safety measures are being implemented, he says.

In mid-January, RAC members similarly recommended extending the earlier clinical hold, but spelled out their recommendation in terms that were more restrictive than FDA's, with RAC members urging a halt to all such gene transfer protocols involving retroviruses and targeting any kind of hematopoietic cell, not merely stem or progenitor cells. This seemingly slight difference between the NIH and FDA advisory messages proved consequential and frustrating for some investigators who are conducting gene transfer clinical trials, including several who pointedly asked RAC in February to ease its restrictions and to provide very precise advice.

For example, Ken Cornetta of Indiana University School of Medicine in Indianapolis, who represents the American Society of Gene Therapy, calls on FDA and RAC to "harmonize" their advice. "Harmony helps our investigators, who need to be compliant with recommendations from and must respond to six or more committees," he says, alluding also to local as well as federal advisory boards such as institutional review boards (IRBs).

Steven Rosenberg of the National Cancer Institute (NCI) at NIH criticizes RAC for blocking clinical trials that might extend the lives of some cancer patients who have exhausted other available treatments. A mid-January advisory from RAC effectively blocked his and several other similar protocols that entail treating patients who have advanced cancer with their own genetically modified T cells, he says.

Such cancer patients appear to be at little risk for developing leukemia, in part because their survival expectations are so bleak and short-term, Rosenberg points out. Moreover, the mechanism underlying leukemia development in the two children in France does not appear applicable in terms of the molecular and cellular manipulations at work in the NCI clinical protocols involving adults with cancer. Nonetheless, he says, "IRBs are very skittish about gene therapy, don't have [comparable] expertise, and consider RAC recommendations as law."

That initially restrictive advice from RAC in mid-January was not offered "lightly," says committee member Diane Wara of the University of California, San Francisco. When committee members learned late in December of the second case of leukemia in France, they realized that an "`n=2' heightened the risk and eliminated the notion that it was chance."

Although that advice may have caused "angst" among investigators conducting gene transfer protocols, it was intended as a stopgap or "place-holder," pending the deliberations of the full committee with outside experts during its special meeting in February, adds Amy Patterson, who directs the NIH Office of Biotechnology Activities, through which RAC operates. By the end of that meeting, RAC members issued a series of recommendations, including one saying that resuming other gene transfer clinical trials, including those such as the one described by Rosenberg that involve treating a patient's hematopoietic cells with retroviral vectors, "may be justified."

Jeffrey L. Fox

Jeffrey L. Fox is the ASM News Current Topics and Features Editor.

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