Tuesday, March 10, 2009

Departing Milano Stazione? ADA-SCID and Gene Transfer

Greetings after a hiatus for teaching, grants, committees, book deadlines, wiping runny noses, and more. Much has happened since my last posting, and in the next two or three weeks, I hope to catch up.

First item on the agenda is a Jan 29 report in New England Journal of Medicine (NEJM) describing successful reconstitution of immune function in eight of ten children receiving gene transfer for adenosine deaminase severe combined immune deficiency (ADA-SCID). The paper follows on a previous report in Science, 2002, and almost certainly counts as gene transfer's greatest clinical accomplishment to date.

I have previously argued in Lancet and Developing World Bioethics, as well as in my forthcoming book, that this study raised important justice concerns because it recruited volunteers from economically disadvantaged settings without clearly fulfilling the requirement, articulated in the Declaration of Helinki, of responsiveness. The NEJM article does not say where subsequent volunteers were recruited, though the fact that all but one new volunteer received PEG-ADA (a very expensive standard of care available only in high-income countries) suggests that later patients were not economically disadvantaged.

Rather than dwell on justice, I'd like to focus on the significance of this study. As indicated, eight of ten children with a life threatening immune disorder had their immune systems reconstituted. Five of these children had T-cell counts that were "above the lower limits of normal." These children were able to enjoy normal social relations parents and other children.

There do not appear to have been any adverse events relating to the gene transfer vector. A major concern was the possibility that gene transfer might trigger a leukemia-like syndrome observed in two X-SCID studies. Blood tests of children in this ADA-SCID study, however, do not evidence of either the leukemia syndrome or its precursors– at least within the time frame of the study (median follow-up of 4 years; range: 1.8-8 years).

So is ADA-SCID gene transfer ready to leave Milan and conquer ADA-SCID?  For children lacking haplo-identical bone marrow donors, maybe so given the morbidity associated with marrow  transplantation. Still, there are lingering concerns. First, though these results are encouraging, risks of malignancy remain unquantified. Second, this gene transfer regime requires several ancillary treatments- like bone marrow conditioning- that expose patients to risk of infection until the gene transfer intervention kicks in. Several volunteers in this study developed infections and neutropenia, for example. In an accompanying editorial in NEJM, Donald Kohn and Fabio Candotti describe several ways that retroviral gene transfer to blood stem cells might be made safer. Last, it is important to remember that ADA-SCID is a multi-system disorder, with neurological, skeletal, and other effects. Though this approach seems to address what is by far the largest cause of morbidity and mortality in children with ADA-SCID, it does eradicate their condition.

The results of Aiuti et al have been widely celebrated in the gene transfer community.  Kohn and Candotti's editorial, for example, is titled "Gene Therapy Fulfilling its Promise."  More than any single gene transfer study I can think of, this one seems to have earned the vindicating headlines. (photo credit: Paolo Margari, Milano Sazione Centrale Ferrovi, 2008)

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