It was like falling off a cliff," Mark L. Batshaw, George Washington University pediatrics chairman, says of the adverse effect Jesse Gelsinger experienced following gene therapy. On Sept. 14, 18 Hospital of the University of Pennsylvania patients received escalating doses of an adenovirus carrying a gene to restore ornithine transcarbamylase (OTC)--an enzyme that, when missing, renders people unable to break down dietary protein. Gelsinger was in the highest-dose group, which nonetheless received subtherapeutic levels of OTC.

In the first 24 hours following infusion of the vector into the liver, all the patients (six groups of three) suffered minor fevers, aches, and pains--typical flulike side effects associated with adenoviral vector treatment. Encouragingly, liver toxicity indicators weren't increasing along with the dose amounts. "We were feeling very good because it looked like there wasn't a dose-response effect," Batshaw, the Phase I study's principal investigator, recalls.

The second day, Gelsinger inexplicably fell...

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