Verma: ‘no standard’ for measuring doses.

Gene-therapy researchers using adenoviral vectors should apply stricter quality controls and more precise monitoring to them because of the ‘narrow window’ that separates their potential efficacy from toxicity, an advisory panel of the US National Institutes of Health concluded provisionally last week.

The Recombinant DNA Advisory Committee (RAC) met to examine the factors that led to the first death attributed directly to gene therapy (see Nature 401, 517; 1999). But the panel also analysed the field as a whole, hearing testimony from a host of researchers who have used the vector in clinical trials.

Many repeatedly mentioned the ‘window’ and the ‘threshold effect’ associated with it — that one dose level may cause little or no adverse effect in patients, but a slightly higher dose may trigger apparently disproportionate reactions. Those reactions include immune responses resulting in influenza-like symptoms and tissue damage to the organ in which the vector was administered.

Both of these occurred after Jesse Gelsinger, an 18-year-old Arizona man, received one of the highest doses of adenoviral vector ever given to a human, to treat a partial defect of the gene that encodes ornithine transcarbamylase, an enzyme that removes ammonia from the liver.

Shortly after receiving the vector, Gelsinger developed a high fever. Within the first day, tests showed that he had suffered liver injury and inappropriate blood coagulation. These symptoms had begun to improve when, on the third day, Gelsinger started to have trouble breathing. His vital organs were failing, and his doctors took him off life support on the fourth day.

Inder Verma, a gene-therapy researcher with the Salk Institute in La Jolla, California, and co-chair of the RAC, said he was struck not with the similarities among groups, but with the differences. Some groups measured the strength of their doses in infectious units, while others used particles per kilogram.

“There was really no standard,” Verma says, which makes measuring the strength and potential toxicity of one vector against another very difficult. Many gene-therapy researchers also seem unclear about their end points, such as how much messenger RNA ends up in targeted cells after gene therapy.

That end point in the experiment that led to Gelsinger's death was one of many “surprising” findings, according to James Wilson, director of the University of Pennsylvania's Institute for Human Gene Therapy, who oversaw the trial.

Before the autopsy, Wilson expected to find the vector concentrated in the liver, because researchers infused it directly through a catheter into the hepatic artery. But the autopsy revealed “significant” amounts in the spleen, lymph nodes, bone marrow and other tissues.

The autopsy also revealed further abnormalities in Gelsinger's bone marrow, indicating that he may have also been infected with a parvovirus, which, when combined with the adenovirus, may have initiated the chain reaction of adverse events. Finally, when Wilson examined the vector used to carry the gene that codes for ornithine transcarbamylase, he discovered duplicate sequences not engineered in the original.

The RAC concluded that such anomalies underscore the need for more careful measurement at all stages of gene-therapy trials. “Toxicities vary from study to study, vector to vector, patient type to patient type,” says RAC member Robert Warren, an oncologist with the University of California at San Francisco.

In its preliminary findings, the RAC advised gene-therapy researchers to take more safety measures, including checking the sequence integrity of a vector before administration, monitoring cytokine levels more closely before and after gene therapy begins, and ensuring that patients have no additional virus lurking in their systems.

Researchers administering adenoviral vectors directly into organs should also consider smaller increases between doses, so that there is less danger when they approach the toxicity threshold.