NEW YORK (GenomeWeb News) — The US Food and Drug Administration should amend the label of a commonly prescribed cancer drug to reflect that a molecular diagnostic used to predict adverse events in some patients is not required for all patients, according to the findings of a recent meta-analysis.
“Not everyone needs a genetic test” before taking the drug, called irinotecan, and the FDA “should modify its prescription guidelines to say so,” researchers at the University of North Carolina at Chapel Hill wrote in an article appearing in the Aug. 28 Journal of the National Cancer Institute.
Irinotecan, also known as Camptosar, is mainly used as a second-line treatment for colorectal cancer, the third most common form of cancer in the US. In the spring of 2005, the FDA altered the drug’s label to recommend oncologists to screen patients for a variation of UGT1A1 gene, called UGT1A1*28, which could make them more susceptible to adverse events linked to the drug. One such adverse event is neutropenia, a blood disorder that limits the body’s production of white blood cells, hobbling immunity.
According to GenomeWeb Daily News sister publication Pharmacogenomics Reporter, the FDA required that irinotecan’s label recommend a "reduction in the starting dose" for patients known to be homozygous for the UGT1A1*28 allele. The new label did not include specific dosing information, or suggest a specific diagnostic test for the allele.
But according to the UNC researchers, a meta-analysis of nine irinotecan studies showed that only patients who received a medium or high dose of the drug had greater risk of developing neutropenia if they had two copies UGT1A1*28. At lower doses, however, “the risk was the same regardless of what UGT1A1 gene the patients had,” the authors said.
"Many institutions saw the FDA's recommendation as authorization to test all patients before treating them with irinotecan, even though many clinicians didn't think it was always necessary since low doses of the drug weren't causing problems," Howard McLeod, senior author of the study and director of the UNC Institute for Pharmacogenomics and Individualized Therapy, said in a statement accompanying the study.
"Our review showed that at low doses, the drug is well tolerated and can be taken by most people," McLeod said. "Testing only becomes essential when the dosage increases and genetics become a larger factor in determining what side effects patients experience.”
As such, the authors recommend that the FDA “amend” irinotecan’s label to describe the association between dose and risk of hematologic toxicity among patients with two UGT1A1*28 genes.