Bad news for GSK and generics manufacturers of Paxil came this week in the form of a new study published in BMJ (the British Medical Journal) linking antidepressant paroxetine (marketed by GSK as Paxil) with increased mortality rates in breast cancer patients over the age of 66.
The observational study, conducted by a team at the Sunnybrook Health Sciences Centre in Toronto, looked at data on 2,430 patients treated with tamoxifen for breast cancer along with a single SSRI (selective serotonin reuptake inhibitor) for depression. The women were all 66 or older, and all were from the province of Ontario. Those taking paroxetine for 25 percent of the duration of their tamoxifen treatment had a 24 percent increase in risk of death from breast cancer. Patients taking paroxetine for 75 percent of the duration saw their risk jump to 91 percent.
The researchers report that these percentages were confined to paroxetine; other SSRIs, like fluoxetine (Prozac) and sertraline (Zoloft) had no effect on mortality rate.
David Juurlink, a member of the research team, explained that the discrepancy comes from paroxetine’s method of reuptake inhibition. While the other SSRIs settle for temporarily inhibiting a particular enzyme (CYP2D6), paroxetine’s effect is not reversible. Unfortunately, CYP2D6 is essential for metabolizing tamoxifen, so with paroxetine permanently inactivating this enzyme, the cancer-fighting drug is essentially neutralized.
“There’s been a suspicion for years about possible negative effects of paroxetine on drugs like these,” Juurlink said. “This is the first study to actually quantify it.”
The data excluded women younger than 66 for the simple reason that data sets are much less complete—or nonexistent—for those populations in Canada. However, Juurlink believes physicians will (and should) discontinue using paroxetine for all patients being treated with tamoxifen, and is confident that further studies on younger populations will yield similar results.
“You’ll soon see warnings from the FDA and Health Canada, which have already been sniffing around this for a while,” he predicted.