In January 2016, a phase I trial of Bial's oral analgesic BIA 10-2474 killed one volunteer and caused brain damage in four others. The fatal effects of the fatty acid amide hydrolase (FAAH) inhibitor may have been caused by off-target activity on several lipases, an international team of researchers now report (Science 356, 1084–1087; 2017).

When the tragedy first occurred, researchers speculated on three general possibilities for why BIA 10-2474 was toxic: errors may have occurred in the manufacturing, handling or administration of the drug; on-target FAAH activity may have led to overstimulation of certain signalling pathways; or off-target activity may have triggered the side effects. Regulatory authorities dismissed the first possibility, and Pfizer's FAAH inhibitor PF-04457845 made it through phase II trials before failing on efficacy rather than safety, calling into question the on-target toxicity.

To understand whether the off-target activity profile of BIA 10-2474 was to blame, researchers used an activity-based proteomic method to look at what other targets the drug binds to in human cells. Several serine hydrolases that are not targeted by PF-04457845 topped the list, they report. Many of these hydrolases modulate cellular lipid metabolism and show substantial expression in the brain. These findings raise “the possibility that disruption of cellular lipid networks may have contributed to the compound's neurotoxicity,” the authors write.

The study did not assess the off-target activity of Johnson & Johnson's experimental FAAH inhibitor JNJ-42165279, which the company put on clinical hold after the BIA 10-2474 disaster.

“The study highlights the general utility of [activity-based proteomic methods] as a versatile chemical proteomic method to assess on-target engagement and off-target activity of covalent drugs to guide therapeutic development,” the authors conclude.