We report the case of a 50-year-old human immunodeficiency virus-positive patient with stage IV KRAS-mutated colorectal cancer who experienced visible muscle twitching in the right lateral triceps brachii from irinotecan administration for which typical supportive care measures were unsuccessful, including the administration of atropine and slowing down the infusion rate. The patient was able to tolerate this reaction and received 20 cycles of irinotecan-based chemotherapy despite experiencing the muscle twitching with every cycle at the same onset, duration, and severity. It is possible that competitive metabolism by concomitant medications metabolized by CYP3A4 or UGT1A1 was responsible for this event. Due to ethical concerns, we were unable to formally assess the drug interaction by discontinuing the suspected interacting medications and re-initiating them to evaluate the effects. A formal pharmacokinetic study may be warranted to better elucidate these potential drug interactions.
Keywords: Drug interaction; UGT1A1; cytochrome P450 3A4; irinotecan; raltegavir.