- Patients receiving any medications or substances that are inhibitors or inducers of CYP450 enzymes are ineligible; these include St. John's wort or hyperforin (potent CYP3A4 enzyme inducer) and grapefruit juice (potent cytochrome P450 CYP3A4 enzyme inhibitor); such substances can significantly increase or decrease the serum level of cobimetinib; as part of the enrollment/informed consent procedures, the patient will be counseled on the risk of interactions with other agents, and what to do if new medications need to be prescribed or if the patient is considering a new over-the-counter medicine or herbal product