Which medication is MOST concerning when starting tazemetostat for a patient also using verapamil?

When considering the use of tazemetostat in conjunction with verapamil, the concern primarily arises from the pharmacokinetic interactions between the two medications. Verapamil is a calcium channel blocker that is known to inhibit certain cytochrome P450 enzymes, particularly CYP3A4. Tazemetostat, an EZH2 inhibitor used in certain cancers, is extensively metabolized by CYP3A4.

When a patient is already on verapamil, the use of tazemetostat can lead to increased serum levels of the latter due to the inhibition of its metabolism. This can increase the risk of adverse effects associated with tazemetostat, potentially leading to toxicity. Therefore, monitoring and consideration of dose adjustment or alternative treatment strategies are essential when initiating tazemetostat in a patient who is taking verapamil.

The potential for QT prolongation, the use of lisinopril, and proton pump inhibitors, while relevant in their own contexts, do not present the same level of concern in this scenario as the interaction between tazemetostat and verapamil. Thus, careful evaluation of the patient's medication regimen, especially concerning verapamil, is vital when starting tazemetostat therapy.