I have looked in several different sources for information on adjusting the dose of etoposide with a patient currently taking Prezcobix. The references all suggest adjusting the etoposide dose but no specific guidelines on by how much. Do you have any resources that give specific guidelines or even anecdotal sources regarding this? Specific chemotherapy regimen being used is ICE. — Name withheld on request

Prezcobix (darunavir/cobicistat) is a combination of 2 antiretroviral medicines used to treat patients with human immunodeficiency virus (HIV). This product inhibits CYP3A and CYP2D6, enzymes responsible for metabolism of many other medicines.

When reviewing drug-drug interactions, I always like to use multiple drug-information references to ensure I obtain information that is as complete as possible. Evaluating these interactions can be difficult, particularly in a clinical situation such as this where changing either therapy is clinically difficult.

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In cases where good alternate treatment options are not available, the patient should be involved in the risk:benefit conversation. Dose adjustments should also incorporate other patient characteristics, such as low albumin (which increases the proportion of unbound etoposide, increasing toxicity), organ dysfunction, performance status, goals of treatment, and other comorbidities. — Lisa A. Thompson, PharmD, BCOP