Drug Interactions between paclitaxel and Stribild

MONITOR: Coadministration with inhibitors of CYP450 2C8 and/or 3A4 may increase the plasma concentrations of paclitaxel, which has been found in vitro to be a substrate of these isoenzymes. Formal clinical drug interaction studies have not been conducted. However, in a case report involving a 77-year-old woman with HER2-positive invasive ductal breast cancer on long-term amiodarone therapy, 4 cycles of paclitaxel (80 mg/m2 weekly) and trastuzumab led to development of increasing abdominal discomfort and skin lesions and a diagnosis of paclitaxel-induced skin toxicity. Switching to reduced dose docetaxel (100 mg or 75 mg/m2 weekly) led to the development of severe skin and mucosal toxicity, requiring hospitalization 8 days after the first docetaxel dose was administered. Analysis of two blood samples taken 9 and 10 days after docetaxel administration showed an approximately fivefold increase in its AUC as well as the presence of paclitaxel in unquantifiable levels, 20 and 21 days after it was last administered. The authors of this case study propose that, in addition to CYP450 3A4 inhibition, CYP450 2C8 and P-glycoprotein inhibition due to amiodarone may also contribute to the interaction.

MANAGEMENT: Clinicians should consider the potential for interaction with drugs that inhibit CYP450 2C8 and/or 3A4 and monitor for evidence of dose-related toxicities of paclitaxel during coadministration, including diarrhea, mucositis, myelosuppression, and peripheral neuropathy.

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MONITOR: Concomitant use of tenofovir with cobicistat may increase the risk for tenofovir-related renal adverse effects, including renal impairment, renal failure, elevated creatinine, and Fanconi syndrome. The mechanism of this interaction has not been described. Cobicistat may decrease estimated creatinine clearance via inhibition of tubular secretion of creatinine; however, renal glomerular function does not appear to be affected. When given concomitantly with cobicistat, the systemic exposure (AUC) and trough plasma concentrations (Cmin) of tenofovir was also increased by 23% and 55%, respectively. However, data are lacking to determine whether concomitant use of tenofovir with cobicistat-containing regimens is associated with a greater risk of renal complications compared with regimens that do not include cobicistat.

MANAGEMENT: Initiation of cobicistat or cobicistat-containing regimens is not recommended in patients with CrCl less than 70 mL/min if any coadministered medicine requires dose adjustment based on renal function (including tenofovir), or is nephrotoxic. If concomitant therapy is necessary, monitoring of renal function is recommended, particularly in patients with risk factors for renal impairment.

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GENERALLY AVOID: Cobicistat may increase the plasma concentrations of antiretroviral agents. The plasma concentrations of cobicistat may also be increased or reduced in the presence of antiretroviral agents. The proposed mechanism is cobicistat inhibition of the CYP450 3A4 isoenzyme, of which antiretroviral agents may be substrates, and the inhibition or induction of CYP450 3A4 by concomitant antiretroviral medications. Cobicistat is a mechanism-based inhibitor and substrate of CYP450 3A4 with no antiretroviral activity of its own. Rather, it is indicated in its capacity as a pharmacokinetic booster of CYP450 3A4 to increase the systemic exposure of some antiretroviral medications such as atazanavir, darunavir, and elvitegravir, which are substrates of this isoenzyme. Concomitant use of other antiretroviral agents with cobicistat may also increase the plasma levels and risk of side effects associated with these medicines. In contrast, concomitant use of cobicistat-boosted atazanavir or darunavir with CYP450 3A4 inducers nevirapine, etravirine, or efavirenz may reduce the plasma concentrations of cobicistat, darunavir, and atazanavir, leading to a potential loss of therapeutic effect and development of resistance to darunavir and atazanavir. Pharmacokinetic data are not available.

MANAGEMENT: Cobicistat is not intended for use with more than one antiretroviral medication that requires pharmacokinetic enhancement, such as two protease inhibitors or elvitegravir in combination with a protease inhibitor. In addition, cobicistat should not be used concomitantly with ritonavir due to their similar effects on CYP450 3A4. According to some authorities, use of the antiretroviral combinations of atazanavir-cobicistat or darunavir-cobicistat concomitantly with the CYP450 3A4 inducers efavirenz, etravirine, or nevirapine is also not recommended. Other authorities consider the administration of atazanavir-cobicistat with efavirenz or nevirapine to be contraindicated. Since dosing recommendations have only been established for a number of antiretroviral medications, product labeling and current antiretroviral treatment guidelines should be consulted.

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