Drug Interactions between cobicistat / elvitegravir / emtricitabine / tenofovir alafenamide and dyphylline / ephedrine / guaifenesin / phenobarbital

GENERALLY AVOID: Coadministration with inducers of P-glycoprotein (P-gp) may decrease the oral bioavailability and plasma concentrations of tenofovir alafenamide (TAF), which is a substrate of the efflux transporter. In 26 healthy study subjects, administration of TAF (25 mg once daily) with the P-gp inducer carbamazepine (300 mg twice daily) decreased TAF plasma concentration (Cmax) and systemic exposure (AUC) by an average of 57% and 55%, respectively, compared to TAF administered alone. It is not known if, and to what extent, tenofovir disoproxil fumarate (TDF), another prodrug of tenofovir, may interact with P-gp inducers. The interaction has not been studied with TDF, and no information is found in the labeling of various products containing TDF, although it has been reported to be a P-gp substrate also.

MANAGEMENT: Given the risk of reduced viral susceptibility and resistance development associated with subtherapeutic antiviral drug levels, concomitant use of tenofovir alafenamide fumarate with P-gp inducers is not recommended. Whether this also applies to tenofovir disoproxil fumarate has not been established.

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CONTRAINDICATED: Coadministration with potent inducers of CYP450 3A4 may significantly decrease the plasma concentrations of cobicistat, which is primarily metabolized by the isoenzyme. In a study of 12 healthy volunteers, coadministration of the potent CYP450 3A4 inducer carbamazepine (200 mg twice daily) with cobicistat-elvitegravir (150 mg-150 mg once a day) reduced the cobicistat systemic exposure (AUC) and peak plasma concentration (Cmax) by 84% and 72%, respectively, and the elvitegravir AUC and Cmax by 69% and 45%, respectively. In addition, the AUC and Cmax of carbamazepine increased by 43% and 40%, respectively, due to inhibition of the CYP450 3A4-mediated metabolism by cobicistat.

MANAGEMENT: Given the risk of reduced viral susceptibility and resistance development associated with subtherapeutic drug levels, concomitant use of antiretroviral regimens containing cobicistat with potent CYP450 3A4 inducers is considered contraindicated.

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GENERALLY AVOID: Coadministration with potent inducers of CYP450 3A4 may significantly decrease the plasma concentrations of elvitegravir, which is primarily metabolized by the isoenzyme. In a study of 12 healthy volunteers, coadministration of the potent CYP450 3A4 inducer carbamazepine (200 mg twice daily) with cobicistat-elvitegravir (150 mg-150 mg once a day) reduced the cobicistat systemic exposure (AUC) and peak plasma concentration (Cmax) by 84% and 72%, respectively, and the elvitegravir AUC and Cmax by 69% and 45%, respectively.

MANAGEMENT: Given the risk of reduced viral susceptibility and resistance development associated with subtherapeutic drug levels, concomitant use of elvitegravir with potent CYP450 3A4 inducers is generally not recommended. In addition, other antiretroviral agents that are typically combined with elvitegravir may be contraindicated for use with potent CYP450 3A4 inducers. Please refer to the manufacturer's labeling for the specific elvitegravir-containing product for more information.

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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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Ephedrine-methylxanthine combinations are used for the treatment of asthma but the efficacy of the combination has been questioned. This combination may lead to increased xanthine side effects. The mechanism is unknown, but may be related to synergistic pharmacologic effects. Patients using this combination should be closely monitored for side effects such as nausea, vomiting, tachycardia, nervousness, or insomnia. If side effects are noted, the dosage of the xanthine may need to be decreased.

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