Drug Interactions between emtricitabine / nelfinavir / tenofovir and Rimactane

CONTRAINDICATED: Coadministration with rifampin may significantly decrease the plasma concentrations of most protease inhibitors (PIs) except ritonavir. The mechanism is rifampin induction of CYP450 3A4, the isoenzyme responsible for the metabolic clearance of PIs. In pharmacokinetic studies, the interaction has been associated with a 75% to 95% reduction in plasma PI concentrations and systemic exposure (AUC).

MANAGEMENT: Given the risk of reduced viral susceptibility and resistance development associated with subtherapeutic antiretroviral drug levels, alternative antimycobacterial agents should be considered in patients already receiving effective PI-containing antiretroviral therapy. For treatment of latent tuberculosis (TB) infection, a nine-month regimen of isoniazid may be considered if feasible. For treatment of HIV-related TB, a regimen that includes rifabutin is generally preferred, as rifabutin appears to be as effective as rifampin but is a much less potent inducer of CYP450 3A4. Nonrifamycin-containing regimens may be suboptimal (higher mortality rates; higher rates of treatment failure and relapse; increased adverse effects; longer treatment duration) and are usually not recommended for HIV-related TB except in patients who are intolerant of rifamycins or infected with a rifamycin-resistant isolate. Alternatively, rifampin may be used at usual dosages in patients receiving an antiretroviral regimen that includes ritonavir 600 mg or 400 mg twice a day in combination with another PI at a reduced dosage. In patients who have not begun antiretroviral therapy at the time TB treatment is initiated, clinicians may also consider using rifampin and postponing antiretroviral therapy. With early HIV disease, it may be reasonable to monitor CD4 cell count and postpone antiretroviral therapy until TB treatment is complete, since there is low risk of HIV disease progression or death during this period. However, the optimal time for starting antiretroviral therapy should be individualized based on initial response to TB treatment and occurrence of side effects. In patients with low CD4 cell counts, clinicians may consider delaying antiretroviral therapy until after the first one or two months of TB therapy, as side effects are common during this multi-drug phase of TB treatment and may overlap with those of antiretroviral medications. Moreover, delaying antiretroviral therapy may ameliorate adherence issues and decrease the frequency and severity of paradoxical reactions (i.e., immune restoration syndromes resembling exacerbation of TB that sometimes occur after initiation of antituberculosis treatment in patients receiving potent antiretroviral therapy). Rifabutin can be substituted approximately 2 weeks before the planned initiation of antiretroviral therapy to allow time for rifampin's enzyme induction effects to wane. In general, treatment of TB in the context of antiretroviral therapy is complex and requires an individualized approach. Experts in the treatment of HIV-related tuberculosis should be consulted, and TB and HIV care providers should work in close coordination throughout treatment.

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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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MONITOR: Coadministration of rifampin with agents known to induce hepatotoxicity may potentiate the risk of liver injury. There are various possible mechanisms related to rifampin-associated hepatotoxicity described in product labeling and medical literature, however no consensus has been made. These include increased mitochondrial oxidative stress, apoptotic liver cell injury (in rodent studies), the development of cholestasis, hepatic lipid accumulation, and elevated toxic metabolites caused by rifampin-mediated induction of cytochrome P450 enzymes. Cases of drug-induced liver injury (including fatal cases) have been reported within the first few days to months following rifampin treatment initiation. Additional data suggests that 1-2% of patients receiving rifampin monotherapy for tuberculosis prophylaxis experience hepatotoxicity. The severity of hepatotoxicity from rifampin ranges from asymptomatic elevations in liver enzymes, jaundice and/or hyperbilirubinemia, and symptomatic self-limiting hepatitis to fulminant liver failure and death. In most cases, liver function recovers upon on discontinuation of rifampin treatment, however, progression to acute liver failure requiring liver transplantation is possible. Known risk factors that may predispose the patient to rifampin related hepatotoxicity include: coadministration with other hepatotoxic agents, alcoholism, existing liver disease, malnutrition, extensive liver tuberculosis, liver adenocarcinoma and biliary tract neoplasm. Clinical data have been reported with concurrent use of rifampin with other antituberculosis agents (e.g. isoniazid, pyrazinamide), acetaminophen, antiretroviral agents (e.g., saquinavir/ritonavir) and halothane. Data with other hepatotoxic agents are limited.

MANAGEMENT: Caution and close clinical monitoring should be considered if rifampin is coadministered with other hepatotoxic medications. In addition, the manufacturer recommends patients with impaired liver function only be given rifampin in cases of necessity and then under strict medical supervision. Some authorities consider rifampin treatment in patients with existing liver injury contraindicated (Canada). In cases where coadministration of rifampin with hepatotoxic agents is required, careful monitoring of liver function, especially ALT and AST, should be done prior to therapy and then every 2 to 4 weeks during therapy. If hepatic damage is suspected, rifampin should be immediately discontinued. Furthermore, if hepatitis is attributed to rifampin in patients with tuberculosis, alternative agents should be considered. Patients should be instructed to contact their physician immediately if they experience symptoms such as itching, weakness, loss of appetite, nausea, vomiting, abdominal pain, yellowing of the eyes or skin or dark urine.

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