Tenofovir disoproxil fumarate (Viread) is a prodrug of tenofovir, a phosphorylated adenosine nucleoside analogue, and is the only available agent of its class . It is converted by cellular enzymes to tenofovir diphos-phate, which competes with deoxyadenosine triphos-phate (dATP) for access to reverse transcriptase and causes chain termination following its incorporation. Tenofovir was approved as part of a combination therapy for HIV in adults who failed treatment with other regimens; it appears to be effective against HIV strains that are resistant to NRTIs. The pharmacokinetic properties of tenofovir are provided in Table 51.2.
Tenofovir is taken once daily and is generally well tolerated, perhaps because it produces less mitochondr-ial toxicity than the NRTIs. Nausea, vomiting, flatulence, and diarrhea occur in 10% or fewer patients. Resistance to tenofovir has been documented, and cross-resistance to NRTIs may occur.
Tenofovir should not be given to patients with renal insufficiency. Its coadministration with didanosine results in increased plasma levels of didanosine that can produce toxicity. Because lactic acidosis and severe hepatomegaly with steatosis have been reported with NRTIs, it is important to monitor patients with known risk factors during treatment with tenofovir.
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