Nonnucleoside Reverse Transcriptase Inhibitors

The NNRTIs inhibit viral reverse transcriptase by binding adjacent to its active site and inducing a conforma-tional change that causes the enzyme's inactivation. When combined with NRTIs or protease inhibitors,

NNRTIs produce additive and possibly synergistic effects against HIV. The pharmacokinetic parameters of these agents are listed in Table 51.3.

All NNRTIs are active against HIV-1 reverse tran-scriptase only and do not require phosphorylation for activation. These agents share certain adverse effects (e.g., rash) and are subject to numerous drug interactions due to their metabolism by and induction of hepatic cytochrome P450 enzymes. NNRTIs may modify plasma levels of protease inhibitors, which are also metabolized by cytochrome P450 enzymes (Table 51.4). The list of drug interactions provided in this text is not all-inclusive; it is necessary to check for all drug interactions when prescribing NNRTIs. These agents should be used with caution in patients with hepatic disease.

When NNRTIs are used alone, resistance develops rapidly as a result of the development of mutations in reverse transcriptase; therefore, monotherapy with these agents is not recommended. Cross-resistance be tween NNRTIs occurs frequently but is not seen between NNRTIs and NRTIs or the protease inhibitors.

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