The non-nucleoside
reverse transcriptase inhibitors (NNRTIs) directly inhibit the
HIV-1 reverse transcriptase (RT) by binding in a reversible and non-competitive manner to the enzyme. The currently available NNRTIs are
nevirapine,
delavirdine, and
efavirenz; other compounds are under evaluation. NNRTIs are extensively metabolized in the liver through
cytochrome P450, leading to pharmacokinetic interactions with compounds utilizing the same metabolic pathway, particularly PIs, whose plasma levels are altered in the presence of NNRTIs. NNRTIs are drugs with a low genetic barrier, i.e. a single mutation in RT genoma induces a high-level of phenotypic resistance, preventing the use of NNRTIs as monotherapy. In naive patients, several trials have shown the value of NNRTIs in combination with
nucleosides and/or
protease inhibitors. Small pilot studies have shown that NNRTIs may be useful as second-line therapy. However, due to the rapid emergence of resistant virus to these compounds in case of incomplete viral suppression, NNRTIs should not be added to current failing antiretroviral regimen. The most common side-effect reported with
nevirapine and
delavirdine is
rash. The incidence of
rash is rather similar under these two compounds, but severe
rash is less frequent with
delavirdine. The most common adverse reactions reported with
efavirenz are central nervous system complaints such as
dizziness.
Rash is reported less frequently than with
nevirapine or
delavirdine, and is usually mild. NNRTIs resistance mutations are located in the amino acid residues aligning the NNRTI-binding "pocket" site. High-level resistance is often associated with a single point mutation which develops within this site (especially
codon groups 100 - 108 and 181 - 190). Patients failing on one NNRTI are very likely to possess multiple NNRTI resistance mutations. NNRTIs should always be used as part of a potent antiretroviral therapy to insure suppression of viral replication, thus circumventing the rapid selection of cross-resistant variants.