Mechanisms of nucleoside analog antiviral activity and resistance during human immunodeficiency virus reverse transcription.

Abstract

Progression to AIDS in human immunodeficiency virus type 1 (HIV-1)-positive individuals is characterized by a slow destruction of the immune system and a depletion of CD4 cells in the peripheral blood. The complex mechanism of CD4 cell disappearance is poorly understood, but this depletion may be attributable in part to a superantigen effect, apoptosis, viral cytopathicity, or combinations of all of these. A rough correlation exists between increases in viral load in lymphoid organs, rate of disease progression, and extent of CD4 cell depletion (118). Anti-HIV chemotherapy with agents such as 39-azido39-deoxythymidine (AZT) has resulted in at least transient decreases in viral load and increases in CD4 cells (66, 166). The anti-HIV drugs currently employed in clinical trials or licensed for AIDS therapy generally fall into three major categories, i.e., nucleoside analogs, nonnucleoside reverse transcriptase (RT) inhibitors, and HIV proteinase inhibitors. This review focuses on the inhibition of HIV-1 RT and reverse transcription by nucleoside analogs and on mechanisms of resistance to nucleoside analogs. AZT possesses activity against a number of retroviruses (83, 115) besides HIV-1 (106). AZT blocks HIV-1 replication at low concentrations; i.e., the effective concentration for 50% percent inhibition (IC50) is approximately 0.01 mM (Table 1). In addition, this drug is largely nontoxic for T lymphocytes; i.e., the cell culture inhibitory dose for 50% inhibition of cell growth (CCID50) is approximately 10 to 50 mM (Table 1) (106, 109). The active form of AZT, i.e., AZT 59-triphosphate (AZT-TP), phosphorylated by cellular nucleoside kinases, is thought to inhibit HIV-1 reverse transcription both as a competitive inhibitor of RT and as a chain terminator of DNA elongation (152). When studied in phase I and phase II clinical trials, AZT treatment resulted in increased numbers of CD4 cells, decreased occurrences of opportunistic infections, and decreased viral loads (20, 34, 42, 173). Many of the initially serious toxic effects of AZT, e.g., bone marrow suppression and anemia, have been alleviated by lowering the dosage and by administering recombinant erythropoietin (41, 134, 163). AZT can cross the blood-brain barrier and may reverse certain neurological abnormalities such as AIDS dementia (52). However, AZT treatment for prolonged periods also resulted in the emergence of drug-resistant viral isolates that displayed normal replication kinetics and up to 100-fold resistance to this drug (89, 138). In addition, a direct correlation between the development of AZT resistance and clinical progression to AIDS and death has been established (33, 108). A number of other dideoxynucleoside triphosphates (ddNTP), analogous to native deoxynucleoside triphosphates (dNTP) but deficient in their 39 hydroxyl group, have also been shown to inhibit HIV replication in CD4 cells. Relatively few of these compounds, e.g., 29,39-dideoxyinosine (ddI or didanosine) and 29,39-dideoxycytidine (ddC or zalcitabine), have IC50s of below 10 mM (Table 1) (32, 104, 105). Derivatives of these analogs, such as the racemic mixture of 29,39-dideoxy-39thiacytidine (BCH-189) and its negative enantiomer (3TC or lamivudine) (25, 26, 142, 150), 29,39-dideoxy-59-fluoro-39thiacytidine (144), 39-fluoro-39-deoxythymidine (FLT), carbocyclic-29,39-didehydro-29,39-dideoxyguanosine (carbovir) (162), 39-azido-29,39-dideoxyuridine, and 29,39-didehydro29,39-dideoxythymidine (d4T) (60), also inhibited HIV infections of CD4 cells and in many cases showed less cytotoxicity and had lower CCID50s than AZT (25, 60, 177). These ddNTPs have all entered all clinical trials, although resistance continues to be a problem (90, 107, 138, 140, 151, 175, 176). The mechanisms of antiviral action by nucleoside analogs in tissue culture are not fully understood. Nucleoside analog triphosphates may block acute infection by HIV and other retroviruses through inhibition of RT and chain termination (112, 178). However, AZT has also been reported to block virus replication in chronically infected cells (139), to interfere with virus maturation (18), and to disrupt syncytium formation (18). However, AZT did not block cell-to-cell HIV transmission (59, 94, 153). In the related feline immunodeficiency virus model, selection of resistance to AZT resulted in mutations outside the RT-coding region, suggesting that viral proteins besides RT may also play a role in resistance (131). Breakthrough HIV replication in CD4 cells was observed despite constant exposure to high AZT concentrations (147). Certain nonnucleoside inhibitors of RT, i.e., phosphonoformate and Nevirapine, and other nucleoside analogs, i.e., carbovir, ddI, FLT, and ddC, may act synergistically with AZT to delay HIV-1 replication in tissue culture (35, 39, 82, 135, 147).