Human immunodeficiency virus resistance to AZT in MOLT4/8 cells is associated with a lack of AZT phosphorylation and is bypassed by AZT-monophosphate SATE prodrugs

Human T lymphoid MOLT4/8 cells were grown continuously for more than 2 years in a medium containing 3'-azido-2',3'-dideoxythymidine (zidovudine; ATT) at a concentration of 250 mu M. These cells, designated MOLT-4/8(r)AZT(250), were used to test the cytotoxic and antiviral activity of AZT. Intracellular accumulation of AZT, expression of the multidrug resistance 1 (MDR-I) gene, thymidine kinase (TK) gene and activity of the TK enzyme In cellular extracts were measured. The results showed that both the cytotoxic and antiviral activity of AZT were significantly lower in MOLT4/8(r)AZT(250) than in MOLT4/8 cells; concentrations required to inhibit 50% production of the p24 human immunodeficiency virus type 1 (HIV-1) antigen of two laboratory strains were at least 100-fold higher in resistant cells. The MDR-1 gene was not expressed in the resistant cells. TK mRNA expression was significantly lower in the resistant than in the sensitive cells. TK enzymatic activity for deoxythymidine phosphorylation was impaired in MOLT4/8(r)AZT(250) cells compared to the sensitive cells.