Zidovudine phosphorylation after short-term and long-term therapy with zidovudine in patients infected with the human immunodeficiency virus

Background: Resistance of human immunodeficiency virus (HN) to zidovudine (AZT) has been associated with mutations in the viral reverse transcriptase gene. However, recent studies suggest that host cellular factors such as a decreased thymidine kinase activity or an increased cellular P-glycoprotein expression may be important. This study compared concentrations of zidovudine monophosphate, zidovudine diphosphate, and zidovudine triphosphate with P-glycoprotein expression in peripheral blood mononuclear cells from patients receiving long-term (>18 months) and shea-term (<2 months) zidovudine treatment. Methods: Ten subjects in the short-term group and 11 subjects in the long-term group with CD4 counts between 300 and 500 received a single oral dose of zidovudine (200 mg) after a 24-hour washout period. Blood samples were collected at 0, 1, 2, 4, and 6 hours, Intracellular nucleotide concentrations were measured by a combined HPLC-radioimmunoassay method, and P-glycoprotein expression was determined by fluorescence activated cell sorting (PACS) analysis with use of the monoclonal mouse antibody MRK-16. Results: Zidovudine monophosphate was the predominant compound, accounting for 73.4%+/-7.1% (SD) of the total phosphates in the long-term treatment group and 74.2%+/-15.0% (SD) in the short-term group. Zidovudine diphosphate accounted for 13.3%+/-3.3% (SD) in the long-term group and 12.5%+/-6.6% (SD) in the short-term group. Zidovudine triphosphate accounted for 13.4%+/-4.1% (SD) in the long-term group and 13.5%+/-8.3% (SI) in the short-term group. Mean peak concentrations for the active zidovudine triphosphate were 0.04+/-0.02 (SD) pmol/10(6) cells in both groups. Comparison of the individual zidovudine phosphate concentrations and P-glycoprotein expression revealed no significant difference in the two patient populations. Conclusions: These data suggest that intracellular phosphorylation does not change over time and that zidovudine does not select for P-glycoprotein expressing cells.