Pharmacodynamic Studies (PD) of Didanosine (ddI) Alone and in Combination with Azidothymidine (AZT) in Human T-cells; a Stochastic Biochemical Approach to Antiretroviral Nucleoside Drug Combination in Inhibiting HIV-reverse Transcriptase (RT)

Didanosine (ddI) is used in the treatment of HIV-1 infection alone and in combination with azidothymidine (AZT). When combined with AZT, patients exhibit improved patterns of surrogate markers after sequential combination regimens of ddI and AZT compared to either drug monotherapy. We have investigated the biochemical mechanism(s) of this synergistic drug combination in human PBMC cells and in human T-cell lines sensitive and resistant to AZT due to lack of thymidine kinase (TK). DdI is preferentially activated to its triphosphate anabolite, ddATP, at 3:1 ratio in human T-lymphocytes compared to monocytes from the same individual. There are no apparent differences in the intracellular concentrations of ddATP in Jurkat/0 and Jurkat/AZT-10, an AZT resistant human T-cell line, when ddI is administered alone or in combination with AZT, hence there appears to be a case of collateral sensitivity. Intracellular increases of AZTTP concentrations in patient's PBMC cells have been determined clinically after AZT alone and in a combination regimen with ddI. A stochastic biochemical model has been developed that estimates the velocity of HIV-RT under uninhibited and inhibited conditions by the active anabolites, AZTTP and ddATP. This model provides a rational explanation for the greater inhibition of HIV-RT in the presence of both inhibitors, AZTTP and ddATP, as compared to the presence of either anabolite triphosphate alone. Expanding this model to describe the inhibition of HIV-RT in the presence of three competitive inhibitors, AZTTP, ddATP and 3TCTP demonstrated that the presence of these HIV-RT inhibitors resulted in an even greater inhibition of this viral enzyme necessary for HIV integration and replication. Hence, a more effective inhibition of HIV-RT enzyme is achieved by the combination of the three drugs, AZT, ddl and 3TC. In an effort to verify this model with experimental data the kinetics of HIV-RT were studied in the absence and after inhibition by AZTTP or ddATP alone, both AZTTP + ddATP or AZTTP + ddATP + 3TCTP. Treatment of HIV-RT with high concentrations of these triphosphate inhibitors, as high as 3Kis, inhibited this enzyme to greater than 90% of untreated control. However, a small percentage of residual HIV-RT, 6%, was uninhibited even after exposure to 3Ki concentrations of each inhibitor. These studies strongly suggested that: 1) AZT plus ddI or AZT plus ddI plus 3TC are synergistic at the active anabolite level against HIV-RT; 2) the combination of the three nucleoside analog drugs (AZT, ddI 3TC) is needed for more effective inhibition of HIV-RT; 3) that the combination of the triphosphates at concentrations much greater than those pharnacologically achieved in T-Cells or PBMC under treatment conditions did not inhibit completely HIV-RT. Hence, the three nucleoside HIV-RT inhibitors must be combined with other classes of antiviral drugs or T-cell specific inhibitor drugs.