A 24-week open-label phase I/II evaluation of the HIV protease inhibitor MK-639 (indinavir)

Objective: To investigate the safety, pharmacokinetics, and activity of the orally bio-available protease inhibitor MK-639.

Design: An open-label Phase I/II trial of medically stable subjects with screening CD4 lymphocyte counts < or = 300 x 10(6)/I and > or = 20,000 HIV RNA copies/ml. Pharmacokinetics were performed at days 1 and 15. In order to better understand the relationships between drug exposure, baseline activity markers, and their changes during the study, mathematical modeling was performed using the traditional sigmoid-Emax relationship of pharmacologic effect and first order inhomogeneous differential equations for a two compartment system.

Results: The five men enrolled had extensive prior nucleoside therapy (mean, 32.6 +/- 25.6 months), a low mean CD4 lymphocyte cell count (CD4 count, 66.1 +/- 61 x 10(6)/I and CD4 percentage, 4.4 +/- 3.1%), high soluble tumor necrosis factor-alpha type II (sTNFII) receptor concentration (6.23 +/- 2.76 ng/ml) and high viral load (5.13 +/- 0.46 log10 RNA copies/ ml; geometric mean, 133,941 copies/ml). The drug was well tolerated at a dose of 600 mg every 6 h. The steady state concentrations Cmax and Cmin were 4.94 +/- 2.16 microM and 0.28 +/-0.1 microM, respectively, which are approximately equal to 50 and 3 times the 95% inhibitory concentration (IC95) for clinical isolates, respectively. The mean increase in CD4 cell count was 143 x 10(6)/ (217% increase ), the mean increase in CD4 percentage was 5.2 percentage points (118%), mean decrease in HIV RNA was 1.55 log10 RNA copies/ml (a geometric mean difference of 130,120 copies/ml or 97% decrease) with a slow upward drift on continued therapy to a mean 0.64 log10 RNA copies/ml decrease by week 24 (a geometric mean difference of 103,084 copies/ml or 77% decrease), and a mean decrease in sTNFII receptors of 2.78 ng/ml (45% decrease). The mean CD4 counts per week as a function of the starting CD4 counts fit a sigmoid-Emax relationship (r2 = 0.998, P < 0.0001) with the return of CD4 cells being strongly related to the number of CD4 cells at baseline. Drug exposure as measured by either the total exposure (area under the concentration/time curve) or as the Cmin gave similar significant relationships to the fractional inhibition of HIV generation (r2 = 0.999, P < 0.0001, and r2 = 0.996, P < 0.0001, respectively).

Conclusions: MK-639 appears to have significant dose-related Antiviral activity and is well tolerated.