Activity of (S)-1-(3-hydroxy-2-phosphonylmethoxypropyl) cytosine against human cytomegalovirus when administered as single-bolus dose and continuous infusion in in vitro cell culture perfusion system

HPMPC [(S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine] is a potent inhibitor of human cytomegalovirus (HCMV) replication as determined by conventional tissue culture methods in which the drug concentration remains constant over time. Previous studies have shown HPMPC to have a long intracellular half-life. Despite its relatively short extracellular half- life, HPMPC might provide significant anti-HCMV activity long after the elimination of the drug by first-order kinetics. We addressed this hypothesis by measuring the activity of HPMPC in a novel cell culture perfusion system. This system allows us to compare the activity of HPMPC when given as a continuous infusion with its activity when given as a single-bolus dose followed by elimination that simulates the drug's in vivo pharmacokinetics. We show that continuous infusions maintaining maximum concentrations (C(max)s) of 0.05, 0.10, 0.31, and 1.0 μg/ml and achieving areas under the drug concentration-time curves (AUCs) of 8.4, 17, 50, and 162 μg · h/ml, respectively, result in 27, 56, 63, and 88% inhibition of viral DNA accumulation, respectively, compared with an untreated control. Single-bolus doses achieving C(max)s of 0.10, 1.25, 3.0, and 7.7 μg/ml with an elimination half-life of 20 h achieved AUCs of 2.4, 32, 78, and 138 μg · h/ml and resulted in 0, 48, 69, and 87% inhibition of HCMV DNA accumulation. Single-bolus doses achieving C(max)s of 3.9 and 12 μg/ml with an elimination half-life of 6.5 h achieved AUCs of 34 and 105 μg · h/ml, respectively, resulting in 15 and 76% inhibition of viral DNA accumulation. Comparison of C(max)-versus-effect curves for these three regimens suggests that maximum concentration is not the only important pharmacokinetic determinant of HPMPC's antiviral activity. Similar comparisons of AUC-versus-effect curves for continuous and bolus dosing suggest that the AUC is an important determinant of antiviral activity for AUCs greater than 100 μg · h/ml. We conclude that single-bolus doses of HPMPC potently inhibit HCMV DNA accumulation but that this activity is more heavily influenced by the AUC than the C(max) at the upper end of the AUC range tested. At lower AUCs, some other parameter may be the primary determinant of antiviral activity. Our cell culture perfusion system provides a novel, efficient, and convenient method for addressing questions relating the effects of constantly changing drug concentrations to antiviral effects.