METABOLIC DIVERSITY AND ANTIVIRAL ACTIVITIES OF ACYCLIC NUCLEOSIDE PHOSPHONATES

The acyclic nucleoside phosphonates S)-1-(3-hydroxy-2-phosphonylmethox ypropyl)cytosine (HPMPC), (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)a denine (HPMPA), and 9-(2-phosphonylmethoxyethyl)adenine (PMEA) inhibit ed herpes simplex virus-1 replication in Vero cells, and the IC50 valu es ranged from 4 mu M (for HPMPC and HPMPA) to 40 mu M (for PMEA). Pre treatment of cells with HPMPC for 12-24 hr induced an effective antivi ral state, and the cells maintained this antiviral state for >7 days. In contrast, much larger amounts (similar to 2.5-5 x IC50 doses) of PM EA or HPMPA were required to establish an antiviral state, which laste d for only similar to 24 or 72 hr, respectively. A 12-hr treatment of the cells with the phosphonates was required for the establishment of optimal antiviral activity; surprisingly, longer durations of exposure to PMEA (but not HPMPA or HPMPC) resulted in diminished antiviral eff ect. We investigated the metabolism of PMEA and HPMPC to determine the cellular basis for these differences. The cellular uptake of HPMPC wa s similar to 8-fold greater than that of PMEA. The levels of the PMEA metabolites PMEA monophosphate and PMEA diphosphate increased for simi lar to 12 hr and plateaued thereafter. PMEA and its metabolites were c leared from the cells with a half-life of 4.9 hr. In contrast, the HPM PC metabolites HPMPC monophosphate (HPMPCp) and HPMPC diphosphate (HPM PCpp) accumulated throughout the 24-hr study period and, at equimolar drug concentrations (25 mu M), reached intracellular levels similar to 2-3-fold greater than those of the PMEA metabolites. HPMPC also diffe red from PMEA in its capacity to generate a phosphodiester metabolite (HMPCp-choline), which was a predominant metabolite in HPMPC-treated c ells. In addition, the rates of disappearance of intracellular metabol ites of the two drugs were significantly different. Thus, the decay of HPMPCpp was quite slow and biphasic (t(1/2) = 24 and 65 hr) and that of HMPCp-choline was monophasic (t(1/2) = 87 hr). Together, these fact ors can explain the differing antiviral potencies seen with PMEA and H PMPC. The possible role of the choline adduct in the expression of ant iviral activity of the drug remains to be elucidated, but the adduct m ay serve as an intracellular store for the long term maintenance of ac tive HPMPCpp in cells. The results also highlight the extent of divers ity in the cellular pharmacology and antiviral activities of the acycl ic nucleoside phosphonates.