An acyclic phosphonate prodrug of HPMPC is effective against VZV in skin organ culture and mice

Varicella zoster virus (VZV) causes chicken pox and shingles and is prevalent worldwide. Acyclovir and penciclovir (and its prodrugs) are first-line treatments for VZV infections, but they are not highly potent against VZV and resistance may arise in immunocompromised people on long-term therapy. HPMPC (cidofovir) is active against VZV, but cidofovir is not approved for treating VZV diseases, is nephrotoxic, and is not orally bioavailable. Here, we present the synthesis and evaluation of USC-373, a phosphonate prodrug of HPMPC with activity against VZV and other DNA viruses. In cultured fibroblasts, it was potent against VZV Ellen laboratory strain and was not overtly toxic, with EC50 of 4 nM and CC50 of 0.20 μM, producing a selectivity index of 50. In ARPE-19 cells, USC-373 was effective against VZV-ORF57-Luc wild type strain and the acyclovir-resistant isogenic strain. In human skin organ culture, USC-373 formulated in cocoa butter and applied topically prevented VZV-ORF57-Luc spread without toxicity. In NuSkin mice with human skin xenografts, one daily dose of 3 mg/kg was effective by the subcutaneous route, and one daily dose of 10 mg/kg was effective by the oral route. Remarkably, a 10 mg/kg oral dose given every other day was also effective. USC-373 was well tolerated and mice did not lose weight or show signs of distress. The prodrug modifications of USC-373 increase the potency and oral bioavailability compared to its parent nucleoside analog, HPMPC.