Metabolism of Cyclopropavir and Ganciclovir in Human Cytomegalovirus-Infected Cells

Human cytomegalovirus (HCMV) is a widespread pathogen that can cause severe disease in immunologically immature and immunocompromised patients. The current standard of therapy for the treatment of HCMV infections is ganciclovir (GCV). However, high incidence rates of adverse effects are prevalent and limit the use of this drug. Cyclopropavir (CPV) is 10-fold more effective against HCMV in vitro than GCV (50% effective concentrations [EC50s]=0.46 and 4.1 μM, respectively) without any observed increase in cytotoxicity (S. Zhou, J. M. Breitenbach, K. Z. Borysko, J. C. Drach, E. R. Kern, E. Gullen, Y. C. Cheng, and J. Zemlicka, J. Med. Chem. 47:566-575, 2004, doi:10.1021/jm030316s). We have previously determined that the viral protein kinase pUL97 and endogenous cellular kinases are responsible for the conversion of CPV to a triphosphate (TP), the active compound responsible for inhibiting viral DNA synthesis and viral replication. However, this conversion has not been observed in HCMV-infected cells. To that end, we subjected HCMV-infected cells to equivalently effective concentrations (∼5 times the EC50) of either CPV or GCV and observed a time-dependent increase in triphosphate levels for both compounds (CPV-TP=121±11 pmol/10(6) cells; GCV-TP=43.7±0.4 pmol/10(6) cells). A longer half-life was observed for GCV-TP (48.2±5.7 h) than for CPV-TP (23.8±5.1 h). The area under the curve for CPV-TP produced from incubation with 2.5 μM CPV was 8,680±930 pmol·h/10(6) cells, approximately 2-fold greater than the area under the curve for GCV-TP of 4,520±420 pmol·h/10(6) cells produced from incubation with 25 μM GCV. We therefore conclude that the exposure of HCMV-infected cells to CPV-TP is greater than that of GCV-TP under these experimental conditions.