This study describes the extent of cross-resistance and interactions for selected inhibitors of human cytomegalovirus (HCMV) DNA synthesis and DNA processing. HCMV isolates resistant to the benzimidazole D-ribonucleoside viral DNA processing inhibitors TCRB and BDCRB were sensitive to BAY 38-4766, a non-nucleoside inhibitor of viral DNA processing. This indicates that these two drug types have distinct interactions with the products of HCMV genes UL56 and UL89 required for viral DNA cleavage and packaging. These virus isolates also were sensitive to ganciclovir (GCV) but slightly resistant to the L-benzimidazole ribonucleoside viral DNA synthesis inhibitor 1263W94. Virus resistant to 1263W94 remained sensitive to BDCRB, GCV, and BAY 38-4766. Examination of drug-drug interactions in cell culture assays measuring inhibition of HCMV replication revealed strong synergism for the combination of BDCRB with 1263W94, and for combinations of 1263W94 with cidofovir (CDV) and foscarnet (PFA), but not with GCV. Combinations of GCV with CDV and PFA were synergistic as well. The combination of GCV with 1263W94 showed additive antiviral interactions, whereas, a combination of BAY 38-4766 with GCV showed antagonism. Interaction of BDCRB with BAY 38-4766 showed a mixed pattern of synergy and antagonism. The antiviral synergy observed between GCV and PFA or CDV serves to validate clinical combination therapies for these drugs. Antagonism seen for BAY 38-4766 with GCV indicates that these two drugs are unlikely to be useful for combination therapies. Notably, 1263W94 demonstrated greater synergy in combination with PFA or CDV than did GCV, suggesting some promise for this benzimidazole L-riboside in such combination therapies.
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