This led to the synthesis of a trichloro analog in Townsend’s laboratory at the University of Utah and later the discovery of its see more activity against HCMV in John’s laboratory. Much work, in both their laboratories at the University of Michigan, established that it and its 2-bromo

analog (BDCRB) have excellent activity against HCMV with very low cytotoxicity. Surprisingly, it was found to be inactive against other herpes viruses and it did not need conversion to a triphosphate to be active against HCMV. Collaborative studies with Karen Biron at Burroughs Wellcome established that, unlike many other anti-virals that inhibit viral DNA synthesis such as ganciclovir (GCV), these compounds acted by a novel mechanism, inhibition of viral DNA processing. It was the viral resistance studies which revealed the viral targets, pUL89 and pUL56. These two proteins, with pUL104, form a complex known as the terminase which cuts newly synthesised HCMV DNA into unit lengths for packaging into virions. Although BDCRB had many desirable properties in

vitro, it had poor pharmacokinetics in mice and monkeys due to hydrolysis of its glycosidic bond; therefore it was not developed for human use. Much additional work in Drach’s and Townsend’s laboratories at Michigan and by Biron’s group at Burroughs Wellcome ultimately buy KRX-0401 led to two potential drug candidates, BDCRB pyranoside and maribavir ( Fig. 2). Both compounds have excellent activity against HCMV, low toxicity, and excellent pharmacokinetics. Clearly, their modes of action differed

markedly from that of GCV. Quite unexpectedly, they have different mechanisms of action. BDCRB pyranoside has a mechanism of action very similar to its parent compound BDCRB, inhibition of DNA processing. In contrast, maribavir inhibits DNA synthesis, albeit indirectly. It is a 2-isopropylamine derivative of BDCRB except that it has the unnatural L-sugar configuration. Its mechanism of action involves inhibition of the viral kinase (pUL97), which phosphorylates another viral protein, pUL44. Phosphorylated pUL44 is necessary for viral DNA synthesis. Thus inhibition of pUL97 by maribavir inhibits viral DNA synthesis. Interestingly, Inositol monophosphatase 1 pUL97 is also the kinase that activates (phosphorylates) GCV. Resistance studies confirmed that a single mutation in UL97, resulting in a mutation in the kinase (Leu397Arg), was necessary and sufficient for resistance to maribavir. In a further study of resistance, virus already resistant to BDCRB was passaged in increasing concentrations of maribavir and resistant virus was isolated. This strain grew at the same rate as the wild-type virus and was resistant to both BDCRB and maribavir. As expected, resistance to BDCRB was due to known mutations in UL56 and UL89. However, no mutations were found in UL97.