Testing the sensitivities of noncognate inhibitors to varicella zoster virus thymidine kinase: implications for postherpetic neuralgia therapy with existing agents.

Varicella zoster virus (VZV), a member of the human herpesvirus family, affects peripheral or cranial nerves and can reactivate years after the primary infection. Thymidine kinase (TK) is essential for VZV replication, and its active site is highly conserved in the herpesvirus family. A number of small-molecule inhibitors have already been successfully developed that target the TK of herpes simplex virus type 1 (HSV-1), which is one of the most prevalent sexually transmitted infections worldwide. In the present study, we attempted to test the sensitivities of HSV-1 TK inhibitors to their noncognate VZV TK by integrating in silico modeling and an in vitro assay. We tested nine representative HSV-1 TK inhibitors, including three FDA-approved drugs and six compounds that are under clinical development. The structures of the complexes of these inhibitor ligands with HSV-1 TK and noncognate VZV TK had been solved previously by X-ray crystallography or were modeled in the present work using a template-based approach. Subsequently, a rigorous quantum mechanics/molecular mechanics (QM/MM) nonbonded analysis that accounted for the Poisson-Boltzmann/surface area (PB/SA) solvent effect was employed to refine the complex structures and, on this basis, to evaluate the binding potencies of these complexes. As might be expected, the QM/MM-PB/SA-derived free energy was shown to be highly correlated with the HSV-1 TK inhibitory activities of the nine inhibitors. Further, it was found that the HSV-1 TK inhibitors exhibit strong binding affinities for their noncognate VZV TK, although they are still more selective for HSV-1 TK than for VZV TK. In order to test the theoretical results obtained from the computational analysis, we performed an in vitro kinase assay to determine the inhibitory potencies of three commercially available antiviral agents, namely penciclovir, ganciclovir, and aciclovir, against their noncognate target VZV TK, resulting in IC50 values of 86, 127, and 150 µM respectively, which are modestly weaker than the corresponding values obtained for HSV-1 TK. In addition, visual structure examination and virtual mutation/deletion analysis suggested that the residue Arg222 is present at the active site of HSV-1 TK but not at the active site of VZV TK, which is the reason for the difference in inhibitor selectivity between HSV-1 TK and VZV TK.