Extended substrate acceptance of herpes simplex virus type 1 thymidine kinase: a new chance for gene and antiviral therapy

Herpes simplex virus type I thymidine kinase (HSV1-TK) has become increasin gly important as a target in medicinal chemistry because of its links to th erapy of viral infection, gene therapy of cancer and allogeneic transplanta tion. These applications are based on the differences in binding properties between the human and the viral enzyme. Several problems have been encount ered in the clinic, e.g. the increase of resistance for antiviral drugs and the immunosuppressive effects of the dosages needed for tumor regression. Thus intensive efforts have been directed towards understanding substrate d iversity to overcome the clinical limitations. In this context, kinetic and thermodynamic studies revealed that substrates bind in compulsory order an d that the binding event is enthalpy driven. The structural evaluation of a ciclovir resistant HSV strains shows that loss of electrostatic interaction s, change in steric accessibility acid modification of the 3D conformation of HSVI-TK are responsible for the encountered resistance. Further crystall ography studies revealed the role of water in substrate binding, the advant age of a fixed ribose ring and that substrate acceptance of HSV1-TK is exte nded to all five nucleobases. The reviewed results give new rationale for t he design of novel prodrugs and engineered HSV1-TK for antiviral and gene t herapy.