Available online at www.sciencedirect.com Antiviral Research 78 (2008) 268–274 Penciclovir is a potent inhibitor of feline herpesvirus-1 with susceptibility determined at the level of virus-encoded thymidine kinase Islam T.M. Hussein 1 , Rebecca V. Menashy, Hugh J. Field ∗ Department of Veterinary Medicine, University of Cambridge, Madingley Road, Cambridge CB3 0ES, UK Received 17 July 2007; accepted 19 October 2007 Abstract Feline herpesvirus-1 (FHV-1) is the causative agent of a severe ocular disease in cats for which a safe potent antiviral chemotherapeutic agent is highly demanded. The sensitivity of FHV-1 to inhibition by three anti-herpetic nucleoside analogues [acyclovir (ACV), penciclovir (PCV) and cidofovir (CDV)] was tested by means of yield reduction assay. ACV showed very poor ability to inhibit FHV-1 replication. At low multiplicity of infection (MOI), both PCV and CDV were nearly equally effective with IC 50 values ranging between 6 and 8 g/ml. However, when the MOI was raised to 3 PFU/cell, the activity of CDV was markedly reduced (IC 50 25 g/ml), while that of PCV remained relatively low (IC 50 10 g/ml). Although FHV-1 is normally insensitive to ACV, it exhibited >1000-fold increase in sensitivity when the thymidine kinase (TK) encoded by herpes simplex virus-1 (HSV-1) was supplied in trans. Furthermore, three PCV-resistant FHV-1 variants selected in vitro were shown to carry mutations in the TK gene. Taken together, these data provided direct evidence that PCV is a potent selective inhibitor of FHV-1 and that the virus-encoded TK is an important determinant of the virus susceptibility to nucleoside analogues. © 2008 Elsevier B.V. All rights reserved. Keywords: Feline herpesvirus; Thymidine kinase; Penciclovir 1. Introduction Feline herpesvirus-1 (FHV-1), a member of the -herpesvirus subfamily, is the most common viral pathogen of domestic cats worldwide. It causes a severe upper respiratory tract and ocular disease in cats characterized by conjunctivitis, profuse ocular and nasal discharges, and in some cases, severe keratitis and corneal ulceration. In kittens, the infection can generalize result- ing in mortality rates of up to 50% (Gaskell and Dawson, 1994; Andrew, 2001; Maggs, 2005). Despite routine vaccination, FHV- 1 disease continues to pose a major threat to feline health, due to the establishment of lifelong neuronal latency interspersed with episodes of viral reactivation and shedding (Gaskell and Willoughby, 1999; Stiles, 2003). Antiviral chemotherapy is a standard practice in the man- agement of herpesvirus infections in humans, and currently there are 11 licensed anti-herpetic compounds available in the ∗ Corresponding author. Tel.: +44 1223 330810; fax: +44 1223 337610. E-mail address: hjf10@cam.ac.uk (H.J. Field). 1 Present address: HIV Drug Resistance Program, NIH/NCI-Frederick, Fred- erick, MD 21702, USA. human clinic (De Clercq et al., 2006). Two important groups of anti-herpetic drugs, which target the viral DNA polymerase, are the acyclic nucleoside analogues [e.g. acyclovir (ACV), penciclovir (PCV) and ganciclovir (GCV)] and acyclic nucleo- side phosphonates [e.g. cidofovir (CDV)] (De Clercq and Hol ´ y, 2005; Field and Whitley, 2005). Herpesvirus-encoded thymi- dine kinase (TK) is the molecular basis for the selective activity of acyclic nucleoside analogues. Studies on the mode of action of ACV have shown that it is preferentially phosphorylated in infected cells by the viral TK to ACV-monophosphate (ACV- MP), which is in turn converted to ACV-triphosphate (ACV-TP) by cellular kinases. ACV-TP is the active form that inhibits herpesvirus DNA replication (Elion, 1993; Coen and Schaffer, 2003). Since the acyclic nucleoside phosphonates already con- tain a phosphonate group, they need only two, instead of three, phosphorylation steps in order to reach the active form, thus they do not depend on the viral TK to exert their antiviral action (De Clercq, 2003). No effective antiviral therapy for the treatment of FHV-1 infections currently exists. Acyclovir, a potent inhibitor of her- pes simplex virus-1 (HSV-1) with an exceptional safety record (Elion, 1993), is the only anti-herpetic drug that has so far received adequate clinical and research attention in veterinary 0166-3542/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.antiviral.2007.10.015