Background: Cidofovir is a nucleotide analogue with antiviral activity against a wide range of DNA viruses, including herpes simplex virus (HSV). In vitro resistance to cidofovir has been reported with use of laboratory HSV strains; clinical failure of cidofovir therapy for HSV disease has also been reported with an isolate that was susceptible by in vitro testing.
Patients and methods: Three patients with HSV disease that was unresponsive to cidofovir therapy had viral isolates obtained and stored; the isolates were analyzed for antiviral susceptibility by an antigen reduction assay (ARA). PCR cloning and automated sequencing were performed for isolates that displayed in vitro resistance. Mutations were identified by comparison with the appropriate HSV consensus sequence.
Results: An HSV type 2 (HSV-2) isolate recovered from patient 2 displayed in vitro cidofovir resistance with an inhibitory concentration of 50% (IC50) of 13.06 microg/mL. HSV type 1 isolates recovered from patients 1 and 3 had elevated IC50s to cidofovir (7.32 and 8.23 microg/mL, respectively); however, these isolates did not meet the cutoff point for resistance according to the ARA. Sequencing of a cidofovir-resistant HSV-2 isolate revealed several DNA polymerase mutations that had not been previously described during in vitro resistance selection.
Conclusions: To our knowledge, this is the first report of cidofovir resistance in HSV developing in vivo. The 3 sequenced clones all contained mutations truncating the pol C' end, suggesting that this region may be critical for cidofovir antiviral activity. In addition, the presence of multiple mutations suggests that the altered DNA polymerase of cidofovir-resistant virus may have introduced additional mutations into the viral genome. Introduction of the mutations identified in wild-type HSV strains is needed before the resistance phenotype can be definitively associated with any of the mutations found. Additional studies are needed to delineate the mechanisms of cidofovir resistance in HSV.