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CLINICAL PHARMACOLOGY
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Microbiology
Mechanism of Antiviral Activity: The antiviral compound penciclovir has in vitro inhibitory activity against herpes simplex virus types 1 (HSV-1) and 2 (HSV-2). In cells infected with HSV-1 or HSV-2 viral thymidine kinase phosphorylates penciclovir to a monophosphate form which, in turn, is converted to penciclovir triphosphate by cellular kinases. In vitro studies demonstrate that penciclovir triphosphate inhibits HSV polymerase competitively with deoxyguanosine triphosphate. Consequently, herpes viral DNA synthesis and, therefore, replication are selectively inhibited.
Antiviral Activity in vitro and in vivo: In cell culture studies, penciclovir has antiviral activity against HSV-1 and HSV-2. Sensitivity test results, expressed as the concentration of the drug required to inhibit growth of the virus by 50% (IC50) or 99% (IC99) cell culture, vary depending upon a number of factors, including the assay protocols. See Table 1.
| Table 1 | ||||
| Method of Assay | Virus Type | Cell Type | IC50 (mcg/mL) | IC99 (mcg/mL) |
| Plaque Reduction | HSV-1 (c.i.) | MRC-5 | 0.2-0.6 | |
| HSV-1 (c.i.) | WISH | 0.04-0.5 | ||
| HSV-2 (c.i.) | MRC-5 | 0.9-2.1 | ||
| HSV-2 (c.i.) | WISH | 0.1-0.8 | ||
| Virus Yield Reduction | HSV-1 (c.i.) | MRC-5 | 0.4-0.5 | |
| HSV-2 (c.i.) | MRC-5 | 0.6-0.7 | ||
| DNA Synthesis Inhibition | HSV-1 (SC16) | MRC-5 | 0.04 | |
| HSV-2 (MS) | MRC-5 | 0.05 | ||
(c.i.) = clinical isolates. The latent state of any herpes virus is not known to respond to any antiviral therapy.
Drug Resistance: Penciclovir-resistant mutants of HSV can result from qualitative changes in viral thymidine kinase or DNA polymerase. The most commonly encountered acyclovir-resistant mutants that are deficient in viral thymidine kinase are also resistant to penciclovir.
Pharmacokinetics
Measurable penciclovir concentrations were not detected in plasma or urine of healthy male volunteers (n= 12) following single or repeat application of the 1% cream at a dose of 180 mg penciclovir daily (approximately 67 times the estimated usual clinical dose).
Generic Name: Penciclovir
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