"Scientists at the National Institutes of Health report they have discovered in mouse studies that a small molecule released in the spinal cord triggers a process that is later experienced in the brain as the sensation of itch.
Mechanism of Action
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).
The systemic absorption of penciclovir following topical administration has not been evaluated in patients < 18 years of age.
Mechanism of Action
The antiviral compound penciclovir has inhibitory activity against herpes simplex virus types 1 (HSV-1) and 2 (HSV-2). In cells infected with HSV-1 or HSV-2, the viral thymidine kinase phosphorylates penciclovir to a monophosphate form that, in turn, is converted by cellular kinases to the active form penciclovir triphosphate. Biochemical studies demonstrate that penciclovir triphosphate inhibits HSV polymerase competitively with deoxyguanosine triphosphate. Consequently, herpes viral DNA synthesis and, therefore, replication are selectively inhibited. Penciclovir triphosphate has an intracellular half-life of 10 hours in HSV-1 and 20 hours in HSV-2 infected cells grown in culture. However, the clinical significance of the intracellular half-life is unknown.
In cell culture studies, penciclovir has antiviral activity against the following herpes viruses: HSV-1 and HSV-2. The antiviral activity of penciclovir against wild type strains grown on human foreskin fibroblasts was assessed with a plaque reduction assay and staining with crystal violet 3 days postinfection for HSV. The median EC50 values of penciclovir against laboratory and clinical isolates of HSV-1 and HSV-2 were 2 μM (range 1.2 to 2.4 μM, n=7) and 2.6 μM (range 1.6 to 11 μM, n=6), respectively.
Penciclovir-resistant mutants of HSV can result from mutations in viral thymidine kinase (TK) and DNA polymerase genes. Mutations in the viral TK gene may lead to complete loss of TK activity (TK negative), reduced levels of TK activity (TK partial), or alteration in the ability of viral TK to phosphorylate the drug without an equivalent loss in the ability to phosphorylate thymidine (TK altered). The median EC50 values observed in a plaque reduction assays with penciclovir resistant HSV-1 and HSV-2 were 69 μM (range 14 to 115 μM, n=6) and 46 μM (range 4 to > 395 μM, n=9), respectively. The possibility of viral resistance to penciclovir should be considered in patients who fail to respond or experience recurrent viral shedding during therapy.
Cross-resistance has been observed among HSV DNA polymerase inhibitors. The most commonly encountered acyclovir-resistant mutants that are deficient in viral thymidine kinase (TK negative) are also resistant to penciclovir.
DENAVIR was studied in two double-blind, placebo (vehicle)-controlled trials for the treatment of recurrent herpes labialis in which otherwise healthy adults were randomized to either DENAVIR or placebo. Therapy was to be initiated by the subjects within 1 hour of noticing signs or symptoms and continued for 4 days, with application of study medication every 2 hours while awake. In both studies, the mean duration of lesions was approximately one-half-day shorter in the subjects treated with DENAVIR (N=1,516) as compared to subjects treated with placebo (N=1,541) (approximately 4.5 days versus 5 days, respectively). The mean duration of lesion pain was also approximately one half-day shorter in the DENAVIR group compared to the placebo group.
Last reviewed on RxList: 9/25/2013
This monograph has been modified to include the generic and brand name in many instances.
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