High-performance liquid chromatographic method for the determination of 9-(4-hydroxy-3-hydroxymethylbut-1-yl)guanine (BRL-39123) in human plasma and urine

Penciclovir Pharmacokinetics and Distribution to the Brain and Muscle of Rats, Studied by Microdialysis

Famciclovir, the oral form of penciclovir, is a potent, highly selective antiherpesvirus agent licenced for the treatment of herpes zoster (shingles). Some herpesviruses are prone to infect the central nervous system. To obtain guidance for the possible treatment of herpes encephalitis it is important to study the extent of transport of antiviral agents into the brain. We have used microdialysis to sample the unbound extracellular concentration of penciclovir in the gastrocnemic muscle (which corresponds directly to plasma free concentrations) and in the brain of rats under halothane anaesthesia. Penciclovir (50 mg kg−1) was given intravenously (i.v.) and samples were taken for 5 h after administration. The AUC (area under the time versus concentration curve) (0–5 h) of penciclovir in the brain was 0.096±0.018 (mean±SEM) of the AUC in muscle while the mean ratio of brain to muscle concentration 5 h post-injection was 0.1 80±0.084. Famciclovir given per os to rat at a dose of 1 20 mg kg−1 resulted in a concentration ratio for penciclovir between brain and muscle of 0.415±0.078 at 5 h after administration, while the AUC ratio (0–5 h) was 0.143±0.012. Both of these are higher than after i.v. injection of penciclovir. Penciclovir and famciclovir were also administrated by i.v. infusion (60 and 80 mg kg−1 h−1 respectively). Famciclovir administration (AUC 0.075±0.025 mmol h L−1) did not increase penciclovir transport to the brain compared with penciclovir administration (AUC 0.163±0.018 mmol h L−1).

Estimates of the pharmacokinetics of famciclovir and its active metabolite penciclovir in young Asian elephants (Elephas maximus)

OBJECTIVE

To determine plasma pharmacokinetics of penciclovir following oral and rectal administration of famciclovir to young Asian elephants (Elephas maximus).

ANIMALS

6 healthy Asian elephants (5 females and 1 male), 4.5 to 9 years old and weighing 1,646 to 2,438 kg.

PROCEDURES

Famciclovir was administered orally or rectally in accordance with an incomplete crossover design. Three treatment groups, each comprising 4 elephants, received single doses of famciclovir (5 mg/kg, PO, or 5 or 15 mg/kg, rectally); there was a minimum 12-week washout period between subsequent famciclovir administrations. Serial blood samples were collected after each administration. Samples were analyzed for famciclovir and penciclovir with a validated liquid chromatography-mass spectroscopy assay.

RESULTS

Famciclovir was tolerated well for both routes of administration and underwent complete biotransformation to the active metabolite, penciclovir. Mean maximum plasma concentration of penciclovir was 1.3 μg/mL at 1.1 hours after oral administration of 5 mg/kg. Similar results were detected after rectal administration of 5 mg/kg. Mean maximum plasma concentration was 3.6 μg/mL at 0.66 hours after rectal administration of 15 mg/kg; this concentration was similar to results reported for humans receiving 7 mg/kg orally.

CONCLUSIONS AND CLINICAL RELEVANCE

Juvenile Asian elephants are susceptible to elephant endotheliotropic herpesvirus. Although most infections are fatal, case reports indicate administration of famciclovir has been associated with survival of 3 elephants. In Asian elephants, a dose of 8 to 15 mg of famciclovir/kg given orally or rectally at least every 8 hours may result in penciclovir concentrations that are considered therapeutic in humans.

Development and validation of a high-performance liquid chromatography–tandem mass spectrometry for the determination of penciclovir in human plasma: Application to a bioequivalence study

We developed and validated a simple high-performance liquid chromatography (HPLC) coupled with positive ion electrospray ionization tandem mass spectrometry (ESI-MS/MS) detection system for determining penciclovir (active metabolite of famciclovir) levels in human plasma using acyclovir as an internal standard (IS). Acquisition was performed in multiple reaction monitoring (MRM) mode by monitoring the transitions: m/z 254.00>152.09 for penciclovir and m/z 226.00>152.09 for IS. The analytes were chromatographed on a Capcellpak MGII C(18) reversed-phase chromatographic column by isocratic elution using 30% methanol and 70% Milli-Q water containing 10 mM ammonium formate (adjusted to pH 3.1 with formic acid). Results were linear over the studied range (0.05-10 microg/ml) with r(2)=0.9999, and the total analysis time for each run was 2 min. Intra- and inter-assay precisions were 2.3-7.8 and 3.7-7.5%, respectively, and intra- and inter-assay relative errors (RE) were 2.0-8.4 and 1.9-9.1%, respectively. The lower limit of quantification (LLOQ) was 0.05 microg/ml. At this concentration mean intra- and inter-assay precisions were 7.8 and 7.5%, respectively, and mean intra- and inter-assay relative errors were 2.2 and 9.1%, respectively. Penciclovir was found to be stable in plasma samples under short-, long-term storage and processing conditions. The developed assay was successfully applied to a bioequivalence study of penciclovir administered as a single oral dose (500 mg as famciclovir) to healthy volunteers.

Mode of antiviral action of penciclovir in MRC-5 cells infected with herpes simplex virus type 1 (HSV-1), HSV-2, and varicella-zoster virus

The metabolism and mode of action of penciclovir [9-(4-hydroxy-3-hydroxymethylbut-1-yl)guanine; BRL 39123] were studied and compared with those of acyclovir. In uninfected MRC-5 cells, low concentrations of the triphosphates of penciclovir and acyclovir were occasionally just detectable, the limit of detection being about 1 pmol/10(6) cells. In contrast, in cells infected with either herpes simplex virus type 2 (HSV-2) or varicella-zoster virus (VZV), penciclovir was phosphorylated quickly to give high concentrations of the triphosphate ester. Following the removal of penciclovir from the culture medium, penciclovir-triphosphate remained trapped within the cells for a long time (half-lives, 20 and 7 h in HSV-2- and VZV-infected cells, respectively). In HSV-2-infected cells, acyclovir was phosphorylated to a lesser extent and the half-life of the triphosphate ester was only 1 h. We were unable to detect any phosphates of acyclovir in VZV-infected cells. (S)-Penciclovir-triphosphate inhibited HSV-1 and HSV-2 DNA polymerase competitively with dGTP, the Ki values being 8.5 and 5.8 microM, respectively, whereas for acyclovir-triphosphate, the Ki value was 0.07 microM for the two enzymes. Both compounds had relatively low levels of activity against the cellular DNA polymerase alpha, with Ki values of 175 and 3.8 microM, respectively. (S)-Penciclovir-triphosphate did inhibit DNA synthesis by HSV-2 DNA polymerase with a defined template-primer, although it was not an obligate chain terminator like acyclovir-triphosphate. These results provide a biochemical rationale for the highly selective and effective inhibition of HSV-2 and VZV DNA synthesis by penciclovir and for the greater activity of penciclovir than that of acyclovir when HSV-2-infected cells were treated for a short time.

The tolerance to and pharmacokinetics of penciclovir (BRL 39,123A), a novel antiherpes agent, administered by intravenous infusion to healthy subjects

The tolerance to and pharmacokinetics of intravenously administered penciclovir (BRL 39,123A), a novel anti-herpes agent, were investigated in 15 healthy male subjects. The volunteers were divided into three groups, receiving either 10, 15 or 20 mg/kg penciclovir by a 60 min constant-rate infusion. Blood samples were taken sequentially up to 48 h after the start of the infusion and urine collections made at appropriate intervals up to 72 h. After a simple solid phase extraction, concentrations of penciclovir in plasma and urine were determined using HPLC with U.V. detection. Mean values of Cmax, corresponding usually with the end of infusion, and of AUC appeared to increase proportionately with dose. Furthermore, there was no evidence that dose significantly affected any individual pharmacokinetic parameter. Penciclovir was distributed into tissues with an overall mean volume of distribution of approximately 1.5 l.kg-1, i.e. approximately double that of body water. It was rapidly eliminated, with a mean total plasma clearance of 39.3 l.h-1, and a mean terminal-phase half-life of 2.0 h. The majority of the dose, approximately 70%, was excreted unchanged in the urine. Mean renal clearance of BRL 39,123 was 28.1 l.h-1, which exceeds normal glomerular filtration rate and approaches renal plasma flow. At all dose-levels, the infusions of penciclovir were well tolerated, with no evidence of drug-related adverse events.