Distribution of the anticonvulsant felbamate to cerebrospinal fluid and brain tissue of adult and neonatal rats.

The concentrations of felbamate (FBM) and its three metabolites were determined in plasma, cerebrospinal fluid (CSF), and brain tissue of adult and neonatal rats that received single oral doses of FBM at amounts varying from 250 to 2000 mg/kg for adults and from 100 to 500 mg/kg for neonates. The increase in plasma Cmax and AUC0-24 with the dose was less than proportional in both age groups. The highest plasma Cmax in adults dosed with 2000 mg/kg was 140.3 micrograms/ml; in neonates dosed with 500 mg/kg it was 257.0 micrograms/ml. The maximum brain concentrations for these dosages were 90.9 and 220.4 micrograms/g, respectively. The average brain/plasma, CSF/plasma, and brain/CSF partition coefficients for the drug were 0.64, 0.55, and 1.16 for adults and 0.83, 0.67, and 1.23 for neonates, respectively. No statistically significant change of the partition coefficients with time or dose was observed (p < 0.05). Very good linear correlations between FBM plasma concentrations and concentrations in CSF and brain tissue were obtained (r2 > 0.98). Only the 2-hydroxy metabolite was present in considerable amounts in plasma and brain tissue of the high-dose groups of both ages.

Isolation and identification of 3-carbamoyloxy-2-phenylpropionic acid as a major human urinary metabolite of felbamate.

A new metabolite of felbamate, isolated from the polar metabolite fraction of a human urine sample, was identified as 3-carbamoyloxy-2-phenylpropionic acid (CPPA) by electron impact and CI/MS of the methyl ester in the isolated fraction. Its presence in the unconjugated free form in two different human urine samples was confirmed by HPLC comparison of retention times with authentic synthetic CPPA and by on-line negative ion HPLC thermospray tandem mass spectrometric techniques. The amount of CPPA in 0-4 hr postdose human urine was estimated to be approximately 12% of the drug-derived substances present in the urine. Some CPPA was also found to be present in dog urine.

Determination of the anticonvulsant felbamate and its three metabolites in brain and heart tissue of rats.

An automated, internal standard high-performance liquid chromatographic method for the simultaneous quantitation of felbamate and its three metabolites in adult and neonatal rat brain and heart tissue homogenates was developed and validated. The homogenates prepared from one part of the tissue and four parts of water were extracted with ethyl acetate, and the extract was evaporated to dryness and redissolved in mobile phase. Separation was accomplished on a Waters Resolve C18, 5 microns, 300 mm x 3.9 mm I.D. column with a mobile phase consisting of 0.01 M phosphate buffer, pH 6.8-acetonitrile-methanol (800:150:50, v/v/v). Eluting peaks were monitored with an ultraviolet detector at 210 nm. The linear range of the assay for felbamate and the metabolites was 0.20-50.00 micrograms/ml of homogenate or 1-250 micrograms/g of brain or heart tissue. The lower limit of quantitation for all four analytes was 0.20 micrograms/ml of homogenate or 1.00 micrograms/g of tissue.

Pharmacokinetics of the new antiasthma and antiallergy drug, azelastine, in pediatric and adult beagle dogs.

The plasma concentrations and relative bioavailability of azelastine hydrochloride (AZ) and its desmethyl metabolite (DAZ) after a single and 10 once-a-day oral doses of 2.5 mg kg-1 AZ were determined in adult male and female and pediatric male and female beagle dogs. The pediatric and adult dogs were 4-6 weeks and 1-2 years of age, respectively. An analysis of variance (ANOVA) was performed on the bioavailability parameters among all groups and between the first and last doses. No statistically significant (p < 0.05) sex-related differences in the bioavailability parameters were observed. The peak concentration (Cmax) of AZ, especially after the first dose, was significantly higher in pediatric dogs than that in adult dogs, whereas following the multiple AZ administrations, the bioavailability parameters for the last dose were similar in the two age groups. The apparent volume of distribution (Vss) of AZ suggested that the drug was extensively distributed into tissue in adult as well as in pediatric dogs. The Vss was considerably smaller in pediatric dogs, which may explain the higher Cmax in this age group. The bioavailability of the metabolite in adult dogs after multiple administration of AZ was higher than that in pediatric dogs. Although some differences in the parameters among the groups are statistically significant, they do not appear to have any biological significance. Therefore, similar AZ dosages may be required in pediatric and in adult populations to achieve optimum therapeutic drug levels.

Pharmacodynamic and pharmacokinetic studies with azelastine in the guinea pig: evidence for preferential distribution into the lung.

The correlation between the antiasthma activity of azelastine and the concentrations of azelastine and its major metabolite, desmethylazelastine, in the blood and lung were investigated in guinea pigs. Blood and lung tissue samples collected at 15 min after aeroallergen (ovalbumin, 0.5 mg/ml, 30 s, 15 psi) challenge, i.e., 2-1/4 h after oral administration of the drug, were analyzed for azelastine and its desmethyl metabolite by specific HPLC assay. Azelastine afforded protection against immediate allergic responses (IAR, characterized by gasping and bronchospasmic convulsions). Based on the reduction of the severity of IAR, the ID50 was 0.04 mg/kg. It also delayed the onset of IAR and prevented mortalities. A positive correlation (r2 = 0.944) between the antiallergic activity and the sum concentrations of azelastine and desmethylazelastine in the lung was observed. Preferential uptake of azelastine and desmethylazelastine by the lung was demonstrated by the mean lung/blood ratio of about 22 and 146, respectively. The selective uptake of azelastine and its active metabolite by the lungs could contribute to its antiasthmatic/antiallergic activity in guinea pigs.

Pharmacokinetics of felbamate in pediatric and adult beagle dogs.

The relative bioavailability and pharmacokinetics of felbamate (FBM) after a single oral dose and after 10 once-daily oral doses of 60 mg/kg were investigated in adult and pediatric dogs of both sexes. The pediatric and adult dogs were aged 4-6 weeks and 1-2 years, respectively. Analysis of variance (ANOVA) was performed on the bioavailability parameters among all groups and between the first and last doses. No sex-related differences in bioavailability and pharmacokinetic parameters were observed. The bioavailability of FBM in pediatric dogs was significantly less as compared with that in adult dogs. Rapid overall elimination of the drug in pediatric dogs appears to be responsible for the lower bioavailability. The bioavailability of FBM after the last dose was also significantly lower than after the first dose for both age groups. No major differences in the rate constant of FBM absorption (ka) and volume of distribution at steady state (VSS) were observed between the two age groups. As with other clinically useful antiepileptic drugs (AEDs), higher doses of FBM may be required in pediatric populations to achieve optimum drug levels, assuming that age-related changes in FBM disposition will also be confirmed in humans.

Pharmacokinetics of azelastine and its active metabolite, desmethylazelastine, in guinea pigs.

Pharmacokinetics of azelastine (AZ) and its major active metabolite desmethylazelastine (DAZ) after iv or po administration of 1 mg/kg [14C]AZ hydrochloride or unlabeled AZ hydrochloride were studied in guinea pigs. Total 14C radioactivity concentrations in blood, plasma, lung, urine, and feces were determined by liquid scintillation counting. AZ and DAZ concentrations in the plasma and lung samples were determined by HPLC methods. For pharmacokinetic modeling, the mean concentrations of AZ and DAZ in plasma were converted to those in blood. Following the iv or po dose, AZ blood concentrations declined biexponentially with the distribution and elimination phases. The open two- and one-compartment models for AZ and DAZ concentrations in blood, respectively, and the open one-compartment model for the two compounds in lung tissue describe the experimental data reasonably well. The apparent volume of distribution of AZ suggested that the drug was widely distributed in the body. The mean lung/blood concentration ratios, which varied from 14.2 to 20.1 in the iv- and po-dosed animals for AZ and from 96.7 to 140.3 in the iv- and po-dosed groups for DAZ, respectively, indicate the capacity of the lung ("target tissue") for preferential uptake of the drug and its active metabolite. The efficiency of the desmethyl metabolite uptake into the lung was about 6-fold greater than that of AZ. The clearance of AZ from the body was faster than that of DAZ following either po or iv administration. The estimated availability of the AZ oral dose in guinea pigs was 0.19, which suggested that AZ was subjected to an extensive hepatic first-pass metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)

Felbamate metabolism in the rat, rabbit, and dog.

Two major and one minor metabolite of felbamate (FBM) as well as unchanged drug were isolated and identified by electron impact and chemical ionization mass spectrometry from rat and dog urine after dosing with [14C]FBM. The metabolites were 2-(4-hydroxyphenyl)-1,3-propanediol dicarbamate (p-OHF), 2-hydroxy-2-phenyl-1,3-propanediol dicarbamate, and 2-phenyl-1,3-propanediol monocarbamate. The metabolites and FBM were excreted mainly in urine, where their sum accounted to 81-94% of the radioactivity in hydrolyzed rat urine samples, 71-82% in rabbit urine samples, and 69-83% in dog urine samples. The amount of metabolites in the conjugated form was estimated to be 20-35% in rat, 20-30% in rabbit, and 10-20% in dog urine. The major biliary metabolite in all three species was p-OHF, whereas the amount of FBM was small. Metabolites found in dog feces were the same as those in the urine.

Felbamate pharmacokinetics in the rat, rabbit, and dog.

Rats, rabbits, and dogs were given single iv or single and multiple oral doses of felbamate ranging from 1.6-1000 mg/kg. Absorption of oral drug was complete in all species. The mean Cmax increased with dose from 13.9 to 185.9 micrograms/ml in rats, from 19.1 to 161.9 micrograms/ml in rabbits, and from 12.6 to 168.4 micrograms/ml in dogs. The tmax also increased with dose from 1-8 hr in rats, 8-24 hr in rabbits, and 3-7 hr in dogs. The plasma elimination half-life for the drug increased with dose from 2-16.7 hr in rats, 7.2-17.8 hr in rabbits, and 4.1-4.5 hr in dogs. A proportional increase in Cmax with dose was observed in all species up to 300-400 mg/kg doses. A biexponential equation fitted the drug plasma concentration vs. time data well. For multiple oral doses of 50 mg/kg or less, projected and observed steady-state concentrations agreed well. Animals dosed with [14C]felbamate eliminated most of the radioactivity in urine (58-87.7%), less in feces (7-23.7%), with considerable amounts in the bile. In rats, radioactivity was readily distributed into tissues and crossed the placenta and blood-brain barrier, but no accumulation in any tissue was observed. The volume of distribution was 131, 54, and 72% of body weight for rats, rabbits, and dogs, respectively. Binding of drug to rat, rabbit, and dog plasma proteins ranged from 22.4-35.9%. The overall plasma clearance of the drug for rats, rabbits, and dogs was 327, 52, and 108 ml.h-1.kg-1, respectively. Renal clearance of unchanged drug accounted for an estimated 20-35% and hepatic clearance due to metabolism for 65-80% of the overall clearance.

Felbamate in vitro metabolism by rat liver microsomes.

Incubation of [14C]felbamate at 37 degrees C for 60 min with liver microsomes from untreated Sprague-Dawley rats converted 10% of the drug to the p-hydroxy (6%) and 2-hydroxy (4%) metabolites. With microsomes from phenobarbital-pretreated rats, 21% of the drug was metabolized to the p-hydroxy (7.5%) and 2-hydroxy (13.5%) metabolites. With microsomes from felbamate-pretreated rats, up to 25% of drug was metabolized to the p-hydroxy (5%) and 2-hydroxy (20%) metabolites. In addition, a small amount of the monocarbamate metabolite was also present, but no other metabolites were formed. Coincubations of [14C]phenytoin with felbamate had no effect on the metabolism of phenytoin, whereas the amount of [14C]felbamate metabolized in the presence of phenytoin decreased by 30-38%.

Automated liquid chromatographic determination of ranitidine in microliter samples of rat plasma.

An improved high-performance liquid chromatographic method with UV detection at 313 nm has been developed for quantitation of ranitidine in 100 microliter of rat plasma over the range 25 to 1000 ng/ml. To each sample were added the internal standard (metiamide) and 2 M NaOH. After dichloromethane extraction, the nitrogen-dried extracts were reconstituted in the mobile phase of 0.01 M phosphate buffer-triethylamine-methanol-water (530:5:390:75 v/v). Chromatography on mu Bondapak C18 with quantitation by peak height ratios showed an analyte recovery of 97%; a limit of detection of 10 ng/ml; a precision of 1-10% and an accuracy of 1-5%. About 90 samples can be processed in 24 h.