Placental transfer and distribution of pinazepam and its metabolite N-desmethyldiazepam in the maternal and fetal rabbit: effect of the stage of gestation.

The distribution of pinazepam and its metabolite N-desmethyldiazepam was studied in fetuses of New Zealand rabbits on the 20th and 27th day of pregnancy. The concentrations of both compounds were also measured in the maternal brain, liver and uterus. Pregnant rabbits were sacrificed at 0.5, 2, 4 and 12 h after intravenous administration of pinazepam (5 mg/kg). The concentrations of pinazepam and N-desmethyldiazepam in various biological specimens were measured by a specific gas-chromatographic procedure. Pinazepam and N-desmethyldiazepam rapidly crossed the placenta. In 20 day old fetuses, comparable concentrations of pinazepam were found in the liver, brain, heart, lungs and kidneys. In contrast, the liver of 27 day old fetuses accumulated pinazepam at concentrations higher than the other tissues. The hepatic extraction of pinazepam, already described in adult rabbits (1), develops prenatally. A preferential accumulation of pinazepam rather than N-desmethyldiazepam was also observed in the maternal uterus. In this tissue the concentrations of pinazepam were 5-10 times higher on the 27th rather than the 20th day of pregnancy. The stage of pregnancy influences the distribution pattern of pinazepam in rabbit fetuses and their mothers.

Placental transfer of pinazepam and its metabolite N-desmethyldiazepam in women at term.

Placental transfer of pinazepam and its metabolite N-desmethyldiazepam was investigated in 25 pregnant women at term. Pinazepam was administered orally as a single (10 mg) dose to 13 women, or in multiple doses of 5 mg daily to 12 women. The dose-delivery interval ranged between 1 and 26 h for the single dose, and the period between the last of the multiple doses and delivery was 1.4 to 24 h. Pinazepam and N-desmethyldiazepam were measured in plasma obtained from the umbilical vein and from the mother, at delivery. Pinazepam was only detectable in plasma after the 10 mg dose. The drug did not reach an apparent equilibrium between fetal and maternal plasma. The average (+/- SEM) cord/maternal ratio of plasma pinazepam concentrations was 0.64 +/- 0.07. N-desmethyldiazepam was detectable on each occasion. Its concentration in the plasma from the cord vein became higher than that in the maternal specimens 1-2 h after administration of the parent drug. Little N-desmethyldiazepam was excreted in breast milk.

Subcellular distribution of styrene oxide in rat liver.

The subcellular distribution of (3H )-styrene-7,8-oxide was studied in the rat liver. The compound was added to liver homogenate to give a final concentration of 2 X 10(-5); 2 X 10(-4) and 2 X 10(-3) M. Subcellular fractions were obtained by differential centrifugation. Most of styrene oxide (59-88%) was associated with the cytosolic fraction. Less than 15 percent of the compound was retrieved in each of the nuclear, mitochondrial and microsomal fractions. A considerable percentage of radioactivity was found unextractable with the organic solvents, suggesting that styrene oxide reacted with the endogenous compounds. The intracellular distribution of this epoxide was also studied in the perfused rat liver. Comparable results with those previously described were obtained. The binding of styrene oxide to the cytosolic protein was investigated by equilibrium dialysis and ultrafiltration. Only a small percentage of the compound was bound to protein.

Subcellular distribution of pinazepam and its metabolite N-desmethyldiazepam in rat liver.

The distribution in subcellular fractions of pinazepam (Pz) and its metabolite N-desmethyldiazepam (N-Dz) was studied after a single oral administration of Pz (20 mg/kg body wt) to rats. Animals were killed 2 min and 6 hr after dosing. The amounts of the parent drug and its metabolite retrieved in the microsomal fraction were small, but significantly higher than those in the nuclear, mitochondrial and soluble fractions. The cytosol contained the lowest percentages of both drugs. No substantial difference was found in the subcellular distribution pattern of both Pz and N-Dz 2 min and 6 hr after administration.

Disposition, distribution, plasma protein binding and biliary excretion of pinazepam after i.p. administration to rat.

The pharmacokinetics, distribution, plasma protein binding and the biliary excretion of pinazepam were studied in the rat. The drug was administered (5 mg/kg) by i.p. injection. The chemical analysis of pinazepam and its metabolites was carried out by a gas-chromatographic method. The parent compound was rapidly absorbed, accumulated into the tissues and converted into N-desmethyldiazepam. The highest plasma levels of the parent compound (367 +/- 13 ng/ml) were found 3 min after administration. The volume of distribution and the clearance of the drug were 1315 ml and 7.23 ml/min respectively. The metabolite was detected in the plasma and tissues 3 min after administration. At this sampling time its concentrations were 76 +/- 16 ng/ml in the plasma and 1081 +/- 68 ng/g in the liver. The decay curve of both pinazepam and N-desmethyldiazepam in the plasma, liver, lung, heart, kidney, brain, and gastrochemius muscle were characterized for their Kel, t 1/2, and AUC values. The tissue AUC to plasma AUC ratios indicated a preferential accumulation of pinazepam over its metabolite in the tissues. The plasma protein binding of pinazepam was measured by dialysis at the equilibrium. Rat plasma proteins bound 89.17 +/- 0.20 percent of the drug. The association constant was 2.60 X 10(3) l/mole and the number of sites 0.44 X 10(-6) sites/g. The biliary excretion of pinazepam and N-desmethyldiazepam was poor.

Disposition, distribution, and liver first-pass effect of pinazepam in the rat.

The disposition, distribution and the hepatic first-pass effect of pinazepam and its metabolite N-desmethyldiazepam was investigated in rats. Pinazepam (20 mg/kg) was orally administered by stomach intubation. Plasma and tissue concentrations of the parent compound and metabolite were measured by GLC analysis. Pinazepam was rapidly absorbed (ka = 2.77 hr-1) and converted into N-desmethyldiazepam, plasma levels of which were higher than those of parent compound shortly after administration. The elimination rate constants were 0.20 hr-1 for pinazepam and 0.69 hr-1 for N-desmethyldiazepam. Liver contained the highest concentrations of both compounds. Brain, lung, heart, and kidney preferentially accumulated pinazepam when compared to the gastrocnemius muscle, which accumulated both compounds poorly. The first-pass effect of pinazepam by the liver was studied by analyzing blood from the portal and hepatic veins in rats. Ten minutes after dosing, the mean plasma concentrations of parent compound and metabolite, respectively, were 367.5 and 22.1 ng/ml in the portal vein and 9.8 and 40.7 ng/ml in the hepatic vein. Therefore, rat liver avidly extracted pinazepam from the blood and rapidly metabolized it into N-desmethyldiazepam.

Elimination kinetics of desmethyldiazepam in two young and two elderly subjects.

Pinazepam (P) (10 mg) was orally administered to two young (22-23 years) and two elderly (74-80 years) healthy volunteers. Also, desmethyldiazepam (DD) (10 mg) was orally administered to the same young volunteers on a separate occasion. P was almost completely converted into DD 24 h after administration. Plasma levels of DD were measured in all subjects. In all cases, the DD concentration-time curve showed a biphasic decay (a first slow decay was followed by a fast one) which closely fitted by a Michaelis-Menten equation. Vmax and km constant were computed for all subjects. The kinetic analysis relative to the young subjects showed that T 1/2 of both the slow and fast decay as well as the area under the curve (AUC) were reduced when P was administered instead of DD. Comparison of DD kinetics between young and elderly subjects showed an increase of half-life in the elderly, while AUC was found unchanged.