Pentachlorophenol toxicokinetics after intravenous and oral administration to rat.

1. The toxicokinetics of pentachlorophenol (PCP) were studied in rats. Doses of 2.5 mg/kg were given i.v. (bolus, five rats) and orally (gastric intubation, five rats). Concentrations in plasma, urine and faeces were measured by capillary g.l.c. with electron-capture detection. 2. After i.v. administration, the clearance and volume of distribution at steady state were 0.026 +/- 0.003 l/h per kg and 0.25 +/- 0.02 l/kg, respectively. These two parameters exhibit low inter-rat variability (coefficients of variation less than 15%). The half-life of the initial decline of PCP plasma concn. was less than 1.3 h, while the second phase half-life was 7.11 +/- 0.87 h. 3. After oral administration the peak plasma concn. (7.3 +/- 2.8 micrograms/ml) occurred between 1.5 and 2 h and absorption was complete (bioavailability = 0.91-0.97). No distinct distribution phase was observed and the elimination half-life was 7.54 +/- 0.44 h. 4. PCP clearance is essentially metabolic since only 5.3 +/- 0.2% dose is eliminated unchanged by the kidney. About 60% dose was recovered in urine, mainly as conjugated PCP and conjugated tetrachlorohydroquinone (TCHQ). 5. For both routes of administration, about 10% dose was recovered in faeces as PCP and/or metabolites, which indicates that biliary excretion contributes to total elimination.

Colon-specific delivery of dexamethasone from a glucoside prodrug in the guinea pig.

Dexamethasone-beta-D-glucoside is a potential prodrug for colonic delivery of the antiinflammatory agent, dexamethasone. The ability of this prodrug to deliver dexamethasone selectively to the colon depends not only on its being slowly absorbed from the alimentary canal, but also on its having chemical and enzymatic stability in the stomach and small intestine. Once reaching the large bowel, it should be quantitatively hydrolyzed to release the active agent. The potential of dexamethasone-beta-D-glucoside for colon-specific delivery of dexamethasone is assessed by determining the rates of its hydrolysis down the alimentary canal of the guinea pig, an animal in which an inflammatory bowel disease model has been developed. The hydrolytic activity is examined in tissues and luminal contents of the stomach, proximal and distal segments of the small intestine, cecum, and colon. For the tissues, the greatest hydrolytic activity is in the proximal small intestine, while the stomach, cecum, and colon have only moderate activity. In contrast, the contents of the cecum and colon show greater activity than the contents of the small intestine and stomach. The luminal contents retained beta-glucosidase activity even after repeated centrifugation and resuspension in a buffer. The activity was unaffected by homogenization. These observations suggest that hydrolytic activity is associated with enzymes located on the surface of luminal cells. The movement and hydrolysis of dexamethasone-beta-D-glucoside down the gastrointestinal tract of the guinea pig are also examined. About 20 to 30% of an oral dose appears to reach the cecum. Here the prodrug is rapidly hydrolyzed to the active drug. From intravenous administration of the prodrug and drug, it is apparent that dexamethasone-beta-D-glucoside is poorly absorbed in the gastrointestinal tract (bioavailability, less than 1%). There is a ninefold selective advantage for delivery of dexamethasone in cecal tissues in the guinea pig under the conditions of this experiment. Thus, there is a potential for a decrease in the usual dose and a concomitant reduction in the systemic exposure to dexamethasone. Because humans have much less glucosidase activity in the small intestine, even greater site-selective delivery to the cecum and colon is expected.

Metabolic studies on phencyclidine: characterization of a phencyclidine iminium ion metabolite.

Studies on the metabolic bioactivation of the psychotomimetic amine phencyclidine have been pursued through the characterization of a new metabolite which is formed via initial cytochrome P-450 catalyzed oxidation of the parent drug to the corresponding iminium species. CI mass spectrometric and diode array UV and 1H NMR spectral analyses provided evidence for the conjugated amino enone compound, 1-(1-phenylcyclohexyl)-2,3-dihydro-4-pyridone. Confirmation of the proposed structure was achieved by comparing the 1H NMR and high-resolution EI mass spectral properties of the metabolic isolate with the corresponding spectra of an authentic synthetic sample. Possible intermediates involved in the formation of the dihydropyridone metabolite from the phencyclidine iminium ion are discussed in terms of structural analogies to reactive intermediates formed in the bioactivation of the nigrostriatal toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP).

Phencyclidine iminium ion. NADPH-dependent metabolism, covalent binding to macromolecules, and inactivation of cytochrome(s) P-450.

The phencyclidine iminium ion (PCP-Im+), a potentially reactive 2,3,4,5-tetrahydropyridinium species, is formed by the cytochrome(s) P-450-catalyzed alpha-carbon oxidation of phencyclidine (PCP), a commonly abused psychotomimetic agent. Incubation of PCP-Im+ with liver microsomes obtained from phenobarbital-induced rabbits resulted in over 50% loss of microsomal N-demethylase activity and 30% reduction in cytochrome(s) P-450 content. These effects were concentration-dependent, irreversible, and exhibited pseudo-first order kinetics, characteristics of a mechanism-based enzyme inactivation process. Incubation of 3H-PCP-Im+ with liver microsomes resulted in covalent binding of radioactive material to macromolecules by a process that also was NADPH-dependent. PCP-Im+ was metabolized by liver microsomes in the presence of NADPH and this metabolism was inhibited by SKF 525A and carbon monoxide. HPLC analysis has led to the preliminary characterization of an oxidized metabolite of PCP-Im+ which also is formed from PCP. These results support the proposal that this tetrahydropyridinium metabolite of PCP is biotransformed in a cytochrome(s) P-450-catalyzed reaction to form reactive species capable of covalent interactions with biomacromolecules.

Cation-exchange high-performance liquid chromatography assay for the nigrostriatal toxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and its monoamine oxidase B generated metabolites in brain tissues.

This paper describes a sensitive (1 pmol/mg tissue) and selective bioanalytical method for the quantitative estimation of the nigrostriatal toxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its monoamine oxidase B generated metabolites, the 1-methyl-4-phenyl-2,3-dihydropyridinium species MPDP+ and the 1-methyl-4-phenylpyridinium species MPP+. The method is based on initial separation of the analytes after treatment of brain tissue homogenates with 5% trichloroacetic acid. The soluble fraction is analyzed directly by cation-exchange high-performance liquid chromatography employing a diode array UV detector. Results obtained with this assay have provided the first evidence for the presence of MPDP+ in the mouse brain following intravenous administration of MPTP.

Metabolism-dependent inactivation of liver microsomal enzymes by phencyclidine.

Incubation of phencyclidine (PCP) with rabbit liver microsomes resulted in NADPH-dependent loss of N-demethylase activity accompanied by reduction in microsomal cytochrome P-450 content. This effect was concentration-dependent, exhibited pseudo-first order kinetics, and was irreversible, thus exhibiting characteristics of "suicide substrate" inhibition. Cyanide ions at low concentrations, which have been used to trap the iminium intermediate of PCP metabolism as its cyano adduct, antagonized the inhibition of N-demethylase by PCP. PCP iminium ions were effective inhibitors of microsomal enzyme activity but required NADPH. These results support our suggestions that iminium ion formation is an intermediary step in the bioactivation of PCP leading to reactive electrophilic species.