William R. Porter

Changes in the metabolic profiles of R- and S-warfarin and R- and S-phenprocoumon as a probe to categorize the effect of inducing agents on microsomal hydroxylases.

Abstract The biotransformation of R- and S-warfarin was examined using liver microsomes prepared from both noninduced rats and rats pretreated with Arochlor 1254, β-napthoflavone (BNF), pregnenolone-16α-carbonitrile (PCN), phenobarbital (PB), and 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD). For comparison, the metabolism of a closely related coumarin anticoagulant, R- and S-phenprocoumon, was determined using the same microsomal preparations. In noninduced microsomes the overall metabolism of warfarin was about five times that of phenprocoumon, with 7-hydroxywarfarin as the principle metabolite followed by the 6-, 4′- and 8-hydroxy derivatives (benzylic hydroxylation was not examined). For phenprocoumon a different regioselectivity was observed with 4′ hydroxylation being the greatest followed by 6-, and 7 and 8 hydroxylation. In the case of warfarin, hydroxylation with noninduced microsomes was either nonstereoselective (4′ hydroxylation) or selective for the R-enantiomer. The metabolic pattern observed for phenprocoumon showed hydroxylation to be either nonstereoselective (7 and 8 hydroxylation) or, in contrast to warfarin, selective for the S-enantiomer. Induction of cytochrome P-450 by Arochlor, BNF, and TCDD produced a similar metabolic pattern for both substrates in which 6 and 8 hydroxylation were greatly increased over control levels. In keeping with the pattern obtained from noninduced microsomes, a reversed stereoselectivity was again observed after induction with these three agents, i.e. warfarin metabolism was selective for the R-enantiomer and phenprocoumon metabolism was selective for the S-enantiomer. Based on cytochrome P-450 levels PCN decreased the metabolism of both substrates while PB had no effect. However, the induction with PB was readily apparent when calculations were performed on a per mg protein basis.

Adsorption of Insulin to the Surface of Peritoneal Dialysis Solution Containers

Continuous ambulatory peritoneal dialysis (CAPD) is becoming widely used in diabetics with end-stage renal disease. One of the proposed advantages of this technique is the ability to administer insulin intraperitoneally. The administration of insulin by this route provides acceptable plasma glucose control with no need for subcutaneous injections. Because adsorption of insulin to glass and polyvinyl chloride (PVC) surfaces is known to occur with intravenous fluid systems, this study was conducted to measure insulin adsorption to dialysis fluid systems. The effects of time, temperature, heparin concentration, dextrose concentration, and insulin concentration in 2-L glass and PVC dialysis solution containers were studied. While all factors significantly affected adsorption to the PVC containers, the effects of heparin and dextrose were considered to be clinically unimportant. The percentage of insulin adsorbed to the PVC surface increased with increasing time and temperature and decreased with increasing insulin concentrations. Under the conditions studied, 7.1% to 9.6% of insulin added to PVC containers was lost within the first minute. Since 15 to 30 minutes is required by most CAPD patients to prepare and instill the dialysate, 10% to 20% of added insulin may be adsorbed to the PVC bad. The adsorption of insulin to the glass surfaces was rapid with 39.6% to 42.8% being lost within the first minute. All factors studied significantly affected adsorption to the glass, but the effects of time, temperature, dextrose, and heparin were considered to be of minor clinical importance.(ABSTRACT TRUNCATED AT 250 WORDS)

Stereochemical aspects of the metabolism of phenprocoumon in rat liver microsomes

Abstract The oxidation of the enantiomers of phenprocoumon catalyzed by noninduced rat hepatic microsomes was determined, and the formation of products was found to be either nonstereoselective or selective for the (S)-enantiomer. Of the seven possible aromatic monohydroxylated metabolites, only the 4′-, 6-, 7- and 8-hydroxy derivatives were detected. These hydroxylation reactions were shown to be cytochrome P-450-dependent by decreased product formation in the presence of the inhibitors CO, metyrapone and SKF-525A (β-diethylaminoethyl 2,2-diphenylvalerate) and by decreased product formation when NADH was substituted for NADPH. Comparison of these results to those obtained earlier for warfarin reveals that major regioselective and stereoselective differences exist in the in vitro metabolism of these two similar coumarin anticoagulants.

Resolution of valproic acid from deuterated analogues and their quantitation in plasma using capillary gas chromatography

Quantitation of valproic acid and a deuterated analogue in the same plasma sample by capillary gas chromatography without mass spectrometry was illustrated. Specificity was accomplished solely with a 60 m X 0.25 mm fused silica WCOT column coated with OV-351. A hexadeutero and two tetradeutero analogues of valproic acid had resolutions of at least 1.2 from valproic acid. Plasma samples were extracted with carbon tetrachloride following the addition of 2-ethylhexanoic acid as the internal standard. The method is sensitive to at least 0.5 microgram/ml and provides the capability of conducting absolute bioavailability and pulsed dosing studies with deuterated drug analogues without a mass spectrometer. The technique was applied to the analysis of plasma samples from dogs simultaneously administered valproic acid and a deuterated analogue.