Inhibition of N-nitrosodimethylamine metabolism in rats by ether anesthesia.

Short-term exposure to diethyl ether strongly inhibits the metabolism of N-nitrosodimethylamine (NDMA). Twenty-six 6-week-old male Fischer 344 rats were exposed to ether vapor until their righting reflex was lost (approximately 2 min). The animals were removed from the ether and NDMA was immediately administered by i.v. bolus injection at a dose of 300 microgram/kg via a cannula surgically inserted 20 h earlier. A second group of 28 rats received injections of NDMA in an identical manner but without ether exposure. In the unanesthetized animals blood levels of NDMA declined with a half-life of 11 min; by contrast essentially constant blood levels of NDMA were observed in ether-treated animals for 120 min after removal from the anesthetic. The apparent total systemic clearance for the 5-h experiment was reduced from 43 ml/min/kg without ether to 5 ml/min/kg with ether. Diethyl ether has been found previously to inhibit the metabolism of other drugs requiring oxidative metabolism but the suppression of clearance documented here appears to be unusually pronounced. It is recommended that ether's potential for altering metabolic rates be carefully considered when planning or interpreting animal experiments.

Low-dose in vivo pharmacokinetic and deuterium isotope effect studies of N-nitrosodimethylamine in rats.

The rates of elimination of N-nitrosodimethylamine (NDMA) and its fully deuterated analogue (N-nitrosodi[2H6]methylamine, [2H6]NDMA) were studied in vivo to explore the origins of the difference in their carcinogenicity. Male Fischer 344 rats, 7.5 weeks of age, were given nitrosamine bolus doses of 1.35 mumol/kg by tail vein injection and 2.02 or 4.05 mumol/kg by p.o. gavage. Animals were sacrificed at various time points from 2.5 to 180 min after i.v. administration or 5 to 120 min after p.o. dosage, and their blood was analyzed for NDMA by gas chromatography-high resolution mass spectrometry. After i.v. injection, blood nitrosamine concentrations declined in an apparently biexponential manner with a terminal half-life of 10 min for NDMA and 12 min for [2H6]NDMA. The apparent total systemic blood clearances for NDMA and [2H6]NDMA were 39 and 26 ml/min/kg, respectively. The apparent steady-state volumes of distribution were nearly identical (297 and 309 ml/kg, respectively). The areas under the curve after 2.02- and 4.05-mumol/kg p.o. doses were proportional to dose. The apparent bioavailability of NDMA was 8%, while that of [2H6]NDMA was 21%. Isotope effects calculated as the ratios of first-pass metabolism, total systemic clearances, bioavailabilities, and intrinsic hepatic clearances were 1.2, 1.5, 2.6, and 3.2, respectively. The isotope effect determined from blood concentrations measured after simultaneous administration of NDMA and [2H6]NDMA by steady-state infusion (each at 1.5 mumol/kg/h) was 2.6 +/- 0.9 (SD). This study thus provides quantitative reference data on the time course of the disappearance of both N-nitrosodimethylamine and its deuterated analogue from blood (over 5 to 8 half-lives) after doses similar to those used to elicit liver tumors in chronic feeding studies, confirms the first-pass effect on their metabolism using direct blood measurements, and permits estimation of their bioavailabilities from actual blood concentrations. The results suggest that elimination pathways not involving alpha-hydroxylation are more important than is currently recognized.

Metabolic denitrosation of N-nitrosodimethylamine in vivo in the rat.

Enzymatic denitrosation is a potentially inactivating metabolic route that has been shown to convert carcinogenic N-nitrosodimethylamine (NDMA) to methylamine (MA) in vitro. To investigate its quantitative course in vivo, groups of 8-week-old male Fischer rats have been given small (8-15 mumol/kg) p.o. or i.v. bolus doses of 14C-labeled NDMA and the subsequent formation of radioactive MA has been monitored by high performance liquid chromatographic analysis of serially collected blood samples from each individual. Adjusting the [14C]MA fluxes observed for the previously measured rates at which MA is itself eliminated from the system after intragastric administration, denitrosation was calculated to represent a rather uniform 21.3 +/- 1.3% (SE) of total NDMA elimination in the four animals studied. By contrast, repetition of the experiment with fully deuterated NDMA (NDMA-d6) revealed a significantly wider variance in the results (39.8 +/- 8.9%). An alternative calculation using values for elimination of i.v. doses of MA and its trideuteromethyl analogue gave an even larger difference for MA formation between NDMA and NDMA-d6, the estimated extents of in vivo denitrosation in this case being 14.5 +/- 0.9% and 48.3 +/- 10.8%, respectively. The results indicate that denitrosation is a major metabolic pathway for NDMA elimination and suggest that deuteration of the carcinogen induces a shift in its metabolism toward increasing denitrosation at the expense of the competing activation pathway. Consequently, denitrosation may be the previously undefined in vivo metabolic route, the existence of which was suggested by the findings that deuteration of NDMA lowered its hepatocarcinogenicity and liver DNA alkylating ability in rats.

Deuterium isotope effect on denitrosation and demethylation of N-nitrosodimethylamine by rat liver microsomes.

In an attempt to elucidate the molecular basis for the decrease in rat liver carcinogenicity and DNA-alkylating ability that accompanies deuteration of N-nitrosodimethylamine (NDMA), NDMA and its fully deuterated analogue ([2H6]NDMA) were incubated with acetone-induced rat liver microsomes. Rates for the competing metabolic routes, denitrosation and demethylation, were determined from colorimetric data on nitrite and formaldehyde generation, respectively. The Vmax calculated for demethylation of NDMA was 7.9 nmol/min/mg, while that for denitrosation was 0.83 nmol/min/mg. Deuteration of NDMA did not significantly change the Vmax for either pathway, but it did increase the Km for demethylation from 0.06 to 0.3 mM. The Km for denitrosation was also increased from 0.06 to 0.3 mM on deuteration, as determined by incubating an equimolar mixture of amino-15N-labeled NDMA with [2H6]NDMA and measuring the methyl[15N]amine:[2H3]methylamine ratio by derivatization-gas chromatography-mass spectrometry. The fact that the Km values for denitrosation were so similar to those for demethylation suggested that the two pathways were catalyzed by the same enzyme. The isotope effects calculated from these data [VmaxH/VmaxD approximately 1 and (Vmax/Km)H/(Vmax/Km)D approximately 5] show that microsomal metabolism of NDMA is not significantly shifted from demethylation to denitrosation on deuteration of substrate and may indicate a low commitment to catalysis for the enzyme. The results are consistent with the view that the metabolism of NDMA is initiated by formation of an alpha-nitrosamino radical which either combines with a hydroxyl radical to form the alpha-hydroxynitrosamine as the initial product of the demethylation pathway or fragments to nitric oxide and N-methylformaldimine as the first products of denitrosation.

In vivo inhibition of oxidative drug metabolism by, and acute toxicity of, 1-aminobenzotriazole (ABT). A tool for biochemical toxicology.

One hour following intravenous pretreatment of rats with 50 mg/kg of the cytochrome P-450 suicide substrate 1-aminobenzotriazole (ABT), the metabolism of phenacetin to acetaminophen is inhibited completely [B. A. Mico et al., Drug Metab. Dispos. 15, 274 (1987)]. Here we report an examination of the time-course of inhibition of phenacetin elimination by ABT, a demonstration of dose-dependent inhibition of phenacetin and antipyrine clearances by ABT, and an examination of the acute toxicity of ABT in rats, as well as the effect of ABT on phenacetin metabolism in beagles. After a 1-, 12-, 24- or 36-hr pretreatment of rats with ABT (50 mg/kg, i.v.), the clearance of phenacetin was decreased 85, 88, 81 and 48%, respectively, from control values. Twelve hours after intraperitoneal pretreatment of rats with 0.3, 1.0, 5.0, 20, and 50 mg/kg of ABT, the total systemic clearance of phenacetin was suppressed 39, 47, 60, 75, and 79%, respectively, from control values. The clearance of intravenously administered antipyrine was decreased 38 and 66% after a 12-hr intraperitoneal pretreatment of rats with 10 or 50 mg/kg of ABT. In rats, no hematological, clinical chemistry, macroscopic, or microscopic abnormalities were detected 1, 2, 3, and 9 days after a single i.v. dose of ABT (50 mg/kg). A 1-hr pretreatment of beagles with ABT (20 mg/kg) decreased the clearance of intravenous phenacetin 50% and completely prevented the formation of acetaminophen. These results demonstrate that ABT pretreatment causes long-lasting inhibition of oxidative drug metabolism without disruption of normal physiological processes. Profound inhibition of oxidation in two species suggests that ABT may have general utility as an inhibitor of oxidative drug metabolism in biochemical pharmacology and toxicology studies.

Inhibition of adrenal cytochromes P450 by 1-aminobenzotriazole in vitro. Selectivity for xenobiotic metabolism.

Studies were done to determine the effects of a P450 suicide inhibitor, 1-aminobenzotriazole (ABT), on adrenal steroid and xenobiotic metabolism. Incubation of guinea pig adrenal microsomes with ABT plus an NADPH-generating system caused a time-dependent decline in total P450 concentrations. The maximal decrease in P450 levels was approximately 35% and was accompanied by an equimolar decrease in heme content. Western blot analyses indicated that ABT had no effect on P450 apoprotein levels. Benzphetamine (BZ) N-demethylase and benzo[a]pyrene (BP) hydroxylase activities were inhibited almost completely by microsomal incubations with ABT. In contrast, neither steroid 17 alpha-hydroxylase nor 21-hydroxylase activity was affected by ABT. The steroid-induced type I spectral change in adrenal microsomes also was not affected by ABT, whereas that induced by BZ was eliminated. Similar studies with adrenal mitochondria indicated that ABT had no effect on mitochondrial P450 concentrations or on mitochondrial steroid metabolism. The results demonstrate that the in vitro actions of ABT on adrenal cytochromes P450 are highly selective for those isozymes that catalyze xenobiotic metabolism. Therefore, ABT should serve as a useful probe for further characterization of adrenal xenobiotic-metabolizing P450 isozymes.

Inactivation of adrenal cytochromes P450 by 1-aminobenzotriazole. Divergence of in vivo and in vitro actions.

Recent investigations demonstrated that administration of 1-aminobenzotriazole (ABT) to rats caused adrenal gland enlargement. Studies were done to pursue the mechanism(s) involved. Preliminary experiments revealed that the adrenal enlargement caused by ABT was associated with a decline in plasma corticosterone concentrations, suggesting inhibition of adrenal steroidogenesis. Indeed, a single injection of ABT (25 or 50 mg/kg body weight) to rats caused concentration-dependent declines (60-80%) in adrenal mitochondrial and microsomal cytochrome P450 (P450) concentrations. The decreases in adrenal P450 levels exceeded those in hepatic microsomes. Accompanying the declines in adrenal P450 concentrations were decreases in steroid hydroxylase activities. Mitochondrial 11 beta-hydroxylase and cholesterol side-chain cleavage activities and microsomal 21-hydroxylase activity were diminished markedly (60-90%) by ABT treatment. In contrast, activity of adrenal 3 beta-hydroxysteroid dehydrogenase-isomerase was not affected by ABT, indicating specificity for P450-dependent reactions. Incubation of adrenal microsomes or mitochondria in vitro with ABT plus an NADPH-generating system had no effect on P450 concentrations or on steroid hydroxylase activities. Similar incubations with hepatic microsomes caused declines in P450 levels and in the rates of P450-mediated xenobiotic metabolism. The results demonstrate that ABT is a potent inhibitor of adrenal steroid hydroxylases in vivo, but the in vitro studies indicate that the mechanism of action differs from that on other P450 isozymes. The absence of inhibitor effects in vitro suggests that an extra-adrenal metabolite of ABT is responsible for the in vivo inactivation of steroidogenic enzymes.

Inhibition of adrenal steroid metabolism by administration of 1-aminobenzotriazole to guinea pigs.

Prior in vitro investigations demonstrated that the P450 suicide substrate, 1-aminobenzotriazole (ABT), was a potent inhibitor of xenobiotic metabolism but had no effect on steroidogenic enzymes in the guinea pig adrenal cortex. Studies were done to determine if ABT administration of guinea pigs in vivo also selectively inhibited adrenal xenobiotic metabolism. At single doses of 25 or 50 mg/kg, ABT effected rapid decreases in spectrally detectable adrenal P450 concentrations. The higher dose caused approx. 75% decreases in microsomal and mitochondrial P450 levels within 2 h. The decreases in P450 were sustained for 24 h but concentrations returned to control levels within 72 h. Accompanying the ABT-induced decreases in adrenal P450 content were proportionately similar decreases in P450-mediated xenobiotic and steroid metabolism. Microsomal benzo(a)pyrene hydroxylase, benzphetamine N-demethylase, 17 alpha-hydroxylase and 21-hydroxylase activities were decreased to 20-25% of control values by the higher dose of ABT. Mitochondrial 11 beta-hydroxylase and cholesterol sidechain cleavage activities were similarly diminished by ABT treatment. Adrenal 3 beta-hydroxysteroid dehydrogenase activity, by contrast, was not affected by ABT, indicating specificity for P450-catalyzed reactions. The results demonstrate that ABT in vivo is a non-selective inhibitor of adrenal steroid- and xenobiotic-metabolizing P450 isozymes. The absence of ABT effects on steroid metabolism in vitro suggests that an extra-adrenal metabolite may mediate the in vivo inhibition of steroidogenesis.

In vivo and in vitro hepatotoxicity and metabolism of acetaminophen in Syrian hamsters.

The purpose of this investigation was to correlate the in vitro and in vivo toxicity of the hepatotoxicant, acetaminophen. Hamsters were pretreated with either phenobarbital (70 mg/kg) or 3-methylcholanthrene (20 mg/kg) or an appropriate vehicle for 3 days. In non-pretreated hamsters, single doses of acetaminophen (200-400 mg/kg i.p.) caused elevations in serum alanine aminotransferase and sorbitol dehydrogenase activities in a dose-related manner. 3-Methylcholanthrene significantly potentiated, while phenobarbital significantly reduced acetaminophen-induced elevations in serum liver enzyme activities. Both phenobarbital and 3-methylcholanthrene significantly reduced acetaminophen plasma T1/2 while only 3-methylcholanthrene increased APAP clearance. Phenobarbital pretreatment increased the urinary excretion of APAP-glucuronide. Exposure of isolated hepatocytes to acetaminophen (0.01-2.0 mM) resulted in concentration-related decreases in hepatocyte viability. Cells from 3-methylcholanthrene-pretreated hamsters were more markedly susceptible to acetaminophen toxicity than cells isolated from non-induced animals. Hepatocytes isolated from phenobarbitol pretreated animals were slightly but significantly more susceptible to acetaminophen toxicity than cells from control animals. Hepatocytes isolated from 3-methylcholanthrene pretreated animals had increased formation of an acetaminophen-glutathione conjugate compared to control. Pre-treatment with either phenobarbital or 3-methylcholanthrene enhanced glucuronidation of acetaminophen in vitro. These data demonstrate a lack of correlation between in vivo hepatotoxicity and in vitro cytotoxicity in that phenobarbital pre-treatment protected hamsters from acetaminophen-induced liver toxicity, but failed to protect hepatocytes exposed to acetaminophen in vitro.

Oxidation of carbon tetrachloride, bromotrichloromethane, and carbon tetrabromide by rat liver microsomes to electrophilic halogens.

In order to determine whether CCl4, CBrCl3, CBr4 or CHCl3 undergo oxidative metabolism to electrophilic halogens by liver microsomes, they were incubated with liver microsomes from phenobarbital pretreated rats in the presence of NADPH and 2,6-dimethylphenol. The analysis of the reaction mixtures by capillary gas chromatography mass spectrometry revealed that 4-chloro-2,6-dimethylphenol was a metabolite of CCl4 and CBrCl3 whereas 4-bromo-2,6-dimethylphenol was a metabolite of CBr4. The formation of the metabolites was significantly decreased when the reactions were conducted with heat denatured microsomes, in the absence of NADPH or under an atmosphere of N2. These results indicate that the chlorines of CBrCl3 and CCl4 and the bromines of CBr4 are oxidatively metabolized by rat liver microsomes to electrophilic and potentially toxic metabolites.

Liquid chromatographic determination of 4-(2-di-N,N-propylaminoethyl)-2-(3H)-indolone in rat, dog, and human plasma with ultraviolet detection.

A sensitive, specific, and accurate assay for 4-(2-di-N,N-propylaminoethyl)-2-(3H)-indolone, 1 (SK&F 101468), in plasma was developed using high-performance liquid chromatography with UV detection. The method involves sample preparation by solid-phase extraction, elution of 1 and the internal standard with a volatile solvent, concentration, and reversed-phase chromatography in the presence of an ion-pairing agent. Using 1 mL of plasma, 5 ng/mL of 1 is detectable and 10 ng/mL of 1 can be quantitated. The recovery of 1 and internal standard from plasma is greater than 95%. The within-day precision of this method at 17.5, 219, and 395 ng/mL is 3.4, 1.3, and 1.5%, respectively. The between-day precision at these concentrations is 6.0, 1.9, and 2.6%, with a mean accuracy of 100.6, 98.3, and 100.2%, respectively. Stability studies indicate that 1 is stable in plasma at -80 degrees C for less than or equal to 180 d.

Functional group metabolism of dopamine-2 agonists: conversion of 4-(2-di-n-propylaminoethyl)-2-(3H)-indolone to 4-(2-di-n-propylaminoethyl)-7-hydroxy-2-(3H)-indolone.

In the previous report, we reported the results of absorption, protein binding, and pharmacokinetics of the dopamine-2 agonists (D2-agonists) 4-(2-di-n-propylaminoethyl)-7-hydroxy-2-(3H)-indolone, N-(2'-hydroxy-5'-[N,N-di-n-propylaminoethylphenyl])methanesulfonamide, and 4-(di-n-propylaminoethyl)-2-(3H)-indolone. Both phenolic compounds, 1 and 2, were subject to more rapid metabolism than the nonphenol 3. In the present study, we investigated the metabolic basis of the differences in the pharmacokinetics of these compounds. In both rats and dogs, the principal urinary metabolite of 1 and 2 was the corresponding glucuronide. In contrast, 3 was first converted to 1 which then was converted to a glucuronide. On the basis of the urinary excretion of 1 and its glucuronide after intravenous administration of 1 and 3, approximately 78% of the dose of 3 in rats and 58% in dogs was converted to 1. The depropyl analogue of 3 was identified as a minor urinary metabolite. 4-(2-Di-n-propylaminoethyl)-7-hydroxy-2-(3H)-indolone was found in the plasma of rats, dogs, and cynomolgus monkeys treated with 3. The concentration of 1 declined in parallel with that of 3 in dogs and monkeys, indicating that the true half-life of 1 is shorter than or equal to that of 3. On the basis of plasma concentrations of 1 in dogs, the apparent conversion of 3 to 1 was 9%.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics of dopamine-2 agonists in rats and dogs.

The absorption, protein binding, blood-to-plasma ratio, renal excretion, and pharmacokinetics of the dopamine-2 agonists (D2-agonists) 4-(2-di-n-propylaminoethyl)-7-hydroxy-2-(3H)-indolone (1), N-(2'-hydroxy-5'-[N,N-di-n-propylaminoethylphenyl])methanesulfonamide (2), and 4-(2-di-n-propylaminoethyl)-2-(3H)-indolone (3) were examined in dogs and rats. On the basis of relative cumulative urinary recoveries of radiolabeled drug, all three compounds are well absorbed in rats and dogs. In dogs, the free fractions in plasma of unchanged 1, 2, and 3, determined by in vitro studies, were 74, 86, and 63%, respectively, and the protein binding was constant with increasing concentration. The blood-to-plasma partition ratios of the respective compounds were 1.22, 1.14, and 1.16 in dogs, and the ratios were constant with increasing concentration. Large differences between species (dogs, rats, and humans) in protein binding and blood-to-plasma ratios were not seen. The clearances (blood or plasma) of 1 and 2 in dogs were significantly greater than the clearance of 3. The clearance of 3 was almost exclusively nonrenal, whereas 13% of 1 and 2 were recovered unchanged in urine. The steady-state volumes of distribution and the distribution and elimination half-lives of the three compounds were not significantly different. Importantly, the mean residence time of 3 (147 min) in dogs was significantly longer than those of 1 (90 min) and 2 (96 min). The results of analogous studies in rats indicate that 1 and 2 are more rapidly metabolized than 3.

Reductive oxygenation of carbon tetrachloride: trichloromethylperoxyl radical as a possible intermediate in the conversion of carbon tetrachloride to electrophilic chlorine.

Under aerobic conditions, rat liver microsomes convert carbon tetrachloride to an electrophilic form of chlorine that is trapped with 2,6-dimethylphenol to form 4-chloro-2,6-dimethylphenol. The mechanism of cytochrome P-450-catalyzed electrophilic chlorine formation from carbon tetrachloride was examined with structure-activity studies of electrophilic halogen formation and chemical and in vitro microsomal studies. 4-Chloro-2,6-dimethylphenol is not formed as a consequence of a reaction of 2,6-dimethylphenoxyl radical with carbon tetrachloride or carbon tetrachloride-induced lipid peroxyl radical formation. Only tetrahalomethanes were found to yield electrophilic halogens. The chemical oxidants hydrogen peroxide, cumene hydropheroxide, sodium periodate, and iodobenzene diacetate did not support electrophilic halogen formation from carbon tetrachloride, carbon tetrabromide, or hexachloroethane in microsomal studies. The addition of superoxide dismutase, catalase, sodium azide, or glutathione to microsomal incubations did not affect the rate of electrophilic chlorine formation, whereas Paraquat completely inhibited the reaction. The radical spin trap phenyl t-butyl nitrone (14 mM) completely inhibited electrophilic chlorine formation. The rate of electrophilic chlorine formation was highest at 2-5% atmospheric oxygen, whereas anaerobiosis completely inhibited electrophilic chlorine formation, and high oxygen tension impaired electrophilic chlorine formation. These results preclude direct oxidation of carbon tetrachloride or a reaction of superoxide anion radical with carbon tetrachloride as the initial step in electrophilic chlorine formation and suggest that the likely initial step is reductive dehalogenation of carbon tetrachloride to trichloromethyl radical which then traps oxygen to form trichloromethylperoxyl radical. Subsequent reaction of trichloromethyl peroxyl radical leads to electrophilic chlorine. These findings may have important implications concerning carbon tetrachloride-induced lipid peroxidation and carbon tetrachloride hepatotoxicity.

The bioavailability and disposition of tiodazosin levulinate in beagle dogs with a comparison to prazosin hydrochloride.

Tiodazosin is a recently developed compound that is currently undergoing investigation for use in hypertension. It is structurally and pharmacologically similar to prazosin, the prototype of the aminoquinazoline class of vasodilators. Lacking an intravenous dosage form for use in human subjects, our study concentrated on the determination of the bioavailability and disposition parameters of tiodazosin in dogs, with a comparison to those parameters for prazosin, obtained from previous work in our laboratory. Tiodazosin was administered to five male beagle dogs at a dose of 1 mg/kg both intravenously and as an oral solution. Plasma and whole blood samples were taken serially over 24 hr and analyzed with a new specific and sensitive HPLC assay. The half-life of tiodazosin was significantly shorter than that of prazosin. The tiodazosin-calculated bioavailability was 3-fold less than predicted, whereas prazosin-calculated bioavailability was the same as predicted. Assumptions necessary to predict the bioavailability of compounds cleared by the hepatic route appear to be incorrect for tiodazosin. Possible mechanisms for the unpredictable low tiodazosin bioavailability in dogs are presented.

Pharmacokinetics of a series of 6-chloro-2,3,4,5-tetrahydro-3-substituted-1H- 3-benzazepines in rats. Determination of quantitative structure-pharmacokinetic relationships.

To aid in the effort to discover novel agents for the treatment of cardiovascular disease, the relationships between pharmacokinetic parameters in the rat and lipophilicity and basicity were studied for a series of 6-chloro-2,3,4,5-tetrahydro-3-substituted-1H-3-benzazepines. Eight compounds, ranging in lipophilicity from log P = 1.64 to 3.50 and basicity from pKa = 6.75 to 9.36, were studied. The compounds were administered iv to rats, and the pharmacokinetic parameters were calculated from the plasma concentration-time curves. Plasma protein binding was determined in vitro using equilibrium dialysis to allow calculation of steady state volume of distribution of unbound drug, Vss,u; and tissue binding. Stepwise regression analysis with each pharmacokinetic parameter as the dependent variable and log P and pKa as the independent variables was performed. In no case was there a significant relationship between a pharmacokinetic parameter and both of the independent variables. Statistically significant linear relationships were found between pKa and Vss and t 1/2z. Lipophilicity was found to correlate with the free fraction in plasma and the free fraction in tissues. The clearance parameters did not correlate with either of the physicochemical parameters. The pharmacokinetics of the one secondary amine in the series were clearly different from those of any of the tertiary amines. The clearance of the secondary amine was lower and the volume of distribution higher than any of the tertiary amines. These results demonstrate that alteration of the lipophilicity of 3-substituted benzazepines does not alter their pharmacokinetics in a predictable fashion but that the pharmacokinetics of secondary amines may be substantially different than tertiary amines.

Potential for metabolic deactivation of carcinogenic N-nitrosodimethylamine in vivo.

Enzymatic cleavage of N-nitrosodimethylamine (NDMA) to nitrite (normally representing about 10% of the total metabolism in vitro) also produces methylamine in yields roughly equimolar to those of nitrite, suggesting that the 'denitrosation' pathway may be responsible for the previously unexplained detection of methylamine as a urinary metabolite of NDMA and, at least in part, for the recovery of less than stoichiometric amounts of dinitrogen in 15N-labelling experiments. We have now followed excretion of labelled methylamine by rats receiving 14C-NDMA as a possible index of the extent of in-vivo denitrosation. Correcting for the proportion of labelled methylamine recovered in the urine following its administration under the conditions used for NDMA, 2.5-10% of the NDMA metabolism in Fischer rats appeared to proceed by a methylamine-forming route. The results are consistent with the conclusion that the metabolism of NDMA is best viewed as a competition between two pathways, with denitrosation diverting a significant proportion of the clearance to a presumably deactivating metabolic route at the expense of the activating alkylation pathway responsible for carcinogenesis.

Charge and lipophilicity govern the pharmacokinetics of glycopeptide antibiotics.

The pharmacokinetics and urinary excretion of nine glycopeptide antibiotics with diverse pIs (3.8 to 8.5) and lipophilicities were studied. The disposition of the aridicin antibiotics and their hydrolysis products were examined in male CD-1 mice after subcutaneous and intravenous administration and compared with the disposition of teicoplanin, ristocetin, and vancomycin. The total systemic clearance, half-life, volume of distribution, and urinary excretion were highly correlated with pIs. In general, as the pI decreased, the clearance, urinary recovery, and volume of distribution decreased, whereas the half-life increased. With those glycopeptides that had similar pIs, clearance decreased and half-life increased with increasing lipophilicity. The urinary recovery of the glycopeptides decreased with decreasing pI and increasing lipophilicity. Because vancomycin (pI = 8.0) is cleared by glomerular filtration, increased binding to serum is the likely mechanism of reduced renal clearance for glycopeptides with low pIs. These results are consistent with previous findings concerning the correlation of physical-chemical properties and the drug disposition of small organic molecules. Results of these studies also indicate that desirable pharmacokinetic properties can be incorporated into glycopeptides through semisynthetic modifications.

Determination of Ro 23-7637 in dog plasma by multidimensional ion-exchange-reversed-phase high-performance liquid chromatography with ultraviolet detection.

Ro 23-7637 (I) is a new drug under development for the treatment of metabolic diseases. A high-performance liquid chromatographic-ultraviolet detection (HPLC-UV) analytical procedure for its analysis in dog plasma was developed and is reported here. Following C18 solid-phase extraction, the sample is applied to a strong cation-exchange column in the first dimension. The analyte and internal standard, Ro 24-4558 (II), are transferred to a base-deactivated C18 reversed-phase column in the second dimension (orthogonal separation mechanism), with UV detection at 254 nm. The reversed-phase solid-phase extraction provides a gross isolation of the drug, based on hydrophobicity. The first-dimension ion-exchange separation allows neutral species and anions to elute with the column void volume, while separating basic species according to pKa. The second dimension provides a high-resolution separation dependent upon the hydrophobicity of the sample species. The rationale for using orthogonal multidimensional chromatography was that an exhaustive examination of reversed-phase and normal-phase columns invariably resulted in co-elution of I with endogenous plasma components, which limited the sensitivity of the method. We have utilized C18 solid-phase extraction, followed by multidimensional HPLC-UV, to develop an accurate and precise analytical method whose limit of quantitation, 5 ng/ml using 0.5 ml of plasma, is determined by inherent detector sensitivity. Increased sensitivity can be readily achieved by using more sample or by using microbore HPLC on the second dimension.