Bexarotene metabolism in rat, dog, and human, synthesis of oxidative metabolites, and in vitro activity at retinoid receptors.

The metabolism of bexarotene, a rexinoid recently approved in the United States for treatment of cutaneous T-cell lymphoma, was studied using liver slices from untreated rats and dogs, liver microsomes from untreated and pretreated rats, and pooled human liver microsomes. Metabolite profiles were examined in bile and plasma from rats and dogs, and plasma from humans treated with bexarotene. Four metabolites, racemic 6-hydroxy-bexarotene, racemic 7-hydroxy-bexarotene, 6-oxo-bexarotene, and 7-oxo-bexarotene, were synthesized and their binding to, and transactivation of retinoid receptors were examined. Qualitatively similar metabolite profiles were observed in the microsomal and liver slice extracts; the predominant metabolites were 6-hydroxy-bexarotene and glucuronides of parent or hydroxylated metabolites. Pretreatment of rats with bexarotene induced hepatic microsomal bexarotene metabolism. The hydroxy and oxo metabolites were observed in plasma of rats, dogs, and humans treated with bexarotene and 6-hydroxy-bexarotene was a major circulating metabolite. The oxidative metabolites were more abundant relative to parent in plasma from humans than from rat or dog. The predominant biliary metabolites in rat and dog were bexarotene acyl glucuronide and a glucuronide of oxidized bexarotene, respectively. Since bexarotene elimination is primarily biliary in these species, these metabolites represent the main bexarotene metabolites in rats and dogs. The binding of synthetic metabolites to retinoid receptors was much reduced relative to parent compound. The metabolites exhibited minimal activity in transactivating retinoic acid receptors and had reduced activity at retinoid X receptors relative to bexarotene. Thus, while there is substantial systemic exposure to the oxidative metabolites of bexarotene, they are unlikely to elicit significant retinoid receptor activation following bexarotene administration.

A Phase I study of LGD1069 in adults with advanced cancer.

LGD1069 [Targretin; 4-[1-(5,6,7,8-tetrahydro-3,5,5,8,8-pentamethyl-2-naphtalenyl) propenyl] benzoic acid] is a novel synthetic retinoid X receptor-selective retinoid that has been recently identified. The goals of this study were to determine the safety, toxicity, pharmacokinetics (PKs), and metabolic profile of LGD1069 in advanced cancer patients. Sixty patients received oral LGD1069 at doses ranging from 5-1000 mg/m2/day with PK sampling performed on days 1 and 15. No dose-limiting toxicities (DLTs) were observed up to the 500 mg/m2/day dose level. DLT observed at and above 650 mg/m2/day included skin desquamation, hyperbilirubinemia, transaminase elevation, leukopenia, and diarrhea. Asymptomatic, dose-related alterations in lipid and thyroid metabolism were also observed. DLTs frequently observed with retinoic acid receptor-selective retinoids and pan agonists, including headache, mucocutaneous toxicity, and hypercalcemia, were not dose-limiting with LGD1069. Day 1 LGD1069 Cmax and area under the curve values increased dose-proportionately up to 800 mg/m2/day. Repeat-dose (day 15) area under the curve values varied between 25 and 105% of day 1 values. Although no objective tumor responses were observed, tumor progression may have been substantially arrested or delayed in non-small cell lung cancer (5 of 16) and in head and neck cancer (1 of 5), as well as other tumor types. At the higher dose levels, the molar concentration of LGD1069 was up to 10-fold higher than observed with other retinoids, yet toxicity was minimal. LGD1069 is an retinoid X receptor-selective retinoid agonist with a more favorable PK and toxicity profile than previously studied retinoids and merits further investigation as a chemopreventive and anticancer agent. On the basis of this Phase I trial, the recommended Phase II dose is 500 mg/m2/day.

Phase I study of 9-cis-retinoic acid (ALRT1057 capsules) in adults with advanced cancer.

9-cis-Retinoic acid (9-cis-RA) and all-trans-RA (ATRA) are naturally occurring hormones. The nuclear receptors that mediate the effects of retinoids are the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). ATRA binds RAR with high affinity but does not bind to RXR, whereas 9-cis-RA, an isomer of ATRA, is a ligand that binds and transactivates both RARs and RXRs. The goals of this study were to determine the safety, tolerability, pharmacokinetics, and metabolic profile of 9-cis-RA in advanced cancer patients. Forty-one patients received oral 9-cis-RA (ALRT1057; Panretin capsules) at doses ranging from 5-140 mg/m2/day. Twenty-six patients were treated once daily with up to 140 mg/m2; a subsequent cohort of 15 patients were treated twice daily (b.i.d.) at 100-140 mg/m2/day (50, 60, and 70 mg/m2 b.i.d.) to evaluate a b.i.d. dosing regimen. Headache was the most frequent adverse event and was dose limiting in 3 of 41 patients. Skin toxicity was the next most common toxicity and was seen in 11 of 41 patients; it was typically mild and limited to skin dryness and erythema. Other toxicities included conjunctivitis, flushing, diarrhea, transaminitis, hypercalcemia, and asymptomatic hypertryglyceridemia. Toxicities were typically dose related, occurred primarily above 83 mg/m2/day, and were not ameliorated by b.i.d. dosing. No tumor responses were observed. The mean day 1 area under the plasma concentration-time curve and peak plasma concentration values were dose-proportional over all dose levels, whereas day 15 area under the plasma concentration-time curve and peak plasma concentration values were nonlinear above 83 mg/m2/day, suggesting that 9-cis-RA induced its own metabolism at doses equal to and above 140 mg/m2/day. 9-cis-RA is a retinoid receptor pan agonist with a more favorable pharmacokinetic and toxicity profile than that observed with previously studied retinoids and merits further investigation.

Effects of retinoid treatment of rats on hepatic microsomal metabolism and cytochromes P450. Correlation between retinoic acid receptor/retinoid x receptor selectivity and effects on metabolic enzymes.

Retinoids are compounds that bind to and activate one or more retinoid receptors to elicit various physiological responses. There are two families of retinoid receptors, i.e. retinoic acid receptors (RAR) and retinoid X receptors (RXR), for which the various synthetic and naturally occurring retinoids have differing selectivities. The synthetic analogs LG100268 and LGD1069 (Targretin) are RXR-selective, whereas ALRT1550 is highly RAR-selective. Naturally occurring all-trans-retinoic acid (Tretinoin) has a degree of selectivity for RAR, whereas ALRT1057 (9-cis-retinoic acid, Panretin) is equally active at RAR and RXR (i. e. a pan-agonist). To evaluate the effects of these compounds on metabolic enzymes, male Sprague-Dawley rats received daily oral doses for 4 days, and liver microsomes were prepared on day 5. As a class, these ligands exerted profound effects on hepatic microsomal metabolic enzyme levels. Those with RAR activity decreased hepatic cytochrome P450 (CYP or P450) levels and in vitro metabolism of the compound of pretreatment, whereas those exerting predominantly RXR activity increased these parameters. A similar relationship was observed when glucuronidation was examined. Hepatic CYP2B1/2 was unaffected and CYP3A was decreased by RAR-selective ALRT1550, whereas both were induced by ligands selective for RXR. However, both RAR- and RXR-selective ligands decreased CYP1A2, whereas they induced CYP4A. Although the mechanisms underlying these effects are not known, these results suggest that RAR- and RXR-binding ligands exert distinct effects on hepatic metabolism, and they indicate the potential for drug-drug interactions, especially involving CYP3A. The nature of such interactions would depend on the RAR/RXR selectivity of the ligand and the P450 isozymes responsible for the metabolism of coadministered drugs.

Initial clinical trial of the retinoid receptor pan agonist 9-cis retinoic acid.

The retinoid response is mediated by families of nuclear receptors, the retinoic acid receptors (RARs), and the retinoid X receptors. All-trans retinoic acid (RA) binds only RARs and induces its own metabolism. In contrast, 9-cis RA is a newly identified agonist for both RARs and retinoid X receptors. We undertook a dose-ranging study to examine the safety, clinical tolerance, and pharmacokinetics of 9-cis RA in patients with advanced cancer. Thirty-four patients received once daily p.o. doses of 9-cis RA (administered as LGD1057) ranging from 5 to 230 mg/m2 for 4 weeks. Pharmacokinetic studies were performed on 28 patients at seven dose levels. 9-cis RA was generally well tolerated. Headache was the most common dose-limiting adverse effect. Other prominent reactions included facial flushing, myalgia, dyspnea, hypertriglyceridemia, and hypercalcemia. Relative to other retinoids, mucocutaneous reactions were mild. No major antitumor responses were observed. Pharmacokinetic analysis revealed that the day 1 area under the plasma concentration x time curves (AUCs) were proportional to the dose. Up through doses of 140 mg/m2, the day 1 AUCs were similar to those on days 15 and 29. At higher doses, however, AUCs tended to decline with repeat dosing. 9-cis RA is a novel compound that exploits a newly identified pathway of retinoid receptor biology that may be relevant to tumor cell proliferation and differentiation. We recommend a dose of 140 mg/m2 for single-agent trials utilizing a once-daily schedule of administration.

Oxidative and reductive metabolism of 9-cis-retinoic acid in the rat. Identification of 13,14-dihydro-9-cis-retinoic acid and its taurine conjugate.

9-cis-Retinoic acid (9-cis-RA), a hormone that binds and activates all known retinoid receptor subtypes, is structurally similar to all-trans-retinoic acid and may share common metabolic fates. Both oral and intravenous doses of 9-cis-RA to rats led to hydroxylation and ketone formation at carbon-4. 9-Cis-RA also isomerized in vivo to 13-cis-retinoic acid, 9-cis, 13-cis-retinoic acid, and all-trans-retinoic acid. After administration of [11-3H]9-cis-RA, the proportion of plasma radioactivity that was volatile increased over time, which suggested that beta-oxidative chain-shortening of 9-cis-RA might occur. An equimolar mixture of [1-13C2H3]9-cis-RA and 9-cis-RA was administered to rats for stable-isotope-labeled metabolite production. A chromatographic peak that had a lambdamax = 290 nm vs. 348 nm for the parent compound, had a retention time similar to the parent, and yielded a 1:1 positive-ion isotope cluster at m/z 303/307 in its mass spectrum. NMR analysis revealed 9-cis and 13,14-dihydro configurations, indicating that 9-cis-RA can be metabolized in rat by reduction to 13,14-dihydro-9-cis-RA. An earlier-eluting HPLC peak that exhibited a lambdamax at 290 nm, and a negative-ion-MS isotope cluster at m/z 408/412 was observed during separations of rat liver extracts. LC/MS/MS analysis revealed product ions for this peak diagnostic for carboxylic acid taurine conjugates. In rats, reduction of 9-cis-RA to 13,14-dihydro-9-cis-RA may represent an initial step leading to beta-oxidation, although available data demonstrate it is conjugated with taurine to form a novel metabolite.

Pharmacokinetics of 9-cis-retinoic acid in the rhesus monkey.

9-cis-Retinoic acid is a naturally occurring biologically active retinoid capable of binding and transactivating both the retinoic acid receptors and the retinoid X receptors. A study was performed to characterize the pharmacokinetics 9-cis-retinoic acid following i.v. bolus administration in the nonhuman primate. Groups of three animals received i.v. bolus doses of 9-cis-retinoic acid of either 50 or 100 mg/m2. Blood and cerebrospinal fluid samples for determination of 9-cis-retinoic acid concentration were obtained prior to and 5, 10, 15, 30, 45, 60, 75, 90, 120, 150, 180, 240, 360, and 480 min following drug administration. The plasma drug concentration profile of 9-cis-retinoic acid was consistent with a first-order elimination process, with a harmonic mean half-life of 31 min, and a mean clearance of 97 ml/min/m2. The pharmacokinetics of 9-cis-retinoic acid were linear over the dose range studied. Plasma concentrations of all-trans-retinoic acid following 9-cis-retinoic acid administration were less than the limit of quantitation (0.1 microM), suggesting that isomerization to all-trans-retinoic acid is not a major metabolic pathway. In contrast to all-trans-retinoic acid, the elimination of 9-cis-retinoic acid did not appear to be capacity limited (saturable). Previous studies in the Rhesus monkey have shown that repeated dosing with all-trans-retinoic acid leads to a reduction of this saturable component of elimination and results in reduced exposure to drug. These studies, in an animal model highly predictive of humans, suggest that declines in plasma concentrations of 9-cis-retinoic acid as a result of its repeat administration at doses up to 100 mg/m2 will not occur.

Biotransformation of lovastatin. V. Species differences in in vivo metabolite profiles of mouse, rat, dog, and human.

Lovastatin is a prodrug lactone whose open-chain 3,5-dihydroxy acid is a potent, competitive inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, the rate-limiting enzyme in cholesterol biosynthesis. The compound undergoes extensive and complex metabolism in animals and humans, with the metabolites excreted predominantly in bile. Radiochromatograms of bile from three human subjects and of bile and liver homogenates from mouse, rat, and dog displayed obvious species differences. Biotransformation of lovastatin occurred by three distinct routes, namely hydrolysis of the lactone ring to yield the pharmacologically active dihydroxy acid, cytochrome P-450-mediated oxidation of the fused-ring system, and beta-oxidation of the dihydroxy acid side chain. The first two reactions occurred in all four species, but the last was observed in mouse and rat only. The P-450 reactions, hydroxylation and a novel dehydrogenation reaction, yielded a 6'-hydroxylated metabolite of the dihydroxy acid and a 6'-exomethylene derivative as major and minor metabolites, respectively, in the bile of rat and dog. Human bile, which contained predominantly polar metabolites, yielded these metabolites in similar proportions only after mild hydrolysis at pH 5.0. In mouse and rat an atypical beta-oxidation of the dihydroxy acid side chain occurred to give a pentanoic acid derivative that was observed in liver homogenates. This metabolite was subsequently conjugated with taurine and excreted in the bile. From these studies, cytochrome P-450 oxidation is the primary route of phase I metabolism for lovastatin in human and dog, but beta-oxidation plays a major metabolic role in rodents.

Dose-dependent stereopharmacokinetics of 5,6-dihydro-4H-4(isobutylamino)thieno(2,3-B)thiopyran-2-sulfonamide-7,7 -dioxide , a potent carbonic anhydrase inhibitor, in rats.

[5,6-dihydro-4H-4(isobutylamino)thieno(2,3-B)thiopyran-2-sul fonamide-7,7- dioxide] (MK-927), a potent carbonic anhydrase inhibitor capable of reducing intraocular pressure after topical application, is currently under investigation for the treatment of glaucoma. The purpose of this study was to characterize the pharmacokinetics of the enantiomers of MK-927 with particular emphasis on the effect of dose on the elimination kinetics. Because the drug resided primarily in erythrocytes, the kinetic analysis was generally performed based on the drug concentration of whole blood. Following iv administration, the rat cleared the (R)(-)-enantiomer more rapidly than the (S)(+)-isomer. The stereoselective difference in elimination kinetics was dose-dependent; total blood clearance of the (R)(-)-enantiomer was approximately 40 times that of the (S)(+)-isomer at 0.05 mg/kg, and about 7-fold at 5 mg/kg. For both enantiomers, the pharmacokinetic parameters remained unchanged when the dose increased from 0.05 to 0.2 mg/kg, while the total blood clearance and apparent volume of distribution increased substantially as the dose exceeded 2 mg/kg. Nevertheless, the terminal half-life for each enantiomer appeared to be dose-independent. The enantiomers were extensively bound to erythrocytes in a stereoselective manner; at low concentrations, the (S)(+)enantiomer was bound 3-fold more strongly than the (R)(-)-enantiomer in vitro and 6-fold in vivo. Clearly, the magnitude of stereoselectivity in the elimination kinetics of MK-927 enantiomers (40-fold) cannot be explained solely by stereoselective binding. Thus, other factors may also contribute to the overall stereoselectivity in the elimination kinetics of MK-927. The dose-dependent kinetics of the enantiomers was probably due to the saturable binding to carbonic anhydrase.

Dose-dependent pharmacokinetics of MK-417, a potent carbonic anhydrase inhibitor, in rabbits following single and multiple doses.

MK-417, a potent carbonic anhydrase inhibitor capable of reducing intraocular pressure after topical application, is currently under investigation for the treatment of glaucoma. The purposes of this study were to characterize dose-dependent pharmacokinetics of MK-417 and to determine the accumulating effect of the drug during chronic topical administration in rabbits. Because the drug resided primarily in the erythrocytes, kinetic analyses were performed on whole blood concentration data. Following i.v. administration, both total blood clearance and apparent volume of distribution for MK-417 increased disproportionately between the low and high dose, while the half-life of the drug appeared to be independent of dose. Total blood clearance and apparent volume of distribution increased from 0.993 +/- 0.224 ml/hr/kg (mean +/- SD) and 88.6 +/- 9.4 ml/kg at a dose of 0.05 mg/kg to 2.73 +/- 0.17 ml/hr/kg and 272 +/- 5.5 ml/kg at a dose of 1 mg/kg. The dose-dependent kinetics of MK-417 are probably due to the saturable binding of carbonic anhydrase. Upon instillation of MK-417 into the eyes, the drug was rapidly and well absorbed. At the low dose of 0.05 mg/kg, the bioavailability varied from 58% to 98.5% with a mean value of 76.5 +/- 20.5%. Prediction of concentrations of MK-417 during chronic topical administration were performed based on the corresponding concentrations after a single topical dose using an overlay technique. Good agreement between the experimental data and the predicted blood concentrations of MK-417 during chronic dosing at 0.05 mg/kg, but not at 1 mg/kg, strongly suggests that linear kinetics apply in the case of the low dose but not in the case of the high dose.

Species-dependent enantioselective plasma protein binding of MK-571, a potent leukotriene D4 antagonist.

The plasma protein binding of the enantiomers of MK-571 was stereoselective and the stereoselectivity was species dependent. The 12 mammalian species studied could be classified into three groups: those that bind the S-(+)-enantiomer to a greater extent than the R-(-)-enantiomer (human, baboon, monkey, cow, dog, and cat); those that bind the R-(-)-enantiomer more extensively (rat, guinea pig, and sheep); and those that show no stereoselectivity (rabbit, hamster, and mouse). The stereoselective binding appears to have no phylogenetic relationship. Using serum albumin instead of plasma, a similar degree of stereoselective binding was observed for human, dog, sheep, and rat, suggesting that albumin is the major binding component for MK-571 enantiomers, and that species differences in stereoselective binding are likely due to structural differences in the albumin molecule. Displacement studies with [14C] diazepam, [14C]warfarin, and [3H]digitoxin indicated that the enantioselective differences in protein binding are most likely due to the differences in binding affinity rather than to different binding sites.

Biotransformation of lovastatin. I. Structure elucidation of in vitro and in vivo metabolites in the rat and mouse.

Structures of in vitro microsomal and in vivo metabolites of lovastatin, a new cholesterol-lowering drug, were elucidated with the combined application of HPLC, UV, fast atom bombardment-MS, and NMR spectroscopy. Liver microsomes from rats and mice catalyzed the biotransformation of lovastatin, primarily at the 6'-position of the molecule, to form 6'-hydroxy-lovastatin and a novel 6'-exomethylene derivative. Hydroxylation at the 6'-position occurred stereoselectively, giving 6'-beta-hydroxy-lovastatin. Stereoselective hydroxylation at the 3"-position of the methylbutyryl side chain and hydrolysis of the lactone group to the corresponding hydroxy acid were the other two pathways of microsomal metabolism. 3'-Hydroxy-iso-delta 4',5'-lovastatin was isolated, but is not believed to be a direct metabolite since 6'-beta-hydroxy-lovastatin rearranges to this compound under mildly acidic conditions. The major metabolites excreted in bile of rats treated with the hydroxy acid form of the drug were identified as the 3'-hydroxy analog and a taurine conjugate of a beta-oxidation product of lovastatin. The pentanoic acid derivative of lovastatin, formed by beta-oxidation of the heptanoic acid moiety, was a major metabolite in livers of mice dosed with the hydroxy acid form of lovastatin. The microsomal metabolites, in their hydroxy acid forms, were active inhibitors of HMG-CoA reductase. The relative enzyme inhibitory activities of hydroxy acid forms of lovastatin, 6'-beta-hydroxy-, 6'-exomethylene-, and 3"-hydroxy-lovastatin were 1, 0.6, 0.5, and 0.15, respectively.

Dose-dependent kinetics of cilastatin in laboratory animals.

Cilastatin, a potent inhibitor of renal dehydropeptidase I, was specifically designed to inhibit renal metabolism of the antibiotic imipenem in order to achieve therapeutically relevant imipenem concentrations in the urinary tract. In this study the elimination kinetics of cilastatin in rats at doses of 5, 10, 20, 50, 100, and 200 mg/kg iv were demonstrated to be dose dependent, with total plasma clearance and non-renal clearance falling from 20.2 +/- 3.1 ml/min/kg and 17.7 +/- 3.3 ml/min/kg (mean +/- S.D.) at the 5 mg/kg dose to 11.4 +/- 1.2 ml/min/kg and 5.30 +/- 1.2 ml/min/kg, respectively, at the 200 mg/kg dose, whereas the volume of distribution of the drug remained unchanged. Since cilastatin is mainly eliminated by renal excretion as well as by N-acetylation, the non-renal clearance may reasonably reflect the N-acetylation process. Thus, the dose-dependent kinetics of cilastatin might be explained, at least partly, by the saturation of the N-acetylation of the drug. The dose-related decrease in the fraction (fm) of cilastatin converted to its N-acetylated metabolite provided further evidence for the saturable N-acetylation. The fm values decreased from 0.915 at the 10 mg/kg dose to 0.626 at the 100 mg/kg dose. Although both the total plasma clearance and non-renal clearance decreased with increasing dose, the dose had an opposite effect on the renal clearance of cilastatin. The renal clearance of cilastatin increased from 2.50 +/- 0.40 ml/min/kg at the lowest dose to 6.10 +/- 0.50 ml/min/kg at the highest dose as the dose increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of experimental diabetes on elimination kinetics of diflunisal in rats.

The effects of insulin-deficient diabetes on the elimination of diflunisal were investigated in streptozotocin-treated rats. Diflunisal, a fluorinated salicylate with nonsteroidal antiinflammatory properties, is eliminated primarily as the ester and ether glucuronides. After an iv injection of a 10 mg/kg dose, diabetic rats cleared diflunisal more rapidly than control rats; time-averaged total body clearances were 1.96 +/- 0.29 and 1.10 +/- 0.12 ml/min/kg, respectively. For a low clearance drug such as diflunisal, changes in the total body clearance can result from changes in the extent of plasma protein binding and/or drug metabolic rate. To determine whether the pronounced changes in elimination clearance in diabetic rats were due to the changes in plasma protein binding or enzyme activity, diflunisal was infused to obtain steady state kinetics. At steady state, the unbound intrinsic clearance increased from 43.4 +/- 16.4 ml/min/kg in the control rats to 82.5 +/- 21.1 ml/min/kg in diabetic rats at a high infusion rate (72 micrograms/min). When the infusion rate was lowered to 4.5 micrograms/min, the respective values for the unbound intrinsic clearance were 353 +/- 101 ml/min/kg and 561 +/- 112 ml/min/kg. Diabetic rats, however, showed no changes in plasma protein binding of diflunisal. The data suggest that the elimination of diflunisal was increased as a result of increased enzyme activity. Insulin treatment appeared to reverse the diabetic effect, suggesting that the effect on drug metabolism was the result of insulin deficiency and not a secondary or nonspecific effect of streptozotocin.(ABSTRACT TRUNCATED AT 250 WORDS)

Renin inhibitors containing hydrophilic groups. Tetrapeptides with enhanced aqueous solubility and nanomolar potency.

Nineteen tetrapeptides containing statine (Sta) and 4-amino-5-cyclohexyl-3-hydroxypentanoic acid (ACHPA) were prepared. Solubility measurements of these compounds were carried out in H2O and in pH 7.4 phosphate buffer solution, and their partition coefficients were determined in a 1:1 1-octanol/sodium phosphate-citric acid buffer system. The tetrapeptides were tested in vitro for their ability to inhibit porcine, canine, and human plasma renins. Four compounds, 6, 12, 14, and 20, were potent inhibitors against all renins tested (IC50 = 10(-9) M). Compound 12 was administered orally to dogs and substantially inhibited plasma renin activity for up to 5 h. The addition of polar groups to the C-terminus of Sta- and ACHPA-containing tetrapeptides renders them soluble in aqueous milieu and provides a valuable tool with which to examine the role of the renin-angiotensin system in physiological and pathological circumstances.

Differential renal handling of angiotensin-converting enzyme inhibitors enalaprilat and lisinopril in rats.

Enalaprilat, the active metabolite of enalapril, and its lysine analogue lisinopril are potent nonsulfhydryl angiotensin-converting enzyme inhibitors. Earlier studies from our laboratories demonstrated that neither drug is significantly metabolized, and both are almost exclusively eliminated by renal excretion. This report compares the renal excretory mechanisms for these structurally related compounds in the rat. After an iv, 1-mg/kg dose, ratios of renal clearance (CLR) of unbound drug to glomerular filtration rate (GFR) for enalaprilat and lisinopril were 2.72 +/- 0.70 and 1.01 +/- 0.18, respectively, suggesting that enalaprilat, but not lisinopril, was actively secreted by the kidneys. Treatment with probenecid and p-aminohippuric acid, potent competitive inhibitors for the renal anionic transport system, caused a profound decrease in the renal clearance of enalaprilat to the level of GFR. The CLR/fu.GFR, where fu is the unbound fraction, became 1.10 +/- 0.09 and 1.25 +/- 0.25, respectively. These results and the fact that quinine, a potent inhibitor for the cationic transport system, had little effect on the renal clearance of enalaprilat indicated that enalaprilat is secreted by the organic anion transport system. On the other hand, probenecid, p-aminohippuric acid, and quinine had no effect on the renal clearance of lisinopril, suggesting that lisinopril is eliminated exclusively by glomerular filtration.

Kinetic studies on the competition between famotidine and cimetidine in rats. Evidence of multiple renal secretory systems for organic cations.

The H2-receptor antagonists famotidine and cimetidine are both basic drugs that are predominantly eliminated by the kidneys. Cimetidine has been shown to inhibit the renal secretion of tetraethyl-ammonium bromide (TEAB) but not p-aminohippuric acid (PAH), suggesting that cimetidine is secreted by an organic cation transport system [Weiner and Roth: J. Pharmacol. Exp. Ther. 216: 516 (1981)]. The present study shows that famotidine behaves like cimetidine in that it also inhibits TEAB but not PAH excretion. Where a high concentration of cimetidine in plasma has an inhibitory effect on the renal excretion of famotidine, the reverse is not true, i.e. high plasma levels of famotidine have no effect on the excretion of cimetidine. Further evidence that additional transport systems are involved in the renal tubular secretion of cimetidine is as follows. Quinine, a potent competitor of the organic cation transport system, inhibits the secretory component of famotidine renal clearance but not that of cimetidine. Probenecid, a classic competitor for the organic anion transport system, inhibits the renal excretion of cimetidine but not famotidine. However, the effect of probenecid is minor and not sufficient to account for other components of cimetidine secretion not affected by famotidine and quinine.

Renin inhibitors. Statine-containing tetrapeptides with varied hydrophobic carboxy termini.

A series of statine-containing tetrapeptides, systematically modified at the carboxy terminus with various hydrophobic aromatic groups, is described. These compounds were tested in vitro for their ability to inhibit porcine, human plasma, and purified human kidney renins. These analogues help to define optimal binding aspects in a region of the enzyme that appears to be specific for spatial arrangement of aromatic groups. Replacement of the metabolically labile Phe amide with nonpeptidal groups proved possible while achieving inhibitory potency in the nanomolar range vs. porcine kidney renin. For the compounds 6i, 6m, and 6o, a large discrepancy in potency between the human plasma and the purified human kidney renin assays was observed. This disparity does not appear to be a consequence of a previously proposed plasma binding component.

Differential effects of phenobarbital on ester and ether glucuronidation of diflunisal in rats.

The relative contribution of ether and ester glucuronidation to diflunisal metabolism was assessed by studying the effects of enzyme inducers, phenobarbital (PB), 3-methylcholanthrene (3-MC) and beta-naphthoflavone (BNF). Treatment with either PB, 3-MC or BNF increased markedly the unbound intrinsic clearance of diflunisal. Saline-treated control rats showed a greater unbound intrinsic clearance of diflunisal than oil-treated controls indicating that repetitive treatment with oil had an effect on enzyme activity. Treatment with 3-MC and BNF appeared to cause a decrease in the biliary clearance of ether and ester glucuronide, but PB had little effect on the biliary clearance of glucuronides. Rats pretreated with PB showed a 3-fold increase in the fractional metabolite formation clearance of ether glucuronide and a 2-fold increase in the fractional metabolite formation clearance of ester glucuronide, suggesting differential effects of PB on ester and ether glucuronidation. A similar trend, but to a smaller extent, was also observed for 3-MC- and BNF-treated rats. These results suggest the possibility of selective induction of multiple forms of UDP-glucuronyltransferase involved in metabolism of diflunisal.