RESUMO
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.
Assuntos
Tretinoína/farmacocinética , Animais , Relação Dose-Resposta a Droga , Meia-Vida , Humanos , Macaca mulatta , Taxa de Depuração Metabólica , Estereoisomerismo , Fatores de Tempo , Tretinoína/sangue , Tretinoína/líquido cefalorraquidianoRESUMO
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.
Assuntos
Antineoplásicos/uso terapêutico , Neoplasias/tratamento farmacológico , Receptores do Ácido Retinoico/agonistas , Fatores de Transcrição/agonistas , Tretinoína/uso terapêutico , Adulto , Idoso , Alitretinoína , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Receptores X de Retinoides , Tretinoína/efeitos adversos , Tretinoína/farmacocinéticaRESUMO
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.
Assuntos
Anticarcinógenos/uso terapêutico , Neoplasias/tratamento farmacológico , Tetra-Hidronaftalenos/uso terapêutico , Adulto , Idoso , Idoso de 80 Anos ou mais , Anticarcinógenos/efeitos adversos , Anticarcinógenos/farmacocinética , Bexaroteno , Feminino , Cefaleia/induzido quimicamente , Humanos , Leucopenia/induzido quimicamente , Masculino , Pessoa de Meia-Idade , Estadiamento de Neoplasias , Neoplasias/metabolismo , Tetra-Hidronaftalenos/efeitos adversos , Tetra-Hidronaftalenos/farmacocinética , Resultado do TratamentoRESUMO
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.
Assuntos
Antineoplásicos/farmacocinética , Antineoplásicos/toxicidade , Neoplasias/tratamento farmacológico , Tretinoína/farmacocinética , Tretinoína/toxicidade , Administração Oral , Adulto , Idoso , Idoso de 80 Anos ou mais , Alitretinoína , Antineoplásicos/administração & dosagem , Cápsulas , Esquema de Medicação , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Tretinoína/administração & dosagemRESUMO
The synthesis and biological activity of a series of inhibitors of angiotensin-converting enzyme (EC 3.4.15.1) are described. Incorporation of the substituted N-carboxymethyl dipeptide design of enalapril (MK-421) into acyl tripeptides and larger peptides yielded potent inhibitors of the enzyme. These can be viewed as substrate analogues in which the carbonyl of the scissile peptide bond is replaced by a CHCO2H group. Several of the analogues described possess inhibitory potency equal to that of enalaprilat (MK-422), but none achieves an increase in potency which would demonstrate additional binding interactions contributed by the extended peptide chain. Application of the design described may be useful for inhibition of other metallopeptidases.
Assuntos
Inibidores da Enzima Conversora de Angiotensina , Dipeptídeos/farmacologia , Inibidores Enzimáticos/síntese química , Enalapril , Relação Estrutura-AtividadeRESUMO
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.
Assuntos
Oligopeptídeos/síntese química , Renina/antagonistas & inibidores , Aminoácidos/farmacologia , Animais , Cães , Humanos , Oligopeptídeos/farmacologia , Solubilidade , Relação Estrutura-Atividade , SuínosRESUMO
Analogues of the renin octapeptide substrate were synthesized in which replacement of the scissile dipeptide with (3S,4S)-4-amino-3-hydroxy-6-methylheptanoic acid (statine, Sta) transformed the substrate sequence into potent, transition-state analogue, competitive inhibitors of renin. Synthesis and incorporation of the cyclohexylalanyl analogue of Sta, (3S,4S)-4-amino-5-cyclohexyl-3-hydroxypentanoic acid (ACHPA), gave the most potent inhibitors of renin yet reported, including N-isovaleryl-L-histidyl-L-prolyl-L-phenylalanyl-L-histidyl-ACHPA-L -leucyl-L- phenylalanyl amide [Iva-His-Pro-Phe-His-ACHPA-Leu-Phe-NH2,3], with renin inhibitions of Ki = 1.6 X 10(-10) M (human kidney renin), IC50 = 1.7 X 10(-10)M (human plasma renin), IC50 = 1.9 X 10(-9)M (dog plasma renin), and IC50 = 2.1 X 10(-8) M (rat plasma renin). This inhibitor 3, containing ACHPA, was 55-76 times more potent vs. human renin than the comparable Sta-containing inhibitor 1 and 17 times more potent vs. dog renin than 1. Inhibitor 3 lowered blood pressure in sodium-deficient dogs, with in vivo potency 19 times that shown by 1, in close agreement with the relative in vitro potencies. Structure-activity results are presented that show the minimal N-terminus for these inhibitors. An ACHPA-containing pentapeptide, N-[(ethyloxy)carbonyl]-L-phenylalanyl-L- histidyl-ACHPA-L-leucyl-L-phenylalanyl amide [Etoc-Phe-His-ACHPA-Leu-Phe-NH2,8], retained subnanomolar inhibitory potency. Molecular modelling studies are described that suggested the design of ACHPA.
Assuntos
Aminoácidos , Oligopeptídeos/farmacologia , Renina/antagonistas & inibidores , Aminoácidos/síntese química , Animais , Ligação Competitiva , Pressão Sanguínea/efeitos dos fármacos , Fenômenos Químicos , Química , Cães , Humanos , Rim/enzimologia , Macaca mulatta , Camundongos , Oligopeptídeos/síntese química , Coelhos , Ratos , Renina/sangueRESUMO
A series of inhibitors of angiotensin converting enzyme (ACE, dipeptidyl carboxypeptidase, EC 3.4.15.1) is described which addresses certain conformational aspects of the enzyme-inhibitor interaction. In this study the alanylproline portion of the potent ACE inhibitor enalaprilat (2) is replaced by a series of monocyclic lactams containing the required recognition and binding elements. In order to more fully assess the lactam ring conformations and the key backbone angle psi as defined in 3 with respect to possible enzyme-bound conformations, a series of model lactams was investigated with use of molecular mechanics. The results point to a correlation between inhibitor potency (IC50) and the computed psi angle for the lowest energy conformation of the model compounds. Thus the psi angle as defined in 3 is an important determinant in the binding of inhibitors to ACE. The inhibition data in conjunction with the computational data have served to define a window of psi angles from 130 degrees to 170 degrees which seems to be acceptable to the ACE active site.
Assuntos
Inibidores da Enzima Conversora de Angiotensina , Enalapril/análogos & derivados , Enalapril/farmacologia , Enalaprilato , Conformação Molecular , Relação Estrutura-Atividade , Difração de Raios XRESUMO
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.
Assuntos
Aminoácidos , Inibidores Enzimáticos/síntese química , Oligopeptídeos/farmacologia , Renina/antagonistas & inibidores , Animais , Inibidores Enzimáticos/farmacologia , Humanos , Concentração de Íons de Hidrogênio , Rim/enzimologia , Relação Estrutura-Atividade , SuínosRESUMO
An extensive series of novel 4-substituted 3-hydroxy-1H-pyrrole-2,5-dione derivatives has been prepared and studied as inhibitors of glycolic acid oxidase (GAO). Compounds possessing large lipophilic 4-substituents are, in general, potent, competitive inhibitors of porcine liver GAO in vitro. Methylation of the nitrogen or the 3-hydroxy substituent reduced potency dramatically, indicating the requirement for the two acidic functions on the 1H-pyrrole-2,5-dione nucleus. In rat liver perfusion studies, with three representative compounds, concentration-dependent inhibition of the conversion of [1-14C]glycolate to [14C]oxalate was observed. Chronic oral administration to ethylene glycol fed rats of the 4-(4'-bromo[1,1'-biphenyl]-4-yl) derivative (83) was shown to effect a significant reduction in urinary oxalate levels over a 58-day period.
Assuntos
Oxirredutases do Álcool/antagonistas & inibidores , Maleimidas/farmacologia , Animais , Fígado/enzimologia , Maleimidas/síntese química , Metilação , Perfusão , Ratos , SuínosRESUMO
A centrifugal gel filtration separation of serum angiotensin converting enzyme (ACE) from a potent stable inhibitor is described. This, together with a 20 hr assay incubation of very dilute enzyme, permitted the assessment of the effects of enalapril maleate treatment on total serum ACE in rats and dogs. Total serum ACE increased in both species after 1 or 2 weeks at 10 mg/kg/day. Serum ACE in rats was more than doubled; whereas the increase was modest in dogs (48 +/- 9% minimum). The effect of the drug on serum ACE combined with inherent variability of ACE precluded use of serum ACE activity as an accurate measure of inhibitor concentration in animals receiving enalapril maleate.
Assuntos
Dipeptídeos/farmacologia , Peptidil Dipeptidase A/sangue , Inibidores da Enzima Conversora de Angiotensina , Animais , Cromatografia em Gel/métodos , Dipeptídeos/isolamento & purificação , Cães , Relação Dose-Resposta a Droga , Enalapril , Masculino , Peptidil Dipeptidase A/isolamento & purificação , Ratos , Especificidade da Espécie , Fatores de TempoAssuntos
Frutose-Bifosfatase/análise , Fígado/enzimologia , Monofosfato de Adenosina , Animais , Cromatografia , Diálise , Ácido Edético , Frutose-Bifosfatase/antagonistas & inibidores , Frutose-Bifosfatase/isolamento & purificação , Temperatura Alta , Concentração de Íons de Hidrogênio , Métodos , Coelhos , EspectrofotometriaAssuntos
Enalapril/toxicidade , Nefropatias/induzido quimicamente , Inibidores da Enzima Conversora de Angiotensina , Animais , Pressão Sanguínea/efeitos dos fármacos , Cães , Relação Dose-Resposta a Droga , Enalapril/farmacologia , Rim/patologia , Rim/fisiopatologia , Nefropatias/patologia , Nefropatias/fisiopatologia , Necrose Tubular Aguda/induzido quimicamente , Necrose Tubular Aguda/patologiaRESUMO
Animal studies (particularly in dogs) on enalapril maleate have served to predict the patterns of absorption and elimination observed in man. Enalapril is more readily absorbed in man than the active inhibitor form MK-422. Estimates of minimum absorption of enalapril are of the order of 60-70%, based on urinary recovery. Metabolism of enalapril to MK-422 appears to be largely a postabsorptive process. From urinary recovery data, a minimum of 43% of a 10-mg dose of enalapril is available as MK-422. Excretion of enalapril and MK-422 is principally renal. The excellent mass balance obtained in human studies precludes extensive metabolism beyond hydrolysis to MK-422. Data in hand suggest that any metabolism other than to MK-422 is of a trace nature.
Assuntos
Inibidores da Enzima Conversora de Angiotensina , Dipeptídeos/metabolismo , Absorção , Animais , Cães , Enalapril , Enalaprilato , Humanos , RatosRESUMO
[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.
Assuntos
Inibidores da Anidrase Carbônica/farmacocinética , Sulfonamidas/farmacocinética , Tiofenos/farmacocinética , Animais , Proteínas Sanguíneas/metabolismo , Inibidores da Anidrase Carbônica/sangue , Cromatografia Líquida de Alta Pressão , Eritrócitos/metabolismo , Meia-Vida , Masculino , Ligação Proteica , Ratos , Ratos Endogâmicos , Estereoisomerismo , Sulfonamidas/sangue , Tiofenos/sangueRESUMO
To provide insight into the reported reduction in the plasma clearance of diflunisal in human renal failure, this investigation evaluated several possible mechanisms for this effect in experimental renal failure. Rats with renal failure, induced by uranyl nitrate or by ureteral ligation, had both a lower plasma clearance and an increased apparent volume of distribution, a pattern resembling that seen in human renal failure. Steady-state diflunisal concentration and unbound fraction were determined in studies during a constant infusion of diflunisal to establish the relationships of concentration, protein binding and intrinsic clearance. The infusion studies revealed that the intrinsic clearance of diflunisal, i.e., the ability of enzyme system(s) to metabolize the drug, was decreased in uremia. Also, plasma protein binding of diflunisal was decreased, which may explain the increase in apparent volume of distribution in uremic rats. The decreased intrinsic clearance of diflunisal in uremic rats may be due partly to saturation of biotransformation process(es) by increasing unbound concentration as a consequence of impairment of plasma protein binding of diflunisal, and partly due to the diminished enzyme activity of glucuronidation by renal failure. The lack of an effect of the esterase inhibitor phenylmethylsulfonyl fluoride on the intrinsic clearance of diflunisal in uremic rats suggested that the reduced intrinsic clearance of diflunisal was not attributable to the systemic enzymatic hydrolysis of the ester glucuronide.
Assuntos
Diflunisal/sangue , Salicilatos/sangue , Uremia/sangue , Animais , Proteínas Sanguíneas/metabolismo , Glucuronatos/metabolismo , Cinética , Masculino , Taxa de Depuração Metabólica , Ligação Proteica , Ratos , Ratos EndogâmicosRESUMO
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)
Assuntos
Cilastatina/farmacocinética , Animais , Biotransformação , Proteínas Sanguíneas/metabolismo , Cilastatina/sangue , Cilastatina/metabolismo , Relação Dose-Resposta a Droga , Meia-Vida , Injeções Intravenosas , Masculino , Probenecid/farmacologia , Ligação Proteica , Ratos , Ratos EndogâmicosRESUMO
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.
Assuntos
Sistema Enzimático do Citocromo P-450/fisiologia , Microssomos Hepáticos/efeitos dos fármacos , Retinoides/farmacologia , Animais , Bexaroteno , Sistema Enzimático do Citocromo P-450/análise , Isoenzimas/análise , Isoenzimas/fisiologia , Masculino , Microssomos Hepáticos/enzimologia , Microssomos Hepáticos/metabolismo , Estrutura Molecular , Ácidos Nicotínicos/farmacologia , Ratos , Ratos Sprague-Dawley , Receptores do Ácido Retinoico/agonistas , Receptores do Ácido Retinoico/metabolismo , Receptores X de Retinoides , Tetra-Hidronaftalenos/farmacologia , Fatores de Transcrição/agonistas , Fatores de Transcrição/metabolismo , Tretinoína/farmacologiaRESUMO
alpha-Isopropylmalate synthase, the first specific enzyme in leucine biosynthesis, was purified approximately 100-fold from extracts of Saccharomyces sp. (strain 60615), the most effective step being specific elution with the feedback inhibitor leucine from a hydroxyapatite column. In the early steps of purification, special care was taken to protect the synthase against proteolytic activities. The apparent molecular weight of the enzyme as determined from gel filtration on a calibrated column was 137,000 in the absence and 121,000 in the presence of leucine. Inhibition by leucine was specific and strongly pH-dependent, with the leucine concentration necessary for half-maximal inhibition increasing about 10-fold as the pH increased from 7.5 to 8.5. Within this pH range, catalytic activity remained almost unchanged. The apparent K(m) values for the two substrates were found to be 16 mum for alpha-ketoisovalerate and 9 mum for acetyl-coenzyme A. K(+) was required for full activity, the apparent K(a) value being 2 mm. Leucine inhibition was of the mixed type, resulting in decreased V(max) and increased apparent K(m) values forboth substrates. Whereas no cooperative effects were observed with either substrate, positive cooperativity was seen with leucine in the presence of saturating substrate concentrations. Leucine and, to a lesser extent, alpha-ketoisovalerate stabilized the purified enzyme against heat-inactivation. The presence of acetyl-coenzyme A, on the other hand, accelerated the inactivation. In subsequent experiments, coenzyme A was recognized as the actual inactivating ligand, being effective even at lower temperatures and in concentrations which were estimated to be in the range of the enzyme concentration.