In vitro and in vivo murine metabolism of spirogermanium.

We report the identification of spirogermanium (SG) metabolites derived from incubation of the drug with a mouse liver microsomal preparation as well as those obtained from the urine of mice injected with the drug. GC/MS data using electron impact and chemical ionization indicate that the major metabolic products appearing in the urine of mice are hydroxylated metabolites resulting from oxidation of the ethyl substituents on germanium. Thermospray (TSP) LC/MS data suggest that these hydroxy metabolites are further oxidized to an acid and a deethylated metabolite that has undergone hydroxylation of the germanium atom. In a separate experiment, human urine from a subject undergoing therapy with SG was subjected to TSP-LC/MS analysis. The SG metabolite pattern observed in the urine from human was similar to that observed in the mouse urine. These results suggest that the metabolic fate of SG in human is qualitatively similar to that found in the mouse.     

In vitro and in vivo studies on the metabolism of tirofiban.

Tirofiban hydrochloride [L-tyrosine-N-(butylsulfonyl)-O-[4-(4-piperidinebutyl)] monohydrochloride, is a potent and specific fibrinogen receptor antagonist. Radiolabeled tirofiban was synthesized with either (3)H-label incorporated into the phenyl ring of the tyrosinyl residue or (14)C-label in the butane sulfonyl moiety. Neither human liver microsomes nor liver slices metabolized [(14)C]tirofiban. However, male rat liver microsomes converted a limited amount of the substrate to a more polar metabolite (I) and a relatively less polar metabolite (II). The formation of I was sex dependent and resulted from an O-dealkylation reaction catalyzed by CYP3A2. Metabolite II was identified as a 2-piperidone analog of tirofiban. There was no evidence for Phase II biotransformation of tirofiban by microsomes fortified with uridine-5'-diphospho-alpha-D-glucuronic acid. After a 1 mg/kg i.v. dose of [(14)C]tirofiban, recoveries of radioactivity in rat urine and bile were 23 and 73%, respectively. Metabolite I and unchanged tirofiban represented 70 and 30% of the urinary radioactivity, respectively. Tirofiban represented >90% of the biliary radioactivity. At least three minor biliary metabolites represented the remainder of the radioactivity. One of them was identified as I. Another was identified as II. When dogs received 1 mg/kg i.v. of [(3)H]tirofiban, most of the radioactivity was recovered in the feces as unchanged tirofiban. The plasma half-life of tirofiban was short in both rats and dogs, and tirofiban was not concentrated in tissues other than those of the vasculature and excretory organs.     

Inhibitory effect of stiripentol on carbamazepine and saquinavir metabolism in human

AIMS: To characterize the in vitro and in vivo inhibitory effect of stiripentol, a new anticonvulsant, on the metabolism of carbamazepine and saquinavir, which are substrates of CYP3A4. METHODS: Human liver microsomes and cDNA-expressed CYP enzymes were used for the in vitro experiments. Pharmacokinetic data from epileptic children and healthy adults were used for the carbamazepine and saquinavir in vivo studies, respectively. RESULTS: Carbamazepine biotransformation to its 10,11-epoxide by human liver microsomes (Vmax = 10.3 nmol min(-1) nmol(-1) P450, apparent Km = 362 microm), cDNA-expressed CYP3A4 (Vmax = 1.17 nmol min(-1) nmol(-1) P450, apparent Km = 119 microm) and CYP2C8 (Vmax = 0.669 nmol min(-1) nmol(-1) P450, apparent Km = 757 microm) was inhibited by stiripentol (IC50 14, 5.1, 37 microM and apparent Ki 3.7, 2.5, 35 microm, respectively). Saquinavir biotransformation to its major metabolite M7 by human liver microsomes (Vmax = 5.7 nmol min(-1) nmol(-1) P450, apparent Km = 0.79 microm) was inhibited by stiripentol (IC50 163 microM, apparent Ki 86 microm). In epileptic children treated with carbamazepine and stiripentol, the plasma concentration ratio of carbamazepine epoxide/carbamazepine was decreased by 65%. The in vivo apparent Ki for stiripentol ranged from 10.5 to 41.4 microm. The pharmacokinetics of saquinavir was not modified by stiripentol in healthy adults. The 95% confidence intervals for the difference for Cmax and AUC of saquinavir between the placebo and stiripentol phase were (-39.8, 39.8) and (-33.2, 112), respectively. CONCLUSIONS: These results showed that stiripentol was a weak inhibitor of saquinavir metabolism both in vitro and in vivo. In contrast, stiripentol is a potent inhibitor of carbamazepine 10,11-epoxide formation in vitro and in vivo in epileptic patients.     

Species differences in clobazam metabolism and antileptazol effect

The antileptazol effect of clobazam lasts longer in the mouse than in the rat. After intraperitoneal injection of clobazam (10 mg kg^?1) plasma and brain concentrations of the drug and its rate of disappearance were similar in both species, whereas the metabolite N demethylclobazam was present at higher concentrations and for longer in the mouse than in the rat. Although the exact contribution of the N-desmethylclobazam to the anticonvulsant effect of clobazam cannot be assessed, the longer duration in mice than in rats seems to be associated with different brain accumulations of the metabolite.     

In vitro and in vivo determination of piperacillin metabolism in humans.

Piperacillin metabolism and biliary excretion are different between humans and preclinical species. In the present study, piperacillin metabolites were characterized in bile and urine of healthy humans and compared with metabolites formed in vitro. Volunteers were administered 2 g of piperacillin IV; blood, urine, and duodenal aspirates (obtained via a custom-made oroenteric catheter) were collected. The metabolism of piperacillin in humans also was investigated in vitro using pooled human liver microsomes and sandwich-cultured human hepatocytes. Piperacillin and metabolites were estimated by high-performance liquid chromatography with tandem mass spectrometry detection. Piperacillin, desethylpiperacillin, and desethylpiperacillin glucuronide were detected in bile, urine, and human liver microsomal incubates. Similar to the in vivo results, desethylpiperacillin was formed and excreted into bile canaliculi of sandwich-cultured human hepatocytes. This is the first report of glucuronidation of desethylpiperacillin in vitro or in vivo. The clinical method employed in this study to determine biliary clearance of drugs also facilitates bile collection as soon as bile is excreted from the gallbladder, thereby minimizing the exposure of labile metabolites to the intestinal environment. This study exemplifies how a combination of in vitro and in vivo tools can aid in the identification of metabolites unique to the human species.     

In vivo and in vitro metabolism of arylamine procarcinogens in acetyltransferase-deficient mice.

Arylamine N-acetyltransferases (NATs) catalyze the biotransformation of a number of aromatic and heterocyclic amines, many of which are procarcinogenic agents. Interestingly, these enzymes are binary in nature, participating in both detoxification and activation reactions, and thus it is unclear what role NATs actually play in either preventing or enhancing toxic responses. The ultimate direction may be substrate-specific and dependent on its tissue-specific metabolism by competing, but genetically variable, drug-metabolizing enzymes. To investigate the effect of N-acetylation on the metabolism of some classical procarcinogenic arylamines, we have used our double knockout Nat1/2(-/-) mouse model to test both in vitro activity and the in vivo clearance of some of these agents. As expected, N-acetylation activity was undetectable in tissue cytosol preparations from Nat1/2(-/-) mice for 4-aminobiphenyl (ABP) and 2-aminofluorene (AF), whereas significant levels were measured in all wild-type tissue cytosols tested, indicating the widespread metabolism of these agents. Nat1/2(-/-) mice displayed a variable response with respect to in vivo pharmacokinetics. AF appeared to be most severely compromised, with a 3- to 4-fold increased area under the curve (AUC), whereas the clearance of ABP was found to be less dependent on N-acetylation, with no difference in ABP-AUC between wild-type and knockout animals. 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine was neither N-acetylated nor was its clearance affected by NAT genotype, signifying a dependence on other drug-metabolizing enzymes. The elucidation of the role that N-acetylation plays in the clearance of procarcinogenic agents is the first step in attempting to correlate metabolism by NATs to toxic outcome prevention or augmentation.     

In vitro and in vivo protective effect of orphenadrine on glutamate neurotoxicity.

The anticholinergic drug orphenadrine is used in the treatment of Parkinson's disease. In this study we evaluate the neuroprotective effects of orphenadrine on excitotoxicity in vivo and in vitro. Orphenadrine prevented the mitochondrial and the cytoplasmic membrane potential decrease evoked by NMDA (100 microM) in rat dissociated cerebellar granule cells showing an IC50 value of 11.6 +/- 4.7 microM (mean +/- SEM, n = 5) and 13.5 +/- 2.3 microM (n = 3), respectively. Orphenadrine was able to protect cerebellar granule cell cultures from glutamate-induced neurotoxicity. Kainic acid (KA, 10 mg/kg)-induced excitotoxicity was evaluated in vivo using the microglial marker peripheral-type benzodiazepine receptor (PBR) and heat shock protein 72 (HSP72) expression in the hippocampus. The Bmax of PBR for control tissues was 589.1 +/- 40.0 fmol/mg protein (n = 4), increasing to 1692.5 +/- 51.6 fmol/mg protein (n = 5) after the KA treatment. Pretreatment with orphenadrine (10 mg/kg) blocked the KA-induced increase in PBR density. As expected, KA-administration induced the expression of HSP72 that was blocked in the orphenadrine + KA-treated rats. We demonstrate that orphenadrine, interacting at the NMDA receptor, is able to prevent the neurotoxicity mediated by activation at glutamate ionotropic receptors.     

In vitro and in vivo correlation of the inhibitory effect of cyclosporin A on the transporter-mediated hepatic uptake of cerivastatin in rats.

Previously, we have shown that the inhibition of the transporter-mediated hepatic uptake of cerivastatin (CER) by cyclosporin A (CsA) could, at least partly, explain a pharmacokinetic interaction between these drugs in humans. In the present study, we have examined the effect of CsA on the in vivo disposition of CER in rats and the in vitro uptake of [14C]CER in isolated rat hepatocytes in an attempt to evaluate the effect of inhibition of transporter-mediated hepatic uptake on the in vivo CER disposition. The steady-state plasma concentration of CER increased 1.4-fold when coadministered with CsA up to a steady-state blood concentration of 4 microM. Studies of [14C]CER uptake into isolated rat hepatocytes showed saturable transport, with the saturable portion accounting for more than 80% of the total uptake. CsA competitively inhibited the uptake of [14C]CER with a Ki of 0.3 microM. The IC50 for the uptake of [14C]CER in the absence and presence of rat plasma was 0.2 and 2.3 microM, respectively. The in vivo hepatic uptake of [14C]CER evaluated by the liver uptake index method was also inhibited by CsA in a dose-dependent manner. On the other hand, CsA did not inhibit the metabolism of [14C]CER in rat microsomes. The in vitro and in vivo correlation analysis revealed that this pharmacokinetic interaction between these drugs in rats could be quantitatively explained by the inhibition of transporter-mediated hepatic uptake. Thus, this drug-drug interaction in rats is predominantly caused by the transporter-mediated uptake process.     

In vitro and in vivo inhibitory effects of disodium cromoglycate on influenza virus infection

Disodium cromoglycate (DSCG) is one of the safest drugs for the prevention of bronchial asthma and allergic rhinitis attacks. The effect of DSCG on acute upper respiratory tract viral infection is still controversial. Here we investigated DSCG inhibition of influenza virus infection in vivo and in vitro. In vivo effects of DSCG on viral infection were assessed using a murine model of respiratory tract infection. Intranasal administration of DSCG protected mice from death induced by infection with influenza virus A/PR/8/34. We analyzed DSCG anti-viral effects in vitro by either (i) treating cells prior to viral adsorption, (ii) treating cells concurrently with viral adsorption, or (iii) treating cells after viral adsorption. DSCG treatment of cells during or after, but not before, viral adsorption significantly inhibited influenza viral infection, indicating DSCG acts on events late in viral infection. DSCG exerts anti-influenza effect both in vitro and in vivo at the doses compatible with treatment for asthma. DSCG marginally inhibited influenza viral neuraminidase and membrane fusion functions, suggesting that DSCG inhibition of viral neuraminidase and fusion activities may partially mediate this anti-influenza effect. Our results indicate that treatment of patients including children with DSCG may take advantages for prevention from influenza virus infection.     

In vitro and in vivo inhibitory effects of propentofylline on cyclic AMP phosphodiesterase activity

1-(5'-Oxohexyl)-3-methyl-7-propylxanthine (propentofylline), a vasoactive agent, was investigated for its in vitro and in vivo inhibitory effects on cyclic AMP phosphodiesterases activity. Soluble cyclic AMP phosphodiesterases from the cerebral cortex, heart muscle, descending aorta and platelet showed two Km values, high and low. The low Km values were in the range of 2.1-3.0 mumol/l and the high Km values were 111, 28.7, 30.2 and 18.7 mumol/l, respectively. Propentofylline inhibited the enzyme from the 4 tissues noncompetitively. Ki values for the low and high Km cyclic AMP phosphodiesterases were 83.4-135 and 107-188 mumol/l, respectively. In the four tissues, the enzyme inhibitory effect of propentofylline was 1/4 to 1/10 times that of 3-isobutyl-1-methylxanthine (IBMX) but 3-9 and 6-17 times as potent as those of theophylline and caffeine, respectively. The in vivo cyclic AMP phosphodiesterase inhibitory effect of propentofylline was determined in mice using an elevation of plasma cyclic AMP levels as a measure. When given both singly and in combination with isoprenaline (isoproterenol), propentofylline was a potent inhibitor of cyclic AMP phosphodiesterase in the case of intravenous administration.     

In vitro and in vivo oxidative metabolism and glucuronidation of anastrozole

AIMS

Little information is available regarding the metabolic routes of anastrozole and the specific enzymes involved. We characterized anastrozole oxidative and conjugation metabolism in vitro and in vivo.

METHODS

A sensitive LC-MS/MS method was developed to measure anastrozole and its metabolites in vitro and in vivo. Anastrozole metabolism was characterized using human liver microsomes (HLMs), expressed cytochrome P450s (CYPs) and UDP-glucuronosyltransferases (UGTs).

RESULTS

Hydroxyanastrozole and anastrozole glucuronide were identified as the main oxidative and conjugated metabolites of anastrozole in vitro, respectively. Formation of hydroxyanastrozole from anastrozole was markedly inhibited by CYP3A selective chemical inhibitors (by >90%) and significantly correlated with CYP3A activity in a panel of HLMs (r = 0.96, P = 0.0005) and mainly catalyzed by expressed CYP3A4 and CYP3A5. The K_m values obtained from HLMs were also close to those from CYP3A4 and CYP3A5. Formation of anastrozole glucuronide in a bank of HLMs was correlated strongly with imipramine N-glucuronide, a marker of UGT1A4 (r = 0.72, P < 0.0001), while expressed UGT1A4 catalyzed its formation at the highest rate. Hydroxyanastrozole (mainly as a glucuronide) and anastrozole were quantified in plasma of breast cancer patients taking anastrozole (1 mg day⁻¹); anastrozole glucuronide was less apparent.

CONCLUSION

Anastrozole is oxidized to hydroxyanastrozole mainly by CYP3A4 (and to some extent by CYP3A5 and CYP2C8). Once formed, this metabolite undergoes glucuronidation. Variable activity of CYP3A4 (and probably UGT1A4), possibly due to genetic polymorphisms and drug interactions, may alter anastrozole disposition and its effects in vivo.     

In vitro inhibitory effect of some flavonoids on rotavirus infectivity

The inhibitory effects of some flavonoids on the infectivity of rotavirus, which predominantly causes sporadic diarrhea in infants and young children, were investigated. Among tested flavonoids, diosmin and hesperidin had the most potent inhibitory activity on rotavirus infection. The fifty percent inhibitory concentration of both compounds was 10 microM. However, their aglycones did not have the inhibitory activity. The rutinose moiety of flavonoids should protect against the invasion of rotavirus into cells.     

头孢他美的体内外抗菌活性

测定头孢他美对临床分离菌株 (包括革兰氏阳性菌、革兰氏阴性菌 )的体内、体外抗菌活性 ,同时测定了头孢他美对产 β-内酰胺酶菌株的最低抑菌浓度及它对 β-内酰胺酶的稳定性。并与头孢噻肟、头孢哌酮、头孢呋辛、头孢曲松等 4个头孢菌素做了比较。结果表明头孢他美具有和它们相似的抗菌活性 ,而且具有很强的β-内酰胺酶稳定性。     

In vitro characterization of clobazam metabolism by recombinant cytochrome P450 enzymes: importance of CYP2C19.

The specific cytochrome P450 (P450) isoforms mediating the biotransformations of clobazam (CLB) and those of its major metabolites, N-desmethylclobazam (NCLB) and 4'-hydroxyclobazam were identified using cDNA-expressed P450 and P450-specific chemical inhibitors. Among the 13 cDNA-expressed P450 isoforms tested, CLB was mainly demethylated by CYP3A4, CYP2C19, and CYP2B6 and 4'-hydroxylated by CYP2C19 and CYP2C18. CYP2C19 and CYP2C18 catalyzed the 4'-hydroxylation of NCLB. The kinetics of the major biotransformations were studied: CYP3A4, CYP2C19, and CYP2B6 mediated the formation of NCLB with Km = 29.0, 31.9, and 289 microM, Vmax = 6.20, 1.15, and 5.70 nmol/min/nmol P450, and intrinsic clearance (CLint) = 214, 36.1, and 19.7 microl/min/nmol P450, respectively. NCLB was hydroxylated to 4'-hydroxydesmethylclobazam by CYP2C19 with Km = 5.74 microM, Vmax = 0.219 nmol/min/nmol P450, and CLint = 38.2 microl/min/nmol P450 (Hill coefficient = 1.54). These findings were supported by chemical inhibition studies in human liver microsomes. Indeed, ketoconazole (1 microM) inhibited the demethylation of CLB by 70% and omeprazole (10 microM) by 19%; omeprazole inhibited the hydroxylation of NCLB by 26%. Twenty-two epileptic patients treated with CLB were genotyped for CYP2C19. The NCLB/CLB plasma metabolic ratio was significantly higher in the subjects carrying one CYP2C19*2 mutated allele than in those carrying the wild-type genotype. CYP3A4 and CYP2C19 are the main P450s involved in clobazam metabolism. Interactions with other drugs metabolized by these P450s can occur; moreover, the CYP2C19 genetic polymorphism could be responsible for interindividual variations of plasma concentrations of N-desmethylclobazam and thus for occurrence of adverse events.     

In vitro and in vivo evaluation of the metabolism and pharmacokinetics of sebacoyl dinalbuphine.

A diester prodrug of nalbuphine, sebacoyl dinalbuphine (SDN), and its long-acting formulation are currently being developed to prolong the duration of nalbuphine. A comparative in vitro hydrolysis study was conducted for SDN in rat, rabbit, dog, and human blood. Both SDN and nalbuphine in blood or plasma were measured by high-performance liquid chromatography. The hydrolysis rates of SDN in blood were ranked as follows: rat > rabbit > human > dog. The rapid formation of nalbuphine in the blood accounted for almost 100% of the prodrug, which supported the contention that nalbuphine is the major metabolite after SDN hydrolysis. The hydrolysis profiles of SDN were similar both in plasma and in red blood cells when compared in the blood. In vitro release results of SDN long-acting formulation showed that the rate-limited step of SDN hydrolysis to nalbuphine in blood is the penetration of SDN from oil into the blood. After intravenous administration of SDN in sesame oil into rats, nalbuphine quickly appeared in plasma and, thereafter, exhibited monoexponential decay. Pharmaceutical dosage forms affecting the drug disposition kinetics were demonstrated after intravenous administration. The AUC of nalbuphine was significantly higher and clearance was significantly lower, without changes in the t(1/2) of nalbuphine after intravenous dosing of SDN in sesame oil when compared with that of intravenous dosing with nalbuphine HCl in rats. Overall, these results suggest that SDN fulfilled the original pro-soft drug design in which the prodrug can rapidly metabolize to nalbuphine, and no other unexpected compounds were apparent in the blood.     

In vitro metabolism and inhibitory effects of atractylenolide II on various hepatic CYPs in HLMs

In the present study, we aimed to investigate the interaction between atractylenolide II (AT-II) and CYP450 enzyme in human liver microsomes, and to lay a theoretical foundation for predicting the possible interaction of AT-II in combination with drugs. The chemical inhibition experiment was carried out with specific inhibitors to clarify the CYP450 subtypes affecting the metabolism of AT-II, and the mechanism, kinetics, and type of inhibition of CYP450 enzyme by AT-II were studied by using the probe-based determination method of human liver microsome system with the related data of IC50 and K_i as evaluation indexes. The metabolism of AT-II was affected by CYP1A2, CYP2C9 and CYP3A4 inhibitors, and the highest inhibition rates were 41.35%, 41.97% and 82.45%, respectively. The IC₅₀ values of AT-II to five subtypes of P450 CYP2C9, CYP1A2, CYP2C19, CYP3A4 and CYP2D6 were 69.7, 84.3, 92.4, 173.8 and 190.1 μmol/L, respectively. The K_i values of AT-II to five subtypes of P450 CYP2C9, CYP1A2, CYP2C19, CYP3A4 and CYP2D6 were 190.6, 179.1, > 200, 72.2 and 66.8, respectively. Among these enzymes, AT-II exhibited non-competitive inhibition on CYP1A2, showed competitive inhibition on CYP2C9 and CYP3A4, and displayed mixed AT-II inhibition on CYP2C19 and CYP2D6. CYP1A2, CYP2C9 and CYP3A4 were involved in the AT-II metabolism, and AT-II exhibited different inhibitory mechanisms and strengths for the five subtypes of CYP450.     

The urotoxic effects of N,N'-dimethylaminopropionitrile. 2. In vivo and in vitro metabolism

The urotoxicity and metabolism of N,N'-dimethylaminopropionitrile (DMAPN) were investigated in male Sprague-Dawley rats. Animals treated with 525 mg DMAPN/kg or equimolar doses of commercially available potential DMAPN metabolites showed varying levels of urinary retention. About 44% of the administered dose of DMAPN was excreted unchanged in 5 days. beta-Aminopropionitrile and cyanoacetic acid were identified as urinary metabolites. The urinary excretion of cyanoacetic acid was nonlinearly proportional to the volume of urine retained in the bladders. In vitro, the metabolism of DMAPN to cyanide, formaldehyde, and cyanoacetic acid was localized mostly in the microsomal fraction of liver, kidney, and urinary bladders. This reaction required NADPH and oxygen for maximal activity. Metabolism of DMAPN was increased in hepatic microsomes obtained from phenobarbital-treated rats (220% of control) and decreased following CoCl1 treatments (73% of controls). Addition of SKF 525-A to the incubation mixtures inhibited the metabolism of DMAPN to formaldehyde (47-64% of controls). Addition of sulfhydryl compounds (glutathione and cysteine) to the incubation mixtures did not affect the rate of these reactions. These findings indicate that DMAPN is primarily metabolized via a cytochrome P450-dependent mixed-function oxidase system and that the urotoxic effects of DMAPN may be related to this metabolism.     

In vivo metabolism of aloemannan.

The metabolism of fluoresceinyl isothiocyanate labeled aloemannan (FITC-AM) was examined by p.o. and i.v. administration in mice at a dose of 120 mg/kg. Analysis of FITC-AM in urine and feces showed that FITC-AM (MW 500 KD) was metabolized into smaller molecules that mainly accumulated in the kidneys. AM was catabolized by the human intestinal microflora to catabolites 1 and 2 with molecular weights of 30 and 10 KD, respectively. Hydrolysis of AM showed hexosamine peaks on HPAE. The findings suggest that the immunomodulation of AM may come from not only neutral polysaccharides but also contaminated hexosamine in AM.     

In vitro and in vivo effects of kelatorphan on enkephalin metabolism in rodent brain

Biologically relevant assays were used to compare the potency of kelatorphan (N-[3(R)-[(hydroxyamino)carbonyl]-2-benzyl-1-oxopropyl]-L-alanine) as inhibitor of the peptidase-induced metabolism of enkephalins to that of bestatin, a non-specific inhibitor of aminopeptidase and thiorphan, a highly potent blocker of the neutral endopeptidase (EC 3.4.24.11) designated as enkephalinase. Kelatorphan almost completely inhibited the formation of the three metabolites [3H]Tyr, [3H]Tyr-Gly and [3H]Tyr-Gly-Gly produced by incubation of [3H][Tyr1,Met5]enkephalin with rat striatal slices. Co-administered with [Met5]enkephalin in mouse brain, kelatorphan was able to prevent by 80% the degradation of the exogenous peptide. Moreover, a mixture of thiorphan (1 microM) and bestatin (20 microM) or kelatorphan alone (20 microM) induced a 2.2 to 2.5-fold increase in endogenous [Met5]enkephalin overflow after evoked depolarization of superfused rat striatal slices. In this assay, kelatorphan was the only compound to increase by 63% the basal level of released [Met5]enkephalin. Kelatorphan was about 100 times less potent than bestatin to inhibit the total rat striatal aminopeptidases, but as efficient (IC50 = 4 X 10(-7) M) as bestatin to inhibit a minor aminopeptidase activity resembling aminopeptidase M. Therefore the reported enhanced analgesic potency of kelatorphan with regard to the association of bestatin and thiorphan is very likely related to its ability to almost completely inhibit enkephalin-degrading enzymes (including the Tyr-Gly releasing peptidase) and to its better selectivity for the biologically relevant aminopeptidase M. Kelatorphan would be a valuable probe, preferable to the association of bestatin and thiorphan, to investigate the physiological functions regulated by a phasic enkephalinergic activity.