In vitro and in vivo binding characteristics of a new long-acting histamine H1 antagonist, astemizole

Binding characteristics of astemizole were studied in vitro in various receptor binding models and in vivo by determining the occupancy of histamine H1 receptors in guinea pig lung and cerebellum. In vitro, astemizole was found to have a high affinity for histamine H1 receptors, but great difficulties were encountered in proving this because of its high affinity for nonspecific binding sites. Since the equilibrium conditions were not reached in vitro, the real affinity of astemizole remains unclear and its receptor profile must be interpreted with caution. Nevertheless, the drug is certainly much more potent on histamine H1 receptors than on serotonin S2 and adrenergic alpha 1-receptors. Moreover, it was found to be devoid of antimuscarinic and antidopaminergic properties. The most striking property of this drug is its extremely slow dissociation rate from H1 receptors when assayed in vitro using [3H]-pyrilamine. Ex vivo experiments were performed in guinea pigs; astemizole was given orally to the animals, and the occupancy of H1 receptors in the lung and the cerebellum was determined in vitro by the [3H]-pyrilamine binding assay. Astemizole was found to occupy H1 receptors in lung at very low doses. Here again the most striking receptor binding property was its very long duration. The occupancy of H1 receptors in lung began to decline only 4-6 days after administration of the drug. However, there was a marked difference between the occupancy of peripheral and central receptors; indeed, in contrast to pyrilamine, astemizole at pharmacological doses did not reach the H1 receptors in the cerebellum, presumably because the drug does not readily cross the blood-brain barrier.     

CL 284,846, a novel sedative-hypnotic: Evaluation of its metabolites for pharmacological activity in vitro and in vivo

CL 284,846, N-[3-3-cyanopyrazolo[1,5-a]pyrimidin-7-yl(phenyl)]-N-ethylacetamide, is a novel non-benzodiazepine sedative-hypnotic with benzodiazepine-like sedative effects, but with less apparent liability for accompanying undesired side effects. Three metabolites of the parent sedative-hypnotic have been isolated and identified, a desethyl metabolite, CL 284,859, a desethyl-5-keto metabolite, CL 345,644, and a 5-keto metabolite, CL 345,905. In experiments to determine the ability of these compounds to displace [³H]-flunitrazepam from rat cortical benzodiazepine receptors, the IC₅₀ values were 205 nM for CL 284,846 compared to 4.5 nM for diazepam as a positive control, and 11 μM for CL 284,859. The other two metabolite, CL 345,644 and CL 345,905, failed to displace [³H]-flunitrazepam. In a second experiment designed to evaluate in vivo receptor-mediated activity, CL 284,846 (3.0 mg/kg; 9.836 μmol/kg) was established as a discriminative stimulus (DS) in rats. While CL 284,846 (0.03ndash;3.0 mg/kg; 0.098–9.836 μmol/kg) showed a doserelated increase in drug-appropriate responding and one dose of triazolam (0.3 mg/kg; 0.875 μmol/kg) substituted as a positive control, all three metabolites (3.0–100.0 mg/kg; 10.3–341.3 μmol/kg for CL 284,859; 6.0–201.2 μmol/kg for CL 345,644; 9.3–311.5 μmol/kg for CL 345,905) failed to substitute for the DS effects of CL 284,846. These results suggest that CL 284,859, CL 345,644, and CL 345,905, the metabolites of the sedative-hypnotic CL 284,846, have no significant neuropharmacological effects at central benzodiazepine receptors and, thus, do not contribute to the activity of the parent compound. © 1994 Wiley-Liss, Inc.     

罗红霉素主要代谢产物的体外抗菌活性研究

临床上广泛使用的 (E) -罗红霉素在人体内有多种代谢途径。在鉴别和制备其代谢产物的基础上 ,采用二倍稀释法 ,选用 3种生物检测实验标准菌株 ,测定了母体药物和 6种主要代谢产物的体外抗菌活性(MIC,MBC)。结果表明 ,(E) -罗红霉素经代谢转化后 ,生成的 (Z) -罗红霉素异构体的活性略下降 ,(E) -红霉素肟的活性无明显改变 ,(E) - O-单去甲罗红霉素的 MIC未改变 ,而对芽孢杆菌的 MBC有所降低 ,(E) - N-单去甲罗红霉素的活性显著降低 ,(E) -脱红霉糖罗红霉素则基本失活。被测药物及代谢物在不同菌株之间的MIC和 MBC变化趋势基本相同 ,MBC较相应的 MIC大 10 0～ 10 0 0倍左右。     

In vivo and in vitro pharmacological characterization of SVT-40776, a novel M3 muscarinic receptor antagonist,for the treatment of overactive bladder

Background and purpose:

Highly selective M₃ muscarinic receptor antagonists may represent a better treatment for overactive bladder syndrome, diminishing side effects. Cardiac side effects of non-selective antimuscarinics have been associated with activity at M₂ receptors as these receptors are mainly responsible for muscarinic receptor-dependent bradycardia. We have investigated a novel antimuscarinic, SVT-40776, highly selective for M₃ over M₂ receptors (Ki = 0.19 nmol·L⁻¹ for M₃ receptor affinity). This study reports the functional activity of SVT-40776 in the bladder, relative to its activity in atria.

Experimental approach:

In vitro and ex vivo (oral dosing) inhibition of mouse detrusor and atrial contractile responses to carbachol were used to study the functional activity of SVT-40776. The in vivo efficacy of SVT-40776 was characterized by suppression of isovolumetric spontaneous bladder contractions in anaesthetized guinea pigs after intravenous administration.

Key results:

SVT-40776 was the most potent in inhibiting carbachol-induced bladder contractions of the anti-cholinergic agents tested, without affecting atrial contractions over the same range of concentrations. SVT-40776 exhibited the highest urinary versus cardiac selectivity (199-fold). In the guinea pig in vivo model, SVT-40776 inhibited 25% of spontaneous bladder contractions at a very low dose (6.97 µg·kg⁻¹ i.v), without affecting arterial blood pressure.

Conclusions and implications:

SVT-40776 is a potent inhibitor of M₃ receptor-related detrusor contractile activity. The absence of effects on isolated atria preparations represents an interesting characteristic and suggests that SVT-40776 may lack unwanted cardiac effects; a feature especially relevant in a compound intended to treat mainly elderly patients.British Journal of Pharmacology (2009) doi:10.1111/j.1476-5381.2008.00082.x     

Beta-blocking actions and the partial agonist activity of bopindolol, a new beta-adrenoceptor antagonist, and its two metabolites

The beta-blocking effects of bopindolol, a new long-acting beta-adrenoceptor antagonist, and its two metabolites, 18-502 and 20-785, were studied in the isolated atria and trachea preparations of the guinea pig. The partial agonist activity (PAA) and the membrane stabilizing activity (MSA) were studied in the pithed rat and the right ventricular papillary muscle preparations of the guinea pig, respectively. Antagonistic effect of bopindolol against isoproterenol was 1.1-2.8 and 14.1 times more potent than propranolol in the atria and the trachea preparations, respectively, while those of 18-502 were 34.7-38.0 (in atria) and 29.0 (in trachea) times more potent than propranolol. beta-Blocking potencies of 20-785 were 50% (in atria) and 10% (in trachea) of those of propranolol. Bopindolol, 18-502 and 20-785 increased the heart rate of the pithed rat dose-dependently. The effects were inhibited by pretreatment of the preparation with propranolol. The onset of action of bopindolol was slower and the duration, longer than those of its two metabolites. Bopindolol and 18-502 were more potent membrane stabilizers than propranolol, while 20-785 produced only a minimal stabilizing effect. These results indicate that not only bopindolol, but also the metabolites of bopindolol, 18-502 and 20-785 are potent beta-blocking agents with PAA. It is presumed that the long duration of action of bopindolol is due to the potent activities of its metabolites 18-502 and 20-785.     

Unbound plasma concentrations,total plasma concentrations,and red blood cell concentrations of thioridazine and its main metabolites: an in vitro study

Summary The relationships between, on the one hand, the unbound plasma concentrations of thioridazine and thioridazine metabolites and, on the other, the total plasma concentrations and the red blood cell concentrations were studied in vitro. The relationships between the unbound and total plasma concentrations were curvilinear, while those between the unbound concentrations and the red blood cell concentrations within a broad range were closer to rectilinear. Profound changes were seen in the relationship between unbound and total plasma concentrations at varying concentrations of ₁-acid glycoprotein, while the rectilinear relationship between the unbound plasma concentrations and the red blood cell concentrations remained essentially unchanged. Send offprint requests to E. Mårtensson at the above addressThe study was supported by the Swedish Medical Research Council (project number B85-21X-0371-12B)     

Salmeterol, a novel, long-acting beta 2-adrenoceptor agonist: characterization of pharmacological activity in vitro and in vivo.

1. Salmeterol, a novel, long-acting beta-adrenoceptor agonist, has been compared with isoprenaline and salbutamol for activity in vitro on a range of beta-adrenoceptor containing preparations from laboratory animals and man, and in vivo for bronchodilator activity in the conscious guinea-pig. 2. Salmeterol, like isoprenaline and salbutamol, relaxed preparations of both guinea-pig trachea (contracted by prostaglandin (PG)F2alpha or electrical stimulation) and human bronchus (contracted by PGF 2 alpha) in a concentration-related fashion. Salmeterol was of similar potency to isoprenaline and more potent than salbutamol on both airway preparations. 3. Relaxant responses of superfused guinea-pig trachea and human bronchus to isoprenaline and salbutamol declined rapidly when the agonists were washed from the tissues, with complete recovery within 10 min, whereas responses to salmeterol were more persistent. In electrically-stimulated guinea-pig trachea preparations, inhibition by salmeterol persisted for periods of up to 12h, despite continuous superfusion with agonist-free medium. However, these persistent responses were rapidly and fully reversed by the beta-adrenoceptor blocking drug, propranolol (0.1 microM). In further studies on guinea-pig trachea, propranolol caused concentration-related parallel, rightward shifts of salmeterol concentration-effect curves, yielding a pA2 of 9.0. The slope of the Schild plot was 1.02. 4. Another beta-adrenoceptor blocking drug, sotalol (10 microM), also fully and rapidly reversed established submaximal responses to salmeterol in superfused guinea-pig trachea. However, after administration of sotalol was stopped, the antagonism waned, and salmeterol responses were reasserted without the addition of further agonist. 5. In the beta 1-adrenoceptor containing preparation, rat left atria, isoprenaline exhibited potent, concentration-related, positive inotropic activity, whereas salbutamol and salmeterol were at least 2000-5000 fold less potent, and appeared to be partial agonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

Antihypertensive activity of OPC-13340, a new potent and long-acting dihydropyridine calcium antagonist, in rats.

The antihypertensive action of OPC-13340, a new dihydropyridine, was studied in rats and compared with the action of nicardipine and other dihydropyridines. OPC-13340 showed more potent and longer hypotensive action than nicardipine when administered either intravenously (i.v.) or orally in normotensive and hypertensive rats. Among 6 compounds tested, (OPC-13340, nifedipine, nitrendipine, nisoldipine, nicardipine and diltiazem), OPC-13340 was the most potent and long-acting when administered orally to spontaneously hypertensive rats (SHR). Tachycardia after administration of OPC-13340 was less or diminished earlier than that of nicardipine. Oral administration of OPC-13340 (3 mg/kg) once daily for 13 days did not cause any rebound phenomena in SHR. The compound inhibited Ca- or K-induced contractions in isolated rat aorta and shortened action potential duration in guinea pig papillary muscle, suggesting Ca channel blocking action. OPC-13340 might be useful as a drug for once-daily therapy of essential hypertension.     

Structural investigation of Cetrorelix,a new potent and long-acting LH-RH antagonist

The new decapeptide SB-75 (INN: Cetrorelix) has been characterized as a potent antagonist of luteinizinghormone releasing hormone (LH-RH). Such derivatives are of great medicinal interest owing to their potential application in areas such as hormone-dependent tumors, uterine fibroids, and in diseases and conditions which result from inappropriate hormone levels or which can be treated by suppression of estrogens. SB-75 is the subject of intensive ongoing clinical evaluation and is an accepted standard for the design of new LH-RH antagonists. We characterized SB-75 by means of modern MS and NMR techniques to demonstrate the significance of both sequencing methods on a complicated unnatural decapeptide. Our structural elucidations with nuclear Overhauser (NOE) experiments revealed clear evidence for a highly flexible molecule with no single predominant conformation even in sodium dodecyl sulfate (SDS) mimicking a cellular membrane.     

In vitro and in vivo alpha-blocking activity of thymoxamine and its two metabolites

The alpha-adrenoceptor potency of thymoxamine and its two metabolites deacetylthymoxamine and demethyldeacetylthymoxamine were determined on the contraction of rat vas deferens induced by noradrenaline, the blood pressure increase induced by noradrenaline given i.v. to dogs and the contraction of the nictitating membrane induced by electrical stimulation in cats. In vivo the three drugs were administered at 6.35 x 10(-6) mol kg-1 intravenously. Deacetylthymoxamine presented nearly the same alpha-blocking activity as the parent drug. This was ascribed in vivo to the rapid deacetylation of thymoxamine. Demethyldeacetylthymoxamine was less active. In vitro its pA2 was 6.20 +/- 0.09 compared with 6.75 +/- 0.20 for thymoxamine and 6.57 +/- 0.13 for deacetylthymoxamine. In vivo, it was inactive in dog and less active than the other two drugs soon after its administration in the cat. The oral LD 50 values in the mouse for the three drugs were respectively 0.81, 0.71 and 1.14 mmol kg-1 for thymoxamine, deacetylthymoxamine and demethyldeacetylthymoxamine.     

In vitro pharmacological profile of SK-896, a new human motilin analogue

SK-896 ([Leu(13)]motilin-Hse) is a new human motilin analogue synthesized by Escherichia coli using a biotechnological method. We investigated the binding of SK-896 to motilin receptors and the contractile effect of SK-896 on smooth muscle preparations isolated from the gastrointestinal tract and various regional organs in order to clarify its in vitro pharmacological profile. SK-896 inhibited the binding of (125)I-human motilin to rabbit gastroduodenal motilin receptors with the same potency as unlabeled human motilin. The IC(50) values of SK-896 and human motilin were 3.5 +/- 1.5 and 3.1 +/- 1.8 nmol/l, respectively. The K(d) of human motilin was 3.0 +/- 1.5 nmol/l, and the Ki of SK-896 was 3.4 +/- 1.5 nmol/l. SK-896 induced contraction of smooth muscle preparations isolated from rabbit duodenum in a concentration-dependent manner. However, there was no effect of SK-896 on duodenal preparations isolated from the dog and the rat. SK-896 thus exhibited species specificity in its contractile effect. We next investigated the effect of SK-896 on various smooth muscle preparations isolated from rabbit gastrointestinal tract, trachea, bladder, gallbladder, uterus, vas deferens and artery. Results showed that SK-896 induced contraction of smooth muscle preparations isolated from gastrointestinal tract, with potencies in the order duodenum > gastric pylorus = jejunum = descending colon > ascending colon >/= ileum. However, there was no effect of SK-896 on smooth muscle preparations from gastric fundus and other regional organs. SK-896 thus exhibited regional specificity in its contractile effect. Moreover, the effects of SK-896 on smooth muscle preparations from rabbit duodenum were the same as those of human motilin, and were not inhibited by pretreatment with tetrodotoxin and atropine but were inhibited by verapamil. These findings indicate that SK-896 has the same pharmacological profile as human motilin. They suggest that SK-896 acts on gastrointestinal smooth muscle isolated from rabbit directly and specifically.     

Determination of glyceryl trinitrate and its two main metabolites in human plasma using a new sensitive gas chromatography method

The aim of the present study was to determine the concentrations of nitroglycerin (glyceryl trinitrate, GTN, CAS 55-63-0) and its two main stable metabolites; 1,2-dinitroglycerin (1,2-glyceryl dinitrate, GDN, CAS 621-65-8) and 1,3-dinitroglycerin (1,3-GDN, CAS 623-87-0) in human plasma using a capillary gas chromatography method with an electron capture detection. Using the GC conditions, linear calibrations were obtained for 1,3-GDN from 0.14 to 3 ng/mL, for 1,2-GDN from 0.06 to 6 ng/mL, and for GTN from 0.01 to 0.3 ng/mL in plasma samples by the following calibration curve equations: [y = 0.1924x - 0.0088 (r = 0.999)], [y = 0.2273x + 0.0164 (r = 0.995)], [y = 17.434x - 0.0751] for 1,3-GDN, 1,2-GDN, and GTN respectively. The calculated limits of quantification values for GTN, 1,2-GDN, and 1,3-GDN were 0.03 ng/mL, 0.2 ng/mL, and 0.15 ng/mL respectively. This method was verified with a bioequivalence study of an Iranian brand of oral sustained release nitroglycerin with an innovator formulation.     

In vitro synergy between cefotaxime and its main metabolite, desacetylcefotaxime

In vitro synergistic interaction between cefotaxime (CTX) and its main metabolite, desacetyl-cefotaxime (DCTX), against 7 species of clinical isolates (23-27 strains per species) was examined. Complete or partial synergy was noted with a 1:1 combination of CTX and DCTX against 22-78% of the Bacteroides fragilis, Staphylococcus aureus, Citrobacter freundii, Pseudomonas cepacia and Enterobacter cloacae isolates examined. Antagonistic effects of the drugs appeared against 11% of Proteus vulgaris and 4% of Serratia marcescens. When combined at various ratios by the checkerboard method and tested against B. fragilis, CTX and DCTX were found to act synergistically, and no antagonism occurred. The combined use of CTX and DCTX exhibited strong bactericidal activity against B. fragilis and inhibited bacterial regrowth. An experiment with concentrations of CTX and DCTX simulating human serum levels after intravenous administration also showed that the coexistence of DCTX augmented bactericidal activity of CTX against B. fragilis and brought inhibitory effects on bacterial regrowth. It is presumed from the present results that clinically applied CTX would have more potent effects than expected from in vitro sensitivity test data.     

新氨基糖苷类抗生素威替米星的体外抗菌活性研究

目的评价威替米星对革兰阴性菌的体外抗菌活性。方法采用琼脂平皿稀释法测定威替米星及其对照药威大霉素、庆大霉素、依替米星、阿米卡星、奈替米星、妥布霉素、哌拉西林、头孢噻肟、环丙沙星对892株革兰阴性菌的体外抗菌作用。结果威替米星对892株临床分离革兰阴性菌具有较强的抗菌活性，对大肠埃希菌、肺炎克雷伯菌、阴沟肠杆菌、产气肠杆菌、弗氏柠檬酸杆菌、差异柠檬酸杆菌、摩氏摩根菌、奇异变形菌、粘质沙雷菌、流感嗜血杆菌、副流感嗜血杆菌和乙酸钙不动杆菌的MIC50值为0．25～1mg／L。对铜绿假单胞菌、假单胞菌、雷氏普罗威登斯菌的MIC50值分别为8、64、4mg／L。威替米星对大肠埃希菌、肺炎克雷伯菌显示浓度依赖性杀菌作用。威替米星对大肠埃希菌和肺炎克雷伯菌的抗菌活性随着pH的升高而增强，而2价金属阳离子可降低其抗菌活性，接种量和血清浓度对其抗菌活性无明显影响。结论威替米星对革兰阴性菌具有较强体外抗菌作用，值得进一步研究和开发。     

In vitro antibacterial activity of prulifloxacin,a new oral fluoroquinolone

We compared antibacterial activity of NM394, which is the active metabolite of a prodrug of new fluoroquinolone prulifloxacin (PUFX), against clinical isolates of bacteria with those of ciprofloxacin (CPFX), levofloxacin (LVFX), gatifloxacin (GFLX), tosufloxacin (TFLX) and fleroxacin (FLRX). 1. NM394 showed a broad-spectrum antibacterial activity against both Gram-positive and Gram-negative bacteria. 2. MIC80 of NM394 for methicillin-sensitive Staphylococcus aureus, Streptococcus pneumoniae and Enterococcus faecalis were 0.5 microgram/ml, 2 micrograms/ml and 4 micrograms/ml, respectively. MIC80 of NM394 for Escherichia coli, Klebsiella pneumoniae and Enterobacter cloacae was lower than 0.06 microgram/ml. MIC80 of NM394 for Serratia marcescens and Pseudomonas aeruginosa were 0.25 microgram/ml and 2 micrograms/ml, respectively. 3. Short-time bactericidal activity of NM394 against P. aeruginosa was stronger than those of CPFX, GFLX, LVFX and TFLX. 4. Short-time bactericidal activity of NM394 at Cmax concentration against 12 strains of P. aeruginosa was stronger than those of CPFX, LVFX, GFLX and TFLX.     

In vitro antiplasmodial activity of Tithonia diversifolia and identification of its main active constituent: tagitinin C

The antimalarial properties of Tithonia diversifolia, an Asteraceae traditionally used to treat malaria, were investigated in vitro against three strains of Plasmodium falciparum. The ether extract from aerial parts of the plant collected in S?o Tomé e Príncipe, demonstrated good antiplasmodial activity (IC 50 on FCA strain: 0.75 microg/ml). A bioassay guided fractionation of this extract led to the isolation of the known sesquiterpene lactone tagitinin C as an active component against Plasmodium (IC 50 on FCA strain: 0.33 microg/ml), but also possessing cytotoxic properties (IC 50 on HTC-116 cells: 0.706 microg/ml).     

In vitro and in vivo pharmacological characterization of J-113397, a potent and selective non-peptidyl ORL1 receptor antagonist

1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl -1, 3-dihydro-2H-benzimidazol-2-one (J-113397) was found to be the first potent nonpeptidyl ORL1 receptor antagonist (K(i): cloned human ORL1=1.8 nM) with high selectivity over other opioid receptors (K(i): 1000 nM for human mu-opioid receptor, >10,000 nM for human delta-opioid receptor, and 640 nM for human kappa-opioid receptor). In vitro, J-113397 inhibited nociceptin/orphanin FQ-stimulated [35S]guanosine 5'-O-(gamma-thio)triphosphate (GTP gamma S) binding to Chinese Hamster Ovary (CHO) cells expressing ORL1 (CHO-ORL1) with an IC(50) value of 5.3 nM but had no effect on [35S]GTP gamma S binding by itself. Schild plot analysis of the [35S]GTP gamma S binding assay and cAMP assay using CHO-ORL1 indicated competitive antagonism of J-113397 on the ORL1 receptor. In CHO cells expressing mu-, delta- or kappa-opioid receptors, J-113397 had no effects on [35S]GTP gamma S binding up to a concentration of 100 nM, indicating selective antagonism of the compound on the ORL1 receptor. In vivo, J-113397, when administered subcutaneously (s.c.), dose-dependently inhibited hyperalgesia elicited by intracerebroventricular (i.c.v.) administration of nociceptin/orphanin FQ in a tail-flick test with mice. An in vitro binding study using mouse brains indicated that J-113397 possesses high affinity for the mouse ORL1 receptor (K(i): 1.1 nM) as well as the human receptor. In summary, J-113397 is the first potent, selective ORL1 receptor antagonist that may be useful in elucidating the physiological roles of nociceptin/orphanin FQ.     

Simultaneous determination of levodopa, its main metabolites and carbidopa in plasma by liquid chromatography

An ion-pair reversed-phase liquid chromatographic method for the simultaneous determination of levodopa, 3-O-methyldopa, 3,4-dihydroxyphenylacetic acid, homovanillic acid and carbidopa in plasma designed for clinical trials performed to study the effect of peripheral catechol-O-methyltransferase inhibitors on the metabolism of levodopa is described. The high sample throughput of over 50 samples per day of the method makes it ideal for the assay of the large number of samples encountered in clinical trials. After protein precipitation with perchloric acid the analytes are completely separated within 15 min and determined down to a plasma concentration of 20 ng ml-1 using amperometric detection at 800 mV relative to an Ag/AgCl reference electrode. For all analytes the within-day precision defined as a relative standard deviation (n = 8) is lower than 7 and 3% at plasma concentrations of 20 and 40 ng ml-1, respectively. As the method is specific and highly reproducible, the most important factor affecting accuracy is the stability of the analytes during storage and analysis.     

Chloroethylnorapomorphine,a proposed long-acting dopamine antagonist: Interactions with dopamine receptors of mammalian forebrain in vitro

N-Chloroethylnorapomorphine (NCA) interacts with homogenates of rat corpus striatum to block subsequent stimulation of cyclic AMP synthesis by 50 μM dopamine (DA) (IC₅₀ 30 μM). The 10-O-methylated (12-monophenolic) analog of NCA is weaker; an O,O′-diester of NCA is inactive. The NCA-induced inhibition is counteracted by coincubation with DA or apomorphine, but not norepinephrine. Blockade by NCA is non-competitive vs. DA and not readily reversed by washing. NCA also competes for binding of ³H-apomorphine to subsynaptosomal calf caudate membranes (IC₅₀ 285 nM) with greater potency than the monophenolic or diester analogs. NCA may represent a unique long-acting antagonist of DA receptors.