In vitro inhibition of human liver drug metabolizing enzymes by second generation antihistamines.

Cetirizine, terfenadine, loratadine, astemizole and mizolastine were compared for their ability to inhibit marker activities for CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP3A4 and for some glucuronidation isoenzymes in human liver microsomes. The most pronounced effects were observed with terfenadine, astemizole and loratadine which inhibited CYP3A4-mediated testosterone 6beta-hydroxylation (IC50 of 23, 21 and 32 microM, respectively) and CYP2D6-mediated dextromethorphan O-demethylation (IC50 of 18, 36 and 15 microM, respectively). In addition, loratadine markedly inhibited the CYP2C19 marker activity, (S)-mephenytoin 4-hydroxylation (Ki of 0.17 microM). Furthermore, loratadine activated the CYP2C9-catalyzed tolbutamide hydroxylation (ca. 3-fold increase at 30 microM) and inhibited some glucuronidation enzymes. Mizolastine appeared to be a relatively weak and unspecific inhibitor of CYP2E1, CYP2C9, CYP2D6 and CYP3A4 (IC50Ss in the 100 micromolar range). Cetirizine demonstrated no effect on the investigated activities. A comparison of the inhibitory potencies of cetirizine, terfenadine, loratidine, astemizole and mizolastine with their corresponding plasma concentrations in humans suggests that these antihistamines are not likely to interfere with the metabolic clearance of coadministered drugs, with the exception of loratidine, which appears to inhibit CYP2C19 with sufficient potency to warrant additional investigation.

In vitro evaluation of potential drug interactions with levetiracetam, a new antiepileptic agent.

Levetiracetam and its carboxylic metabolite (AcL) were tested for their potential inhibitory effect on 11 different drug metabolizing enzyme activities using human liver microsomes. The following specific assays were investigated: testosterone 6beta-hydroxylation [cytochrome P-450 3A4 (CYP3A4)], coumarin hydroxylation (CYP2A6), (R)-warfarin hydroxylation (CYP1A2), (S)-mephenytoin hydroxylation (CYP2C19), p-nitrophenol hydroxylation (CYP2E1) tolbutamide hydroxylation (CYP2C9), dextromethorphan O-demethylation (CYP2D6), epoxide hydrolase and UDP-glucuronyltransferase (UGT) toward paracetamol (UGT1*6), ethinyloestradiol (UGT1*1), p-nitrophenol (UGT(pl 6.2)), and valproic acid. None of these activities were affected by levetiracetam or AcL added at concentrations up to 1 mM. Additionally, primary cultures of rat hepatocytes were used to assess a potential inducing effect of levetiracetam on CYPs. Phenobarbital (2 mM), beta-naphtoflavone (40 microM), dexamethasone (1 microM), and phenytoin (up to 300 microM) were tested as positive controls. When added to cells for 48 h, all the positive controls increased 7-ethoxycoumarin O-deethylase activity demonstrating the inducibility of CYPs in the present culture conditions. By contrast, levetiracetam did not affect the activity up to 1 mM. The highest levetiracetam concentrations examined in the above in vitro studies are well in excess of those measured in the plasma of patients receiving therapeutic doses. It is thus concluded that levetiracetam is unlikely to produce pharmacokinetic interactions through inhibition of CYPs, UGTs, and epoxide hydrolase. Furthermore, based on the in vitro assays with rat hepatocytes, it could be speculated that levetiracetam does not act as a CYP inducer.

Observation of hepatotoxic effects of 2-n-pentylaminoacetamide (Milacemide) in rat liver by a combined in vivo/in vitro approach.

Milacemide or 2-n-pentylaminoacetamide hydrochloride, a new glycine derivative, was found to cause elevations of plasma transaminases in patients suffering from severe depression and Alzheimer's disease. However, no signs of liver toxicity were observed during the course of earlier conducted subchronic and chronic in vivo studies in rodents and cynomolgus monkeys. In this study an in vivo/in vitro approach has been proposed to detect early alterations in key metabolic and functional liver capacities. Milacemide was administered by continuous i.v. infusion for 7 days to male Sprague-Dawley rats using subcutaneously implanted osmotic pumps. Doses were given of 0, 250 and 500 mg/kg per day. Body weight and food intake were recorded and at day 7 of exposure, Milacemide concentration, glucose, urea, triglycerides and cholesterol levels and alanine (ALT) and aspartate aminotransferase (AST) activities were measured in plasma. Non-esterified fatty acids were determined in serum. On day 8, after overnight fasting, hepatocytes were isolated. A portion of the cells derived from untreated animals (no osmotic pumps) were cultured in a primary monolayer and exposed in vitro to different Milacemide concentrations. The xenobiotic biotransformation capacity of the isolated hepatocytes was studied by measuring the cytochrome P450 content, ethoxycoumarin-O-deethylase (ECOD), pentoxyresorufin-O-deethylase (PROD), ethoxyresorufin-O-deethylase (EROD), aldrin epoxidase (AE), epoxide hydrolase (EH) and glutathione S-transferase (GST) enzyme activities. Triglycerides, cholesterol and phospholipid contents were measured on the isolated cells. At plasma concentrations of 43 and 130 microM Milacemide, the ALT activity was unchanged or significantly decreased, whereas the AST activity was increased in both cases. Other clinical chemistry parameters remained unchanged. Weight gain was significantly lower in rats treated with the high Milacemide dose. In addition, decreased food consumption was observed in all treated animals leading to significantly lower food efficiency factors for the rats treated with the high dose. Milacemide had a specific inhibitory effect on xenobiotic biotransformation: ECOD activity decreased to 60% of the control value for both Milacemide doses, PROD activity remained unaffected whereas EROD activity decreased to 65% of the control value. A decrease was also observed at the highest drug concentration for AE (to 41%), EH (to 65%), cytochrome P450 content (to 80%) and GST (to 85%). At 500 mg Milacemide kg/day, hepatocyte triglycerides levels increased 3.1-fold while cholesterol and phospholipid levels remained unaffected. Electron and light microscopy on total liver and isolated hepatocytes indicated a concentration-dependent accumulation of lipid droplets, the occurrence of numerous vacuoles in the cytoplasm and other structural abnormalities. When the cultured hepatocytes of control animals (without osmotic pumps) were exposed to Milacemide, the appearance of vacuoles and myeloid bodies could be confirmed in vitro. The results of this study using an in vivo/in vitro approach clearly show potential hepatotoxic properties of Milacemide, an effect not observed in conventional toxicity studies.

Effect of folate supplementation on clinical chemistry and hematologic changes related to bidisomide administration in the rat.

In a chronic toxicity study in the rat, bidisomide administered as a dietary admixture produced a dose-related lowering of reticulocytes and leucocytes. Plasma alanine aminotransferase activity was increased at 300 mg/kg and decreased at 900 mg/kg. The potential mechanisms of these effects were investigated by comparing the responses in groups of male Sprague-Dawley rats receiving a control diet, or 300 or 1200 mg/kg/day bidisomide. Subsets of these groups were co-treated subcutaneously with folinic acid or with a vitamin B1, B6, B12 complex. Subsets of control and 300 mg/kg groups were maintained on a 20-25% feed restriction regimen for 3 months, to mimic the depression in body weight gain observed in animals receiving 1200 mg/kg. Body weight gains were significantly reduced at 1200 mg/kg and in all feed-restricted animals. Plasma and liver alanine aminotransferase (ALT) and plasma aspartate aminotransferase (AST) levels were also reduced at this dose level. At 300 mg/kg, plasma transaminases, glutamate dehydrogenase (GLDH) and sorbitol dehydrogenase (SDH) activities were increased. These changes were prevented in animals receiving folinic acid supplementation. Plasma glucose, triglycerides, and unsaturated and total iron binding capacities were decreased, while plasma iron levels tended to increase, mainly at the high dose. Vitamin supplementation prevented a decrease in reticulocyte counts at 300 mg/kg. Bidisomide increased urinary formimino-glutamic acid (FIGLU) excretion but did not affect methylmalonic acid (MMA) or taurine excretion. The effect on FIGLU at 1200 mg/kg was prevented by folinic acid co-treatment. Absolute liver weight was lowered at both dose levels and in feed-restricted animals. However, the relative liver weights were unaffected. Thymidine kinase and thymidylate synthase activity of the bone marrow cells were not altered by the bidisomide treatment. Except for the increase in plasma transaminase, GLDH and SDH levels at 300 mg/kg, changes in clinical chemistry parameters are considered to result mainly from nutritional restrictions. Changes in hematologic parameters appear to be related to the combination of decreased feed consumption (leukocytes) and decreased availability or utilization of folates (reticulocytes). This alteration, however, did not affect DNA synthesis in bone marrow. The prevention by folinic acid, but not by feed restriction, of the elevation of liver enzymes at 300 mg/kg is an intriguing, yet unexplained finding. There was no evidence that bidisomide affected B6 and B12 availability.

A simple method for performing routine histopathological examination of the cardiac conduction tissue in the dog.

In standard toxicity studies, the cardiac conduction tissue is not systematically sampled and examined for histopathological changes. Most methods described use serial sectioning perpendicular to the long axis of the sinoauricular node (SAN) and atrioventricular node (AVN). Dozens of slides are needed to allow examination of a significant portion of the SAN and AVN. A simple method was developed to be used in routine histopathologic examination of the dog heart. With a plane parallel to the external wall and the upper edge of the right auricle, the SAN and its arterial supply were easily sectioned and examined. The frontal plane parallel to the interventricular crest was the most appropriate plane for observing a large portion of the AVN and the bundle of His. Based on these results, the heart of 240 dogs from toxicology studies were successfully sampled and processed utilizing this technique. An average of 5 slides per node was needed to perform a satisfactory examination of each of the SANs and AVNs.

Fasting for 24 h reveals liver microsteatosis after continuous i.v. infusion of milacemide in the rat.

Milacemide (2-n-pentylaminoacetamide) hydrochloride was administered by continuous i.v. infusion for up to 7 days, at 300 and 600 mg/kg per day to male Sprague-Dawley rats. This was intended to provide high and sustained exposure to evaluate the effect of a preterminal 24-h fast on liver lipid content. Liver lipid content, as assessed by triglyceride concentration and histopathology, was not different in saline controls or rats infused with up to 600 mg/kg per day for up to 7 days, when they had access to food up to sacrifice. When the rats were fasted for 24 h before sacrifice, milacemide produced microsteatosis in the periportal and midzonal areas. The effect was significant after 2 days of infusion at 600 mg/kg per day and increased in intensity with duration of administration. After 7 days of infusion, at 600 mg/kg per day, liver triglycerides increased by more than 4-fold in rats fasted for the last 24 h. No other differences from the controls were observed at light microscopy or in liver protein content and AST activity. Liver ALT activity was decreased by 28% and plasma ALT activity by 23%. Plasma triglyceride levels were lowered by milacemide, in both fasted and fed rats. This study demonstrates that fasting for 24 h triggers the development of liver microsteatosis in rats exposed to milacemide. Fasting has been previously described to increase liver microsteatosis after administration of sodium valproate, 4-en valproate and pentenoic acid in the rat. These findings might help to identify the mechanism of the hepatic effects of milacemide.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative metabolism of SC-42867 and SC-51089, two PGE2 antagonists, in rat and human hepatocyte cultures.

1. The metabolism of SC-42867 and SC-51089, two PGE2 antagonists, was studied in cultured rat and human hepatocytes. Both compounds possess an 8-chlorodibenzoxazepine moiety, but differ from each other by the nature of the side chain connected to the nitrogen atom. SC-42867 and SC-51089 and their in vitro metabolites were separated by reversed-phase hplc. The major metabolites of both compounds were identified by mass spectrometry (ms) analysis. 2. SC-42867 was metabolized on the tricyclic moiety only. Oxidative N-dealkylation with opening of the oxazepine ring was the major metabolic pathway obtained in rat hepatocytes. The metabolic profile obtained in cultured human hepatocytes was comparable with that of cultured rat hepatocytes. However, the compound was metabolized to a much lower extent by the human cells. 3. SC-51089 was extensively metabolized by both cultured rat and human hepatocytes. Human cells metabolized this compound quite differently than cultured rat hepatocytes. Aromatic hydroxylation with consequent glucuronidation and sulphation were the main metabolic pathways observed in cultured human hepatocytes. Oxidative N-dealkylation with opening of the oxazepine ring and consequent glucuronidation was the major metabolic pathway observed in rat hepatocytes. Further metabolism occurred, in contrast with the human hepatocytes, mainly on the side chain. 4. The present in vitro results are compared with data of previous in vivo studies performed in rat.

Deoxyuridine suppression test on isolated rat bone marrow cells and the in vitro effect of bidisomide.

The deoxyuridine suppression test was performed on isolated rat bone marrow cells in order to study the effect of bidisomide, a new Class I antiarrhythmic agent, on folate-dependent DNA synthesis. Methotrexate and 5-fluorouracil, two known inhibitors of DNA synthesis, were included in the study to validate the test system. Methotrexate and 5-fluorouracil, at a concentration of 5.5 mum, decreased thymidine incorporation into DNA by way of the de novo pathway (thymidylate synthase activity). The salvage pathway of DNA synthesis (thymidine kinase activity), however, was not affected by these anticancer drugs. Bidisomide up to 1 mm did not affect the folate-dependent thymidylate synthase activity, nor the thymidine kinase activity of isolated rat bone marrow cells.

The novel neuropsychotropic agent milacemide is a specific enzyme-activated inhibitor of brain monoamine oxidase B.

The novel neuropsychotropic agent milacemide hydrochloride (2-n-pentylaminoacetamide HCl) is a highly selective substrate of the B form of monoamine oxidase (EC 1.4.3.4; MAO). Under the in vitro conditions used in the present study, milacemide acts as an enzyme-activated, partially reversible inhibitor of MAO-B. A reversible inhibition of MAO-A activity is also observed at high concentrations. The inhibitory activity of milacemide is significantly greater for MAO-B. In vivo, after single or repeated oral administration, a specific inhibition of MAO-B is apparent in brain and liver, with a lack of inhibition of the MAO-A activity. In contrast to the irreversible inhibitory action of L-deprenyl, the recovery of MAO-B activity in vivo after milacemide administration is significantly faster, a result suggesting that it is a partially reversible inhibitor. The selective inhibitory effect of milacemide for MAO-B in vivo is confirmed by its potentiation of phenylethylamine-induced stereotyped behavior, whereas vasopressor responses to tyramine were not affected. These observations suggest that milacemide could enhance dopaminergic activity in the brain and could be used as therapy for Parkinson's disease in association with L-3,4-dihydroxyphenylalanine.

Arterial reactivity, blood pressure, and plasma levels of atrial natriuretic peptides in normotensive and hypertensive rats: effects of acute and chronic administration of atriopeptin III.

We compared acute and chronic effects of atriopeptin III in normotensive and spontaneously hypertensive rats. Atriopeptin III relaxed isolated aortae and intrarenal microarteries but not coronary and mesenteric microarteries of normotensive rats. Effects on arterial smooth muscle were comparable in hypertensive and normotensive rats and were not affected by long-term treatment of the animals with the peptide. Acute administration of atriopeptin III (4-400 nmol/kg, intravenously) reduced systolic blood pressure in conscious spontaneously hypertensive and renal hypertensive rats but not in normotensive rats. In spontaneously hypertensive rats, nephrectomy increased the sensitivity to and the duration of the acute antihypertensive effect. Renal subcellular fractions rapidly inactivated atriopeptin III in vitro. This atriopeptinase activity was comparable for normotensive and spontaneously hypertensive rats and was not affected by long-term treatment of the rats with the peptide. Continuous administration of low doses of atriopeptin III (0.4 and 4.0 nmol/kg/h, intravenously (i.v.) during 7 days) caused a progressive reduction in systolic blood pressure in spontaneously hypertensive but not in normotensive rats. It did not affect plasma levels of aldosterone or renin and resulted in less than a doubling of the plasma levels of atrial natriuretic peptides. These findings confirm that atrial natriuretic peptides preferentially relax the renal microvasculature. They demonstrate that although atriopeptin III comparably relaxes arterial smooth muscle of normotensive and spontaneously hypertensive rats, both acute and chronic administration of the peptide preferentially lower blood pressure in hypertensive rats. Rather than contributing to the effects on blood pressure, the kidneys modulate the duration of action of atrial natriuretic peptides.

Smooth muscle relaxing, acute and long-term blood pressure lowering effects of atriopeptins. Structure-activity relationship.

The relationship between the arterial relaxing, acute and long-term blood pressure lowering effects of atriopeptins were analyzed. We therefore evaluated effects of atriopeptin (103-126), i.e., atriopeptin III, and compared them to those of selected analogs. Atriopeptin (103-126) relaxed isolated aortas (1-30 nM) and lowered systolic blood pressure in conscious spontaneously hypertensive rats when high doses (4-400 nmol/kg i.v.) were administered acutely or low doses (0.4-4.0 nmol/kg/hr i.v.) were administered chronically. The relaxing effect was not affected by amino-terminal extension with up to four amino acids and by carboxy-terminal deletion of one amino acid. Further deletions lowered the relaxing potency. The observed maximal amplitude of the acute blood pressure lowering was reduced drastically by carboxy-terminal deletion of even one amino acid. This was prevented by substitution of the carboxy-terminal acid by an amide. Although not affecting the amplitude, amino-terminal extension with Arg or Arg-Arg prolonged markedly the duration of action of the acute effect. Chronic administration of atriopeptin (103-126), (103-125) and (103-123) affected blood pressure similarly. These findings illustrate different structure-activity relationships for the smooth muscle relaxing, acute and long-term antihypertensive activities of atriopeptins. From this it is suggested that the carboxy-terminal amino acids modulate differently the biological activities of atriopeptins, that amino-terminal amino acids modulate the duration of action of the acute in vivo effects and that the long-term antihypertensive effects of atriopeptins are not related directly to their arterial smooth muscle relaxing and acute blood pressure lowering effects.

In vitro vasorelaxing activity of suloctidil.

The vasorelaxing effect of suloctidil was evaluated in isolated rat and rabbit aorta and in isolated rabbit mesenteric and saphenous artery. Suloctidil inhibited contractions induced by increasing extracellular calcium in depolarized arteries, mainly in a competitive way. In the rat aorta, the pA2 value was 7.50 for suloctidil, while pA2 values of 9.96, 7.90 and 8.10 were obtained for nifedipine, cinnarizine and verapamil, respectively. Suloctidil more potently inhibited calcium-induced contractions in small arteries (mesenteric and saphenous artery), than in the aorta. Suloctidil also reduced the tonic component of the responses to norepinephrine. In contrast to the effects on calcium-induced contractions, the effects of suloctidil on norepinephrine-induced responses was mainly noncompetitive. In addition, and unlike cinnarizine and verapamil, high concentration of suloctidil also reduced the phasic component of contractile responses to norepinephrine. Furthermore, unlike nifedipine, verapamil, diltiazem and cinnarizine, suloctidil was devoid of a negative inotropic effect in spontaneously beating guinea-pig atria. In conclusion, suloctidil behaves as a Ca2+-channel blocker in arteries and displays an additional mode of action that could include receptor-operated Ca2+-channels or an intracellular site of action. In addition, suloctidil was found to affect small arteries more than the aorta, and not to affect the atria.

Effect of suloctidil on rat liver.

The effect of suloctidil (120 mg/kg body weight PO for 3 weeks) on rat liver was investigated using biochemical and morphological methods: enzymatic activities characteristic of the main cellular compartments were used as biochemical markers of hepatocyte function and morphometry was applied to investigate morphological changes. No sign of hepatotoxicity was evidenced after suloctidil treatment (liver weight; cytochrome c oxidase; glucose 6-phosphatase; NADPH-cytochrome c reductase; D-amino acid oxidase; urate oxidase; fatty acid oxidation; peroxisomal number, volume and size distribution). Suloctidil increased catalase activity by 22% without morphologically detectable changes in the peroxisomes. After suloctidil treatment, slightly increased mitochondrial volume fraction and slightly decreased mitochondrial number were noted without significant changes in cytochrome c oxidase. Clofibrate, at the same dose, increased NADPH-cytochrome c reductase, catalase, acylCoA oxidase, mitochondrial and peroxisomal number and volume fraction, and decreased urate oxidase activity.

Antihypertensive activity of tibalosine (CP 804 S) in the rat. Possible involvement of a central alpha 1-adrenergic receptor blockade.

The effect of tibalosine (CP 804 S) on systolic blood pressure and heart rate of unanaesthetized normotensive, spontaneously hypertensive (SHR) and DOCA-salt and Goldblatt hypertensive rats has been examined. After a single oral dose, tibalosine (1.9 to 15 mg/kg) elicited dose-dependent reductions in blood pressure in the four models tested. These reductions are accompanied by a tachycardia except in the SHR where no variation in heart rate is observed. The same result is obtained in SHR after oral, i.v. or i.c.v. administration. After repeated treatment (9 to 14 weeks), the blood pressure lowering effect of tibalosine (10 mg/kg, p.o.) is observed only in hypertensive rats. No variation in heart rate is observed in normotensive, DOCA-salt and Goldblatt rats. A significant bradycardia is observed in the SHR. The antihypertensive effect of tibalosine in SHR is suppressed by naloxone, like that of clonidine and unlike that of prazosin. The present study suggests that the antihypertensive activity of tibalosine is at least partly centrally mediated.

Inhibition by suloctidil of [3H] nitrendipine binding to cerebral cortex membranes.

[3H] Nitrendipine binds specifically with high affinity and high capacity (KA congruent to 0.20 +/- 0.01 nM, Bmax = 4.4 +/- 0.3 pM/g tissue, means +/- S.E.M.; N = 4) to guinea-pig cerebral cortex membranes. Suloctidil fully inhibits [3H] nitrendipine binding with a Ki value of 0.45 microM. The interaction between suloctidil and the putative Ca2+ channels is allosteric as shown by competition experiments performed in the presence of D 600 or diltiazem. Comparison of the activity of some close analogs of suloctidil provides evidence for the importance of the amino group and of the hydrophobic amino substituent in the interaction of suloctidil and the putative Ca2+ channel. It is suggested that part of the previously reported blockade of Ca2+ entry induced by suloctidil is due to a blockade of the Ca2+ channels.

Comparison of [3H]nitrendipine binding to heart membranes of normotensive and spontaneously hypertensive rats.

We compared the binding properties of [3H]nitrendipine in heart membranes from 9- and 24-week-old spontaneously hypertensive rats (SHR) and their normotensive controls (WKY). In native membranes of 9-week-old SHR and WKY and of 24-week-old WKY, [3H]nitrendipine binds to a single class of binding sites with high affinity (Kd, approximately equal to 0.20 nM) and high capacity (Bmax, approximately equal to 96 fmol X mg protein-1). By contrast, in membranes of 24-week-old, SHR, Kd and Bmax increased significantly to 0.30 nM and 137 fmol X mg protein-1, respectively. Addition of 1 mM Ca2+ had no effect on [3H]nitrendipine binding in WKY but decreased the Kd value to 0.18 nM in 24-week-old SHR. Bmax remained 40% above the value in WKY. Inhibition and activation of specific binding by nifedipine and dilitiazem are identical in both strains. We conclude that the differences in [3H]nitrendipine binding in hearts of aged SHR are due to a decreased Ca2+ content and might reflect a functional adaptation.

Receptor binding profile of tibalosine, a new antihypertensive.

The ability of 1-(2,3-dihydro-5-benzo[b]thienyl]-2-(4-phenylbutylamino)-1- propanol-(erythro) (tibalosine, CP 804 S), a new antihypertensive agent, to interact with 11 different receptors has been studied by binding assays. Tibalosine interacts specifically with alpha- and beta-adrenergic receptors and calcium channel binding sites. Ki values (nmol/l) for inhibition of specific binding are 26, 1000, 1000 and 770 respectively for the alpha 1-adrenergic, beta 1- and beta 2-adrenergic receptors and calcium channel binding sites. The interaction with adrenergic receptors is stereoselective since CP 804 S/T, the threo derivative of tibalosine, exhibits Ki values 8 to 10 times higher than those of tibalosine. The selectivity ratio between alpha 1- and alpha 2-adrenergic receptors is about 400. Tibalosine and CP 804 S/T interact with the low affinity binding sites labelled by 3H-WB 4101 in rat brain membranes. These binding sites for putative Ca2+ channel inhibitors are present in rat heart membranes and have the same characteristics as those in rat brain membranes. These results are discussed in relation with the mechanism of tibalosine antihypertensive effect.

Anticonvulsant activity of milacemide.

The anticonvulsant activity of a new drug, milacemide (2-(pentylamino)-acetamide), has been studied in animal models of convulsions like those induced by bicuculline, pentylenetetrazol, picrotoxin, strychnine, inhibitors of GABA synthesis as 3-mercaptopropionic acid, allylglycine, isoniazid and thiosemicarbazide and electroshock. Milacemide is particularly effective in inhibiting the convulsions induced by bicuculline. The ED50 is 5.7 mg/kg by oral route and the activity lasts for more than 48 hr. It is less active against pentylenetetrazol and only marginally active against electroshock. It has not be found active against the other types of convulsions. Milacemide has a low toxicity (LD50: 2585 mg/kg in the mouse) and alters the behaviour of mouse, rat and monkey, only at high doses (greater than or equal to 1000 mg/kg). Milacemide seems to be specially free of sedative potential.