Burimamide and metiamide which have been described previously (Black, Duncan, Durant, Ganellin & Parsons, 1972; Black, Duncan, Emmett, Ganellin, Hesselbo, Parsons & Wyllie, 1973) are histamine H2-receptor antagonists. This communication describes some aspects of the pharmacology of cimetidine (N-cyano-N′-methyl-N″[2-(5- methyl-4-imidazolyl-methylthio)ethyl] guanidine; SK&F 92334), a new H2-receptor antagonist. In vitro the compound antagonizes the actions of histamine on isolated guinea-pig atrium and isolated electrically-stimulated rat uterus with KB values of 7.9 × 10−7m and 8.1 × 10−7m respectively, corresponding to pA2 values of 6.1 on each tissue. At very high concentrations cimetidine antagonizes the actions of isoprenaline on atrium and uterus and the actions of histamine and carbachol on isolated guinea-pig ileum but the results are not consistent with competitive antagonism at β-adrenoceptors, histamine H1-receptors or muscarinic receptors. The effects of cimetidine on gastric acid secretion have been studied in a number of preparations. The results are summarized in Table 1. In all preparations cimetidine was approximately equiactive in inhibiting histamine and pentagastrin-stimulated acid secretion but less effective in inhibiting carbachol-stimulated secretion. Basal secretion was also inhibited. In Heidenhain pouch dogs the blood levels to give 50% inhibition of maximally-stimulated gastric secretion (EC50) were approximately 1–2 µm and the half-life of the compound about one hour. In male human volunteers cimetidine given intravenously has been shown to inhibit histamineor pentagastrin-stimulated gastric secretion with an EC50 of about 2.5 µm and a half-life of about two hours. In chronic toxicity studies metiamide has been shown at high doses to produce kidney damage and agranulocytosis in some dogs (Brimblecombe, Duncan & Walker, 1973). In tests so far carried out cimetidine at equivalent doses has not shown similar toxicity.

1 Cimetidine has been shown to have low acute toxicity in dogs and rodents. Repeated-dose studies of up to 24 months' duration in rodents at doses up to 950 mg day kg -1 showed few adverse effects. Liver weight was consistently increased at the highest dose and testis, prostate and seminal vesicle weights were reduced in a dose- and time-related fashion. Cimetidine was not carcinogenic in the rat. 2 In tests of up to 1 year's duration in dogs two animals receiving 504 mg day kg -1 had to be killed before the end of the study. They had degenerative changes in the liver and renal tubular nephrosis. These and other dogs at 504 mg day -1 kg -1 had elevated serum transaminases. No such changes were seen at 366 mg day -1 kg -1 or less. Prostate weights were reduced in a dose-and time-related fashion. In a 7-year study in dogs, specifically designed for the purpose, no changes of the stomach mucosa were seen during regular biopsy. 3 Although shown to be a mild anti-androgen, cimetidine produced no significant adverse effects in reproductive studies. 4 The large body of evidence that cimetidine is not a risk for gastric cancer is reviewed. 5 Over 30 million patients have so far been treated with cimetidine and the prediction from the animal studies that it would be an extremely safe therapeutic agent has been borne out in practice.

Three groups of patients studied after operations which had cured their duodenal ulcer were compared with a control group (no operation, n = 8). The surgical procedures included: proximal gastric vagotomy (n = 7), truncal vagotomy and pyloroplasty (n = 7), truncal vagotomy and antrectomy (n = 8). Samples of gastric juice were aspirated half hourly or hourly over 24 hours for measurement of pH, counts of all identified bacteria, nitrite and total N-nitrosocompounds. Although the pH over 24 hours was significantly higher after proximal gastric vagotomy (p less than 0.05) and truncal vagotomy and antrectomy (p less than 0.001) than controls, there was no difference between truncal vagotomy and pyloroplasty and controls. Counts of nitrate reducing bacteria over 24 hours were also significantly higher after truncal vagotomy and antrectomy than controls (p less than 0.1) but no differences were observed between the other groups. Only after truncal vagotomy and antrectomy was nitrite over 24 hours significantly increased compared with controls (p less than 0.01). Despite these higher values after truncal vagotomy and antrectomy, there was no significant difference in total N-nitrosocompounds between any of the four groups. Whereas bacterial counts and nitrite increased with pH, no correlation was found between total N-nitrosocompounds and pH. These results provide no evidence that exposure to total N-nitrosocompounds is increased after operations for duodenal ulcer.

Eight healthy subjects were studied half-hourly or hourly for 24 h periods before, during, and after cimetidine treatment. No significant differences in intragastric bacterial counts or bacterial species or in intragastric nitrite or N-nitroso-compound concentrations were found as a result of cimetidine treatment. Bacterial counts and nitrite concentrations tended to increase with pH, but N-nitroso-compound concentrations did not. This study provides no evidence that cimetidine treatment may increase the risk of gastric carcinoma by raising N-nitroso-compound concentrations.

The effects of some muscarinic antagonists, namely, N‐ethyl‐2‐pyrrolidylmethyl‐cyclopentylphenyl glycollate (PMCG), N‐methyl‐4‐piperidyl‐phenylcyclohexyl glycollate (PPCG, racemate and R and S enantiomers) and 4′‐N‐methyl‐piperidyl‐l‐phenyl‐cyclopentane carboxylate (G3063) on organ o‐phosphate (sarin, soman)‐ and carbamate (neostigmine)‐induced twitch augmentation have been studied in cat soleus muscle. The results of a preliminary study comparing the potency of sarin and soman in inhibiting the acetylcholinesterase activity of muscle in relation to the effect on the maximal twitch response indicated that there is not a simple relationship between degree of enzyme inhibition by these drugs and alteration of muscle function. The muscarinic antagonists studied were capable of preventing or reversing sarin‐, soman‐ or neostigmine‐induced twitch augmentation. Doses sufficient to give complete protection from the effects of the anticholinesterase agents had little or no effect on the twitch response of normal muscle. The protective action of these muscarinic antagonists is dose‐dependent but independent of known antagonist actions at muscarinic receptors. The effects of some local anaesthetics (lignocaine, prilocaine, cinchocaine, procaine) and other membrane stabilizers (quinine, ketamine, chlorpromazine, triflupromazine) were compared with those of the muscarinic antagonists in an attempt to elucidate the mode of action of these acetylcholine antagonists. The evidence is insufficient to exclude the involvement of a membrane stabilizing action.

1. Experiments in mice with Cycloheximide (CXM) helped to elucidate some of the underlying mechanisms involved in learning and memory.

The peculiar sensitivity of the nervous system to a wide variety of chemical substances indicates the need to introduce techniques that are capable of detecting, at an early stage, untoward effects on that system. Behavioral techniques, suitable for use in acute and chronic toxicity tests and in reproduction toxicity studies, are discussed. Simple, noninvasive techniques can be used to indicate deviations from normal behavior and deficits in the special senses. More complex electrophysiological techniques may be required to investigate these effects in more detail.

Screening for behavioral toxicity or neurotoxicity has been in use for many decades; however, only in the past 20 years has this become a standard practice in toxicology and safety pharmacology. Current screening batteries, such as the functional observational battery and Irwin screen, are derived from protocols used in pharmacology, toxicology, and psychology. Although there are a range of protocols in use today, all focus on detailed observations and specific tests of reflexes and responses. Several neurological functions are typically assessed, including autonomic, neuromuscular, and sensory levels, as well as levels of activity and excitability. The tests have been shown to be valid in detecting expected effects of known neurotoxicants and are reliable and reproducible when compared across laboratories. Regardless of the specific protocol used, proper conduct and statistical analyses of the data are critical. Interpretation is based on the information from individual end points and the profile or pattern of effects observed. With continued refinement, behavioral screening methods will continue to be important tools with which to protect human health in the future and to support safe human testing of new pharmaceutical compounds.

Irwin/FOB testing is routinely conducted to investigate the neurofunctional integrity of laboratory animals during preclinical development of new drugs, however, the study design frequently varies to meet specific needs. Representatives of several European-based pharmaceutical companies performed a “state-of-the-art” assessment of how they conduct their CNS safety evaluation using Irwin/FOB tests.This assessment consisted of (1) a survey of current/historical practice, (2) an evaluation of historical studies with reference compounds (amphetamine, chlorpromazine) to determine intercompany reproducibility of results, and (3) an interlaboratory test using reference compounds (MK-801, chlorpromazine) to determine whether partially standardized conditions (animals, sex, doses, vehicles, administration route, observation time points, systemic exposure) might reduce variability of results.Our survey revealed several similarities, e.g., main endpoints of home cage and openfield observations, species, and positive control substances, but also a high level of heterogeneity between different companies with regard to behavioral endpoints during handling and reflex testing, scoring, group size, and timing of studies. Analysis of heterogeneously designed historical studies with amphetamine and chlorpromazine showed the anticipated behavioral changes, albeit with quantitative variability, and identified more robust (e.g., activity, posture, muscle tone, startle reflex, body temperature) and less robust (piloerection, stereotypical behavior, palpebral closure, respiration) Irwin/FOB parameters. A partially standardized interlaboratory test with MK-801 and chlorpromazine showed the expected behavioral changes and principally confirmed the historically-based more/less robust Irwin/FOB parameters, however, it also showed exposure variability and did not show a markedly reduced quantitative variability of behavioral results.Our survey and intercompany test results demonstrate certain heterogeneity in design and conduct of Irwin/FOB tests by pharmaceutical companies. Although the general behavioral profiles for the reference compounds were consistently found, quantitative variability of results remained even under partially standardized conditions. This suggests the importance of a high level of standardization with regard to the Irwin/FOB test modification used, scoring system, and observer training, in order to achieve an improved intercompany comparability of Irwin/FOB results.

The concept of two classes of histamine receptor, H1 and H2, is introduced and the chemical derivation of histamine H2-receptor antagonists is outlined briefly. Starting from the structure of histamine, chemical modification led eventually to burimamide, the first described histamine H2-receptor antagonist. Further stepwise modifications ultimately afforded metiamide and cimetidine. In vitro studies show that cimetidine is a specific competitive histamine H2-receptor antagonist. In vivo, it is a potent inhibitor of histamine-stimulated gastric acid secretion in rats and dogs after both intravenous and oral administration. It is equally potent as an inhibitor of pentagastrin-stimulated secretion. The evidence suggests that cimetidine inhibits gastric acid secretion through blockade of histamine H2-receptors in the gastric mucosa. Cimetidine has been shown to have low acute toxicity. Repeated dose studies of up to 24 months in rats and up to 12 months in dogs have been carried out and the results are presented and discussed. There is no known toxic effect which would limit the usefulness of cimetidine in man.

Burimamide and metiamide are two histamine H2-receptor antagonists. Evidence is presented that indicates the competitive nature and the specificity of the antagonism. Metiamide is about ten times more potent than burimamide and is also more effective than burimamide when given orally. Both compounds inhibit gastric secretion and the evidence is consistent with this inhibition being due to competitive antagonism of H2 receptors in the gastric mucosa. Burimamide, unlike metiamide, causes release of catecholamines even at dose levels that are just sufficient to produce H2-receptor antagonism. Burimamide, but not metiamide, has alpha-adrenoceptor blocking activity. In certain models for inflammation, particularly rat paw edema induced by compound 48/80, burimamide in combination with the H1-receptor antagonist mepyramine shows anti-inflammatory activity. This may, in part, be associated with the catecholamine-releasing properties of the compound. Metiamide is less active in this respect.

A study has been made of the effect of histamine H2-receptor antagonists on thermal injury in rat bind paws produced by immersion in water at 56°C for 30 seconds. Cimetidine caused dose-dependent suppression of thermal oedema when administered before the injury, and when given after the injury also suppressed thermal oedema if given before the response was complete. Two other histamine H2-receptor antagonists, burimamide and metiamide, had similar effects. The suppression of thermal oedema by cimetidine was due to a reduction in both the hyperaemia and increased vascular permeability to albumin caused by injury. Pretreatment with mepyramine, a histamine H1-receptor antagonist, with the 5-hydroxytryptamine (5-HT) antagonists, methysergide and 2-bromolysergic acid diethylamide, and with the prostaglandin synthesis inhibitors, indomethacin and aspirin, did not suppress thermal oedema.