Action of the new H2-antagonist, DA 4577, on different in vitro and in vivo preparations

In vitro studies on the metabolic fate of mifentidine, a novel histamine H2-receptor antagonist

The in vitro metabolism of mifentidine and several of its metabolites was studied using hepatic microsomes from seven animal species. The effects of potential enzyme inducers, inhibitors and activators were also studied. Mifentidine metabolites identified and characterised were: 4-imidazolylphenylamine (amine), 4-imidazolylphenyl-formamide (formamide), the urea derivative of mifentidine (urea) and the imidazole-hydroxylated derivative of the amine (i-OH-amine), along with three unidentified metabolites, M1, M2 and M3. Evidence for the presence of the amine, formamide, urea and i-OH-amine was obtained by comparison with authentic reference compounds: (i) HPLC retention times; (ii) UV spectra; and (iii) MS spectra of metabolites. The postulated intermediates are: carbinolimine (for formamide, amine, i-OH-amine and urea formation); formamide (for amine and i-OH-amine formation); amine (for i-OH-amine formation), and nitrone (for urea formation). One 'metabonate' of mifentidine was also identified, namely the nitro analogue of the amine. A possible prerequisite for the formation of this nitro is the corresponding hydroxylamine or nitroso compound. Cytochromes P450I and P450II were shown to be involved in the in vitro microsomal biotransformation of mifentidine, but the involvement of the flavin monooxygenase system was not proven.

Inhibitory effects of H2-receptor antagonists on cytochrome P450 in male ICR mice

1. The present study was undertaken to examine the effects of H2-receptor antagonists including newly developed mifentidine derivatives, IY-80843 and IY-80845, on cytochrome P450(P450) in vitro and in vivo. 2. Initially, 3-methylcholanthrene-, phenobarbital-, ethanol- and dexamethasone-induced liver microsomes were prepared from male ICR mice to study in vitro effects of above chemicals on ethoxyresorufin O-deethylase(EROD), pentoxyresorufin O-dealkylase(PROD), p-nitrophenol hydroxylase and erythromycin N-demethylase(ERDM) activities, respectively. It was found that histamine, cimetidine and famotidine were not inhibitory to four enzyme activities. Meanwhile, mifentidine slightly inhibited EROD and PROD activities and its derivatives IY-80843 and IY-80845 strongly inhibited PROD, EROD and ERDM activities. 3. Prolongation of hexobarbital-induced sleeping time was determined in male ICR mice to confirm in vitro inhibitory effects of mifentidine and its derivatives in vivo. It was observed that cimetidine, mifentidine, IY-80843 and IY-80845 caused dose-dependent increases in the sleeping time, indicating the inhibition of P450 responsible for hexobarbital metabolism. 4. It was concluded that mifentidine and its derivatives are P450 inhibitors and that our newly synthesized IY-80843 is most inhibitory. 5. The present results indicate that mifentidine and its derivatives not only antagonise the H2-receptor but also inhibit P450 enzymes.

Changes in the ionic environment may alter the kind of antagonism of some histamine H2-receptor blockers in the guinea pig papillary muscle

The effect of changes in the composition of the bathing medium on the effect of histamine and histamine H2-receptor antagonists was investigated in the isolated guinea pig papillary muscle. Ringer or Krebs-Henseleit solutions were used as nutrient fluids. They mainly differed with respect to pH and to Mg2+ and H2 PO4- content. Whereas the effect of histamine was not altered by ionic changes, the antagonism by some H2 blockers was different in the two nutrient solutions. The insurmountable antagonism elicited by high concentrations (greater than or equal to 10(-6) M) of famotidine, oxmetidine and mifentidine in Ringer solution was converted to surmountable when these drugs were tested in Krebs-Henseleit solution. Conversely, the antagonism induced by ranitidine was surmountable in both solutions, and that induced by high amounts of Ioxtidine was insurmountable in both nutrient fluids. Results obtained in Ringer solution were not modified by pH adjustments or by the addition of ions present in Krebs-Henseleit medium. These results suggest that the interaction of histamine with H2 receptors in the guinea pig papillary muscle was not influenced by alterations in the ionic composition of the nutrient fluid, whereas the antagonism may be critically dependent on the ionic environment.

Pharmacokinetics of mifentidine after single and multiple oral administration to healthy volunteers

1. The pharmacokinetics of mifentidine, a new long acting histamine H2-receptor antagonist, were studied using two protocols. 2. In one study, on 5 different days six normal male subjects were given 2.5, 5, 10, or 20 mg mifentidine or placebo orally 60 min before starting a 3 h continuous gastric aspiration during which time blood samples were taken for measurement of mifentidine concentration. 3. The area under the curves of mifentidine plasma levels (AUC) vs time for the four doses was linearly related to the dose for each individual subject (r = 0.972, P less than 0.001). After doses of 2.5, 5, 10 and 20 mg, mifentidine reduced hydrogen ion output by respectively 36, 37, 60 and 75% and secretory volume by 1, 17, 40, and 47%. The effects at the two highest doses were statistically significant. AUC was correlated positively with the percentage reduction in hydrogen ion output (r = 0.802, P less than 0.001) and volume (r = 0.834, P less than 0.001) over a 3 h period. 4. In the second study, the pharmacokinetics were evaluated after once-daily treatment for 14 days in seven subjects given 10 mg and in seven others subjects given 20 mg. 5. After multiple dosing, renal clearance was similar for the two doses (11.6 +/- 2.11 l h-1 for the low dose and 17.0 +/- 2.0 l h-1 for the high dose). Plasma half-life (t1/2 lambda 2) was 16.0 +/- 3.0 h after the 10 mg dose and 11.9 +/- 1.2 h after 20 mg.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacology of the novel H2 antagonist famotidine: in vitro studies

The novel antiulcer drug famotidine was found to be a potent and selective inhibitor of histamine H2 receptors. Its activity on different parameters involving H2 receptors was higher than that of other compounds of the family: pA2 values were 8.33, 7.86 and 7.83 in the guinea pig atria, guinea pig papillary muscle and isolated rat gastric secretion, respectively. Apart from quantitative differences, famotidine differed from the other compounds, since it caused a competitive antagonism only at low concentrations and an unsurmountable antagonism at higher concentrations. The duration of the inhibitory action on the "in vitro" gastric secretion resembled that of cimetidine and ranitidine. Famotidine was highly effective (approximately 10 times as potent as ranitidine) also on the rat uterus (unsurmountable antagonism) and on the guinea pig gallbladder (pA2 value = 7.71). Famotidine was apparently devoid of non-specific effects concerning the gastrointestinal motility even at very high concentrations (10(-4) M). In this respect, famotidine appeared to be more selective than cimetidine and ranitidine at the H2 receptor level. The high potency, the peculiarity of the antagonism and the lack of side-effects on a number of isolated preparations, indicate this H2 antagonist as a very peculiar member of the group.

Ranitidine potentiates ileum contractions caused by GABA and electrical stimulation

GABA-evoked contractions of the guinea pig ileum were significantly potentiated by the histamine H2-receptor antagonist ranitidine in concentrations above 10 microM. To help define the mechanism of this interaction, the present study compared the effects of ranitidine on contractile responses of the guinea pig ileum to GABA, acetylcholine (A Ch) and electrical stimulation of intrinsic cholinergic neurons. Ranitidine, at concentrations that potentiated responses to GABA, also potentiated contractions induced by transmural electrical stimulation. The ability of ranitidine to amplify these latter responses was antagonized by atropine. Contractile responses to exogenous A Ch, however, were unaffected by ranitidine at any concentration. These results suggest that prejunctional, rather than postjunctional mechanisms, are of primary importance in the interaction between ranitidine and GABA.

Histamine H2-receptor agonists and antagonists on pancreatic exocrine secretion of the dog

The use of H2-receptor agonists and antagonists allowed us to establish that histamine H2-receptors are present in pancreatic exocrine tissue and their stimulation caused a dose-dependent increase in pancreatic juice. The fact that H2-antagonists from one side and aminoguanidine from the other were unable to modify basal levels of pancreatic secretion, seems to minimize a role for H2-receptors in the regulation of pancreatic secretion. On the other hand H2-antagonists modified ceruletide-induced secretion in different ways according to the different molecules. Ranitidine strongly potentiated whereas cimetidine, oxmetidine and mifentidine slightly inhibited the effect of ceruletide. The stimulatory effect of eserine and the inhibitory effect of atropine indicate a cholinergic interference in the action of ceruletide. Therefore the potentiating effect of ranitidine may be related to its cholinomimetic action and the inhibitory effect of the other H2-antagonists may be connected with an anticholinergic effect. However, the potentiating effect of aminoguanidine on ceruletide-induced secretion may indicate a possible role for histamine in the response to ceruletide.

Pharmacological profile of mifentidine: a novel H2-receptor antagonist

Mifentidine, a representative compound of a novel class of H2-antagonists, has been investigated for its ability to interact with H2-receptors and to inhibit gastric acid secretion. Affinity estimates (KB) of mifentidine obtained from in vitro studies on cardiac and gastric mucosal histamine (H2) receptors were in the 20-50 nM range. Mifentidine appeared to be endowed with strong anti-secretory properties against histamine-stimulated secretion in the anaesthetized rat and in the conscious dog. Distinct features of mifentidine were considerable bioavailability and duration of anti-secretory effect.

Histamine H2 receptor antagonists may modify dog intestinal motility independently of their primary action on the H2 receptors

Various histamine H2 receptor antagonists were tested for their possible effects on intestinal motility of the dog both "in vivo" and "in vitro". They were found to behave quite differently from one another: ranitidine exerted a small atropine-sensitive stimulatory effect by itself and strongly potentiated the stimulatory action of compounds which interfere with the cholinergic system thus confirming its cholinomimetic direct and/or indirect activities. Cimetidine and mifentidine were ineffective even at very high doses. Oxmetidine exerted an inhibitory effect independently of the kind of the stimulant agent and this suggested a direct effect on the smooth muscle. The different effects elicited by the above H2 antagonists indicate that their action on the intestinal motility is completely independent of the H2 blockade. This was furtherly confirmed by the lack of effect shown by the H2 receptor selective agonists, dimaprit and impromidine. The motor effects of ranitidine were, so far, the only ones which have been encountered, in different conditions, also in humans.

Action of mifentidine and ranitidine on the isolated rat uterus

The new H2-antagonist mifentidine (compound marked DA 4577) was tested for its inhibitory effect on the relaxation induced by histamine on the rat uterus and was compared with the well known H2-blocker ranitidine. Mifentidine was shown to be more effective than ranitidine (about 10 times). However whereas ranitidine behaved as a "classical" competitive antagonist, mifentidine at concentrations of 10(-7) M, caused a remarkable depression of the maximum response to histamine. This "unsurmountable" antagonism may connected with a tight binding of the compound to the receptor with a consequent low degree of dissociation. Ranitidine, but not mifentidine, at concentrations of 10(-5) M was able to potentiate the stimulatory effect of acetylcholine thus confirming also in the uterus its cholinergic-like effects so far observed mainly in the gastrointestinal tract.

Action of histamine and of some H2-antagonists on gastric secretion 'in vitro'

The effect of histamine and of some H2-antagonists on isolated gastric mucosal preparation from immature (14-18 days) rats, was investigated. Basal secretion varied, in our experimental conditions, between 1.06 and 3.54 mumol cm-2 h-1, reaching higher values (approximately 4.6 mumol cm-2 h-1) only in a small percentage of animals (10%). Histamine exerted a concentration-dependent stimulation of acid secretion in concentrations varying between 2 X 10(-6) and 1.6 X 10(-4) M. The response to histamine was competitively antagonized by ranitidine (pA2 value = 6.78) and by 4(5)-(4- isopropylaminomethyleniminophenyl ) imidazole (compound marked DA 4577) (pA2 value = 7.37). Oxmetidine acted as a competitive antagonist only for concentrations as low as 10(-8) M; higher concentrations (10(-7) and 10(-6) M) determined a non-competitive inhibition. Ranitidine and compound marked DA 4577 did not affect basal secretion up to concentrations of 3 X 10(-4) M. On the contrary oxmetidine exerted a concentration-dependent inhibition starting from 10(-5) M. Since in our experimental conditions the role of calcium ions in the regulation of basal secretion could not be established, the mechanism of action of oxmetidine was not completely clarified, even if an interference in the utilization of calcium ions may be suggested. In any case it is deemed of interest that this H2-antagonist was the only compound capable of inducing a reversible complete inhibition of basal acid secretion (only KSCN, in very high concentrations, had a similar behaviour).