Reversible inhibitors of the gastric (H+/K+)-ATPase. 5. Substituted 2,4-diaminoquinazolines and thienopyrimidines.

Quinazolines bearing a secondary 4-(arylamino) substituent demonstrate an SAR for inhibition of the gastric (H+/K+)-ATPase different from the previously described 3-acylquinolines, suggesting that, although these compounds are also K(+)-competitive, they probably bind to the enzyme in a different orientation. Compounds bearing a tertiary 4-(arylamino) substituent, however, in particular 4-(N-methylarylamino), appear to possess an SAR quite similar to the 3-acylquinolines. We show that this arises from the effect of the N-methylation, which is to orientate the 4-(arylamino) substituent syn to C5, analogous to the 3-acylquinolines. Compounds possessing both a 4-(N-methylarylamino) substituent and a 2-(arylamino) substituent proved to be very potent, K(+)-competitive inhibitors of K(+)-stimulated ATPase activity with Ki values down to 12 nM. Some compounds also proved to be effective inhibitors of stimulated acid secretion in both the rat and dog when dosed intravenously. However, although a number of these demonstrated activity after oral administration in the dog, the level and variability precluded further evaluation.

Reversible inhibitors of the gastric (H+/K+)-ATPase. 3. 3-substituted-4-(phenylamino)quinolines.

Previously, gastric (H+/K+)-ATPase inhibitors such as 2 have been prepared as analogues of 1a on the presumption that the 3-carbethoxy substituent plays a key role in establishing the orientation of the 4-arylamino group. In this paper we explore further the contribution made to activity by the quinoline 3-substituent. We show that, for compounds bearing such a substituent, only a particular combination of properties provides high activity, both in vitro and as inhibitors of gastric acid secretion in vivo. The ability of the substituent to affect activity by restricting rotation about the Cquin-N bond through a combination of both a pi-electron withdrawal and hydrogen bonding is supported by the current study. However, high activity is only achieved if the effect of this group on the quinoline pK(a) is kept to a minimum. 3-Acyl substituents provide an optimum combination of electronic properties. From this series, compound 17c (SK&F 96067) was shown to be a potent inhibitor of histamine-stimulated gastric acid secretion after oral dosing in the Heidenhain pouch dog and was selected for further development and evaluation in man.

Structure-activity studies of pyridinylalkyl-isocytosine H1-receptor antihistamines and identification of an active conformation.

For a series of 2-(pyridin-2-ylbutylamino)-5-(3-picolinylmethyl)pyrimidin- 4(1H)-ones (isocytosines) substituted in the pyridine 3-position, activity as H1- and H2-receptor histamine antagonists appears to correlate with the size of the 3-substituent. Steric interaction between the substituent and the butyl chain is explored by conformational analysis using Molecular Mechanics and Molecular Orbital calculations on 2-propyl- and 2-propyl-3-methyl-pyridines; it appears that the substituent may alter activity by changing the preferred conformation of the drug. This observation is extended by synthesis of a semirigid bicyclic analogue wherein 3-methylpyridinylbutyl is replaced by tetrahydroquinolinylpropyl. This compound is 2-3 times more potent as an H1-receptor antagonist confirming that a trans/trans conformation favours activity. Although derived from an H2-antagonist structure, the compound is not an antagonist at histamine H2 receptors thus proving that the conformational requirements are different at H1 and H2 receptors.

2-[[(4-Amino-2-pyridyl)methyl]sulfinyl]benzimidazole H+/K+-ATPase inhibitors. The relationship between pyridine basicity, stability, and activity.

The benzimidazole sulfoxide class of antisecretory H+/K+-ATPase inhibitors need to possess high stability under neutral physiological conditions yet rearrange rapidly at low pH to the active sulfenamide 2. Since the initial reaction involves internal nucleophilic attack by the pyridine nitrogen, control of the pyridine pKa is critical. In this paper we show that by utilizing the powerful electron-donating effect of a 4-amino substituent on the pyridine, moderated by the electron-withdrawing effect of a 3- or 5-halogen substituent, a combination of high potency (as inhibitors of histamine-stimulated gastric acid secretion) and good stability under physiological conditions can be obtained. Furthermore, the role of the steric interaction between the 3/5-substituents and the 4-substituent in modifying the electron-donating ability of the 4-amino group is exemplified, and additional factors affecting stability are identified. One compound, in particular, 2-[[(3-chloro-4-morpholino-2- pyridyl)methyl]sulfinyl]-5-methoxy-(1H)-benzimidazole (3a, SK&F 95601), was chosen for further development and evaluation in man.