Assessment of gastric antisecretory effects of phenoxyisobutyrate derivatives in the rat and the mouse

Role of peroxisome proliferator-activated receptors alpha and gamma in gastric ulcer: An overview of experimental evidences

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors belonging to the nuclear hormone receptor superfamily. Three subtypes, PPARα, PPARβ/δ, and PPARγ, have been identified so far. PPARα is expressed in the liver, kidney, small intestine, heart, and muscle, where it activates the fatty acid catabolism and control lipoprotein assembly in response to long-chain unsaturated fatty acids, eicosanoids, and hypolipidemic drugs (e.g., fenofibrate). PPARβ/δ is more broadly expressed and is implicated in fatty acid oxidation, keratinocyte differentiation, wound healing, and macrophage response to very low density lipoprotein metabolism. This isoform has been implicated in transcriptional-repression functions and has been shown to repress the activity of PPARα or PPARγ target genes. PPARγ1 and γ2 are generated from a single-gene peroxisome proliferator-activated receptors gamma by differential promoter usage and alternative splicing. PPARγ1 is expressed in colon, immune system (e.g., monocytes and macrophages), and other tissues where it participates in the modulation of inflammation, cell proliferation, and differentiation. PPARs regulate gene expression through distinct mechanisms: Ligand-dependent transactivation, ligand-independent repression, and ligand-dependent transrepression. Studies in animals have demonstrated the gastric antisecretory activity of PPARα agonists like ciprofibrate, bezafibrate and clofibrate. Study by Pathak et al also demonstrated the effect of PPARα agonist, bezafibrate, on gastric secretion and gastric cytoprotection in various gastric ulcer models in rats. The majority of the experimental studies is on pioglitazone and rosiglitazone, which are PPARγ activators. In all the studies, both the PPARγ activators showed protection against the gastric ulcer and also accelerate the ulcer healing in gastric ulcer model in rats. Therefore, PPARα and PPARγ may be a target for gastric ulcer therapy. Finally, more studies are also needed to confirm the involvement of PPARs α and γ in gastric ulcer.

Ciprofibrate stimulates the gastrin-producing cell by acting luminally on antral PPAR-alpha

The lipid-lowering drug ciprofibrate stimulates gastrin-producing cells in the rat stomach without lowering gastric acidity. Although suggested to be a luminal action on antral peroxisome proliferator-activated receptor-alpha (PPAR-alpha), the mechanism is still not fully elucidated. Gastric bypass was surgically prepared in male Sprague-Dawley rats. Gastric-bypassed and sham-operated rats were either given ciprofibrate (50 mg.kg(-1).day(-1) in methocel) or vehicle alone for 7 wk. PPAR-alpha knockout (KO) and wild-type (WT) mice were either given ciprofibrate (500 mg.kg(-1).day(-1) in methocel) or vehicle alone for 2 wk. The concentration of gastrin in blood was analyzed. Antral G cell density and gastrin mRNA abundance were determined by using immunostaining and Northern blot analysis. Ciprofibrate did not raise plasma gastrin or G cell density in gastric-bypassed rats, although the gastrin mRNA level was slightly increased. In contrast, ciprofibrate induced hypergastrinemia, a 50% increase in G cell density, and a threefold increase in gastrin mRNA in sham-operated rats. In PPAR-alpha KO mice, ciprofibrate did not raise G cell density or the gastrin mRNA level. The serum gastrin level was reduced by ciprofibrate. In WT mice, ciprofibrate induced hypergastrinemia, a doubling of G cell density, and a threefold increase in gastrin mRNA. Comparing animals dosed with vehicle only, PPAR-alpha KO mice had higher serum gastrin concentration than WT mice. We conclude that the main effects of ciprofibrate on G cells are mediated from the antrum lumen, and the mechanism is dependent on PPAR-alpha. The results indicate that PPAR-alpha may have a role in the physiological regulation of gastrin release.

The role of investigative toxicology in pharmaceutical industry

In summary, the benefits of a mechanistic toxicology approach include increased understanding of the processes underlying primary pathology, the development of predictive capability, enhanced decision making and improved responsiveness to regulatory agencies. The overall objective of toxicology in the pharmaceutical industry is risk benefit assessment in order to support product registration. This requires both that studies be conducted in accordance with regulatory guidelines and that the biological processes underlying pathology be understood. These should be regarded as inseparable components of the risk assessment process.

Neuroendocrine cell hyperplasia and neuroendocrine carcinoma of the rodent fundic stomach

Certain substances when given orally to rats have effects on the neuroendocrine cells of the fundic stomach. Such compounds also have effects on acid or its secretion, which is to a greater or lesser extent suppressed, with a consequent rise in serum gastrin, followed by an increase in the number of histamine-secreting ECL cells. These changes are seen with the histamine H2 receptor antagonists loxtidine, SKF 93479, ICI 162,846 and ranitidine; with the hypolipidaemic agents clofibrate, ciprofibrate and benzofibrate; with sodium bicarbonate and pentagastrin; and with omeprazole, a potent inhibitor of the parietal cell proton pump mechanism. Changes in the pH of the rat stomach stimulate the neuroendocrine G cells of the pylorus to secrete gastrin, which acts on the ECL cells of the fundus causing the production of histamine, which in turn stimulates the parietal cell. This sequence leads to an excess of circulating gastrin, which is detectable within 5 days. Subsequently increases in the number of ECL cells occur, the hyperplasia being related to hypergastrinaemia and the degree of acid suppression. The hyperplastic response is rapid, being so obvious with loxtidine at 39 days that there is good reason to suppose it could well be detected earlier. Using omeprazole, hyperplasia was found at 28 days after oral doses of 140 mg/kg/day. In order to get an equivalent degree of acid suppression with ranitidine it was necessary to deliver 420 mg/kg/day by subcutaneous infusion using an osmotic minipump, when hyperplasia occurred. Interestingly, only omeprazole produced a hyperplastic response of G cells. Such results reflect the covalent binding of omeprazole to the proton pump as opposed to the competitive binding of ranitidine to the histamine H2 receptor site. In addition to ECL cell hyperplasia there is ample evidence from lifetime studies in rats and mice that neoplasia may result. Neuroendocrine carcinomas (carcinoids) of the rat fundic stomach have been observed with loxtidine, omeprazole, SKF 93479 and ICI 162,846. They are seen late in the 2-year rat studies and are most unlikely to have arisen purely as an extension of the hyperplastic response. It is possible that the prolonged disturbance of gastric homoestasis resulting from achlorhydria result in the production of a carcinogen or carcinogens, in which event it is not too surprising, in view of the neuroendocrine hyperplasia, that the tumours seen are neuroendocrine carcinomas.(ABSTRACT TRUNCATED AT 400 WORDS)

The importance of pharmacokinetic and receptor studies in drug safety evaluation

The importance of pharmacokinetic and receptor studies in the preclinical and clinical safety evaluation of candidate drugs is reviewed with reference to a number of recently developed drugs. Different aspects of the relationships between pathways of metabolism, pharmacokinetics, receptor interactions, and drug toxicity are illustrated. The failure of animal toxicity studies to predict drug toxicity in humans, due to species differences in metabolism and pharmacokinetics, is illustrated by reference to the anti-inflammatory antiviral terpenoid carbenoxolone, the antiasthmatic candidate drug FPL 52757, and the cardiotonic drug amrinone. The false prediction of adverse effects in man from toxicity manifested in experimental animals, due to species differences in pharmacokinetics or receptor activities, is exemplified with reference to the antiepileptic valproic acid, the hypolipidemic drug ciprofibrate, the antipeptic ulcer drug, omeprazole, and the progestogen lynestrenol. Finally, the importance of adequate, repeat-dose, clinical pharmacokinetic studies in patients as distinct from healthy volunteers to evaluate any effect of the disease state, in the elderly and the young to examine the effects of age, and in sufficiently large populations to detect genetic anomalies and idiosyncrasies is illustrated by reference to the anti-rheumatoid drug benoxaprofen, the antiangina drug perhexiline, and the diuretic tienilic acid.

Clofibrate raises human 24 h intragastric acidity but does not affect plasma gastrin concentration

1. We studied the effects of acute oral dosing with clofibrate (500 mg four times daily) on 24 h intragastric acidity and plasma gastrin concentration in 12 healthy female subjects. 2. The 24 h integrated intragastric acidity rose from 429 mmol l-1 h (95% CI 296-479) before dosing to 527 mmol l-1 h (95% CI 385-664) on the day of dosing (+23%; P = 0.041), but no change was observed in the 24 h integrated plasma gastrin concentration: 420 pmol l-1 h (95% CI 282-499) before and 389 pmol l-1 h (95% CI 249-489) during dosing (P = 0.182). 3. We conclude that clofibrate has no acute antisecretory effect on the human stomach, and that human gastrin-induced enterochromaffin-like cell proliferation is unlikely with this drug.

The trophic effects of gastrin on fundic neuroendocrine cells of the rat stomach

The histomorphological effect of multidose administration of 6 mg/kg pentagastrin b.d. for 5 weeks, and 1000 mg/kg sodium bicarbonate b.d. for 13 weeks, on the rat fundic mucosa has been examined. Sodium bicarbonate induced a significant hypergastrinaemia (plasma gastrin concentrations were 370.5 pg/ml in the control versus 642.6 pg/ml in sodium bicarbonate-treated rats after 13 weeks, P less than 0.01). Both treatment regimens induced fundic neuroendocrine cell hyperplasia. The cellular proliferation that occurred following hypergastrinaemia of endogenous or exogenous origin suggests that systemic gastrin concentrations play a major role in the control of fundic neuroendocrine cell populations.

Gastric morphological changes including carcinoid tumors in animals treated with a potent hypolipidemic agent, ciprofibrate

Oral administration of ciprofibrate, a potent hypolipidemic compound, to rats for 2 or more weeks at doses of 20 mg/kg/day or more resulted in hypertrophy and increased eosinophilia of the oxyntic cells in the gastric mucosa. Ultrastructural evaluation revealed small secretory canaliculi with small microvilli in these cells, changes consistent with the inhibition of acid secretion. After longer administration (e.g., greater than 2 months at 20 mg/kg/day), hyperplasia of the neuroendocrine cells (in particular, the enterochromaffin-like cells) was present in the fundic mucosa of the stomach. After life-time (2-year) administration at 10 mg/kg/day, neuroendocrine cell hyperplasia was accompanied by formation of malignant carcinoid tumors in the fundus of 5 of 59 male and 1 of 60 female rats. In contrast, administration of ciprofibrate to mice at 20 mg/kg/day for 2 months was not associated with oxyntic or neuroendocrine cell changes, a finding consistent with the lack of gastric carcinoid tumors in a 2-year mouse study. Similarly, no significant changes were induced in the marmoset stomach by doses as high as 100 mg/kg/day for 6 months. These findings are consistent with the hypothesis that the formation of gastric carcinoid tumors following ciprofibrate administration is a phenomenon that occurs specifically in those species such as the rat where this compound has significant gastric antisecretory activity.