The effect of ciprofibrate on gastric secretion in the rat

Proton pump inhibitors, enterochromaffin-like cell growth and Helicobacter pylori gastritis

In both rodents and humans the development of gastrin-promoted gastric argyrophil enterochromaffin-like cell carcinoids requires the involvement of a genetic factor inherent to multiple endocrine neoplasia syndrome or of type A autoimmune chronic atrophic gastritis. Prolonged severe hypergastrinaemia acting on non-gastritic mucosa, as in Zollinger-Ellison syndrome patients, results in diffuse argyrophil enterochromaffin-like cell hyperplasia but, as a rule, does not produce tumours. Combination of chronic atrophic gastritis (mostly related to Helicobacter pylori infection) with hypergastrinaemia frequently causes linear and micronodular hyperplasia of argyrophil cells, whereas carcinoids are exceptional. No tumours or pre-neoplastic lesions have been observed in patients treated long-term with proton pump inhibitors, apart from rare cases in patients with combined Zollinger-Ellison and multiple endocrine neoplasia syndromes. A moderate increase in the incidence of argyrophil cell clustering, with or without hyperplasia, probably results from the parallel evolution of ulcer-associated Helicobacter gastritis into chronic atrophic gastritis. Eradication of H. pylori with a combination of proton pump inhibitors and antibiotics suppresses gastritis and prevents ulcer recurrence.

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.

Potentiating hypergastrinemic effect by the peroxisome proliferator ciprofibrate and omeprazole in the rat

BACKGROUND

Profound inhibition of gastric acid secretion induces enterochromaffin-like (ECL) cell carcinoids due to hypergastrinemia. Peroxisome proliferators also lead to hypergastrinemia and ECL cell carcinoids but without reducing gastric acidity. Since the peroxisome proliferator ciprofibrate is still in use as lipid-reducing agent, and proton pump inhibitors are among the most commonly used drugs, we found it of interest to evaluate both the effect of a combination of these drugs on serum gastrin and the expression of gastrin and somatostatin mRNA in antral mucosa.

METHODS

The drugs were given by gastric gavage once daily for 4 weeks to female rats. Blood was drawn by vein puncture before and at the end of the 4-week period for determination of gastrin by radioimmunoassay. At death the stomachs were removed, the antral mucosa homogenized, and the density of gastrin and somatostatin mRNA determined by Northern blot, using 32P-labelled probes.

RESULTS

Omeprazole dosing increased serum gastrin 4-fold, ciprofibrate 5-fold, and the combination 24-fold. Serum gastrin during ciprofibrate dosing increased gradually, reaching significance after 14 days. Antral gastrin mRNA density increased similarly to the increase in serum gastrin, whereas antral somatostatin mRNA tended to be reduced in the omeprazole and increased in the ciprofibrate-dosed rats.

CONCLUSION

A potentiating hypergastrinemic effect of the peroxisome proliferator ciprofibrate and the inhibitor of gastric acid secretion omeprazole is shown, indicating different mechanisms of action.

Effects of hypochlorhydria and hypergastrinemia on structure and function of gastrointestinal cells. A review and analysis

Since hypochlorhydria can induce hypergastrinemia, and gastrin has a trophic effect on some gastrointestinal cells, states that cause elevated plasma gastrin levels are of interest in terms of effects on cell growth and function. This article reviews the relationship between gastric mucosal cells during periods of acid stimulation and inhibition and analyses the effects of hypochlorhydria and hypergastrinemia on gastric and colonic cells and tumors. Hypochlorhydria releases the inhibitory effect of antral gastrin cells, inducing them to release gastrin in the presence of peptides or amino acids in the gastric lumen or in response to antral distension. Gastrin stimulates the oxyntic mucosa, which may lead to hyperplasia of enterochromaffin-like cells, resulting in enterochromaffin-like carcinoid tumors in aged rats and, rarely, in patients with chronic atrophic gastritis or gastrinomas. In addition to hypergastrinemia, other factors appear to be required for the progression of enterochromaffin-like hyperplasia to carcinoids; genetic factors may be involved. Gastrin elevations due to antisecretory drug therapy are indirectly proportional to the degree of acid inhibition and are reversible upon cessation of therapy. The gastrin levels during omeprazole therapy are similar to those caused by gastric vagotomy. Available evidence does not support a relationship between hypergastrinemia and the occurrence or growth of gastric carcinoma or colonic tumors.

Toxicokinetic and mechanistic considerations in the interpretation of the rodent bioassay

When chemicals that are nongenotoxic in conventional assays produce increases in tumor incidence in rodents in chronic bioassays, the determination of the significance of these data for human safety is a challenging task. An important first step in this process is consideration of available data on the mechanism of action and biological properties of the chemical as well as pharmacokinetic and metabolism data in the species showing the response. In recent years, there has been an increase in the understanding of so-called "secondary mechanisms" of carcinogenesis (e.g., thyroid tumors in rats following exposure to enzyme inducers). Application of these data may assist in determination of human risk. There are 2 important questions that will be explored and developed: Are there biological effects produced in the test species that could explain the increase in tumor incidence, and will these effects be reproduced in humans? What is the exposure to the chemical that is associated with the increase in tumors, and how does this relate to exposure in humans?

Enterochromaffin-like (ECL) cells and their growths: relationships to gastrin, reduced acid secretion and gastritis

ECL cells are argyrophilic endocrine cells of the stomach. Their distribution is species specific, however they are consistently located in the oxyntic mucosa and, in particular, in very close contact with the adenomeres of acidopeptic glands. ECL cells store histamine and are considered a key element in the mechanisms of gastric acid secretion as controlled by gastrin stimulus. Their peculiar anatomical disposition and secretory properties strongly suggest that ECL cells exert their function by a paracrine mechanism, i.e. by releasing histamine in the extracellular spaces surrounding acid-producing parietal cells. ECL cell activity is strongly stimulated by gastrin, which, once applied as a long-standing stimulus, also exerts a potent proliferating effect. Long-lasting hypergastrinaemia has been demonstrated to elicit ECL cell proliferation in laboratory animals, inducing ECL cell hyperplasia, dysplasia and ECL cell tumours, i.e. argyrophilic gastric carcinoids. However, in experimental rodents it is believed that hypergastrinaemia is not per se a stimulus capable of inducing ECL cell transformation, a predisposing genetic background being required for tumour development in endocrine organs. In man, long-standing hypergastrinaemia exerts the same proliferative pressure on ECL cells and is associated with hyperplasia with or without dysplastic changes and carcinoid development. Clinical evidence suggests that other factors, both genetic and environmental, are required to induce ECL cell transformation and carcinoid development. For this reason human gastric argyrophilic ECL carcinoids are subdivided into three main groups depending on their clinical background: (1) gastric carcinoids in patients with chronic atrophic gastritis; (2) gastric carcinoids in patients with Zollinger-Ellison and multiple endocrine neoplasia type 1 syndrome (MEN-ZES); and (3) solitary, sporadic gastric carcinoids. The clinical assessment of carcinoid-bearing patients is strongly recommended for better diagnosis and management of patients.

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)

Partial gastric corpectomy results in hypergastrinemia and development of gastric enterochromaffinlike-cell carcinoids in the rat

Studies in the rat have shown that partial gastric corpectomy, in which about 75% of the acid-producing oxyntic mucosa was removed, leads to markedly reduced acid secretion and a feedback increase in the plasma gastrin levels. Ten weeks after operation, the gastric enterochromaffin (ECL)-like cell density in the remaining part of the oxyntic mucosa had increased significantly. In the present study, the effects on the gastric ECL cells of lifelong persistent hypergastrinemia induced by partial (75%) corpectomy have been investigated. Seventy-five partially corpectomized rats and 40 control rats were investigated for plasma gastrin and oxyntic mucosal changes in a 124-week study. The partially corpectomized rats showed increased plasma gastrin levels after the operation; the mean increase compared with the controls was almost 10-fold during the entire study. The remaining oxyntic mucosa of the partially corpectomized rats differed from that of control rats in two respects, showing first general hypertrophy and second a marked hyperplasia of argyrophil ECL cells. The degree and incidence of these changes increased towards the end of the study, i.e., in the aging rats. An age-related increase in ECL-cell density occurred spontaneously also in the control rats but to a lesser extent than in the partially corpectomized group. ECL-cell carcinoids were found in the oxyntic mucosa of 26 of the 75 partially corpectomized rats. The first carcinoid was found 78 weeks after the beginning of the study. Six rats with carcinoids (23%) were found before week 104 (2 years) and the remainder, 20 (77%), were discovered later. No carcinoid tumor was found in the control rats. It is concluded that lifelong hypergastrinemia induced by partial corpectomy leads to the development of ECL-cell carcinoids in the oxyntic mucosa of some rats towards the end of their life span. This observation strongly supports the hypothesis that the gastric ECL-cell carcinoids found in rats treated with antisecretory drugs are caused by long-standing hypergastrinemia developing secondary to inhibition of gastric acid secretion.

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 comparative pharmacokinetics and gastric toxicity of bezafibrate and ciprofibrate in the rat

1. The comparative gastric toxicology and pharmacokinetics of two phenoxyisobutyrate derivatives have been evaluated in the Fischer rat. 2. After oral administration of single daily doses for 7 days, the plasma elimination half-life for bezafibrate was rapid (t1/2 of 4-5 h) in comparison to ciprofibrate (t1/2 of 76 h). 3. The area under the plasma drug concentration versus time curve (AUC) 0-24 (micrograms.h/ml +/- SD) for bezafibrate (dose 125 mg/kg per day) was 1553 +/- 334, which was less than half the value of 3748 +/- 358 achieved by ciprofibrate (10 mg/kg per day) after 7 days. 4. Oral administration of ciprofibrate at 10 mg/kg every 48 h produced similar sustained plasma concentrations to those achieved by bezafibrate 125 mg/kg dosed every 12 h. The AUC 0-48 values (micrograms.h/ml +/- SD) achieved were 5124 +/- 450 for bezafibrate compared to 4207 +/- 240 for ciprofibrate. 5. In chronic oral multidose studies with ciprofibrate and bezafibrate, similar gastric toxicity (neuroendocrine cell hyperplasia) occurred in the rat when dose regimens were adjusted to compensate for the pharmacokinetic differences between these two drugs.

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.

A review of the effects of long-term acid inhibition in animals

Studies with H2-receptor antagonists have revealed a trophic effect on the gastric mucosa - an effect which has been ascribed to hypergastrinaemia secondary to acid inhibition. Such hyperplasia of oxyntic mucosal cells has also been demonstrated in chronic toxicity studies following profound, long-standing inhibition of gastric acid secretion with omeprazole. The central role of gastrin in this effect was clearly demonstrated in the omeprazole studies, as antrectomy prevented this effect in both rats and dogs. The hyperplasia was fully reversible in both species. The close correlation between serum gastrin and hyperplasia of enterochromaffin-like (ECL) cells in the rat oxyntic mucosa has been demonstrated in a large number of experiments using different means to induce hypergastrinaemia, including administration of exogenous gastrin, treatment with antisecretory drugs and partial fundectomy. The hyperplasia of ECL cells was fully reversible even after 1 year of sustained gastric acid inhibition following treatment with a high dose of omeprazole. Marked long-standing hypergastrinaemia explains the findings of gastric ECL cell carcinoids in the life-long rat toxicity studies with both omeprazole and other inhibitors of gastric acid secretion.

Species variation in gastric toxicity following chronic administration of ciprofibrate to rat, mouse, and marmoset

Comparative oral toxicity studies with ciprofibrate have been undertaken in the mouse, rat, and marmoset for up to 26 weeks. Chronic administration of ciprofibrate (20 mg/kg/day) produced a prolonged, modest, but statistically significant hypergastrinemia in the rat. Morphological changes in the rat stomach included increased eosinophilia and hypertrophy of oxyntic cells after 2 or more weeks treatment and hyperplasia of the neuroendocrine (NE) cells after 8 weeks treatment. In contrast, only a transient hypergastrinemia was induced, but not sustained in the mouse at the same dose level over an 8-week time period. No morphological changes were detected in the stomach of this species. In the marmoset treatment, up to 80 mg/kg/day for 26 weeks failed to induce hypergastrinemia and no significant alterations in gastric NE cells were detected.