Allium Cepa

Pharmacodynamics. Mechanism of action. Pantoprazole is a substituted benzimidazole that inhibits secretion of hydrochloric acid in the stomach by specifically blocking the proton pumps of parietal cells. Pantoprazole is converted to its active form in the acidic environment of parietal cells, where it inhibits the enzyme H⁺/K⁺-ATPase, thereby blocking the final step of hydrochloric acid production in the stomach. The inhibition is dose-dependent and affects both basal and stimulated acid secretion. Most patients achieve symptom relief within 2 weeks. Administration of pantoprazole, as with other proton pump inhibitors (PPIs) and H₂-receptor antagonists, reduces gastric acidity and consequently increases gastrin secretion proportionally to the reduction in acidity. The increase in gastrin secretion is reversible. Since pantoprazole binds the enzyme distal to the cell receptor, it can inhibit hydrochloric acid secretion regardless of stimulation by other substances (acetylcholine, histamine, gastrin). The effect is the same with both oral and intravenous administration. Fasting gastrin levels increase during pantoprazole use. With short-term use, they do not exceed the upper limit of normal in most cases. During long-term treatment, gastrin levels double in most cases. Excessive increases occur only in isolated cases. As a result, a slight to moderate increase in specific endocrine (ECL) cells in the stomach (similar to adenomatoid hyperplasia) is observed in a small number of cases during long-term treatment. However, according to studies conducted to date, formation of neuroendocrine tumour precursor cells (atypical hyperplasia) or neuroendocrine tumours of the stomach, which were observed in animal studies, have not been found in humans. Based on the results of animal studies, an effect of long-term (more than one year) pantoprazole treatment on endocrine parameters of the thyroid gland cannot be excluded. During treatment with antisecretory medicinal products, serum gastrin levels increase in response to the reduction of acid secretion. Additionally, chromogranin A (CgA) levels increase due to reduced gastric acidity. Elevated CgA levels may interfere with diagnostic investigations for neuroendocrine tumours. Available published data suggest that PPI therapy should be discontinued between 5 days and 2 weeks prior to CgA measurements. This allows CgA levels to return to the normal range, which may be falsely elevated after PPI treatment. Pharmacokinetics. Absorption. Pantoprazole is rapidly absorbed, and peak plasma concentrations are achieved after a single oral dose of 40 mg. The maximum serum concentration of approximately 2–3 μg/mL is reached on average 2.5 hours after administration; the concentration remains at a constant level after repeated dosing. Pharmacokinetic properties do not change after single or repeated administration. In the dose range of 10 to 80 mg, pantoprazole pharmacokinetics in plasma remain linear with both oral and intravenous administration. The absolute bioavailability of the tablets has been established at approximately 77%. Concomitant food intake does not affect AUC (area under the concentration-time curve) or maximum serum concentration, and therefore does not affect bioavailability. Only the variability of the lag time increases with concomitant food intake. Distribution. Pantoprazole serum protein binding is approximately 98%. The volume of distribution is approximately 0.15 L/kg. Biotransformation. The substance is metabolised almost exclusively in the liver. The main metabolic pathway is demethylation by CYP2C19 followed by sulphate conjugation; other metabolic pathways include oxidation by CYP3A4. Elimination. The terminal elimination half-life is approximately 1 hour, and clearance is 0.1 L/hr/kg. A few cases of delayed elimination have been reported. Due to the specific binding of pantoprazole to parietal cell proton pumps, the elimination half-life does not correlate with the much longer duration of action (inhibition of acid secretion). The major portion of pantoprazole metabolites is excreted renally (approximately 80%); the remainder is excreted in faeces. The main metabolite in both serum and urine is desmethylpantoprazole conjugated with sulphate. The half-life of the main metabolite (approximately 1.5 hours) is not much longer than the half-life of pantoprazole. Special patient populations. Poor metabolisers. Approximately 3% of Europeans have a functional deficiency of CYP2C19 enzyme activity; they are termed poor metabolisers. In these individuals, pantoprazole metabolism is likely catalysed mainly by CYP3A4. After a single 40 mg dose of pantoprazole, the mean area under the plasma concentration-time curve was approximately 6-fold greater in poor metabolisers compared with subjects with functionally active CYP2C19 (extensive metabolisers). The mean peak plasma concentration was increased by approximately 60%. These findings do not affect the dosing of pantoprazole. Renal impairment. No dose reduction is recommended when pantoprazole is administered to patients with impaired renal function (including patients on dialysis). As in healthy subjects, the half-life of pantoprazole is short. Only very small amounts of pantoprazole are dialysed. Although the main metabolite has a moderately longer half-life (2–3 hours), elimination remains rapid and accumulation does not occur. Hepatic impairment. Although in patients with hepatic cirrhosis (Child-Pugh classes A and B) the half-life increases to 7–9 hours and AUC increases 5- to 7-fold, the maximum serum concentration increases only slightly — by 1.5-fold compared with healthy volunteers. Elderly patients. The slight increase in AUC and Cmax in elderly volunteers compared with younger volunteers is also of no clinical significance.