Influence of cisapride on the pharmacokinetics and antihypertensive effect of sustained-release nifedipine

Acetaminophen absorption kinetics in altered gastric emptying: establishing a relevant pharmacokinetic surrogate using published data

Acetaminophen has been used as a tool for clinical and nonclinical experimental designs that evaluate gastric emptying because acetaminophen is not absorbed in stomach but efficiently absorbed from the small intestine. Published pharmacokinetic data of acetaminophen in subjects with normal gastric emptying vs. impaired gastric emptying (i.e., morphine treatment) were evaluated to select a key surrogate. Using Caverage (average concentration), computed from the exposure within the first hour, individual rank distribution was plotted across different studies. Caverage was highly correlated with Cmax (maximum concentration) in subjects with normal gastric emptying (R(2) = .7532) but not in those where gastric emptying was impaired (R(2) = .0213). The 50th percentile value of the distribution pattern of 1/Caverage in acetaminophen+morphine-treated group (coincided with the first shift in the slope) was considered as the cutoff point to figure out the impaired gastric emptying. The individual rank distribution plots for 1/Caverage across different studies supported similar trends in subjects with normal gastric emptying but showed a distinct distribution pattern in the cohort of impaired gastric emptying. Caverage, calculated within the first hour of dosing of acetaminophen (average concentration at 0-1 hour, C0-1havg), can be used as a key surrogate to distinguish the effects of gastric emptying on the absorption of acetaminophen. A 4 μg/mL C0-1havg of acetaminophen (dose: 1.5 g) may be used as cutoff point in future clinical investigations of acetaminophen to clarify the role of gastric emptying.

Cisapride raises the bioavailability of paracetamol by inhibiting its glucuronidation in man

The effect of cisapride on plasma concentrations of paracetamol was investigated with respect to hepatic metabolism. Paracetamol (1 g) together with cisapride (7.5 mg) or placebo was orally administered to five healthy male volunteers. Venous blood samples were taken before and after administration. Plasma paracetamol and its glucuronide and sulfate conjugates were measured by HPLC. The pharmacokinetic variables were calculated from the plasma concentration-time curves of each volunteer. The area under the plasma paracetamol concentration-time curve from 0 to 180 min (mean +/- s.d.) increased from 1875.0 +/- 112.8 micrg min mL(-1) (placebo coadministration) to 2238.8 +/- 125.8 microg min mL(-1) (cisapride coadministration) (P < 0.01). The mean maximum plasma paracetamol concentration(18.2 microg mL(-1))with placebo was reached 30 min after administration, whereas mean maximum plasma paracetamol concentration (21.2 microg mL(-1)) with cisapride occurred 45 min after administration. The plasma paracetamol concentrations with cisapride were significantly greater at 45 to 120 min after administration compared with placebo. Plasma paracetamol glucuronide conjugate concentrations with cisapride were decreased at 15 to 60 min compared with placebo (P< 0.05), whereas plasma paracetamol sulfate conjugate concentrations did not change significantly. Hence the coadministration of paracetamol with cisapride reduced plasma paracetamol glucuronide concentrations and increased plasma paracetamol concentrations, presumably due to inhibition of paracetamol metabolism via paracetamol glucuronyltransferase. Thus, care is necessary when paracetamol and cisapride are coadministered.

Selection of drugs to treat gastro-oesophageal reflux disease: the role of drug interactions

Gastro-oesophageal reflux disease is probably the most common acid-peptic disease in Western countries, and the successful treatment of mild to moderate disease with pharmacotherapy has become commonplace. A large number of effective drugs are now available, and so the decision-making process for physicians increasingly relies on considerations other than pure efficacy. Cost, adverse effects and drug interactions have therefore become important, particularly in the most vulnerable patients - children, the elderly and patients who are ill and are taking medications that may influence the efficacy of antireflux therapy. Important drug interactions with antacids include the prevention of the absorption of antibacterials such as tetracycline, azithromycin and quinolones. H2 antagonists, proton pump inhibitors and prokinetic agents undergo metabolism by the cytochrome P450 (CYP) system present in the liver and gastrointestinal tract. Cimetidine is an inhibitor of CYP3A and it may cause significant interactions with drugs of narrow therapeutic range and low bioavailability that are metabolised by these enzymes. The gastroparietal proton pump inhibitors lansoprazole, omeprazole and pantoprazole are all primarily metabolised by a genetically polymorphic enzyme, CYP2C19, that is absent from approximately 3% of Caucasians and 20% of Asians. These drugs may also interact with CYP3A, but to a lesser extent. Interactions with prokinetic agents carry the greatest potential for harm. Metoclopramide is a dopamine antagonist that may cause extrapyramidal effects when administered alone at high concentrations, or when coadministered with antipsychotic agents such as haloperidol or phenothiazines. Cisapride is clearly able to prolong the electrocardiographic QT interval and cause lethal ventricular arrhythmias when its metabolism is slowed by interaction with inhibitors of CYP3A, such as erythromycin, ketoconazole or itraconazole.

Drug, meal and formulation interactions influencing drug absorption after oral administration. Clinical implications

Drug-drug, drug-formulation and drug-meal interactions are of clinical concern for orally administered drugs that possess a narrow therapeutic index. This review presents the current status of information regarding interactions which may influence the gastrointestinal (GI) absorption of orally administered drugs. Absorption interactions have been classified on the basis of rate-limiting processes. These processes are put in the context of drug and formulation physicochemical properties and oral input influences on variable GI physiology. Interaction categorisation makes use of a biopharmaceutical classification system based on drug aqueous solubility and membrane permeability and their contributions towards absorption variability. Overlaying this classification it is important to be aware of the effect that the magnitudes of drug dosage and volume of fluid administration can have on interactions involving a solubility rate limits. GI regional differences in membrane permeability are fundamental to the rational development of extended release dosage forms as well as to predicting interaction effects on absorption from immediate release dosage forms. The effect of meals on the regional-dependent intestinal elimination of drugs and their involvement in drug absorption interactions is also discussed. Although the clinical significance of such interactions is certainly dependent on the narrowness of the drug therapeutic index, clinical aspects of absorption delays and therapeutic failures resulting from various interactions are also important.