摘要:

The &bgr;-adrenoceptor antagonists have been widely used clinically for over 20 years and their pharmacokinetics have been more thoroughly investigated than any other group of drugs. Their various lipid solubilities are associated with differences in absorption, distribution and excretion. All are adequately absorbed, and some like atenolol, sotalol and nadolol which are poorly lipid-soluble are excreted unchanged in the urine, accumulating in renal failure but cleared normally in liver disease. The more lipid-soluble drugs are subject to variable metabolism in the liver, which may be influenced by age, phenotype, environment, disease and other drugs, leading to more variable plasma concentrations. Their clearance is reduced in liver disease but is generally unchanged in renal dysfunction.All the &bgr;-adrenoceptor antagonists reduce cardiac output and this may reduce hepatic clearance of highly extracted drugs. In addition, the metabolised drugs compete with other drugs for enzymatic biotransformation and the potential for interaction is great, but because of the high therapeutic index of &bgr;-adrenoceptor antagonists, any unexpected clinical effects are more likely to be due to changes in the kinetics of the other drug.Because satisfactory plasma concentration effect relationships have been difficult to establish for most clinical indications, and little dose-related toxicity is seen, plasma &bgr;-adrenoceptor antagonist concentration measurement is usually unnecessary.The investigation of the clinical pharmacokinetics of the &bgr;-adrenoceptor antagonists has added greatly to our theoretical and practical knowledge of pharmacokinetics and made some contribution to their better clinical use.

ISSN:0312-5963    

出版商:ADIS    

年代:1987

数据来源: ADIS

摘要:

The number of studies on drug interactions with cimetidine has increased at a rapid rate over the past 5 years, with many of the interactions being solely pharmacokinetic in origin. Very few studies have investigated the clinical relevance of such pharmacokinetic interactions by measuring pharmacodynamic responses or clinical endpoints. Apart from pharmacokinetic studies, invariably conducted in young, healthy subjects, there have been a large number ofin vitroandin vivoanimal studies, case reports, clinical observations and general reviews on the subject, which is tending to develop an industry of its own accord. Nevertheless, where specific mechanisms have been considered, these have undoubtedly increased our knowledge on the way in which humans eliminate xenobiotics. There is now sufficient information to predict the likelihood of a pharmacokinetic drugdrug interaction with cimetidine and to make specific clinical recommendations.Pharmacokinetic drug interactions with cimetidine occur at the sites of gastrointestinal absorption and elimination including metabolism and excretion. Cimetidine has been found to reduce the plasma concentrations of ketoconazole, indomethacin and chlorpromazine by reducing their absorption. In the case of ketoconazole the interaction was clinically important. Cimetidine does not inhibit conjugation mechanisms including glucuronidation, sulphation and acetylation, or deacetylation or ethanol dehydrogenation. It binds to the haem portion of cytochrome P-450 and is thus an inhibitor of phase I drug metabolism (i.e. hydroxylation, dealkylation). Although generally recognised as a nonspecific inhibitor of this type of metabolism, cimetidine does demonstrate some degree of specificity. To date, theophylline 8-oxidation, tolbutamide hydroxylation, ibuprofen hydroxylation, misonidazole demethylation, carbamazepine epoxidation, mexiletine oxidation and steroid hydroxylation have not been shown to be inhibited by cimetidine in humans but the metabolism of at least 30 other drugs is affected. Recent evidence indicates negligible effects of cimetidine on liver blood flow. Cimetidine reduces the renal clearance of drugs which are organic cations, by competing for active tubular secretion in the proximal tubule of the kidney, reducing the renal clearances of procainamide, ranitidine, triamterene, metformin, flecainide and the active metabolite N-acetylprocainamide. This previously unrecognised form of drug interaction with cimetidine may be clinically important for both parent drug, and metabolites which may be active. Cimetidine does not alter plasma protein binding of other drugs, but reduces the volumes of distribution of labetolol, lignocaine (lidocaine), imipramine and pethidine (meperidine) by unknown mechanisms. Cimetidine increases the plasma concentrations of drugs in a wide range of therapeutic classes.A number of physiological, pathological and drug-related factors alter the degree of inhibition of hepatic drug clearance by cimetidine. In certain patients with already depressed drug clearance (e.g. the elderly, the cirrhotic), cimetidine will further decrease drug clearance to a potentially dangerous extent. This reduction in drug clearance is greater following enzyme induction by rifampicin or phenytoin or in smokers, although findings in the latter group have been inconsistent. Cimetidine will not fully attenuate the induction of drug metabolism by the above agents.The degree of inhibition of drug metabolism by cimetidine is of the order of 10 to 20% with a daily dosage of 300 to 400mg, 20 to 30% with 400 to 800mg, 30 to 40% with 800 to 1600mg: with daily dosages greater than 2000mg, the inhibition is between 40 and 50%, depending upon the substrate used. The onset of inhibition is rapid: maximum inhibition occurs 24 hours after starting cimetidine, and is maintained for at least 30 days if cimetidine is continued. The recovery rate is also rapid and clearance rates return to baseline 2 to 3 days after stopping cimetidine, depending on the half-life of the interacting drug; in the case of warfarin, plasma concentrations will not return to the precimetidine level for at least 7 days.Because of the large number of drugs which can potentially interact with cimetidine, the physician should suspect a drug interaction when an abnormal response is encountered in any patient coprescribed cimetidine. Toxicity may occur for drugs with a narrow therapeutic index, e.g. theophylline, phenytoin, warfarin and the majority of the antiarrhythmic, antidepressant and antipsychotic drugs for which clinical evidence of the drug interactions has been reported. These patients can be managed by: (a) reducing the dose of the interacting drug; (b) selecting a drug of similar therapeutic efficacy that does not interact with cimetidine; or (c) selecting other antiulcer drugs which do not interact. The need for cimetidine or other antiulcer therapy should also be assessed.Although cimetidine interacts with a large number of drugs, reports of incidents of drug toxicity are uncommon. This may be due to the fact that physicians are well aware of those drugs with a narrow therapeutic index which interact clinically with cimetidine and have taken appropriate action, or the fact that the majority of drugs have a wide therapeutic index, so that a 50% increase in plasma concentration would not be deleterious to the patient.

ISSN:0312-5963    

出版商:ADIS    

年代:1987

数据来源: ADIS

摘要:

Several investigations which have taken treatment time into account have shown that the pharmacokinetic parameters, the therapeutic efficacy and even the toxicity of a large number of products may vary according to the administration schedule. The present study was carried out in order to evaluate any circadian changes in pharmacokinetic parameters of ketoprofen, a new non-steroidal anti-inflammatory drug (NSAID).This randomised crossover study consisted of a single oral dose of ketoprofen 100mg administered to 8 healthy male volunteers, mean age 27.2 years, at 07.00 hours, 13.00 hours, 19.00 hours or 01.00 hours in 4 study periods during the first 3 months of the year. The order of administration was randomised, with each subject acting as his own control. A total of 14 blood and 4 urine samples were taken over a 12-hour period.The peak plasma concentration was twice as high after drug administration at 07.00 hours (13.4 ± 1 mg/L) than after other administration times (13.00 hours: 6.9 ± 1; 19.00 hours: 7.2 ± 0.7; 01.00 hours: 6.3 ± 0.5 mg/L)[p < 0.001].The time to reach peak concentration was much longer after drug administration at 01.00 hours (135 ± 16.7 min) than at 07.00 (73.1 ± 14.1 min), 13.00 (75 ± 16.5 min) or 19.00 hours (82.5 ± 12.7 min)[p < 0.05].The lag time was significantly longer at 01.00 hours than at 13.00 hours (p < 0.01). The absorption rate constant after treatment at 01.00 hours was less than at the other times of administration (p < 0.05). The bodyweight-corrected area under the curve (AUC0-12) was greater after 07.00 hours than after 13.00 (p < 0.01) or 19.00 hours (p < 0.05) and greater after 01.00 hours than after 13.00 hours (p < 0.05). The elimination half-life was significantly longer after administration at 01.00 hours than after 19.00 hours (p < 0.05), while the total clearance was lowest at 07.00 hours. Cosinor analysis demonstrated statistically significant circadian rhythms for all pharmacokinetic parameters described above.The amount of ketoprofen eliminated in the urine was delayed, and was significantly greater after the administration at 01.00 hours than 07.00 hours or 19.00 hours (p < 0.01).The relationship between absorption, diffusion and/or elimination mechanisms of the drug are discussed.

ISSN:0312-5963    

出版商:ADIS    

年代:1987

数据来源: ADIS