摘要:

The alignment of drug metabolism and pharmacokinetic departments with drug discovery has not produced a radical improvement in the pharmacokinetic properties of new chemical entities. The reason for this is complex, reflecting in part the difficulty of combining potency, selectivity, water solubility, metabolic stability and membrane permeability into a single molecule. This combination becomes increasingly problematic as the drug targets become more distant from aminergic seven-transmembrane-spanning receptors (7-TMs). The leads available for aminergic 7-TMs, like the natural agonists, are invariably small molecular weight, water soluble and potent. Even moving to 7-TMs for which the agonist is a peptide invariably produces lead matter that is less drug-like (higher molecular weight and lipophilic). The role of drug metabolism departments, therefore, has been to guide chemistry to obtaining adequate, rather than optimal, pharmacokinetic properties for these ‘difficult’ drug targets.A consistent belief of many researchers is that a high value is placed on optimal, rather than adequate, pharmacokinetic properties. One measure of value is market sales, and when these are examined no clear pattern emerges. Part of the success of amlodipine in the calcium channel antagonist sector must be due to its excellent pharmacokinetic profile, but the best-selling drugs among the angiotensin antagonists and β-blockers have a much greater market share than other agents with better pharmacokinetic properties. Clearly, many other factors are important in the successful launch of a medicine, some reflected in the manner the compound is developed and the subsequent structure of the labelling.Overall, therefore the presence of drug metabolism in drug discovery has probably contributed most by allowing ‘difficult’ drug targets to be prosecuted, rather than by guiding medicinal chemists to optimal pharmacokinetics. These ‘difficult’ target candidates become successful drugs when skilfully developed. There is no doubt that skilful development relies heavily on drug metabolism and pharmacokinetic departments, in this case those with a clinical rather than a preclinical orientation.

ISSN:0312-5963    

出版商:ADIS    

年代:2002

数据来源: ADIS

摘要:

Tegaserod, a selective serotonin (5-hydroxytryptamine; 5-HT) 5-HT4receptor partial agonist, is indicated in patients with irritable bowel syndrome (IBS) who identify abdominal pain or discomfort and constipation as their predominant symptoms. Tegaserod at dosages of 1 to 12 mg/day exerts pharmacodynamic actions in the upper and the lower gastrointestinal tract, accelerating small bowel and colonic transit in patients with IBS.Tegaserod is rapidly absorbed following oral administration; peak plasma concentrations (Cmax) are reached after approximately 1 hour. Absolute bioavailability is about 10% under fasted conditions. Food reduces the bioavailability of tegaserod by 40 to 65% and the Cmaxby 20 to 40%. Systemic exposure to tegaserod is not significantly altered at neutral gastric pH compared with the fasted state (pH 2). Tegaserod is approximately 98% bound to plasma proteins, primarily to α1-acid glycoprotein, and has a volume of distribution at steady-state of 368 ± 223L.Tegaserod is metabolised mainly via two pathways. The first is a presystemic acid-catalysed hydrolysis in the stomach followed by oxidation and conjugation which produces the main metabolite of tegaserod, 5-methoxyindole-3-carboxylic acid glucuronide (M 29.0). This metabolite has negligible affinity for 5-HT4receptors and is devoid of promotile activity. The second is direct glucuronidation which leads to generation of three isomericN-glucuronides. The plasma clearance of tegaserod is 77 ± 15 L/h, with an estimated terminal half-life of 11 ± 5 hours following intravenous administration. Approximately two-thirds of the orally administered dose of tegaserod is excreted unchanged in faeces, with the remainder excreted in urine, primarily as M 29.0. The pharmacokinetics of tegaserod are dose-proportional over the range 2 to 12mg given twice daily for 5 days, with no relevant accumulation.The pharmacokinetics of tegaserod in patients with IBS are comparable to those in healthy individuals, and similar between men and women. No dosage adjustment is required in elderly patients or those with mild to moderate hepatic or renal impairment. Tegaserod should not be used in patients with severe hepatic or renal impairment.No clinically relevant drug-drug interactions with tegaserod have been identified.In vivodrug-drug interaction studies with theophylline [a cytochrome P450 (CYP) 1A2 prototype substrate], dextromethorphan (a CYP2D6 prototype substrate), digoxin, warfarin and oral contraceptives have indicated no clinically relevant interactions and no requirement for dosage adjustment.

ISSN:0312-5963    

出版商:ADIS    

年代:2002

数据来源: ADIS

摘要:

Rapid-acting insulin analogues such as insulin lispro and insulin aspart produce a more physiological profile of insulin activity than does conventional regular human insulin because of their unique pharmacokinetics. These insulin analogues are absorbed rapidly from the subcutaneous injection site, resulting in a better matching of the appearance of insulin in the circulation with nutrient absorption from the intestine. In addition, they are shorter-acting than regular human insulin, thus decreasing the risk of late postprandial hypoglycaemia due to inappropriate hyperinsulinaemia. Because self-prepared mixtures of these rapid-acting insulin analogues with longer-acting insulins such as neutral protamine Hagedorn (NPH) insulin have been shown to be clinically useful, and because manufactured fixed-ratio mixtures of regular human insulin and NPH already represent a large proportion of insulin use, manufactured fixed-ratio mixtures of insulin lispro and a sustained-release insulin known as NPL have been developed (insulin lispro mixtures).NPL is a protamine-based insulin lispro formulation with pharmacokinetics and glucodynamics comparable to those of human NPH insulin. NPL was developed for use within insulin lispro mixtures because an exchange between soluble insulin lispro and protamine-bound human insulin within human NPH precludes prolonged storage of mixtures of these insulins. An insulin lispro mixture consisting of 25% insulin lispro and 75% NPL is now commercially available. This preparation is intended primarily as an alternative to human insulin 30/70, which is commonly used within a twice-daily injection regimen. A mixture containing 50% insulin lispro and 50% NPL is also available.The rapid activity of insulin lispro is maintained within insulin lispro mixtures, allowing injection just prior to a meal, a convenience that is not available with commercial mixtures of regular human insulin and human NPH insulin, which should be injected 30 to 45 minutes prior to meals. As with insulin lispro itself, the rapid action of insulin lispro within the insulin lispro mixtures also results in a smaller increase in blood glucose levels after meals than with comparable human insulin mixtures. In addition, data from two studies have shown that when Mix25 is injected prior to the evening meal the incidence of nocturnal hypoglycaemia is decreased in comparison with the same dose of human insulin 30/70. The combined rapid and prolonged insulin activity provided by insulin lispro mixtures has been defined both in healthy subjects without diabetes and in patients with diabetes.

ISSN:0312-5963    

出版商:ADIS    

年代:2002

数据来源: ADIS

摘要:

Rapacuronium is an aminosteroidal nondepolarising neuromuscular blocking agent (NMBA). Its neuromuscular blocking effects have a different time course to those of most currently available agents. It also has lower potency than many of the other NMBAs. In doses consistent with short to medium duration of action, rapacuronium has rapid and complete onset. In some doses it gives tracheal intubating conditions that compare favourably with those produced by suxamethonium (succinylcholine) during rapid sequence induction of anaesthesia. Tracheal intubating conditions improve as dose increases, but adverse effects (including potentially severe bronchospasm) become more prominent.Rapacuronium has an active metabolite that is at least as potent as the parent compound and is eliminated much less efficiently. Consequently, the time course of action of rapacuronium is prolonged after multiple doses or an infusion. Its potency is similar across age ranges and its time course after single doses is little altered in patients with hepatic or renal insufficiency. At least in part because of its active metabolite, rapacuronium is highly cumulative in renal failure.In keeping with its rapid onset and short to medium duration of action, rapacuronium has a more rapid clearance than most other NMBAs. Values for clearance are in the range 0.26−0.67 L/h/kg, with most studies giving a value of approximately 0.45 L/h/kg. There is some evidence that clearance declines marginally with advanced age, and it is also reduced in children. A typical value for steady-state volume of distribution is 0.3 L/kg. This is similar to that of many other NMBAs, but is small compared with many other drugs, as expected with a highly polar compound. Pharmacokinetic parameters do not appear to differ markedly in hepatic insufficiency, but clearance is reduced by approximately 30% in renal failure.Rapacuronium equilibrates very rapidly between the plasma and the site of effect. This is the principal explanation behind its unusually rapid onset. It also appears to have a similar potency at the larynx compared with the adductor pollicis; most other NMBAs are less effective at the larynx.Because it gives rapid onset in a dose consistent with brief duration of action, it was hoped that rapacuronium might be a suitable alternative to suxamethonium. It does not have the problems associated with suxamethonium, but its use is associated with bronchospasm, the incidence of which is dose-related. Rapacuronium has been withdrawn from sale because of this adverse effect, and its future availability is uncertain.

ISSN:0312-5963    

出版商:ADIS    

年代:2002

数据来源: ADIS

摘要:

Maturation of drug systemic clearance mechanisms during the postnatal period produces dramatic and rapid changes in an infant's capacity to eliminate drugs. A tentative general mathematical model describing the ontogeny of hepatic cytochrome P450 (CYP) enzyme-mediated clearance and renal clearance due to glomerular filtration in the first 6 months of life was elaborated from age-specificin vitrohepatic microsomal activity data (normalised to amount of hepatic microsomal protein) for enzyme-specific probe substrates andin vivoprobe substrate data for glomerular filtration (normalised to bodyweight), respectively. The model predicts an age- and clearance pathway-specific Infant Scaling Factor (ISF) for the first 6 months of life. The ISF reflects functional maturation of a specific clearance pathway (normalised to bodyweight) relative to adult values. Therefore, the ISF directly correlates adult clearance values with an infant's capacity to eliminate drugs. Substitution of appropriate model parameter estimates and the age of the infant into the model provides an estimated ISF value, which may then be used to predict the contribution of a particular clearance pathway to total systemic clearance in the infant when adult systemic clearance values are known.The model was tested for its ability to predict infant systemic clearance of drugs whose elimination is principally mediated by a single CYP enzyme or by glomerular filtration. The model performed reasonably well for CYP1A2 and CYP3A4, but poorer predictions were obtained for CYP2D6 and CYP2C because of lack of model complexity and/or inadequate hepatic microsomal activity data to fully describe the maturational process of functional enzyme. For renal clearance due to glomerular filtration, data normalised to bodyweight (kg) showed a limited maturational trend, suggesting that adult renal clearances normalised to bodyweight might reasonably predict infant renal clearances in the first 6 months of life. The model provided reasonable predictions of renal clearance due to glomerular filtration in the infant.

ISSN:0312-5963    

出版商:ADIS    

年代:2002

数据来源: ADIS

摘要:

BackgroundMethotrexate is the most efficient anticancer drug in osteosarcoma. It requires individual exposure monitoring because of the high doses used, its wide interpatient pharmacokinetic variability and the existence of demonstrated relationships between efficacy, toxicity and serum drug concentrations.ObjectiveTo develop a maximuma posteriori(MAP) Bayesian estimator able to predict individual pharmacokinetic parameters and exposure indices such as area under the curve (AUC) for methotrexate from a few blood samples, in order to prevent toxicity and facilitate further studies of the relationships between efficacy and exposure.MethodsMethotrexate population pharmacokinetics were estimated by a retrospective analysis of concentration data from 40 children and young adults by using the nonparametric expectation maximisation method NPEM. A linear two-compartment model with elimination from the central compartment was assumed. Individual pharmacokinetic parameters and AUC were subsequently estimated in 30 other young patients, using MAP Bayesian estimation as implemented in two programs, ADAPT II and an inhouse program Winphar®.ResultsThe pharmacokinetic parameters used in the model were the volume of the central compartment (V1) and the transfer constants (k10, k12and k21). The mean values (with percentage coefficient of variation) obtained were: 18.24L (54.1%) and 0.41 (42.3%), 0.0168 (68.7%), and 0.1069 (61.3%) h−1, respectively. Bayesian forecasting enabled nonbiased estimation of AUC and systemic clearance using a schedule with two sampling times (6 and 24 hours after the beginning of the infusion) and either program. Collection of a third sample at 4 hours improved the precision.ConclusionThe Bayesian adaptive method developed herein allows accurate estimation of individual exposure to methotrexate and can easily be used in clinical practice.

ISSN:0312-5963    

出版商:ADIS    

年代:2002

数据来源: ADIS

摘要:

ObjectiveTo describe the relationship between plasma magnesium (Mg2+) concentration and blood pressure response in pregnant women with preeclampsia.MethodsFifty-one preeclamptic women were studied after receiving two consecutive magnesium sulfate infusions (120 mg/kg for 1 hour and 24 mg/kg for 5 hours). Mg2+concentration and systolic/diastolic blood pressure were measured at 0, 0.5, 1, 2, 4, 6, 7, 9, 11, 13 and 15 hours after the beginning of the first infusion. A population pharmacokinetic-pharmacodynamic model was fitted to the data with the computer program NONMEM.ResultsPharmacokinetics were described by a two-compartment model. Population parameter estimates were 5.0 L/h for body clearance (CL), 24L for central volume (Vc), 25L for peripheral volume (Vp) and 5.6 L/h for intercompartment clearance (Q). The interindividual variability in CL, Vc, Vpand Q was 39, 26, 38 and 59%, respectively. The mean population estimates for systolic (diastolic) blood pressure were 36.8 (27.8) mm Hg for the maximum decrease (Emax), 0.75 (0.88) mmol/L for the Mg2+concentration (above baseline) eliciting half-maximum effect (EC50) and 0.76 (0.5) h−1for the equilibrium rate (keo) of the effect compartment model.ConclusionMg2+concentrations within the range (2−4 mmol/L) proposed for treatment of preeclampsia produce greater than half-maximal lowering of systolic and diastolic blood pressure.

ISSN:0312-5963    

出版商:ADIS    

年代:2002

数据来源: ADIS