The bioavailability in man of marketed brands of chlorpropamide

Bioavailability of different preparations of paracetamol

1 Plasma drug concentrations in eight volunteer subjects following oral administration of six preparations of paracetamol have been studied. 2 Significant interindividual differences in plasma paracetamol concentration were found within 1 h but there were no consistent differences between the different drug preparations. The results indicate that all the preparations have equivalent bioavailability. 3 In four subjects, tested with the same preparation on two occasions, the findings suggest that drug absorption may differ from one occasion to another.

The relationship between the pharmacokinetics and pharmacodynamic effects of oral hypoglycaemic drugs

Oral hypoglycaemic drugs have widely differing pharmacokinetic properties. Possible pharmacodynamic benefits include greater efficacy and fewer adverse effects. In general, it has not been possible to demonstrate unequivocal differences in clinical efficacy between the sulphonylureas during long term use, although there are clear differences in potency. These differences have been emphasised to the extent that the term 'second-generation' has been used for the most potent sulphonylureas, but there is little to suggest that potency is of any therapeutic significance. Trials to study differences in efficacy have rarely been of acceptable design. They have often used fixed doses of drugs, begging the question of whether true potency ratios have been established for chronic treatment. They have rarely involved substantial numbers of patients in double-blind crossover studies with a suitable washout period. Trials which show that there is a clear relationship between drug concentrations in blood and drug effects (whether therapeutic effects or adverse effects such as severe hypoglycaemia) are generally lacking. Qualitative and semiquantitative analysis of adverse effects supports the concept that drugs with a long half-life (e.g. chlorpropamide), renally excreted active metabolites (e.g. acetohexamide) or unusual properties (e.g. glibenclamide, which accumulates progressively in islet tissue) are more likely to cause prolonged hypoglycaemia, which may be fatal. The major adverse effect of treatment with biguanides is lactic acidosis, and this probably occurs more commonly in patients treated with phenformin than those treated with metformin because of pharmacogenetic variation in phenformin metabolism. The available evidence therefore favours the use of drugs with a short elimination half-life which are extensively metabolised and which have no active metabolites.

Bioavailability of chlorpropamide

1. The serum profiles of chlorpropamide obtained following single doses of two tablet preparations and from a suspension formulation have been compared in healthy volunteers. The amounts of chlorpropamide absorbed from the three formulations were similar but the drug was absorbed more rapidly from the suspension than from either tablet formulation. There was a small but therapeutically insignificant difference in the rate of absorption of the drug from the two tablets. 2. Changes in blood glucose concentrations were found to be related to the drug serum profile characteristics of the formulations.

Plasma concentrations, bioavailability and dissolution of chlorpropamide

The bioavailability of chlorpropamide from two new formulations (Melitase tablets) has been compared to that from a reference formulation which is currently in clinical use as a hypoglycaemic agent. In both rate and extent of bioavailability, all three formulations may be considered equivalent, providing allowances are made for differences in drug content. With 95% confidence, the mean bioavailability of chlorpropamide from the new formulations was within about 16% of the mean from the reference formulaion, and formulation-related differences were not statistically significant. Although all three formulations were shown to have similar dissolution profiles, dissolution of chlorpropamide was pH-dependent in vitro. Dissolution was almost complete during 30 min at pH 7.2, but only 40%-60% had dissolved during 90 min at pH 2.0. A peak mean concentration of 22.7 mug/ml was reached 3 h after administration of 2 x 100 mg tablets of the new formulation and peak mean concentrations of 26.8 mug/ml and 27.4 mug/ml were reached 3 h and 4 hours after administration of one 250 mg tablet of the new formulation and one 250 mg tablet of the reference formulation respectively. Formulation-related differences of mean plasma concentrations (after scaling for equal doses of 250mg) were not significant and each formulation provided similar plasma concentrations at corresponding times after administration. Statistically significant subject-related differences in all the parameters of bioavailability were shown by analyses of variance.