METABOLISM OF ANTIDIABETIC SULFONYLUREAS IN MAN. I. BIOLOGICAL HALF-LIVES OF THE P-ACETYLBENZENESULFONYLUREAS, U-18536 AND ACETOHEXAMIDE AND THEIR METABOLITES

Pharmacologically active drug metabolites: impact on drug discovery and pharmacotherapy

Metabolism represents the most prevalent mechanism for drug clearance. Many drugs are converted to metabolites that can retain the intrinsic affinity of the parent drug for the pharmacological target. Drug metabolism redox reactions such as heteroatom dealkylations, hydroxylations, heteroatom oxygenations, reductions, and dehydrogenations can yield active metabolites, and in rare cases even conjugation reactions can yield an active metabolite. To understand the contribution of an active metabolite to efficacy relative to the contribution of the parent drug, the target affinity, functional activity, plasma protein binding, membrane permeability, and pharmacokinetics of the active metabolite and parent drug must be known. Underlying pharmacokinetic principles and clearance concepts are used to describe the dispositional behavior of metabolites in vivo. A method to rapidly identify active metabolites in drug research is described. Finally, over 100 examples of drugs with active metabolites are discussed with regard to the importance of the metabolite(s) in efficacy and safety.

Pharmacokinetics of oral antihyperglycaemic agents in patients with renal insufficiency

This paper reviews the effects of renal insufficiency on the pharmacokinetics of oral antidiabetic drugs. Of the 3 groups of drugs currently available for the treatment of non-insulin-dependent diabetes mellitus (NIDDM), the sulphonylureas and metformin are, in general, well-tolerated and generally safe. In patients with chronic renal insufficiency, however, care must be exercised in the use of many of these drugs, as accumulation, either of the active drug or of active metabolites, can lead to serious adverse effects such as hypoglycaemia or, with metformin, lactic acidosis. The sulphonylurea drugs, to a greater or lesser degree, are metabolised in the liver to a variety of active or inactive compounds which, in general, are excreted by the kidneys. In addition, varying amounts of parent compound may depend on renal elimination. As a result, sulphonylurea drugs such as tolazamide, acetohexamide, chlorpropamide and glibenclamide (glyburide) are more likely to cause significant hypoglycaemia, as the metabolism of these drugs, compared with other commonly prescribed sulphonylureas, can lead to the accumulation of either the parent drug or the active metabolite in the presence of renal insufficiency. Tolbutamide, glipizide, gliclazide and gliquidone are much less likely to cause hypoglycaemia as their metabolites are either inactive or have minimal hypoglycaemic potency. Metformin is dependent on renal excretion and is not significantly metabolised. As a result, caution is required when treating patients with renal insufficiency where metformin accumulation can occur, with the danger of lactic acidosis. Although the correlation between creatinine clearance (CLCR) and total oral clearance of drug is weaker than the correlation between CLCR and renal clearance (CLR) of metformin, it is clear that renal insufficiency is associated with most cases of metformin-induced lactic acidosis. For this reason, clinicians in general would regard a raised plasma creatinine as a contraindication to metformin treatment. Acarbose, an alpha-glucosidase inhibitor, and a relatively new agent for treating NIDDM, is likely to be safe in patients with impaired renal function, as the drug is not significantly absorbed from the gut, but data on this subject are lacking.

The management of diabetes mellitus in older individuals

Nearly 50% of individuals with type II diabetes mellitus are over the age of 65 years. There are numerous reasons to maintain blood glucose levels below 11.1 nmol/L (200 mg/dl) in older persons, and there are a number of changes often seen with advancing age that persons, and there are a number of changes often seen with advancing age that may interfere with the management of diabetes mellitus, e.g. hypodipsia, anorexia, visual disturbance, altered renal and hepatic function, depression, impaired basoreceptor response and multiple medications. Hyperglycaemia appears to produce cognitive impairment which may lead to poor compliance. It is often difficult to manipulate diet in older people, and in fact dietary changes can lead to severe protein energy malnutrition. High maximum voluntary oxygen intake has been correlated with increased glucose disposal, but there is little evidence that physical exercise can improve diabetic control in the elderly. Oral sulphonylurea hypoglycaemic agents are extremely useful in the treatment of diabetes in these patients, but it should be remembered that they are more liable to develop hypoglycaemia than are younger diabetics. The role of metformin in the management of older diabetic patients is poorly studied. Many older persons can cope well with insulin therapy, but those with visual disturbances often make errors when drawing up insulin and require special attention. Combination therapy of insulin with oral hypoglycaemic agents is not recommended in this group of patients, and serum fructosamine is preferred to glycated haemoglobin to monitor control. Successful management of elderly diabetic patients thus requires an interdisciplinary team approach.

Pharmacokinetic-pharmacodynamic relationships of oral hypoglycaemic agents. An update

Oral hypoglycaemic drugs, sulphonylureas and biguanides, occupy an important place in the treatment of Type II (non-insulin-dependent) diabetic patients who fail to respond satisfactorily to diet therapy and physical exercise. Although the precise mechanisms of action of these compounds are still poorly understood, there is sufficient agreement that sulphonylureas have both pancreatic and extrapancreatic effects, whereas biguanides have predominantly extrapancreatic actions. By using labelled compounds or measuring the circulating concentrations, the main pharmacokinetic properties of oral hypoglycaemic agents have been assessed and, in some cases, their pharmacokinetic-pharmacodynamic relationships have been evaluated. A correlation between diabetes control and plasma sulphonylurea or biguanide concentrations is generally lacking at the steady-state, with the possible exception of long-acting agents; after either oral or intravenous dosing, the reduction of plasma glucose is usually related to the increased circulating drug concentrations. The toxic effects of oral hypoglycaemic drugs are more frequent in the elderly and in the presence of conditions that may lead to drug accumulation or potentiation (increased dosage, use of long-acting compounds, hepatic and renal disease, interaction with other drugs); however, a relationship between toxic effects and drug plasma levels has been reported only for biguanides.

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.

GLC determination of acetohexamide and hydroxyhexamide in biological fluids

A sensitive and specific GLC assay was developed for acetohexamide and hydroxyhexamide, its major metabolite, in plasma and urine. The assay uses tolbutamide as a mass internal standard. Compounds are extracted from acidified plasma or urine with toluene, converted to methylated derivatives with dimethyl sulfate, and measured by GLC using a flame-ionization detector. With GLC-mass spectrometry, the compounds measured are the N-methylsulfonamides resulting from GLC pyrolysis. Plasma and urine data are presented from a bioavailability study to demonstrate the utility of this method.