Influence of food and age on the single-dose kinetics and effects of tolbutamide and chlorpropamide

Sulfonylurea Blockade of KATP Channels Unmasks a Distinct Type of Glucose-Induced Ca2+ Decrease in Pancreatic β-Cells

OBJECTIVES

This study aimed to explore how sulfonylurea blockade of KATP channels affects the early Ca signals for glucose generation of insulin release.

METHODS

Cytoplasmic Ca was measured with ratiometric microfluorometry in isolated mouse islets loaded with Fura-PE3.

RESULTS

After sulfonylurea blockade of the KATP channels (50 μM-1 mM tolbutamide or 1 μM-1 mM gliclazide), increase of glucose from 3 to 20 mM resulted in suppression of elevated Ca during a 3- to 5-minute period. The Ca decrease was shorter after inhibition of the Na/K pump with ouabain (10 and 100 μM) but prolonged when the α2A adrenoceptors were activated with clonidine (1 and 10 nM) or epinephrine (10 nM). Inhibition of the sarco/endoplasmic reticulum Ca-ATPase pump with 10 μM cyclopiazonic acid counteracted the action of 10 nM clonidine, making the Ca decrease shorter than in controls. Extended superfusion of islets with a medium containing 20 mM glucose and 1 mM tolbutamide sometimes resulted in delayed appearance of Ca oscillations mediated by periodic interruption of elevated Ca.

CONCLUSIONS

Increase of glucose generates prompt suppression of cytoplasmic Ca in β-cells lacking functional KATP channels. Activation of α2A adrenoceptors markedly prolongs the period of glucose-induced Ca decrease, an effect counteracted by cyclopiazonic acid.

Effects of sulfonylurea on the secretion and disposition of insulin and C-peptide

In an attempt to examine the influence of sulfonylurea on the secretion, disposition and effect of insulin, 9 diabetic patients were studied during three one-month medications with (a) chlorpropamide (t1/2 greater than 24 h) once daily, (b) glipizide (t1/2 2-4 h) once daily, and (c) glipizide in divided dosage. The food intake of each patient was identical during each examination period. Blood concentrations of C-peptide, insulin, glucose, and drugs were determined before and after breakfast and lunch on the 4th day of each examination period. As expected, once-daily administration of glipizide led to higher after-breakfast concentrations of the drug than when the dose was divided. However, the C-peptide changes following breakfast were similar both during these two treatments and also during chlorpropamide, indicating that the amounts of insulin released from the pancreas were equivalent. In spite of this, glipizide once daily yielded 60-70% more insulin in systemic blood following breakfast than did the two other treatments. Reasonably, this signifies that the hepatic extraction of insulin was reduced during once-daily glipizide, allowing more insulin to reach systemic circulation. In addition, this was found to promote a more effective utilization of glucose following breakfast. Following lunch, the C-peptide release, the plasma insulin increase and the blood glucose reduction were greater when glipizide was given in divided dosage than when once-daily glipizide or chlorpropamide was employed. This occurred even though the after-lunch concentration of glipizide in systemic blood was lower rather than higher during divided than during once-daily administration. This supports the notion that the effect of orally administered sulfonylurea is determined not only by its concentration in systemic blood but also by its gastroenterohepatic appearance. Glipizide may offer greater therapeutic flexibility than chlorpropamide, but further studies are required to define the optimum choice and use of sulfonylureas.

Human variability for metabolic pathways with limited data (CYP2A6, CYP2C9, CYP2E1, ADH, esterases, glycine and sulphate conjugation)

Human variability in the kinetics of a number of phase I (CYP2A6, CYP2C9, CYP2E1, alcohol dehydrogenase and hydrolysis) and phase II enzymes (glycine and sulphate conjugation) was analysed using probe substrates metabolised extensively (>60%) by these routes. Published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and available data on subgroups of the population (effects of ethnicity, age and disease) were abstracted using parameters relating primarily to chronic exposure [metabolic and total clearances, area under the plasma concentration time-curve (AUC)] and acute exposure (C(max)). Interindividual differences in kinetics for all these pathways were low in healthy adults ranging from 21 to 34%. Pathway-related uncertainty factors to cover the 95th, 97.5th and 99th centiles of healthy adults were derived for each metabolic route and were all below the 3.16 kinetic default uncertainty factor in healthy adults, with the possible exception of CYP2C9*3/*3 poor metabolisers (based on a very limited number of subjects). Previous analyses of other pathways have shown that neonates represent the most susceptible subgroup and this was true also for glycine conjugation for which an uncertainty factor of 29 would be required to cover 99% of this subgroup. Neonatal data were not available for any other pathway analysed.

Tolbutamide stimulation of pancreatic beta-cells involves both cell recruitment and increase in the individual Ca(2+) response

Individual pancreatic beta-cells are functionally heterogeneous. Their sensitivity to glucose is variable, so that the proportion of active cells increases with the glucose concentration (recruitment). We have investigated whether sulphonylureas also recruit beta-cells, by measuring cytoplasmic Ca(2+) ([Ca(2+)](i)) - the triggering signal of insulin secretion - in single cells and clusters of cells prepared from mouse islets. In 4 mM glucose, the threshold concentration of tolbutamide inducing a [Ca(2+)](i) rise was variable (5 - 50 microM). The proportion of responsive cells and clusters therefore increased with the tolbutamide concentration, to reach a maximum of 90% of the cells and 100% of the clusters. This recruitment occurred faster when the glucose concentration was increased from 4 to 5 mM (EC(50) of approximately 14 and approximately 4 microM tolbutamide respectively). Within responsive clusters little recruitment was observed; when a cluster was active, all or nearly all cells were active probably because of cell coupling. Thus, tolbutamide-induced [Ca(2+)](i) oscillations were synchronous in all cells of each cluster, whereas there was no synchrony between clusters or individual cells. Independently of cell recruitment, tolbutamide gradually augmented the magnitude of the [Ca(2+)](i) rise in single cells and clusters. This increase occurred over a broader range of concentrations than did recruitment (EC(50) of approximately 50 and 25 microM tolbutamide at 4 and 5 mM glucose respectively). Tolbutamide (10 microM) accelerated the recruitment of single cells and clusters brought about by increasing glucose concentrations (range of 3 - 7 mM instead of 4 - 10 mM glucose), and potentiated the amplification of the individual responses that glucose also produced. In conclusion, both metabolic (glucose) and pharmacologic (sulphonylurea) inhibition of K(+)-ATP channels recruits beta-cells to generate a [Ca(2+)](i) response. However, the response is not of an all-or-none type; it increases in amplitude with the concentration of either glucose or tolbutamide.

Tissue-specific effects of sulfonylureas: lessons from studies of cloned K(ATP) channels

Sulfonylureas stimulate insulin secretion in type-2 diabetic patients by blocking ATP-sensitive (K(ATP)) potassium channels in the pancreatic beta-cell membrane. This effect is mediated by the binding of the drug to the sulfonylurea receptor (SUR) subunit of the channel. K(ATP) channels are also present in other tissues, but often contain different types of SUR subunits (e.g., SUR1 in beta-cells, SUR2A in heart, SUR2B in smooth muscle). The sensitivity of these different types of K(ATP) channels to sulfonylureas is variable: gliclazide and tolbutamide block the beta-cell, but not the cardiac or smooth muscle, types of K(ATP) channel. In contrast, glibenclamide blocks all three types of channel with similar affinity. The reversibility of the drugs also varies, with tolbutamide and gliclazide being reversible on all three types of K(ATP) channel, while glibenclamide is reversible on cardiac, but not beta-cell, K(ATP) channels. This review summarizes current knowledge of how sulfonylureas act on the different types of K(ATP) channel found in beta-cells and in extrapancreatic tissues, and discusses the implications of these findings for their use as therapeutic agents.

Alterations of insulin secretion from mouse islets treated with sulphonylureas: perturbations of Ca2+ regulation prevail over changes in insulin content

1. To determine how pretreatment with sulphonylureas alters the beta cell function, mouse islets were cultured (18 - 20 h) without (controls) or with (test) 0.01 microM glibenclamide. Acute responses to glucose were then determined in the absence of glibenclamide. 2. Test islets were insensitive to drugs (sulphonylureas and diazoxide) acting on K+-ATP channels, and their [Ca2+]i was already elevated in the absence of stimulation. 3. Insulin secretion was increased in the absence of glucose, and mainly stimulated between 0 - 10 instead of 7 - 20 mM glucose in controls. The maximum response was halved, but this difference disappeared after correction for the 45% decrease in the islet insulin content. 4. The first phase of glucose-induced insulin secretion was abrogated because of a paradoxical decrease of the high basal [Ca2+]i in beta cells. The second phase was preserved but occurred with little rise of [Ca2+]i. These abnormalities did not result from alterations of glucose metabolism (NADPH fluorescence). 5. In islets cultured with 50 microM tolbutamide, glucose induced biphasic increases in [Ca2+]i and insulin secretion. The decrease in the secretory response was matched by the decrease in insulin content (45%) except at maximal glucose concentrations. Islets pretreated with tolbutamide, however, behaved like those cultured with glibenclamide if tolbutamide was also present during the acute functional tests. 6. In conclusion, treatment with a low glibenclamide concentration causes long-lasting blockade of K+-ATP channels and rise of [Ca2+]i in beta cells. Glucose-induced insulin secretion occurs at lower concentrations, is delayed and is largely mediated by a modulation of Ca2+ action on exocytosis. It is suggested that glucose regulation of insulin secretion mainly depends on a K+-ATP channel-independent pathway during in vivo sulphonylurea treatment.

Repaglinide pharmacokinetics in healthy young adult and elderly subjects

In this open-label, single-center, pharmacokinetic study of repaglinide, 12 healthy volunteers (6 men, 6 women) were enrolled in each of 2 groups (total, 24 volunteers). One group consisted of young adult subjects (18 to 40 years), and the other group consisted of elderly subjects (> or = 65 years). On day 1, after a 10-hour fast, all 24 subjects received a single 2-mg dose of repaglinide. Starting on day 2 and continuing for 7 days, subjects received a 2-mg dose of repaglinide 15 minutes before each of 3 meals. On day 9, subjects received a single 2-mg dose of repaglinide. Pharmacokinetic profiles, including area under the curve, maximum concentration (Cmax), time to Cmax, and half-life, were determined at completion of the single-dose and multiple-dose regimens (days 1 and 9, respectively). Trough repaglinide values were collected on days 2 through 7 to assess steady state. The single-dose and multiple-dose pharmacokinetic variables of serum repaglinide were not significantly different between young adult and elderly subjects. Repaglinide was well tolerated in both groups. Hypoglycemic events occurred in 5 young adult and 5 elderly subjects. This study demonstrates that the pharmacokinetics of repaglinide are similar in healthy young adult and elderly subjects.

Non-insulin-dependent diabetes mellitus in the elderly

The prevalence of non-insulin-dependent diabetes mellitus dramatically increases with age. Older diabetic subjects have an increased frequency of complications from diabetes compared with their younger counterparts and higher morbidity and mortality rates compared with age-matched non-diabetic controls. Elderly patients with diabetes are generally treated following the same approach as in younger patients: dietary therapy first, followed by oral hypoglycaemic agents and ultimately insulin. However, several specificities should be pointed out. Changes associated with ageing may affect the pharmacokinetics and pharmacodynamics of both sulphonylureas (increasing the risk of severe hypoglycaemia) and biguanides (increasing the risk of lactic acidosis). The best insulin regimen in old age is not known, but a twice-daily injection of a pre-mixed insulin preparation is usually recommended. Goals of therapy must be realistic and not cause disabling side-effects. The general practitioner plays a crucial role in the care of elderly diabetic patients, but access to a multidisciplinary specialized team may be necessary.

Oral antidiabetic drug use in the elderly

Non-insulin-dependent diabetes mellitus (NIDDM) is a metabolic disease that is common in the elderly, and is characterised by insulin insufficiency and resistance. Measures such as bodyweight reduction and exercise improve the metabolic defects, but pharmacological therapy is the most frequently used and successful therapy. The sulphonylureas stimulate insulin secretion. Metformin and troglitazone increase glucose disposal and decrease hepatic glucose output without causing hypoglycaemia. Acarbose is a dietary aid that spreads the dietary carbohydrate challenge to endogenous insulin over time. These pharmacological agents, either alone or in combination, should improve blood glucose regulation in patients with NIDDM.

Diabetes in the elderly

Diabetes affects at least 20% of the population over the age of 65. Half of these patients are unaware that they have the disease. Diabetes in middle-aged subjects is characterized by an impairment in glucose induced insulin release, increased fasting hepatic glucose output and resistance to insulin mediated glucose disposal. In contrast, diabetes in the elderly is primarily associated with insulin deficiency. The presentation of diabetes in the aged is often non-specific. The elderly have an increased frequency of complications from diabetes. They are particularly susceptible to hypoglycaemia, because of reduced awareness of hypoglycaemic warning symptoms and altered release of counterregulatory hormones. Although no data are yet available from randomized controlled trials, there is abundant epidemiological evidence to suggest that adequate control of blood glucose can be expected to reduce the risk of long-term complications. A team approach is ideal for the management of the elderly patient with diabetes. Little data is available on which to base a diet and exercise prescription for elderly patients. Gliclazide appears to be the sulphonylurea of choice in the aged because it is associated with a lower frequency of hypoglycaemic reactions. Urine glucose testing is unreliable, and capillary glucose monitoring is preferred. Fructosamine may prove to be superior to haemoglobin A1C for monitoring long-term control.

Problems and pitfalls of sulphonylurea therapy in older patients

The prevalence of non-insulin-dependent diabetes mellitus (type II) increases with age, so that approximately half of all known patients in English-speaking countries are over 65 years of age. There is no reason to believe that the criteria for blood glucose control should be any less stringent for elderly patients unless they have a limited life expectancy. Sulphonylurea drugs remain an effective means of achieving blood glucose control after failure of dietary therapy alone in older patients. However, changes in normal metabolism of drugs with age and the development of other pathologies in elderly patients make it important that these drugs are prescribed with care. Severe symptomatic hypoglycaemia is the most serious adverse effect of sulphonylurea drugs and this becomes progressively more likely with increasing age, depending primarily on the substantial reduction of renal function with normal aging. Other adverse effects are much less commonly of clinical importance. To minimise the risk of hypoglycaemia, it is important that patients receive closely supervised dietary management with education about their disease for at least 3 months before sulphonylurea drugs are prescribed. In elderly patients a short-acting agent with no active metabolites should be used. As patients become older, those receiving long-acting agents can be changed to short-acting agents before problems arise. If blood glucose control appears satisfactory on treatment, then symptoms of hypoglycaemia should be sought. If control is poor, then the criteria for introduction of insulin, with appropriate education, do not differ from those in younger patients.

Hepatic drug metabolism and aging

Although there is considerable variation in the effect of age on drug biotransformation, the metabolism of many drugs is impaired in the elderly. Age-related physiological changes, such as a reduction in liver mass, hepatic metabolising enzyme activity, liver blood flow and alterations in plasma drug binding may account for the decreased elimination of some metabolised drugs in the elderly. It is difficult, however, to separate an effect of aging from a background of marked variation in the rate of metabolism due to factors such as individual metabolic phenotype, environmental influences, concomitant disease states and drug intake. The prevailing data suggest that initial doses of metabolised drugs should be reduced in older patients and then modified according to the clinical response. In most studies the elderly appear as responsive as young individuals to the effects of compounds which induce or inhibit the activity of cytochrome P450 isozymes. Concurrent use of other agents, which induce or inhibit drug metabolism, mandates dose adjustment as in younger patients. Many questions remain unanswered. For instance, limitations of in vitro studies prevent any firm conclusion about changes in hepatic drug metabolising enzyme activity in the elderly. With aging, some pathways of drug metabolism may be selectively affected, but this has not been adequately scrutinised. The possibility that metabolism of stereoisomers may be altered in the elderly has not been adequately tested. The effect of aging on the distribution of polymorphic drug metabolism phenotypes is still not established, despite potential implications for disease susceptibility and survival advantage.

The effects of ingestion time of gliclazide in relationship to meals on plasma glucose, insulin and C-peptide levels

The effect of altering the timing of gliclazide administration in relation to a meal was studied in ten type 2 (non-insulin dependent) chronically treated diabetics. Gliclazide was given 30 min before, at the start of and 30 min after breakfast or omitted altogether. Plasma gliclazide was present at greater than 2 mg/l throughout the study periods. Administration at 30 min after the meal significantly delayed the time to peak for plasma gliclazide. No significant difference was noted in plasma glucose, insulin or c-peptide patterns with any protocol. It is concluded that, in clinical practice, with chronically treated diabetics the timing of gliclazide ingestion in relation to meals is not critical.

Control of insulin secretion by sulfonylureas, meglitinide and diazoxide in relation to their binding to the sulfonylurea receptor in pancreatic islets

Sulfonylureas inhibit an ATP-dependent K+ channel in the B-cell plasma membrane and thereby initiate insulin release. Diazoxide opens this channel and inhibits insulin release. In mouse pancreatic islets, we have explored whether other targets for these drugs must be postulated to explain their hypo- or hyperglycaemic properties. At non-saturating drug concentrations the rates of increase in insulin secretion declined in the order tolbutamide = meglitinide greater than glipizide greater than glibenclamide. The same rank order was observed when comparing the rates of disappearance of insulin-releasing and K+ channel-blocking effects. The different kinetics of response depend on the lipid solubility of the drugs, which controls their penetration into the intracellular space. Allowing for the different kinetics, the same maximum secretory rates were caused by saturating concentrations of tolbutamide, meglitinide, glipizide and glibenclamide. A close correlation between insulin-releasing and K+ channel-blocking potencies of these drugs was observed. The relative potencies of tolbutamide, meglitinide, glipizide and glibenclamide corresponded well to their relative affinities for binding to islet-cell membranes, suggesting that the binding site represents the sulfonylurea receptor. The biphasic time-course of dissociation of glibenclamide binding indicates a complex receptor-drug interaction. For diazoxide there was no correlation between affinity of binding to the sulfonylurea receptor and potency of inhibition of insulin secretion. Thus, opening or closing of the ATP-dependent K+ channel by diazoxide or sulfonylureas, respectively, appears to be due to interaction with different binding sites in the B-cell plasma membrane. The free concentrations of tolbutamide, glipizide, glibenclamide and diazoxide which are effective on B-cells are in the range of therapeutic plasma concentrations of the free drugs. It is concluded that the hypo- and hyperglycaemic effects of these drugs result from changing the permeability of the ATP-dependent K+ channel in the B-cell plasma membrane.

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.

Oral hypoglycemic agents

The sulfonylureas remain the most important oral agents, although their chronic hypoglycemic actions are still unexplained and the evidence on their relative efficacy is inconclusive. Data on relative safety suggest that chlorpropamide is the most toxic sulfonylurea but glyburide causes dangerous hypoglycemia as often as chlorpropamide. For many patients, good blood glucose control will be achieved by taking tolbutamide or another sulfonylurea 30 minutes before breakfast and the main evening meal. The biguanide metformin, which is as safe as glyburide, is of use in treating overweight diabetic patients who do not have cardiovascular, hepatic, or renal dysfunction.

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.

Clinical pharmacology of sulfonylureas

Sulfonylureas seem to have similar mechanisms of action, including an acceleration and increase of insulin secretion, an increase of the systemic availability of insulin, and probably indirectly, an increase of insulin action. Sulfonylureas may postpone the development of impaired glucose tolerance (IGT) to manifest non-insulin-dependent diabetes mellitus (NIDDM), and all NIDDM subjects should benefit from sulfonylurea treatment except those in whom insulin secretion has been attenuated. The most effective use is the combination of diet restriction and sulfonylurea introduced in NIDDM subjects soon after transition from IGT to NIDDM. A simple screening procedure has been devised to find the subjects at this early stage. Newer sulfonylureas, such as glipizide and glyburide, are more potent than the older ones, such as tolbutamide and chlorpropamide. During chronic treatment, glipizide and glyburide seem to be equally effective in reducing blood glucose levels, and they do so without causing a chronic elevation of insulin secretion, signifying that they do not increase the risk of pancreatic B cell exhaustion. Glipizide has rapid and complete absorption, as well as a rapid distribution and elimination. This may explain why it is less liable than other sulfonylureas to provoke long-lasting hypoglycemia, which is the major danger when using sulfonylureas. Despite its rapid elimination, 7.5 to 15 mg glipizide can be administered once daily without loss of therapeutic efficacy. This may be due in part to enterohepatic recirculation of the drug in response to meals. The therapeutic efficacy is increased if glipizide is received half an hour before breakfast.

Pharmacokinetics of chlorpropamide in epileptic patients: effects of enzyme induction and urine pH on chlorpropamide elimination

The effects of liver enzyme induction and of urine pH on the pharmacokinetics of chlorpropamide have been studied. A single oral dose of chlorpropamide 250 mg was administered to 8 patients on antiepileptic drugs (phenytoin, carbamazepine) and to 8 healthy volunteers. The half-life of chlorpropamide was significantly shorter in the patients (34.4 h) than in the healthy volunteers (50.2 h), but the difference between the groups in the half-life of antipyrine was even more pronounced (5.1 vs 11.4 h). The clearance and volume of distribution of total chlorpropamide were significantly higher in the patients (2.99 ml X h-1 X kg-1 and 126 ml X kg-1) than in the healthy volunteers (1.60 ml X h-1 X kg-1 and 106 ml X kg-1). The unbound fraction of chlorpropamide in serum was also higher in the patients (5.7%) than in the healthy subjects (4.4%). Neither the volume of distribution nor the clearance of the free fraction of chlorpropamide differed significantly between the groups. There was a significant correlation between the half-lives of chlorpropamide and antipyrine, and the half-life of chlorpropamide also had at least as good an inverse correlation with the urinary excretion of unchanged chlorpropamide. The renal clearance of chlorpropamide correlated well with urine pH and was almost 100-fold higher at pH 7 than at pH 5. Both the metabolic and renal clearances of chlorpropamide are important in its elimination. At urine pH higher than 6.5-7, the renal clearance of chlorpropamide represents more than half its total clearance regardless the degree of induction of liver enzymes.

Effect of sucralfate on the absorption and pharmacokinetics of chlorpropamide

This study compared the pharmacokinetics of chlorpropamide (C) when administered alone, or in combination with sucralfate (S) to evaluate whether a drug-drug interaction exists between these two agents in vivo. A two-way, randomized, cross-over study was performed in 12 healthy male volunteers who received 250 mg C alone or were pretreated with S qid for two days and then received a single 250-mg dose of C with S on day 3 and continued to take sucralfate throughout the day while serial blood samples were drawn. High-performance liquid chromatography determination of plasma concentrations found there to be no statistically significant differences in maximum concentration, time to maximum concentration, elimination rate constant, or area under the concentration-time curve from 0 to 96 hours. However, there was a statistically significant difference in the area under the curve from 0 to infinity data (P less than .05). The authors conclude that there appears to be no drug interaction between sucralfate and chlorpropamide when given concurrently; however, a trend towards less drug availability was seen that may warrant a future multiple-dose study to further evaluate the significance of this finding.

Effect of tolbutamide and chlorpropamide on acetaldehyde metabolism in two inbred strains of mice

The mechanisms by which chlorpropamide and tolbutamide disrupt acetaldehyde metabolism were studied in C57BL and DBA mice. Acute po administration of varying doses of tolbutamide or chlorpropamide 2.5 hr before a 3.0 g/kg ip dose of ethanol to C57BL and DBA mice resulted in significant elevations of blood acetaldehyde when measured 2.5 hr after ethanol dosing. Dose-response analysis revealed a significant (p less than .05) difference in ED50 values for the elevated blood acetaldehyde response to tolbutamide in DBA (60 mg/kg) and C57BL (100 mg/kg) mice. The ED50 value for potentiation by chlorpropamide of blood acetaldehyde concentration was similar (23 to 32 mg/kg) in both inbred strains. At higher doses of chlorpropamide, DBA mice displayed elevations of blood acetaldehyde nearly threefold greater than those measured in C57BL mice treated identically. Measurements of aldehyde dehydrogenase (ALDH) in hepatic subcellular fractions, obtained from both inbred strains treated with 100 mg/kg tolbutamide or chlorpropamide prior to a 3.0 g/kg dose of ethanol, revealed a 50 to 80% inhibition of the low-Km ALDH present in mitochondria. Chlorpropamide and tolbutamide did not inhibit ALDH in vitro, suggesting that metabolites of these hypoglycemic agents may be responsible for the genotypic-dependent alterations in in vivo acetaldehyde oxidation.

Effect of food and tablet age on relative bioavailability and pharmacodynamics of two tolbutamide products

Relative bioavailability and pharmacodynamics of tolbutamide from two different commercially available tablet products have been evaluated in healthy subjects in a single-dose crossover study. "Fresh" tablets and tablets aged by exposure to 98% relative humidity for 3 d at ambient temperature were studied. Aging was found to differentially affect both the rate and extent of absorption for the two products. Differences were reflected by log AUC (generic product AUC 10% lower than the product of the innovator, p = 0.047), peak concentration (generic product 27% lower than the product of the innovator, p = 0.0001), mean absorption time (generic product 119% longer than the product of the innovator, p = 0.0008), and mean residence time (generic product 17% longer than the product of the innovator, p = 0.011). Aged product from the innovator produced statistically significantly higher serum tolbutamide concentrations for the first 8 h postdose and a greater glucose depression than aged generic product. Administration of unaged tablets with food produced differences in the rate of absorption, manifested in time-to-peak (generic product 69% later than the product of the innovator, p = 0.006), peak concentration (generic product 18% lower than the product of the innovator, p = 0.001), and mean absorption time (generic product 104% greater than the product of the innovator, p = 0.007), which resulted in statistically significantly higher tolbutamide concentrations for the product of the innovator than for the generic product for the first 3 h postdose.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions affecting drug absorption

The influence of drug-drug and drug-food interactions affecting the absorption of orally administered medication is reviewed. Drug-drug interactions can be classified in terms of indirect effects by one drug on gastrointestinal tract physiology influencing the absorption of other drugs, or direct interactions involving altered pH, adsorption, absorption, or chelation. Most, but not all, drug-drug interactions result in reduced or delayed systemic drug availability. Drug-food interactions may result in reduced, delayed, or increased systemic drug availability. The absorption of only a small number of drugs is unaffected by concomitant food intake. The degree of interaction and whether it positively or negatively affects drug absorption depends on a number of factors including the physical and chemical nature of the drug, the formulation, the type of meal, and the time interval between eating and dosing. Mechanisms of drug-food interactions are not well characterised. They clearly involve both direct and indirect factors in a similar fashion to drug-drug interactions, but indirect factors probably predominate. Reduced or delayed drug absorption is generally attributed, at least in part, to delayed stomach-emptying due to food. Increased absorption may also result from delayed stomach-emptying facilitating greater drug dissolution before it passes from the stomach into the small intestine. Increased bioavailability of some drugs, e.g. propranolol, metoprolol and labetalol, may be related to reduced presystemic clearance. The potential clinical implications of drug-drug and drug-food interactions must be taken into account with oral medications in order to minimise variations in systemic drug availability and hence in clinical efficacy.

Oral hypoglycaemic agents. An update

Despite the availability of oral hypoglycaemic agents for nearly 30 years, their precise mode of action and role in the management of diabetes mellitus remains poorly defined and controversial. They are regarded by many, though not all, clinicians as helpful adjuncts in the treatment of patients with non-insulin-dependent diabetes who have failed to respond satisfactorily to an adequate programme of dietary treatment. Their initial effectiveness is greatest in those patients who have had diabetes for less than 5 years, are overweight at the time of initiation of therapy, and whose fasting blood glucose levels are not unduly raised (less than 200 mg/dl). If they are receiving treatment with insulin and a shift to oral compounds is contemplated, success in the changeover is more likely if the daily dose has been less than 20 to 30 units daily. While their efficacy in maintaining adequate glycaemic control over the short term in responsive patients is unquestioned, the long term benefit of oral hypoglycaemic agents in reducing morbidity and mortality of late complications remains to be substantiated. In this regard, where long term efficacy is difficult to quantify, physician vigilance for chronic toxicity assumes a special importance. Notwithstanding the potential for interaction between sulphonylureas and numerous other drugs, significant adverse effects are uncommon. Hypoglycaemia is the major health concern associated with the use of sulphonylureas, and lactic acidosis has been the major problem with biguanides. Careful patient selection is thus the key to ensuring efficacy and avoiding toxicity. Recent evidence suggests that while the insulinotropic action of the sulphonylureas may explain the short term hypoglycaemic effect of these compounds, their reported action in enhancing insulin sensitivity, both at the receptor and post-receptor levels, more likely accounts for the long term maintenance of improved carbohydrate tolerance. The relatively new ('second generation') sulphonylurea compounds have not been shown to possess clearly defined advantages over the older preparations; the potentially beneficial effects of gliclazide on the microangiopathic changes of diabetes require considerable further evaluation.

Bioavailability of tolazamide from tablets: comparison of in vitro and in vivo results

The relative bioavailability of tolazamide was determined, in healthy male volunteers, from four different tablet formulations manufactured by direct compaction or granulation processes and the results were compared with in vitro disintegration and dissolution values. Serum tolazamide levels were determined by a high-pressure liquid chromatographic method developed in this laboratory. Serum tolazamide levels from the formulation that gave rise to rapid absorption were described by one-compartment model kinetics with a mean absorption half-time of 1.0 hr and an elimination half-life of 4.6 hr. Peak serum levels occurred at 3.3 hr after drug administration. Marked differences were observed in drug bioavailability from the four tablets, and the mean cumulative relative fraction of dose absorbed was 1.0, 0.42, 0.75, and 0.91 from Formulations A, B, C, and D, respectively. The hypoglycemic effect was closely related to serum tolazamide levels. Disintegration times did not predict in vivo tolazamide bioavailability. Dissolution rates provided an approximate rank order correlation with in vivo absorption but failed to be predictive among formulations. Currently available in vitro tests do not accurately predict tolazamide in vivo bioavailability characteristics among different formulations and manufacturing processes but may be useful to ensure lot-to-lot uniformity in bioavailability for a given formulation and specific method of manufacture.

The effect of food on the bioavailability and pharmacodynamics of tolbutamide in diabetic patients

The effect of food on the rate and extent of absorption of tolbutamide in diabetic patients was studied by varying the time of drug administration in relation to the ingestion of a standard meal. Serum levels of tolbutamide, insulin and glucose and related bioavailability parameters were compared following the administration of a single dose of tolbutamide 0.5 g to diabetic patients 30 min prior to and immediately before a standardized meal. A placebo dosage form was also administered to determine baseline glucose and insulin response to the meal. The 700 calorie standard meal was composed of 41% carbohydrate, 18% protein, and 41% fat. Administration of the drug with the meal resulted in a 6% (statistically significant) decrease in the extent of absorption, as determined by measurement of the area under the tolbutamide serum level-time curve from zero to infinity. Serum levels of tolbutamide were also significantly higher 0.5 h after drug administration when the drug was taken with the meal. Except for these two minor effects, no other differences between the drug treatments were observed in any other parameters of tolbutamide absorption or in the postprandial glucose and insulin serum levels. Therefore, the small differences found were judged to be clinically meaningless. These findings demonstrate that administration of tolbutamide 0.5 g tablets 30 min prior to or with a standard meal results in equivalent therapeutic actions.

Improved effect of glibenclamide on administration before breakfast

In an attempt to assess whether intake of glibenclamide before meals would improve its therapeutic capacity, the present investigation compared the effect of glibenclamide 2.5 mg t.i.d. given before and together with meals. In addition, these effects were compared with that of glibenclamide given as a single morning dose of 7.5 mg. The subjects studied were six Type 2 diabetics not previously exposed to sulphonylurea drugs. Irrespective of dosage and mode of administration, addition of glibenclamide to a standardized breakfast, lunch and dinner enhanced plasma IRI concentrations and reduced blood glucose concentrations as compared to administration of meals without the drug. The different modes of glibenclamide administration did not differ significantly with respect to IRI responses. However the blood glucose reduction after breakfast was significantly greater when glibenclamide 2.5 mg had been given before the meal than when 2.5 or 7.5 mg were given with the meal; a similar, but non-significant tendency was observed after lung; no consistent difference was seen after dinner. Food intake did not affect glibenclamide kinetics. It appears that administration of glibenclamide 2.5 mg before breakfast improved glucose utilization following the breakfast load, due to earlier attainment of an effective concentration of glibenclamide.

Influence of food intake on the absorption and effect of glipizide in diabetics and in healthy subjects

The influence of a standardized breakfast on the single dose (5 mg) kinetics and effects of glipizide was examined in 9 healthy volunteers and in 14 diabetics not previously exposed to a sulfonylurea. In the volunteers, glipizide caused an increase in plasma insulin and a reduction in blood glucose both during continued fasting and when the drug was taken with the breakfast. Food intake did not influence the peak concentration, the elimination half-life or the bioavailability of the drug. However, food intake significantly delayed the absorption of glipizide by about 0.5 h. In the patients. glipizide produced a significant increase in plasma glucose in response to the meal. Starting at breakfast and for 45 min thereafter serum glipizide concentrations were significantly higher when the drug was taken 0.5 h before the meal, than when ingested concurrently with it. With the former treatment, the increase in plasma insulin occurred earlier and the blood glucose reduction was pronouncedly greater than with the latter treatment. As the absorption of glipizide may be delayed by concurrent breakfast, this may help to explain, why the administration of glipizide 0.5 h before breakfast led to a more appropriate relation between the serum concentration of the drug and the metabolic impact of the meal, thereby promoting more appropriate insulin release and better glucose disposition than after concurrent intake of the drug and breakfast.