“Cocktail” Approaches and Strategies in Drug Development: Valuable Tool or Flawed Science?

There is an increasing interest in the simultaneous administration of several probe substrates to characterize the activity of multiple drug-metabolizing enzymes, the so-called "cocktail" approach. However, this method remains controversial and is being investigated more extensively. No general consensus has emerged on the applicability of this approach in clinical investigation and during drug development. The objective of the article is to review this important yet specialized technique, as well as its merits, drawbacks, and potential application in drug development. Among the two-, three-, four-, five-, and six-drug in vivo cocktails previously evaluated in humans, a variety of substrate probe combinations have been studied. Some probe combinations have been validated not to interact in vivo and have been useful in characterizing drug-drug interaction potential and metabolic enzyme induction in humans. For drug candidates that affect two or more in vitro pathways or are potential gene inducers, the use of a cocktail approach may facilitate the rapid delineation of the drug candidate's drug interaction potential. It may also offer the potential of providing clear guidance on safely conducting larger clinical studies and limiting comedication restrictions to only those likely to be clinically relevant.

Clinical pharmacokinetics of nateglinide: a rapidly-absorbed, short-acting insulinotropic agent

The prevalence and medical and economic impact of type 2 diabetes mellitus is increasing in Western societies. New agents have been developed that act primarily to reduce postprandial glucose excursions, which may be of particular significance now that postprandial glucose excursions are known to be correlated with cardiovascular morbidity and mortality. Nateglinide is a phenylalanine derivative that blocks K+ channels in pancreatic beta-cells, facilitating insulin secretion. Nateglinide sensitises beta-cells to ambient glucose, reducing the glucose concentration needed to stimulate insulin secretion. The pharmacokinetics of nateglinide are characterised by rapid absorption and elimination, with good (73%) bioavailability. Nateglinide is more rapidly absorbed when given 0-30 minutes prior to meal ingestion than if given during the meal. Nateglinide is extensively metabolised, primarily by cytochrome P450 2C9, and eliminated primarily by the kidney. Nateglinide pharmacokinetics are linear over the dose range 60-240 mg. No significant pharmacokinetic alterations occur in renally impaired patients, in the elderly, or in mildly hepatically impaired patients. Nateglinide administered prior to meals stimulates rapid, short-lived insulin secretion in a dose-dependent manner, thus decreasing mealtime plasma glucose excursions. Its effects on insulin secretion are synergistic with those of a meal. With increasing nateglinide doses, the risk of hypoglycaemia also increases, but its incidence is low. Even if a meal is missed, and the patient skips the dose of nateglinide (as recommended in the event of a missed meal), the incidence of subsequent hypoglycaemia remains low compared with long-acting agents. The postprandial insulinotropic effects of nateglinide are more rapid than those of repaglinide and more rapid and greater than those of glibenclamide (glyburide), while producing less prolonged insulin exposure and less risk of delayed hypoglycaemia. Further investigation is required to determine if nateglinide inhibition of postprandial glucose excursions will help to prevent diabetic complications or preserve pancreatic beta-cell function.

Tegaserod pharmacokinetics are similar in patients with severe renal insufficiency and in healthy subjects

Tegaserod (HTF 919), a selective 5-HT4 receptor partial agonist with promotile activity throughout the gastrointestinal tract, is in development for the treatment of irritable bowel syndrome. In an open-label, parallel-group study, the pharmacokinetics of a single 12-mg oral dose of tegaserod in patients with severe renal insufficiency requiring hemodialysis were compared with data obtained from healthy subjects matched for age, weight, height, and gender (n = 10, both). The pharmacokinetics of tegaserod were similar in both groups (AUC(0h-tz), ng.h/ml: 14.6 +/- 8.5 vs. 14.3 +/- 7.1; Cmax, ng/ml: 4.6 +/- 2.3 vs. 5.1 +/- 2.2; tmax, h: 1.0, for both). Tegaserod had similar tolerability in renally impaired patients and healthy volunteers, with adverse events largely related to the gastrointestinal pharmacological actions of the drug. Therefore, no dose adjustment of tegaserod is necessary for patients with renal insufficiency.

Pharmacokinetics of nateglinide in renally impaired diabetic patients

Treatment of hyperglycemia in patients with diabetes mellitus and renal insufficiency is complicated by altered pharmacokinetics of hypoglycemic agents. This study evaluated the pharmacokinetic profile and safety of nateglinide, an amino acid derivative that improves early phase insulin secretion and reduces mealtime glucose excursions. This open-label, single-dose, two-center study included patients (mean age = 57 +/- 10 years) with type 1 or 2 diabetes with impaired renal function (IRF) (n = 10) or with renal failure undergoing hemodialysis (n = 10). Both groups were compared with age-, sex-, height-, and weight-matched healthy controls (n = 20). All participants received a single 120-mg dose of nateglinide immediately before breakfast. Pharmacokinetic and safety evaluations were undertaken up to 48 hours postdose. All 40 subjects completed the study. Plasma nateglinide concentrations increased rapidly in patients undergoing dialysis and matched healthy subjects (tmax = 0.95 vs. 0.78 h, respectively) and was comparable with patients with IRF and matched healthy subjects (tmax = 0.80 vs. 0.65 h, respectively). There were no statistically significant differences for Cmax or AUC0-t between the groups. Nateglinide was eliminated rapidly in all groups (t1/2 = 1.9-2.8 h). There was no correlation between the level of renal function and systemic exposure. There was a low extent of renal excretion of nateglinide in healthy subjects (11%) and diabetic patients with IRF (3%). Nateglinide was well tolerated. These data suggest that nateglinide is suitable for use in diabetic patients with IRF or with renal failure undergoing dialysis. Given the comparable absorption and elimination profiles of nateglinide in renally impaired and healthy subjects, no dose adjustment appears necessary in the renally impaired.

Importance of early phase insulin secretion to intravenous glucose tolerance in subjects with type 2 diabetes mellitus

Insulin secretion is impaired in type 2 diabetes with the early response being essentially absent. The loss of this early insulin secretion is hypothesized to be important in the deterioration of glucose tolerance. To determine whether enhancement of the early-phase insulin response can enhance glucose tolerance, we administered 1) 120 mg nateglinide, an insulinotropic agent that enhances early insulin secretion; 2) 10 mg glyburide, which enhances the later phases of insulin secretion; or 3) placebo in random order to 21 subjects with type 2 diabetes (14 males and 7 females; aged 59.2 +/- 2.1 yr, x +/- SEM; body mass index 29.7 +/- 1.0 kg/m(2); fasting plasma glucose 8.1 +/- 0.1 mM). beta-Cell function was quantified as the incremental area under the curve for different time periods for the 5 h following iv glucose administration and glucose tolerance as the glucose disappearance constant (Kg) from 10 to 60 min. Insulin release commenced immediately after nateglinide administration, even before glucose injection, but this was not observed with glyburide. Both nateglinide and glyburide enhanced glucose-induced insulin release, compared with placebo (area under the curve -15-300 min: nateglinide 23,595 +/- 11,212 pM/min, glyburide 54,556 +/- 15,253 pM/min, placebo 10,242 +/- 2,414 pM/min). The profiles of insulin release demonstrated significant enhancement of release between -15 and 30 min for nateglinide, compared with glyburide and between 60 and 300 min for glyburide over nateglinide. Kg increased by 15% with nateglinide (0.87 +/- 0.04%/min), but it did not increase significantly with glyburide (0.79 +/- 0.04%/min), compared with placebo (0.76 +/- 0.04%/min). The enhancement of insulin release by glyburide resulted in a lower minimal glucose concentration with glyburide (3.8 +/- 0.2 mM), compared with nateglinide (5.0 +/- 0.2 mM) and placebo (5.9 +/- 0.2 mM). Thus, enhancement of the early phase of insulin secretion improves iv glucose tolerance, whereas delaying it by 30 min results in a slower rate of glucose disappearance for the first 2 h after iv glucose administration. Further, the differences in the kinetics of nateglinide and glyburide action results in continued insulin release with glyburide despite the fact that glucose levels have returned to basal, thus resulting in a further reduction in glucose levels and a lower nadir.

The effects of tegaserod (HTF 919) on the pharmacokinetics and pharmacodynamics of digoxin in healthy subjects

Tegaserod (HTF 919) is a highly specific 5-HT4 receptor partial agonist that exhibits promotile activity throughout the gastrointestinal tract and is under development for the treatment of functional gastrointestinal motility disorders. The present study was designed to assess the effect of multiple doses of tegaserod on the single-dose pharmacokinetics and pharmacodynamics of digoxin, a commonly prescribed agent for congestive heart failure. The study was an open-label, randomized, two-period crossover design of 12 healthy subjects. One treatment included digoxin treatment alone; the other treatment included a combined digoxin and tegaserod treatment. On day 1 of the digoxin treatment period, subjects received a single 1 mg oral dose of digoxin. In the combined tegaserod/digoxin treatment period, subjects received a single oral dose of 1 mg digoxin after 3 days of tegaserod (6 mg bid). After coadministration of tegaserod, systemic exposure to digoxin was decreased; mean AUC decreased by 11.9% (p < 0.05) relative to digoxin alone. Cmax was decreased by about 15% (p < 0.05). The 0.5-hour difference in the median tmax between the two treatments was not statistically significant. Because the steady-state trough concentration of digoxin (C(SS,min)) correlates with pharmacological effects, C(SS,min) for digoxin alone and in combination with tegaserod was simulated based on both parametric compartmental modeling and nonparametric superpositioning approaches. The predicted arithmetic mean C(SS,min) for combination therapy is 86% to 89% of that following digoxin alone. Likewise, the predicted arithmetic mean steady-state peak concentration (C(SS,min)) and AUC at steady state during a dosing interval (AUC(SS,tau)) have a similar decrease. This extent of decrease in systemic exposure of digoxin at steady state is unlikely to be clinically relevant. Administration of tegaserod (6 mg bid) was well tolerated, both alone and in combination with a single dose of digoxin. There were no pharmacodynamic changes in ventricular rate and QT interval following coadministration of tegaserod with digoxin. The 1.5-hour and 2-hour postdose plasma concentrations of tegaserod on days 3 and 4 confirmed adequate exposure. In conclusion, dose adjustment for digoxin is unlikely to be needed when tegaserod is coadministered.

Tegaserod coadministration does not alter the pharmacokinetics of theophylline in healthy subjects

Tegaserod (HTF 919), a selective 5-HT4 receptor partial agonist, is in development for the treatment of functional gastrointestinal motility disorders. Tegaserod has been found to inhibit cytochrome P-450 (CYP) 1A2, for which theophylline is a prototype substrate. This study was designed to assess the effect of tegaserod on the single-dose pharmacokinetic and safety profile of theophylline. Eighteen subjects were enrolled in a randomized, open-label, two-period crossover study. After an overnight fast, subjects were randomized to receive one of two treatments: (1) a single dose of controlled-release formulation of theophylline (Theo-Dur, 600 mg) on day 1 or (2) a single dose of tegaserod (6 mg) on day 1, concomitant administration of tegaserod (6 mg) and theophylline (600 mg) on the morning of day 2, followed by an additional dose of tegaserod (6 mg) 12 hours later. Four to 10 days later, the subjects received the alternative treatment regimen. The pharmacokinetic parameters of theophylline, including AUC, Cmax, and t(1/2lambda z), were similar for both treatment regimens, although the tmax of theophylline was statistically different between the treatments. Except for a decrease in partial metabolic formation clearance from theophylline to 1-methyluric acid, which is unlikely to be clinically relevant, there were no statistically significant differences in renal clearance of theophylline and partial metabolic formation clearances following the combined treatment compared with theophylline alone. The results of the current study indicate that no dose adjustment is required when drugs metabolized via CYP1A2 are coadministered with tegaserod.

Pharmacokinetics and metabolism of nateglinide in humans

The pharmacokinetics and metabolism of nateglinide were studied in six healthy male subjects receiving a single oral (120 mg) and intravenous (60 mg) dose of [14C]nateglinide in randomized order. Serial blood and complete urine and feces were collected for 120 h post dose. Nateglinide was rapidly (approximately 90%) absorbed, with peak blood and plasma concentrations at approximately 1 h post dose. The maximal plasma concentrations of radioactivity (6360 ngEq/ml) and nateglinide (5690 ng/ml) were comparable, and plasma radioactivity concentrations were about twice those of blood at all times. Oral bioavailability was 72%, indicating only a modest first-pass effect. After either dose, plasma nateglinide concentrations declined rapidly with elimination half-lives of 1.5 to 1.7 h and plasma clearance of 7.4 l/h. Plasma radioactivity was eliminated more slowly with half-lives of 52 and 35 h in plasma and blood, respectively, after the oral dose. The contribution of this more slowly eliminated component to the AUC(0-infinity) was minor. Nateglinide was extensively metabolized, with excretion predominantly (84-87%) in urine. Only approximately 16% of the dose was excreted unchanged in urine after either dosing route. The major metabolites were the result of oxidative modifications of the isopropyl group. Three of these were monohydroxylated, two of which appeared to be diastereoisomers. Additionally, one metabolite with an unsaturation in the isopropyl group and two diol-containing isomers were identified. Glucuronic acid conjugates resulting from direct glucuronidation of the carboxylic acid were also present. The major metabolite in plasma and urine was the result of hydroxylation of the methine carbon of the isopropyl group.

Mealtime glucose regulation with nateglinide in healthy volunteers: comparison with repaglinide and placebo

OBJECTIVE

This study was designed to compare the pharmacodynamic effects of single doses of nateglinide (A-4166), repaglinide, and placebo on mealtime insulin secretion and glycemic control in healthy subjects.

RESEARCH DESIGN AND METHODS

Fifteen healthy volunteers participated in this open-label five-period crossover study. They received single 10-min preprandial doses of 120 mg nateglinide, 0.5 or 2 mg repaglinide, or placebo or 1 min preprandially of 2 mg repaglinide. Subjects received each dose only once, 48 h apart. Pharmacodynamic and pharmacokinetic assessments were performed from 0 to 12 h postdose.

RESULTS

Nateglinide induced insulin secretion more rapidly than 2 and 0.5 mg repaglinide and placebo (10 min preprandial), with mean rates of insulin rise of 2.3, 1.3, 1.15, and 0.8 microU x ml(-1) x min(-1), respectively, over the 0- to 30-min postmeal interval. After peaking, insulin concentrations decreased rapidly in the nateglinide-treated group and were similar to placebo within 2 h postdose. After 2 mg repaglinide, peak insulin concentrations were delayed and returned to baseline more slowly than with nateglinide treatment. Nateglinide treatment produced lower average plasma glucose concentrations in the 0- to 2-h postdose interval than either dose of repaglinide and placebo (P < 0.05 vs. 0.5 mg repaglinide and placebo). Plasma glucose concentrations returned more rapidly to predose levels with nateglinide treatment than with either dose of repaglinide. Treatment with repaglinide produced a sustained hypoglycemic effect up to 6 h postdose.

CONCLUSIONS

In this single-dose study in nondiabetic volunteers, nateglinide provided a more rapid and shorter-lived stimulation of insulin secretion than repaglinide, resulting in lower meal-related glucose excursions. If similar results are observed in diabetes, nateglinide may produce a more physiological insulin secretory response with the potential for a reduced risk of postabsorptive hypoglycemia.

Population PK and PK/PD modelling of microencapsulated octreotide acetate in healthy subjects

AIMS

To develop a population model that can describe the pharmacokinetic profile of microencapsulated octreotide acetate in healthy cholecystectomized subjects. To investigate the correlation between serum IGF-1 and octreotide concentration.

METHODS

A population pharmacokinetic analysis was performed on octreotide data obtained following a single dose of 30 mg microencapsulated octreotide acetate intramuscularly. The relationship between serum IGF-1 concentration and octreotide concentration was effectively described by a population pharmacokinetic/pharmacodynamic model.

RESULTS

The pharmacokinetic profile of octreotide was characterized by an initial peak of octreotide followed by a sustained-release of drug. Plateau concentration were sustained up to day 70, and gradually declined to below the detection limit by day 112. A one-compartment linear model was constructed which consisted of two absorption processes, characterized by KIR and KSR, rate constants for immediate-release and sustained-release, respectively, with first-order elimination (Ke; 1.05 h-1). The surface, unencapsulated drug was immediately absorbed into the central compartment with first-order absorption (KIR; 0.0312 h-1), while the microencapsulated drug was first released in a zero-order fashion into a depot before being absorbed into the central compartment with first-order absorption (KSR; 0.00469 h-1) during a period of tau (1680 h). Body weight and gender were important covariates for the apparent volume of distribution. The type of formulation was an important covariate for KIR but had no effect on KSR. An inhibitory Emax population pharmacokinetic/pharmacodynamic model could adequately describe the relationship between IGF-1 (expressed as percent baseline) and octreotide concentration. Baseline IGF-1 concentration was found to be a significant covariate for the baseline effect (E0). A relationship between GH concentration and octreotide concentration was not established.

CONCLUSIONS

The pharmacokinetic profile of microencapsulated octreotide acetate was effectively described by the derived population model. The relationship between IGF-1 and drug concentration could be used to guide optimization of therapeutic octreotide dosage regimens.

Single-dose pharmacokinetics of nateglinide in subjects with hepatic cirrhosis

This single-dose, open-label, parallel-group study compared the pharmacokinetics and tolerability of 120 mg doses of nateglinide, a physiologic mealtime glucose regulator for type 2 diabetes, in 8 subjects with cirrhosis and 8 matched healthy subjects. In both groups, plasma concentration peaked in a median of 0.5 hours, and mean terminal elimination half-lives were comparable. Mean +/- SD pharmacokinetic parameters in cirrhotic versus healthy subjects were slightly different (Cmax, 7.7 +/- 4.9 vs. 5.6 +/- 1.3 micrograms/ml; AUC(0-t), 18.5 +/- 7.5 vs. 14.2 +/- 2.1 micrograms.h/ml, respectively). Mean apparent total clearance and mean renal clearance in both groups were comparable. Mean protein-bound fractions were equivalent; binding appeared unaltered by metabolites. One cirrhotic and 2 healthy subjects each reported one adverse event. No statistically significant or clinically relevant alteration in pharmacokinetic parameters of nateglinide resulted from hepatic dysfunction, and it was well tolerated; therefore, adjustment of nateglinide dosage is not required in subjects with mild to moderate cirrhosis.

Mealtime glucose regulation by nateglinide in type-2 diabetes mellitus

OBJECTIVES

Pharmacodynamic effects of nateglinide, a novel antidiabetic agent, were investigated in patients with type-2 diabetes mellitus.

METHODS

Ten patients participated in this single-center, double-blind, crossover study. Plasma glucose and insulin levels were measured over 24 h following five 7-day treatment periods with nateglinide (30, 60, or 120 mg) or placebo given three times daily before breakfast, lunch, and dinner. A fifth treatment consisted of 120 mg nateglinide four times daily, with the fourth dose given before an evening snack.

RESULTS

Taken 10 min before meals, doses of 30-120 mg nateglinide caused dose-dependent increases in plasma insulin levels that were significantly greater than with placebo. Higher doses were more effective and had a longer duration of action than lower doses. Nateglinide was also significantly better than placebo in lowering plasma glucose levels; the 60-mg and 120-mg doses were similarly effective and superior to the 30-mg nateglinide treatment. Following the fourth 120-mg dose, the glucose-lowering effects of treatment were maintained through the night. No serious adverse events occurred during the study. There were no events of hypoglycemia and no clinically meaningful changes in safety parameters.

CONCLUSIONS

Nateglinide produced rapid, short-lived, dose-related increases in plasma insulin that significantly lowered mealtime glucose excursions compared with placebo with no incidence of hypoglycemia. The decrease in mealtime glucose levels produced a significant improvement in overall 24-h glycemia.

Pharmacokinetics, pharmacodynamics, and safety of microencapsulated octreotide acetate in healthy subjects

The pharmacokinetics, pharmacodynamics, and safety of the marketed formulation of microencapsulated octreotide acetate were evaluated in an open-label study in 22 healthy cholecystectomized subjects. Each subject received a single 30 mg dose of microencapsulated octreotide acetate intramuscularly (i.m.). Concentrations of octreotide, growth hormone (GH), insulin-like growth factor binding protein 3 (IGFBP-3), and insulin-like growth factor 1 (IGF-1) as well as clinical safety were evaluated over a period of 112 days (16 weeks). After the injection, mean serum octreotide concentration initially increased rapidly, reached the maximum (Cmax, day 1 = 0.96 +/- 0.25 ng/ml) approximately 1.5 hours after dosing, and declined thereafter until 24 hours postdose (Cmin, 24 h = 0.088 +/- 0.093 ng/ml). The octreotide concentration then increased and started a sustained release from day 7 onward. Plateau concentrations were maintained through day 70 and gradually declined to below the lower limit of quantification (LLOQ) by day 112. The plateau height (Cplateau (2-112d, 60%)) was 1.68 +/- 0.88 ng/ml, and the duration (delta plateau, 60%) was 30.2 +/- 15.7 days. The integrated concentration-time curve, AUC0-112d, was 2819 +/- 782 (ng.h/ml), and the apparent half-life (t1/2) was 169 hours. To assess the variability, the drug concentrations were determined hourly for 8 hours on day 28, and the mean octreotide concentration, Cavg, day 28' was 1.55 +/- 1.26 ng/ml. The suppression of IGF-1 was statistically significant compared to the baseline (p < 0.05) through day 63; however, there were no appreciable changes in GH and IGFBP-3 concentrations after a single injection of microencapsulated octreotide acetate. Simulation of a 28-day dose schedule suggested that steady-state octreotide concentrations would be reached by the third injection with steady-state concentrations about twofold greater than the first injection. There were no serious adverse events or clinically meaningful changes in vital signs, ECGs, or laboratory evaluations observed in this study, indicating that the 30 mg i.m. dose of microencapsulated octreotide acetate was well tolerated in this population.

Improved control of mealtime glucose excursions with coadministration of nateglinide and metformin

OBJECTIVE

Nateglinide, a new short-acting D-phenylalanine derivative for treating type 2 diabetes, reduces mealtime blood glucose excursions by physiologic regulation of insulin secretion. This study evaluated the pharmacokinetic and pharmacodynamic interactions of nateglinide and metformin in subjects with type 2 diabetes.

RESEARCH DESIGN AND METHODS

A total of 12 type 2 diabetic subjects with the following baseline characteristics were enrolled: age, 56 +/- 13 years; BMI, 28.7 +/- 4.5 kg/m2; HbA1c, 8.4 +/- 1.3%; and fasting plasma glucose 13 +/- 2.8 mmol/l. All subjects had been previously treated with glyburide and were switched to metformin monotherapy for 3 weeks before study start. Subjects then randomly received, in combination with 500 mg metformin, either 120 mg nateglinide or placebo before meals for 1 day, followed by the alternate treatment 7 days later. After 1 week of washout from both drugs, subjects received 1 day of open-label nateglinide treatment. Plasma concentrations of glucose, insulin, nateglinide, and metformin were assessed frequently during inpatient periods.

RESULTS

Postmeal plasma glucose levels were significantly lower in subjects treated with nateglinide plus metformin than in those treated with either drug alone (P < 0.001), especially after lunch and dinner. Coadministration of nateglinide and metformin did not affect the pharmacokinetics of either drug. All treatments were safe and well tolerated.

CONCLUSIONS

Combination therapy with nateglinide and metformin was more effective than either treatment alone and did not result in any pharmacokinetic interactions. Coadministration of nateglinide and metformin appears to be an excellent option for treating patients with type 2 diabetes not controlled with monotherapy.

Synergistic effects of nateglinide and meal administration on insulin secretion in patients with type 2 diabetes mellitus

This study assessed the synergistic effects of nateglinide (a non-sulfonylurea D-phenylalanine derivative) and meals on insulin secretion in 24 patients with type 2 diabetes. Oral doses of 60 and 180 mg or 120 and 240 mg were administered to two cohorts of subjects 10 min before meals (or fasting) three times daily for 7 days, with washout intervals between treatment periods. Dose-dependent increases in plasma insulin occurred, with the peak effect within 2 h after treatment. Significantly greater insulin secretion was observed when nateglinide was taken before a meal compared to nateglinide given in the fasted state or in response to just the meal. Nateglinide lowered plasma glucose concentrations significantly vs. placebo at all doses, and doses of 120 and 240 mg were more effective than 60 mg (P < 0.05). Adverse event rates were similar for nateglinide and placebo, and no hypoglycemic episodes or serious adverse events were reported during the study. Nateglinide (120 mg) was the maximum effective dose in this study and was shown to be a safe and well tolerated therapy for control of mealtime glucose excursions in patients with type 2 diabetes. Results indicate that a synergistic interaction occurs between nateglinide and elevated mealtime plasma glucose concentrations to stimulate insulin secretion.

Effect of meal timing not critical for the pharmacokinetics of tegaserod (HTF 919)

This study assessed the pharmacokinetic profiles of administering tegaserod (HTF 919) at different time intervals with respect to a meal. It was a randomized, open-label, two-phase, five-period crossover study. In the first phase, 18 healthy subjects received a single 12 mg oral dose of tegaserod administered either 30 or 15 minutes prior to the start of the 600-calorie, fat-rich breakfast. In the second phase, subjects received a single 12 mg oral dose of tegaserod 1 minute before, 2.5 hours after the start of meal, or with a continued 4-hour postdose fast. Safety assessment and plasma samples for the determination of drug concentration were obtained for 24 hours postdose. Noncompartmental analysis results indicated that the AUC of tegaserod was reduced by almost half under fed conditions compared to the fasted condition. Exploratory analyses were implemented to further investigate the absorption characteristics of tegaserod under different fed conditions. A numerical deconvolution approach was used to obtain the tegaserod oral absorption versus time profiles under both fasted and fed conditions. The tegaserod oral absorption versus time profiles were then fitted by NONMEM to a model containing two absorption phases. Based on the absorption analyses, we found that the reduction in the bioavailability of tegaserod under fed conditions was primarily due to a decrease in the extent of absorption and less so to a decrease in the absorption rate(s). Therefore, although the timing of administration of food does not appear to significantly alter the pharmacokinetics of tegaserod, the administration of food reduces the AUC by approximately 50%.

The effect of food on the oral bioavailability and the pharmacodynamic actions of the insulinotropic agent nateglinide in healthy subjects

Nateglinide (Starlix, SDZ DJN 608 or A-4166), a new insulinotropic agent, is intended to be administered prior to a meal in order to improve early insulin release in non-insulin-dependent diabetes mellitus patients. The effects of a meal on the oral bioavailability and pharmacodynamic actions of nateglinide were investigated. Twelve healthy male subjects completed this randomized, single-dose, four-way crossover study in which each subject received a 60 mg dose of nateglinide 10 minutes before the start of and immediately after a high-fat breakfast meal. In addition, each subject received a single 30 and 60 mg dose of nateglinide underfasting conditions. Plasma and urine concentrations of nateglinide were determined by an HPLC method while plasma glucose and insulin concentrations were measured by standard immunoassay methods. Compared to the fasted state, administration of nateglinide 10 minutes before the meal was associated with an increase in the rate of absorption (12% increase in Cmax and 52% decrease in tmax), while there was no significant effect on the extent of absorption (AUC). Alternatively, when nateglinide was given after the meal, a food effect was observed that was characterized by a decrease in the rate of absorption: 34% decrease in Cmax and a 22% increase in tmax but no significant effect on AUC. Nateglinide was rapidly eliminated with plasma t 1/2 = 1.4 hours. Its plasma renal clearance, 20.7 ml/min, appears to be due mostly to active tubular secretion. However, only 13% to 14% of the dose is recovered as nateglinide in the urine. The 30 and 60 mg tablets were dose proportional in terms of both AUC and Cmax; both tmax and t 1/2 were dose independent. Regardless of timing, the combination of a meal and nateglinide produced a larger increase in insulin levels than did nateglinide alone. Meal-related glucose excursions were eliminated when nateglinide was taken prior to the meal. Thus, the rapid onset/short duration stimulation of insulin release by nateglinide should allow good control of prandial hyperglycemia while limiting exposure to hyperinsulinemia.

Distribution of growth hormone receptor messenger ribonucleic acid containing and lacking exon 3 in human tissues

The extent of expression of the GH receptor (GHR) in human tissues is largely unknown. In some cell lines and placenta, the GHR gene generates two different mRNAs by alternative splicing of exon 3, one coding for a full-length receptor (GHR + 3) and the other for a receptor isoform that lacks exon 3 (GHR-3), with deletion of amino acid residues 7-28. To determine the distribution of the GHR and the relative abundance of its two isoforms in man, we studied a variety of tissues obtained at autopsy by reverse transcription and polymerase chain reaction (PCR) amplification, using isoform-specific primers. The nature of the PCR products was verified by restriction analysis and DNA sequencing. The relative proportions of the two GHR isoforms were determined by competitive PCR using a 32P-labeled anti-sense primer and a mixture of both isoform-specific sense primers in equimolar amounts. Electrophoretic bands corresponding to the amplification products were excised and counted, or quantitated by laser densitometry. Restriction analysis and sequencing of the amplified products were consistent with their predicted sequence. Both GHR transcripts were found in all 19 tissues tested, but their relative proportions varied depending on the tissue and, to a lesser extent, between subjects. They ranged from a preponderance of GHR-3 (kidney, bladder, adrenal, and brain stem) to a predominance of GHR + 3 (skeletal muscle and liver). We conclude that the GHR gene is widely expressed in human tissues. Both GHR + 3 and GHR-3 transcripts are present, but their relative proportions depend on the tissue and, possibly, the metabolic status. The physiological significance of the existence of two human GHR forms remains to be elucidated.

Familial neurohypophyseal diabetes insipidus associated with a signal peptide mutation

We studied the pathophysiology, natural history, and genetic basis of familial neurohypophyseal diabetes insipidus (FNDI) in a caucasian kindred. Twelve members had polyuria and a deficiency of plasma vasopressin (AVP), which progressed in severity over time. Another had normal urine volumes and plasma AVP when first tested at age 3 yr, but developed severe FNDI a year later. For unknown reasons, one man had a normal urine volume despite severe AVP deficiency and a history of polyuria in the past. When the AVP-neurophysin-II gene was amplified and sequenced, exon 2/3 was normal, but 7 of 12 clones of exon 1 contained a base substitution (G-->A) predicting a substitution of threonine for alanine at the -1 position of the signal peptide. Restriction analysis found the mutation in all 14 affected members, but in none of the 41 controls or 19 adult members with normal urine volumes and plasma or urinary AVP (lod score = 5.7). The mutation was also found in 2 infants in whom AVP was normal when tested at 6 and 9 months of age. We hypothesize that a mutation in exon 1 of the AVP-neurophysin-II gene causes FNDI in this kindred by making an abnormally processed precursor that gradually destroys vasopressinergic neurons.

Vitamin D binding protein: genomic structure, functional domains, and mRNA expression in tissues

The vitamin D binding protein (DBP), alternatively known as Gc-globulin, is a member of the albumin (ALB) and alpha-fetoprotein (AFP) gene family. The rat DBP gene is expressed at high levels in liver and at moderate levels in kidney, testis, abdominal fat, and yolk sac. Very low levels of DBP as well as ALB and AFP transcripts can be detected in all other tissues studied by the reverse transcriptase/polymerase chain reaction technique. During development, liver DBP gene transcripts are detectable at 14 days of gestation and levels rise gradually until adulthood in parallel with ALB. DBP present on the surface of U937 monocyte-derived cells is acquired from serum, suggesting cell surface binding sites for DBP. The rat DBP gene has been cloned and characterized. It spans 35 kb and contains 13 exons and 12 introns. The DBP gene contains two fewer exons than the ALB or AFP genes, accounting for the shortest size of its mRNA and protein product. Its 5'-flanking region contains a high degree of structural similarity to both ALB and AFP promoters.

The vitamin D-binding protein, alpha-fetoprotein, albumin multigene family: detection of transcripts in multiple tissues

The serum vitamin D-binding protein (DBP), a member of a multigene family that includes alpha-fetoprotein (alpha FP) and albumin (ALB), is expressed at high levels in the adult liver. Although several studies have demonstrated the presence of DBP on the cell surface of immunocytes and cytotrophoblasts, evidence of extra-hepatic DBP synthesis remains inconclusive. Using Northern blot analysis of RNA from multiple rat tissues, the DBP transcripts were present in the expected high levels in the adult liver. In addition, DBP mRNA was found in the adult kidney, testis, abdominal fat, and 18-day fetal yolk sac. The concentrations of DBP mRNA in these tissues is 100-1000-fold less than in the liver. Polymerase chain reaction amplification (PCR) of DBP, alpha FP, and ALB cDNAs, reverse transcribed from a wide variety of rat tissue RNA samples, was performed to examine their respective tissue-specific patterns of gene transcripts. The tissues in which DBP gene transcripts were noted by Northern analysis were paralleled at 20 cycles of PCR with liver much greater than kidney, yolk sac, testis, and abdominal fat. Similarly at 20 cycles, alpha FP-amplified fragments were detected in the yolk sac much greater than brain, testes, uterus, liver, and placenta; and ALB-amplified fragments in liver much greater than testis, uterus, placenta, and yolk sac. Unexpectedly, at 32 cycles of PCR, DBP, alpha FP, and ALB mRNAs could be detected at very low levels in all tissues examined. To examine the nature of the DBP mRNA from these nonhepatic sources, a Sprague-Dawley kidney cDNA library was screened, and a single cDNA recombinant was identified. This clone contained a full length DBP cDNA with 14 nucleotide and 3 predicted amino acid differences from the Fischer liver DBP cDNA reported previously. We conclude that the kidney and liver DBP mRNAs are transcription products of the same DBP locus and that DBP, alpha FP, and ALB genes display different spectra of tissue distribution. The unexpected finding that all three spliced RNA transcripts could be detected in at least trace quantities in all tissues studied suggests a leaky level of transcription by this multigene family or their transcription at low levels by an unidentified stromal cell common to all organs studied.

Vitamin D binding protein sequesters monomeric actin in the circulation of the rat

Plasma vitamin D binding protein (DBP) may scavenge actin released during cell lysis. We examined the plasma disappearance and tissue appearance of 125I-DBP, 125I-G-actin, and the DBP-G-actin complex after their intravenous administration to rats. The plasma disappearance of DBP and DBP-actin were indistinguishable, with rapid initial (t1/2 = 2.6 h) and slower second (t1/2 = 7 h) slopes. After 125I-G-actin (nanomole) injection, plasma disappearance paralleled that of DBP and DBP-actin. All injected actin was associated with DBP, without evidence of free actin, actin-gelsolin complexes or actin oligomers. Tissue appearances of 125I-apo-DBP (apo) or holo-DBP were similar, with highest accumulations in perfused liver, kidney, and skeletal muscle. Although more complex phenomena (plasma entry of F-actin and intracellular actin binding proteins) would occur in vivo after cell lysis, our results suggest a role for DBP in the sequestration and disposition of actin monomers in the circulation.

Detection of vitamin D binding protein on the surface of cytotrophoblasts isolated from human placentae

Vitamin D binding protein (DBP), a Mr 56,000-58,000 alpha 2-glycoprotein, is the major serum protein involved in the transport of vitamin D sterols. Recently it has been suggested that DBP may also be involved in immunoglobulin G binding to cells. Because the trophoblast is involved in the transport of molecules such as vitamin D and immunoglobulin G to the fetus, we asked whether DBP could be detected on the surface of human placental trophoblast cells. Cytotrophoblasts purified from human term placentae were fixed and made permeant with Triton X-100 and examined by indirect immunofluorescence after incubation with a monoclonal antibody to DBP. Greater than 90% of these cells stained positively, whereas no staining was observed with nonimmune antiserum. The presence of DBP on/in the surface of cytotrophoblasts could also be demonstrated by fluorescent cytometry. When cell surface-associated proteins of cytotrophoblasts were radioiodinated, a Mr 57,000 radiolabeled protein could be immunoisolated from the cell lysate with a purified monospecific polyclonal antibody to DBP. Immunoisolation of this radiolabeled protein was prevented by the addition of excess unlabeled human DBP to the cell lysate before incubation with antibody. This Mr 57,000 radiolabeled protein could also be isolated by affinity chromatography selecting for proteins that bind to globular actin. When cytotrophoblasts were incubated with [35S]methionine for 3 or 18 h, active synthesis of DBP could not be demonstrated by immunoisolation techniques. These studies demonstrate the presence of DBP on the surface of well washed, human cytotrophoblasts. This DBP may be maternally derived, since active synthesis of DBP could not be demonstrated.

Characterization of a monoclonal antibody to human serum vitamin D binding protein (Gc globulin): recognition of an epitope hidden in membranes of circulating monocytes

We have developed a murine hybridoma cell line that secretes a monoclonal antibody directed to the serum human vitamin D binding protein (hDBP), a 58,000-dalton alpha-globulin with a high avidity for 25-hydroxycholecalciferol and globular actin. This immunoglobulin G1 kappa-light chain antibody was produced by the fusion of the spleen cells from BALB/c mice, immunized with purified hDBP, with SP2/0-AG4 myeloma cells. The antibody was easily removed from the supernatant of hybridoma cultures or mouse ascites fluid by Protein A affinity chromatography. Apparent serum monospecificity was demonstrated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and isoelectric focusing gels transblotted to nylon membranes and overlayed with purified MAK 89 antibody and radioiodinated Protein A. The affinity of the antibody is high [dissociation constant (Kd) = 2.6 X 10(-11) M]. Parallel displacement of tracer by hDBP and human serum was observed. The sera from various species displaced the hDBP tracer in the following potency: monkey more than cat more than dog more than guinea pig. RIAs for DBP from several species are feasible with this antibody. This antibody does not, in contrast to polyclonal anti-hDBP antiserum, bind to viable monocytes. However, the MAK 89 antibody does bind to the membranes of well washed, fixed, and permeant circulating monocytes. Surface membrane radioiodination of monocytes and immunoprecipitation of the detergent lysates with the antibody demonstrates a protein with molecular weight equivalent to hDBP. The epitope recognized, therefore, appears to be hidden in the viable cells, suggesting an intimate and intricate association of the hDBP and monocyte plasma membrane.

Comparison of inhibition of bone resorption and escape with calcitonin and dibutyryl 3',5' cyclic adenosine monophosphate

Both parathyroid hormone (PTH) and calcitonin (CT) can increase the concentration of cyclic 3',5' adenosine monophosphate (cAMP) in fetal rat bone in organ culture. Moreover, dibutyryl cAMP (dbcAMP) can both stimulate and inhibit 45Ca release from such bones depending on dose and experimental conditions. In this study we compared dbcAMP and CT for their effects on bones pretreated with PTH. Both compounds produced transient inhibition of bone resorption followed by escape. Escape from dbcAMP was independent of prostaglandin synthesis, since it occurred both in the presence and absence of indomethacin, a prostaglandin cyclo-oxygenase inhibitor.