Red blood cell sorbitol lowering effects and tolerance of single doses of AL 1576 (HOE 843) in diabetic patients

Pharmacokinetic-pharmacodynamic modelling of DP-1904, a novel thromboxane synthetase inhibitor in rabbits, based on an indirect response model

A new imidazole derivative, DP-1904, produces a selective, potent and long-acting inhibition of thromboxane A2 (TXA2) syntheses and platelet aggregation. This study was designed to investigate the pharmacokinetics and pharmacodynamics (PK/PD) of DP-1904. DP-1904 disappeared from plasma with a half-life of 20 min after i.v. dosing, and the bioavailability after oral dosing was approximately 70%. The level of serum TXB2, which is a pharmacological marker for thromboxane synthetase inhibition, was measured to characterize the pharmacodynamics of DP-1904. A marked reduction of serum TXB2 was exhibited within 1 h after both i.v. and oral doses, reflecting the rapid onset of action of DP-1904. Serum TXB2 returned to the basal level much more slowly after oral dosing than after i.v. dosing, due to the longer half-life after oral dosing. An Emax model was employed to fit the pharmacological data after oral dosing, and IC50 and Emax values were estimated to be 5.0 ng/ml and 81%, respectively. In order to test its predictability, the PK/PD model was then used to predict a pharmacological profile after i.v. dosing; good agreement between the observed and predicted values was achieved. Thus, the present modelling procedure may be useful for optimizing the therapeutic regimens of DP-1904.

Tolrestat pharmacokinetic and pharmacodynamic effects on red blood cell sorbitol levels in normal volunteers and in patients with insulin-dependent diabetes

OBJECTIVES

To examine the effect of diabetes mellitus on the pharmacokinetics of tolrestat and to investigate its effect on red blood cell sorbitol levels according to a new pharmacodynamic model for this class of drugs.

METHODS

Single and multiple doses of tolrestat (200 mg/twice a day) were administered to 12 patients with insulin-dependent (type I) diabetes and 12 healthy volunteers in an open parallel trial.

RESULTS

Tolrestat disposition was characterized by first-order absorption and biexponential disposition: In normal subjects the terminal disposition half-life (t1/2) was 13 +/- 3 hours (mean +/- SD) and the apparent oral clearance (CL/F) was 48 +/- 9 ml/hr/kg, similar to the values in patients with type I diabetes mellitus (t1/2 = 14 +/- 4 hours; CL/F = 55 +/- 10 ml/hr/kg). Red blood cell sorbitol concentrations, which declined because of tolrestat's inhibition of aldose reductase, were characterized by an indirect-response model including a 50% inhibition constant (IC50) for production of sorbitol by aldose reductase. The removal of sorbitol (kout) was slower in patients with diabetes. The plasma IC50 averaged 2.0 +/- 1.3 micrograms/ml in normal subjects and 2.5 +/- 1.9 micrograms/ml in patients with diabetes. IC50 values expressed in free (unbound) concentrations (fu = 0.64%), which ranged from 12 to 16 ng/ml, were similar to the in vitro IC50 for inhibition of sorbitol accumulation in human red blood cells.

CONCLUSIONS

Tolrestat pharmacokinetics were similar in normal subjects and in patients with diabetes; however, the patients with diabetes had higher baseline sorbitol levels (11 versus 5 nmol/ml for normal subjects) and slower sorbitol efflux rates. The proposed biochemically realistic, dynamic model characterized well the red blood cell sorbitol response patterns after administration of single and multiple doses of tolrestat.

Comparison of four basic models of indirect pharmacodynamic responses

Four basic models for characterizing indirect pharmacodynamic responses after drug administration have been developed and compared. The models are based on drug effects (inhibition or stimulation) on the factors controlling either the input or the dissipation of drug response. Pharmacokinetic parameters of methylprednisolone were used to generate plasma concentration and response-time profiles using computer simulations. It was found that the responses produced showed a slow onset and a slow return to baseline. The time of maximal response was dependent on the model and dose. In each case, hysteresis plots showed that drug concentrations preceded the response. When the responses were fitted with pharmacodynamic models based on distribution to a hypothetical effect compartment, the resulting parameters were dose-dependent and inferred biological implausibility. Indirect response models must be treated as distinct from conventional pharmacodynamic models which assume direct action of drugs. The assumptions, equations, and data patterns for the four basic indirect response models provide a starting point for evaluation of pharmacologic effects where the site of action precedes or follows the measured response variable.

Recent clinical experience with aldose reductase inhibitors

Since 1981 a number of aldose reductase inhibitors (ARIs) have been extensively investigated in clinical trials for the treatment or prevention of diabetic complications. In general, the results from these trials have varied from no effect to improvement. In part, the inconclusive results are due to differences in the study designs. Investigation of some ARIs has been discontinued because of lack of efficacy (ponalrestat) or major toxicity (sorbinil). Of the ARIs that have been evaluated, only tolrestat is currently undergoing clinical investigation. Although the most recent studies have shown promising results, the role of ARIs in the treatment or prevention of secondary complications of diabetic neuropathy will be determined in ongoing or future clinical trials.

Capillary gas chromatographic-electron-capture assay for the aldose reductase inhibitor imirestat in lens and plasma

A sensitive and selective gas chromatographic-electron-capture assay was developed for the determination of the aldose reductase inhibitor imirestat in lens and plasma. The method involves solid-phase extraction of drug and internal standard from the plasma specimen or lens sample homogenate using "Baker"-10 SPE extraction columns followed by derivatization with pentafluorobenzyl bromide and further purification. Derivatives of drug and internal standard were separated on a fused-silica capillary column and analyzed using a 63Ni electron-capture detector. The limit of detection was 2.5 ng per lens or ml of plasma. The method was used to evaluate the pharmacokinetics of imirestat in human subjects and to quantitate imirestat in animal lens tissue following topical ocular administration.