Indirect-response modeling of desmopressin at different levels of hydration

The objective of the present study was to investigate the pharmacokinetics (PK) and pharmacodynamics (PD) of desmopressin in healthy male subjects at different levels of overhydration. Also, we examined if an indirect-response model could be related to renal physiology and the pharmacological action of desmopressin. Eight healthy male subjects participated in this open, randomized crossover study with three periods. Each subject was orally water loaded (0 to 20 ml.kg-1 body weight) on 3 study days in order to achieve three different levels of hydration. After the initial water load, urine was voided every 15 min and the volumes were measured. To ensure continuous overhydration the subjects replaced their fluid loss with drinking-water. When a steady-state diuresis was achieved after approximately 2 hr, 0.396 microgram of desmopressin was administered intravenously as a bolus injection. Blood was sampled and urine was collected at intervals throughout the study day (10 hr). An indirect-response model, where desmopressin was assumed to inhibit the elimination of response, was fit to the urine osmolarity data. There were no statistically significant effects of different levels of hydration, as expressed by urine flow rate at baseline, on the estimates of the PK and PD model parameters. The calculated terminal half-lives of elimination (t1/2 beta) ranged between 2.76 and 8.37 hr with an overall mean of 4.36 hr. The overall means of plasma clearance and the volumes of distribution of the central compartment (Vc) and at steady state (Vss) were estimated to be 1.34 (SD 0.35) ml.min-1.kg-1, 151 (SD28) ml.kg-1, and 386 (SD 63) ml.kg-1, respectively. High urine flow rate, indicating overhydration, produced a diluted urine and thus a low osmolarity at baseline (R0). The effect of the urine flow rate on the urine osmolarity at baseline was highly significant (p < 0.0001). The mean values for IC50 and the sigmodicity factor (gamma) were 3.7 (SD 1.2) pg ml-1 and 13.0 (SD 3.5), respectively. In most cases when there was a high urine flow rate at baseline, the model and the estimated PD parameters could be related to the pharmacological action of desmopressin and renal physiology. Thus, the indirect-response model used in this study offers a mechanistic approach of modeling the effect of desmopressin in overhydrated subjects.