Diltiazem pharmacokinetics in the rat and relationship between its serum concentration and uterine and cardiovascular effects

Abstract

1 The kinetics of diltiazem were investigated in ovariectomized (ovx) non-pregnant and intact late pregnant anaesthetized rats following a bolus i.v. injection (2 mg kg⁻¹) and during a 180 min i.v. infusion (50 μg kg⁻¹ min⁻¹ and 100 μg kg⁻¹ min⁻¹). Uterine contractions, mean blood pressure and heart rate were measured in the non-pregnant rats. 2 Measurement of serum diltiazem concentrations after bolus i.v. injection in ovx non-pregnant rats showed a biexponential decay with time from which the following parameters were calculated: volume of distribution area (V_(area)) − 256 ±46 ml; rate constants k₁₂ − 0.46 ± 0.10 min⁻¹; k₂₁ − 0.09 ± 0.01 min⁻¹; k_el − 0.13 ± 0.03 min⁻¹; elimination clearance − 3.2 ± 0.3 ml min⁻¹; distribution t_1/2 (t_1/2α) − 1.4 ± 0.3 min; elimination t_1/2 (t_1/2β) − 61.2 ± 13.0 min. In pregnant rats, a biexponential decay was also observed with similar parameters to those in non-pregnant animals except for markedly increased V_(area) − 1004 ± 184 ml; k_el − 0.54 ± 0.16 min⁻¹ and elimination clearance − 14.8 ± 2.3 ml min⁻¹. 3 Measurement of serum diltiazem concentrations during infusion yielded the following parameters in non-pregnant ovx rats: V_(ss) − 79 ± 10 ml; rate constants k₁₂ − 1.02 ± 0.21 min⁻¹; k₂₁ − 0.03 ± 0.01 min⁻¹; k_el-0.39 ± 0.06 min⁻¹; elimination clearance-7.8 ± 1.2 ml min⁻¹. In pregnant rats a marked increase was observed in k_el − 1.25 ± 0.38 min⁻¹ and elimination clearance − 36.4 ± 13.8 ml min⁻¹. 4 An immediate reduction in uterine contractions, mean blood pressure and heart rate was observed after bolus i.v. injection of diltiazem with a return towards control values as serum diltiazem concentrations declined. There were significant correlations between the inhibition of the 3 parameters and the log serum concentrations of diltiazem. Serum concentration-response curves indicated IC₅₀ values of 0.5 μg ml⁻¹ for inhibition of uterine contractions, 0.7 μg ml⁻¹ for reduction in blood pressure and 1.2 μg ml⁻¹ for reduction in heart rate. There were maintained reductions in the integral of uterine contractions, mean blood pressure and heart rate during infusion. 5 The metabolite desacetyldiltiazem was rarely detected after i.v. bolus injection and was not found in 5/13 rats infused with diltiazem, yet significant inhibition of uterine contractions was observed in all rats. Diltiazem was 3.2 fold more potent than desacetyldiltiazem as an inhibitor of contractions of the rat isolated uterus. 6 These findings indicate that the inhibition of uterine contractions is due to a direct action of diltiazem, and not the metabolite desacetyldiltiazem, and suggest only a slight selectivity for uterine inhibition compared to cardiovascular effects.