Isradipine is a calcium channel-blocking agent of the dihydropyridine type, used in the treatment of hypertension. A terminal half-life of 8-9 hr has been reported, in several pharmacokinetic studies after oral administration of isradipine. In a yet unpublished study a much shorter half-life was observed, and the present trial was therefore conducted in order to estimate the half-life after intravenous administration of isradipine. The bioavailability was estimated as well. In a randomised cross-over design ten healthy young volunteers were given either isradipine orally or an intravenous infusion. The two study periods were separated by at least 3 days. Blood samples for measurement of isradipine concentration were collected for 10-12 hr after administration and half-life and bioavailability were estimated. Mean terminal half-life after intravenous administration was calculated to be 2.8 hr, and the bioavailability to be 0.28. None of the 10 subjects suffered from side effects. In the present intravenous study the half-life of isradipine seems to be of much shorter than demonstrated in previous oral studies.Isradipine, a dihydropyridine calcium channel-blocking agent, elicits potent vasodilation of the systemic vascular bed without causing any significant cardiodepression (van Wijk 1989). Clinical experience has shown that isradipine administered orally in doses from 1.25 to 2.5 mg b.i.d. as tablets or 5.0 mg once daily in a sustained release formulation has an effect on blood pressure equal to that of other antihypertensive drugs (Fitton & Benfield 1990;Christensen et al. 1991).The terminal half-life of isradipine after oral studies is reported to be about 8.8 hr (Fitton & Benfield 1990). In yet unpublished studies we have observed a significantly shorter half-life (2.5 hr) following intravenous administration of isradipine to patients with pregnancy-induced hypertension. To elucidate if this apparent discrepancy was caused by methodological differences (oral versus intravenous administration) or due to physiological differences induced by pregnancy we conducted this trial to evaluate isradipine kinetics after intravenous infusion and oral administration to healthy volunteers.
Materials and MethodsTen healthy normotensive volunteers, (3 women, 7 men) mean age 24.2 years, and mean weight 71.9 kg were included in the study (table 1). All the subjects had serum concentration values of haemoglobin, potassium, sodium, creatinine and liver enzymes within normal range. below 110/60 mmHg, if blood had been donated within the last 2 months, or if drugs of any kind were used. All volunteers gave written informed consent, and the study was approved by the Local Ethics Committee, the Danish Health authorities and was performed in agreement with the Helsinki declaration.Design. In a randomised cross-over fashion the volunteers were given isradipine as an infusion as well as orally. Half of them started with an infusion of isradipine followed by oral treatment, while the other half were given isradipine orally fol...