Combination of Nifedipine and Doxazosin in Essential Hypertension

Pharmacodynamic and pharmacokinetic interactions have been reported when an alpha 1-antagonist is combined with a calcium antagonist. We evaluated the clinical usefulness of the combination of nifedipine (20 mg twice daily, b.i.d.) and doxazosin (2 mg once daily, o.d.) in hypertensive patients in whom blood pressure (BP) control was suboptimal after doxazosin (group A) or nifedipine (group B) as monotherapy and investigated the underlying kinetic and dynamic interactions, including changes in vascular responsiveness to i.v. infusions of angiotensin II (ANGII) and phenylephrine (PE). The combination was well tolerated and associated with further significant reductions in BP. After 4 weeks of combined therapy, average supine BP over 8 h was 122/77 in group A and 137/80 in group B as compared with 140/86 and 150/88 mm Hg, respectively, during monotherapy + placebo. The combination attenuated both phenylephrine and ANG-induced pressor responses: e.g., the mean PD15 values (dose of agonist required to increase systolic BP by 15 mm Hg) for group A at 1.5-3 h were 3.5 micrograms/kg/min for PE and 7.5 ng/kg/min for ANGII as compared with 2.9 and 2.3, respectively, during treatment with doxazosin and placebo. There was no evidence of a significant kinetic interaction between the two drugs and, in particular, addition of nifedipine had no effect on the steady-state kinetics of doxazosin. In conclusion, doxazosin and nifedipine are an effective antihypertensive combination in patients who require treatment with more than one drug.