Variable metabolism of pinacidil: lack of correlation with the debrisoquine and trimethylamine C- and N-oxidative polymorphisms

Clinical pharmacology of potassium channel openers

Opening of K+ channels in cell membranes with resulting increase in K+ conductance, shifts the membrane potential in a hyperpolarizing direction towards the K+ equilibrium potential. Hyperpolarization reduces the opening probability of ion channels involved in membrane depolarization and excitation is reduced. K+ channel openers are believed to hyperpolarize smooth muscle cells by a direct action on the cell membrane. The best known members of the group are cromakalim, nicorandil and pinacidil, but several new compounds are being evaluated. In addition, it has recently been shown that also clinically well-known drugs like, e.g. diazoxide and minoxidil exhibit K+ channel opening properties. Nicorandil and new compounds containing nitro groups have a dual mechanism of action, also activating guanylate cyclase, an effect that contributes to their cardiovascular effect profile. K+ channel openers have a wide range of effects. Some of their properties and actions are summarized, and their present applications and/or potential for future application, in e.g. hypertension, angina pectoris, asthma, bladder instability, and several other disorders are discussed. It is concluded that K+ channel openning represents an interesting pharmacological principle with many potential clinical applications. However, most available drugs do not seem to have a sufficient tissue selectivity to be useful therapeutic alternatives. Before the potential of the new members of the group on clinical trials can be properly evaluated, clinical experiences are needed.

Metabolism of verapamil in a family pedigree with deficient N-oxidation of trimethylamine

The oxidative N-dealkylation of verapamil has been studied in a family of five members with two propositi with an inherited deficiency of trimethylamine N-oxidation (fish-odour syndrome). The results were assessed for possible co-segregation of the trimethylamine N-oxidation phenotype and any observed deficiency in oxidative N-dealkylation. The general pattern of metabolism of verapamil in the five subjects studied was similar to that reported in earlier investigations. Moreover, there were no differences between the two affected subjects and other family members with respect to the metabolic pattern. It is concluded that there is no functional segregation with respect to the mechanisms controlling trimethylamine N-oxidation and verapamil N-dealkylation.

Genetic polymorphism of sparteine/debrisoquine oxidation: a reappraisal

Polymorphic oxidation of the sparteine/debrisoquine-type has been shown to account for much of the interindividual variation in the metabolism, pharmacokinetics and pharmacodynamics of an increasing number of drugs, including some antiarrhythmic, antidepressant and beta-adrenoceptor antagonist agents. Impaired hydroxylation of these drugs results from the absence of the enzyme cytochrome P450IID6 in the livers of poor metabolisers, who constitute 6% to 10% of Caucasian populations. The clinical importance of the phenomenon has to be explored further and for most sparteine/debrisoquine-related substrates there is a need for controlled prospective studies to define the consequences to the patient of impaired or enhanced drug oxidation.

Pinacidil. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic potential in the treatment of hypertension

Pinacidil is an orally administered antihypertensive drug that acts via direct relaxation of vascular smooth muscle to produce peripheral vasodilatation and a reduction in blood pressure without significant direct effects on cardiac electrophysiology. Pinacidil is unrelated to other antihypertensive drugs in clinical use, either in structure or mechanism of action. It belongs to a new class of agents called 'potassium channel openers' which act via potassium efflux to hyperpolarize cell membranes, indirectly causing a net reduction in intracellular calcium that leads to relaxation of vascular smooth muscle. Pinacidil is indicated in the management of essential hypertension. In clinical trials of up to 1 year duration, pinacidil administered twice daily in a controlled release capsule formulation has been shown to achieve adequate blood pressure control both in previously untreated patients and in those with blood pressure inadequately controlled by beta-adrenoceptor blocking drugs or thiazide diuretics. In long term (up to 1 year) comparative studies pinacidil was at least as effective as hydralazine, prazosin or nifedipine in maintaining blood pressure control. Pinacidil may also have a potential use in the treatment of patients with secondary renal hypertension. Clinical trials to date have usually allowed the addition of a thiazide diuretic and/or beta-adrenoceptor blocking drug to enhance the efficacy of pinacidil and/or to reduce the incidence of adverse effects. The main adverse effects of pinacidil treatment, which result from its peripheral vasodilator activity, are headache, oedema, palpitations and tachycardia. Although the overall incidence of adverse effects is quite high, they are usually mild, transient in nature and respond to a reduction in dose. Nevertheless, these effects may occasionally be severe, necessitating withdrawal from therapy. Thus, pinacidil is an effective antihypertensive drug for the treatment of mild to moderate essential hypertension. Despite its novel mechanism of action pinacidil causes adverse effects typical of peripheral vasodilators; during long term use with twice daily administration of the controlled release capsule formulation, the addition of a diuretic is often necessary to attenuate peripheral oedema and maintain adequate control of blood pressure. Further long term controlled trials are needed to determine the precise role of pinacidil relative to that of the angiotensin converting enzyme (ACE) inhibitors and calcium channel blocking drugs.