Icilio Cavero

The potassium channel opener cromakalim (BRL 34915) activates ATP-dependent K+ channels in isolated cardiac myocytes

In cardiac myocytes, cromakalim (BRL 34915), a potassium channel opener, activates a time-independent K+ current exhibiting poor voltage-sensitivity. This effect of cromakalim is antagonized by low concentrations of glibenclamide, a specific blocker of ATP-dependent K+ channels in cardiac cells. Direct recording of the activity of K+ channels in inside-out membrane patches, confirmed that cromakalim is a potent activator of ATP-dependent K+ channels in cardiac myocytes.

The pharmacology of prazosin, a novel antihypertensive agent

During the past few years a large amount of pharmacological and physiological evidence has been obtained in favor of two distinct types of α-adrenoceptors. As a working hypothesis, it is feasible to assume that both α1- and α2-adrenoceptors are abundant on the vascular effector site, whereas the α-adrenoceptors (the blockade of which increases norepinephrine release) predominate at the level of peripheral sympathetic nerve endings. Prazosin is a novel, selective antagonist of α1-adrenoceptors and can be considered an important advancement both pharmacologically and therapeutically since this compound in contrast to classical α-adrenoceptor blocking agents, is effective for the treatment of high blood pressure. Prazosin lacks direct myorelaxant properties and, unlike many vasodilators, in doses lowering blood pressure it does not produce undesirable increases in heart rate and plasma renin activity. Prazosin has proved to be a very useful pharmacological tool since it has permitted us the furtherance of our knowledge with respect to the subclassification of receptors mediating the effects produced by α-adrenoceptor agonists, particularly clonidine. Pharmacokinetic and metabolic studies on prazosin given orally indicate that in animals and in man this compound has a low bioavailability, short half life and undergoes extensive biotransformation. The most common clinical use of prazosin is as an antihypertensive agent and is often given in association with established blood pressure lowering drugs. Recently, it was shown to be useful in the treatment of congestive heart failure, but for this application tolerance has been described. Generally, patients treated chronically with prazosin suffer only minor unwanted effects. This is in contrast to past experience with traditional α-adrenoceptor antagonist. The most serious side effect of prazosin is known as the “first dose phenomenon” which can sometimes lead to syncope. However, it can be avoided if prazosin therapy is initiated with minimally effective doses and individually tailored to obtain the desired antihypertensive effect. Presently, the interesting clinical profile of prazosin is attributed to its novel property of being a selective antagonist of postsynaptic α1-adrenoceptors. Howeverm this is probably an over simplification since some therapeutic observations are not entirely consistent with results which would have been expected for a selective α1-adrenoceptor. For example, prazosin, like the classical antagonists, would be expected to produce sexual dysfunction but, in fact, does not to any significant degree. Future studies with new chemical structures sharing the pharmacological profile of prazosin will clarify the real role of the selectivity towards α1-adrenoceptors in the therapeutic success of prazosin.

Drugs that prolong QT interval as an unwanted effect: assessing their likelihood of inducing hazardous cardiac dysrhythmias.

Medicinal products that, as an unwanted effect, prolong the QT interval of the electrocardiogram (ECG) can trigger episodes of polymorphic ventricular dysrhythmias, called torsades de pointes, which occasionally culminate in sudden death. The accurate measurement of QT interval requires the adoption of appropriate criteria of recording, measurement and data processing. Traditionally, QT interval is standardised to a reference heart rate of 60 beats/min by using the Bazett algorithm. However, this correction method can bias observed QT intervals in either direction. The ECG reflects cardiac electrical currents generated by ions (Na+, K+ and Ca2+) entering and leaving the cytosol mainly via transmembrane channels. Na+ and Ca2+ carry inward depolarising currents (INa, ICa) whereas K+ carries outward repolarising currents (Ito, IKr, IKS and IK1). Sometimes, a prolonged QT interval is a desired drug effect but, more commonly it is not, and reflects abnormalities in cardiac repolarisation heralding torsades de pointes. Furthermore, the potential torsadogenic activity of drugs is favoured by concurrent cardiac risk factors (old age, female gender, bradycardia, electrolyte imbalances, cardiac diseases etc.) which reduce cardiac repolarisation reserve. The evaluation of the cardiac safety of drug candidates can be started by determining their potency as IKr blockers in cloned Human Ether-a-go-go Related Gene (HERG) channels expressed in mammalian cells. Compounds passing successfully this test (desirable cardiac safety index >> 30, calculated as ratio of IC50 against IKr over ED50 determined in an efficacy test) should be further investigated in other relevant human cardiac ion currents, in in vitro animal heart preparations and finally in in vivo pharmacodynamic models. The decision as to whether the potential benefit of a new drug outweighs the cardiac risk inherent in its therapeutic use should be made in the light of the condition that it is expected to treat and with reference to alternative drug therapies. If a drug represents a unique therapeutic advance, non-clinical and clinical signals of unsatisfactory cardiac safety may not constitute sufficient grounds to abandon its development. However, if the drug offers only marginal benefits over existing therapies, decisions concerning its possible development should be taken by corporate policy makers.

QT interval prolongation by non-cardiovascular drugs: issues and solutions for novel drug development.

In 1997, the Committee for Proprietary Medicinal Products (CPMP) issued a document concerning the potential of non-cardiovascular drugs to cause prolongation of the QT interval of the electrocardiogram. This article reviews several aspects of this complex problem, including a preclinical strategy (in vitro electrophysiology in human cardiac cells and in vivo pharmacologically validated conscious dogs) to satisfy the expectations of the CPMP. In particular, the discussion stresses the danger of drugs prolonging the QT interval in patients with concurrent cardiac risk factors and the need for rigorous clinical testing to determine the risk of fatal cardiac events for drugs with the propensity to prolong QT.

Peripheral dopamine receptors, potential targets for a new class of antihypertensive agents: Part I: Subclassification and functional description

The dopamine receptors of the peripheral cardiovascular system are not a pharmacologically uniform population. A number of studies indicate that they belong to at least two distinct subtypes for which it is proposed to adopt the name DA1- and DA2-dopamine receptors in an attempt to follow the nomenclature presently in fashion for several vascular receptors. Typical DA1-dopamine receptors are those occurring postjunctionally in the renal and mesenteric arterial beds where their stimulation mediates direct smooth muscle relaxation. Typical DA2-dopamine receptors are those present on postganglionic sympathetic neurons (axonal varicosities and perhaps ganglionic cell bodies) where their excitation leads, under appropriate physiological conditions, to a reduction of the neural release of norepinephrine. The latter effect can manifest itself by a passive fall in vascular resistance and heart rate. Other populations of dopamine receptors not yet well characterized pharmacologically but of theoretical interest as additional potential target sites for cardiovascular drugs might be present on nephrons and in the adrenal cortex. Their stimulation can mediate a natriuretic effect and a reduction of aldosterone release, respectively. The pharmacological evidence favoring the subclassification of cardiovascular dopamine receptors into two distinct subtypes is reviewed. Furthermore, the main agonists and antagonists of these receptors and the complexity of their pharmacological profile are mentioned. Part II of this minireview will be dedicated to the description of the sites and mechanisms of the antihypertensive action of dopamine receptor agonists.

Cardiovascular and biological effects of K+ channel openers, a class of drugs with vasorelaxant and cardioprotective properties

Several new chemical entities (RP 52891, cromakalim and its derivatives) are potent and specific openers of vascular K+ channels. This mechanism is also shared, at least partially, by drugs such as minoxidil, diazoxide, pinacidil and nicorandil. The opening of plasmalemma K+ channels produces loss of cytosolic K+. This effect results in cellular hyperpolarization and functional vasorelaxation. In normotensive or hypertensive rats, K+ channel activators decrease aortic blood pressure (by producing a directly mediated fall in systemic vascular resistance) and reflexly increase heart rate. The former effect is not modified by specific blockers of classical vascular receptors but it is completely antagonized by the hypoglycemic sulphonylurea, glibenclamide, an established blocker of ATP-regulated K+ channels. K+ channel openers produce selective coronary vasodilatation and afford functional and biochemical protection to the ischemic myocardium. This salutary effect is mediated via cardiac K+ channel modulation and may result from an improved myocardial oxygen balance in the ischemic region. K+ channel openers increase plasma renin activity in animals as well as in man. However, only diazoxide, but not cromakalim or RP 52891, lowers plasma insulin concentration. The dose of glibenclamide entirely blocking the latter effect is over 50-fold smaller than that antagonizing the hypotensive and hyper-reninemic responses to diazoxide. In conclusion, K+ channel activators are potent vasorelaxant and cardioprotective agents possessing an original mechanism of action which is the opening of plasmalemma ATP-regulated K+ channels. Their clinical use as antihypertensive agents may be accompanied by undesirable effects (characteristic of peripheral vasodilators) which are likely to be attenuated or avoided by controlled release formulations. However, inasmuch as low doses of K+ channel openers may be sufficient to produce selective coronary artery dilatation and cardioprotection, these compounds could be of particular value in treating patients with coronary artery disease efficaciously and possibly without adverse cardiovascular effects.

Endothelin - a new family of endothelium-derived peptides with widespread biological properties

Endothelin (ET) is a novel family of three isopeptides (ET-1, ET-2, ET-3) each containing twenty-one amino acids and two disulfide bonds. Initially isolated from the supernatant of cultured porcine aortic endothelial cells, ET is stored as a preproform and released through an unusual proteolytic cleavage. In general, ET-1, ET-2, ET-3 differ quantitatively but not qualitatively in their biologic activity. ET have potent contractile activity in a variety of isolated tissues including arteries veins, trachea, duodenum urinary bladder and uterus. In vivo, ET possesses potent vasodilator and vasoconstrictor properties. Although the mechanisms mediating the hemodynamic effects of ET are not entirely clarified, recent evidence indicates a role for endothelium-derived relaxant factor (EDRF), protein kinase C and extracellular calcium. Moreover, ET appears to produce inflammation and bronchoconstriction through the formation of arachidonic acid metabolites via the cyclooxygenase pathway. The presence of ET binding sites in blood vessels and in several organ systems suggests ET may have important regulatory functions, which remain to be determined.

“Calcium antagonists”: A class of drugs with a bright future. Part II. Determination of basic pharmacological properties

This minireview discusses some simple pharmacological tests useful in detecting biological activity (screening), characterizing mechanisms of action and predicting possible therapeutic applications for calcium antagonists in general and calcium slow channel blockers in particular. In smooth muscle preparations these agents inhibit mechanical effects evoked by K+-depolarization which selectively opens voltage-operated calcium channels (VOC) to allow extracellular Ca++ into the cytosol. In contrast, any inhibition of receptor-mediated responses by calcium antagonists appears to depend on the transduction system and the specific cellular mechanism (e.g. VOC opening consequent to partial depolarization) activated by the receptor and, evidently, on ancillary pharmacological properties of the studied compound. For instance, whereas calcium slow channel blockers antagonize contractions produced by norepinephrine and K+-depolarization in the rat isolated portal vein, they inhibit effectively only the latter response in the rabbit aorta. This apparent discrepancy may be accounted for by the different pool of Ca++ mobilized in the two tissues by norepinephrine. Agents (e.g. diphenylalkylamines, calmodulin blockers) that impair the interaction of Ca++ with intracellular proteins produce effects which are less specific than those of slow channel blockers. Currently, the pharmacological profile of calcium antagonists can be appropriately defined by studying their effects on radioligand (dihydropyridine) binding, radioactive calcium movements through biological membranes, electrophysiological parameters in cardiac and vascular smooth muscle and on various in vivo cardiovascular preparations. Together, these approaches allow a functional classification of new calcium antagonists in relation to already known compounds and some hypotheses on their potential clinical applications. Finally, desirable pharmacokinetics and pharmacological properties for novel calcium antagonists are mentioned. This point will be further explored in the forthcoming minireview which will deal with the clinical applications of calcium antagonists.

Peripheral dopamine receptors, potential targets for a new class of antihypertensive agents: Part II: Sites and mechanisms of action of dopamine receptor agonists

Relatively selective dopamine receptor agonists, like bromocriptine, lergotrile, pergolide and N,N-di-n-propyl-dopamine, lower arterial pressure in conscious spontaneously hypertensive rats and in several anesthetized animal preparations. This effect has been attributed to stimulation of dopamine receptors since it can be specifically antagonized by several dopamine receptor blocking agents (domperidone, haloperidol, pimozide, sulpiride). The two main mechanisms which can theoretically intervene in the antihypertensive effects of dopamine agonists are direct smooth muscle relaxation mediated by stimulation of post junctional DA1-dopamine receptors and the reduction of the neural release of norepinephrine resulting from activation of of DA2-dopamine receptors on ganglionic bodies or sympathetic nerve terminals. Other accessory mechanisms of undoubted interest might be a natriuretic effect or a decrease of aldosterone release. On the basis of the presently available pharmacological results in experimental animals, it is not unreasonable to advance the hypothesis that agonists of DA1- and DA2-dopamine receptors produce cardiovascular changes most compatible with an antihypertensive activity being due to a fall in peripheral resistance. However, before any of these compounds can become of therapeutic interest further research in this field is necessary to explore whether it is possible to minimize or even entirely avoid certain unwanted effects (vomiting, nausea, endocrinological alterations) that appear to be intimately associated particularly with those agents stimulating the DA2-dopamine receptors subtype. A more thorough pharmacological characterization of human dopamine receptors would be useful to provide an insight into whether novel chemical approaches can solve some of these problems. Finally, the ideal profits of future dopamine receptor agonists aimed at the treatment of elevated arterial pressure is discussed.

Comprehensive in vitro Proarrhythmia Assay, a novel in vitro/in silico paradigm to detect ventricular proarrhythmic liability: a visionary 21st century initiative

Introduction: The Comprehensive in vitro Proarrhythmia Assay (CiPA) is a novel safety screening proposal intended to replace the 2005 regulatory strategy recommended by the International Conference of Harmonization S7B guideline. Areas covered: CiPA consists of three components. The first assay evaluates candidate drug effects on key cardiac ion channels. Then, simulations test whether the channel dataset yields proarrhythmic markers on a computationally reconstructed human ventricular cardiomyocyte action potential. Finally, the relevance of in silico conclusions is verified by determining the electrical activity of human stem cell-derived ventricular cardiomyocytes. Expert opinion: The CiPA initiative is intended to move safety pharmacology from a predominantly traditional pharmacodynamics approach to in silico and in vitro drug toxicity assessment. In practice, CiPA assays will have to be compliant with regulatory safety pharmacology tenets. The latter will necessitate international consensus on assay protocols, method standardization and validation and, thus, is likely to involve protracted discussions to achieve agreement. As such, full CiPA implementation by July 2015, as currently envisaged, to supplant E14 guidance for a thorough QT/QTc interval study as prerequisite for noncardiac drug marketing approval, appears to be difficult. Nevertheless, safety stakeholders should do their best to validate and implement the CiPA initiative in the shortest possible time.