Racial differences in drug responses--a comparative study of trimazosin and alpha 1-adrenoceptor responses in normotensive Caucasians and West Africans

Why do hypertensive patients of African ancestry respond better to calcium blockers and diuretics than to ACE inhibitors and β-adrenergic blockers? A systematic review

BACKGROUND

Clinicians are encouraged to take an individualized approach when treating hypertension in patients of African ancestry, but little is known about why the individual patient may respond well to calcium blockers and diuretics, but generally has an attenuated response to drugs inhibiting the renin-angiotensin system and to β-adrenergic blockers. Therefore, we systematically reviewed the factors associated with the differential drug response of patients of African ancestry to antihypertensive drug therapy.

METHODS

Using the methodology of the systematic reviews narrative synthesis approach, we sought for published or unpublished studies that could explain the differential clinical efficacy of antihypertensive drugs in patients of African ancestry. PUBMED, EMBASE, LILACS, African Index Medicus and the Food and Drug Administration and European Medicines Agency databases were searched without language restriction from their inception through June 2012.

RESULTS

We retrieved 3,763 papers, and included 72 reports that mainly considered the 4 major classes of antihypertensive drugs, calcium blockers, diuretics, drugs that interfere with the renin-angiotensin system and β-adrenergic blockers. Pharmacokinetics, plasma renin and genetic polymorphisms did not well predict the response of patients of African ancestry to antihypertensive drugs. An emerging view that low nitric oxide and high creatine kinase may explain individual responses to antihypertensive drugs unites previous observations, but currently clinical data are very limited.

CONCLUSION

Available data are inconclusive regarding why patients of African ancestry display the typical response to antihypertensive drugs. In lieu of biochemical or pharmacogenomic parameters, self-defined African ancestry seems the best available predictor of individual responses to antihypertensive drugs.

Gender, ethnicity, and genes in cardiovascular disease. Part 2: implications for pharmacotherapy

Women are underrepresented in clinical trials. Lower doses of beta-blockers are required for Southeast Asians. ACE and ARB's are teratogenic in the second trimester. Torsades de Pointes is more common in women related to a longer QT-interval. Lower dose OCPs decrease the risk of MI, stroke and thrombosis. HRTs are not effective for CAD prevention.

Influence of race or ethnicity on pharmacokinetics of drugs

Review of the current literature on racial differences in pharmacokinetics of drugs supports the premise that only pharmacokinetic processes which are biologically or biochemically mediated have the potential to exhibit differences between racial or ethnic groups. Thus, the pharmacokinetic factors which can be expected to potentially exhibit racial differences are (1) bioavailability for drugs which undergo gut or hepatic first-pass metabolism, (2) protein binding, (3) volume of distribution, (4) hepatic metabolism, and (5) renal tubular secretion. Absorption (unless active), filtration at the glomerulus, and passive tubular reabsorption would not be expected to exhibit racial differences. As is evident from this review, there are relatively few drugs for which there is information on ethnic or racial differences in pharmacokinetics. Thus it is often necessary to try to predict whether such differences might exist. Taking into consideration the above factors and evaluation of the pharmacokinetic characteristics of the drug, it should be possible to identify those drugs most likely to exhibit differences in their pharmacokinetics. For example, a drug which is eliminated entirely by the kidneys through filtration and reabsorption and is not highly bound to plasma proteins (or is bound to albumin) is highly unlikely to exhibit racial differences in its kinetics. Conversely, a drug which undergoes significant gut and/or hepatic first-pass metabolism and is highly bound to AGP is much more likely to exhibit kinetic differences between racial groups. A discussion of the impact of racial differences in kinetics on drug response or racial differences in drug efficacy, toxicity, or pharmacodynamics (concentration-response relationship) is beyond the scope of this review. However, a number of the papers described above also evaluated differences in pharmacodynamics or response. Among the comparisons of Chinese and Caucasians, these include the papers on propranolol, morphine, nifedipine, triazolam, diazepam, and omeprazole. For those studies comparing differences in blacks and Caucasians, responses or pharmacodynamics were also determined in the studies of propranolol, trimazosin, and methylprednisolone. Interested readers are also referred to the review by Wood and a more recent review by Kitler for additional discussion of ethnic/racial differences in pharmacodynamics/drug response.

Inter-ethnic differences in xenobiotic metabolism

The aim of safety assurance procedures is to determine a level of intake (the acceptable daily intake (ADI) or tolerable daily intake (TDI)) that is without adverse health effects in the human population. The majority of studies on inter-ethnic differences in xenobiotic metabolism have concentrated in the incidence of expression of the poor metaboliser phenotype for a number of drug metabolising enzymes. Such ethnic differences can result in different incidences of individuals at higher risk, but this would not affect the safety assurance/risk assessment outcome unless poor metaboliser status was not recognised in the database used for the initial assessment and calculation of ADI or TDI. Of far greater importance are ethnic differences which result in population differences in the mean values, and/or the extent of variability within the population, for key kinetic parameters such as the internal dose or area under the plasma concentration time curve (AUC). There are few studies on inter-ethnic differences in sensitivity and most of these relate to in vivo differences for therapeutic/pharmacological agents, so that the reported data reflect both kinetic and dynamic variability. Inter-ethnic differences are limited in extent and well within the uncertainty factor of 10 used for human variability.

Impaired beta 2-adrenergic agonist-induced venodilation in Indians of Asian origin

OBJECTIVES

Vascular responsiveness to infusions of vasoactive substances varies between ethnic groups. Indians of Asian origin are a rapidly growing ethnic group in the United States but have not been extensively studied. We sought to determine whether there was any difference in venous responsiveness to a local infusion of vasoactive substances between Indians of Asian origin and white subjects.

METHODS

We used the dorsal hand vein compliance technique to construct full dose-response curves to the beta 2-agonist isoproterenol (2 to 270 ng/min) in hand veins preconstricted with phenylephrine in 11 young white subjects and in 11 young Asian Indian subjects. In addition, six subjects in each group were randomly selected to have full dose-response curves to nitroglycerin (0.006 to 1485 ng/min) generated.

RESULTS

The maximal response (E(max)) to isoproterenol was smaller in Asian Indians (33.9% +/- 41.1% in Asian Indians versus 107.0% +/- 60.1% in white subjects; p < 0.01). There was no difference in the log of the dose that produced half-maximal venodilation [log(ED50)] between the two groups (1.10 +/- 0.57 in Asian Indians versus 1.15 +/- 0.50 in white subjects). However, nitroglycerin infusion produced similar responses for both the E(max) and the log(ED50) between the two groups.

CONCLUSION

These results indicate that differences may exist in beta-adrenergic responsiveness among white subjects and Indians of Asian origin. Therapy for diseases that use beta-adrenergic responses, such as hypertension, must take into account these differential vascular responses because they may affect their efficacy in Asian Indians.

Ethnic differences in drug disposition and responsiveness

Interethnic differences are important factors accounting for interindividual variations in drug responsiveness. However, these differences in drug response have been a relatively neglected area of investigation, so that similar doses are prescribed to different ethnic populations without consideration of interethnic pharmacokinetic and pharmacodynamic variation. With the increased recognition of genetically determined polymorphism in metabolising ability as an important factor in drug disposition, concern has developed for the importance of individualising drug dose to account for racial differences. The recognition of these differences in drug disposition and responses calls into question the failure of drug licensing authorities to demand information on dosage, efficacy and toxicity in different ethnic groups, and to accept data from limited ethnic groups such as Caucasians. This article reviews the evidence for ethnic differences in drug disposition and sensitivity and should encourage further investigations to elucidate the extent of such differences, their causes and their therapeutic impact.

New alpha 1-adrenergic receptor antagonists for the treatment of hypertension: role of vascular alpha receptors in the control of peripheral resistance

The pharmacology, clinical efficacy and safety of new alpha-adrenergic receptor antagonists for the treatment of hypertension was reviewed (Table XIV). Although all these agents block alpha 1 receptors, some of them have additional effects on histamine, serotonin, dopamine, and alpha 2 receptors. These other actions account for the differences in the side effect profiles observed, i.e., increased incidence of central nervous system side effects found with indoramin, ketanserin, and urapidil, as well as for some additional beneficial effects of ketanserin (i.e., antiplatelet aggregation activity). The magnitude of BP reduction observed with antagonists of alpha 1-adrenergic receptors is modest. In most studies, the degree of BP reduction is comparable to that of prazosin, but less than that achieved with thiazide diuretics, beta-receptor antagonists, or methyldopa. Studies on the comparative efficacy and safety of new alpha 1 antagonists with converting enzyme inhibitors or calcium-channel blockers are not available. In general, alpha 1 antagonists produce greater reductions in standing than in supine BP, an effect due to the venodilatory action of these drugs. New alpha 1 antagonists appear to have equal efficacy in black and white hypertensive individuals. Their comparative efficacy and safety in young vs elderly hypertensive individuals requires further investigation. No information about the possible development of tolerance during treatment with new alpha 1 blockers was encountered. The effects of alpha 1 antagonists on HR are variable and depend on how long after the oral dose the measurements were obtained. In most studies, no significant HR changes are noticed for readings obtained 24 hours post dose; whereas tachycardia has been observed at the time of peak hypotension. Since alpha 1 antagonist-induced tachycardia is most likely of reflex nature, i.e., mediated to an increase in sympathetic activity, the increased HR may be associated with increases in myocardial contractility and in myocardial oxygen consumption. Consequently, a 24-hour HR monitoring during treatment with alpha 1 antagonists should be required for evaluation of new agents. The hemodynamic, humoral, and hormonal effects of the newer alpha 1-receptor antagonists are comparable to those of prazosin. The most consistent finding is a reduction in total peripheral resistance associated with either no change or with only small increases in cardiac index. These agents have been shown either not to change or to increase renal blood flow.(ABSTRACT TRUNCATED AT 400 WORDS)