Is there a relationship between the physical properties and cardioselectivity of beta-adrenoceptor antagonists?

Bopindolol: pharmacological basis and clinical implications

Bopindolol, a non-selective antagonist of beta 1- and beta 2-adrenoceptors (ARs), has been found by pharmacological, molecular biological techniques and molecular modeling to have several unique properties. Bopindolol produces sustained blockade of beta 1- and beta 2-ARs, has intrinsic sympathomimetic as well as membrane stabilizing actions, inhibits renin secretion, and interacts with 5-HT receptors. Also, our recent molecular modeling studies identified possible interaction sites between bopindolol and beta-AR subtypes. The reviewed studies support our findings that bopindolol is non-selective for beta 1- and beta 2-ARs, has low affinity for beta 3-AR subtype and has pharmacological properties that are likely to be beneficial in the treatment of cardiovascular diseases.

Lipid solubility of a series of drugs and its relevance to fatal poisoning

For 11 commonly used drugs, the n-octanol/water partition coefficient (pH 7.4 temperature 37 degrees C) and solubility in n-octanol were determined. The drugs tested were chlorpromazine, amitriptyline, trazodone, dextropropoxyphene, diltiazem, dibucaine, amethocaine, procaine, quinidine, acetylsalicylic acid and paracetamol. For eight of the drugs, the relative lipid saturation corresponding to a fatal plasma concentration was estimated from the two parameters determined above and the median fatal blood concentrations reported in the literature. For five of those eight drugs, the estimated relative saturation in the lipid phase fell within the range 0.001-0.004 which is close to relative saturation figures in aqueous and vapour phases already published for chemicals possessing a non-specific or physical mechanism of toxicity. Since this is determined largely by their lipid solubility, it is probable that accumulation in the lipid phase is an important determinant of the lethal toxicity of drugs and chemicals with a non-specific mechanism of toxicity.

Lipophilic beta-adrenoceptor antagonists stimulate cholesterol biosynthesis in human skin fibroblasts

The effect of a series of beta-adrenoceptor antagonists on cholesterol biosynthesis was studied in vitro in normal human skin fibroblasts. Some, but not all, of the drugs studied stimulated the incorporation of [2-14C]-acetate into cell sterols in a dose-dependent manner. This effect was unrelated to beta-blocking potency, selectivity for beta 1 or beta 2 adrenoceptors and partial agonistic activity of the drugs, thus ruling out a beta-receptor mediated mechanism. A positive, statistically significant correlation was found, however, between the drug lipophilicity and the stimulation of sterol biosynthesis. Propranolol, the most effective agent in increasing [2-14C]-acetate incorporation into cellular sterols, also enhanced the conversion of 3-hydroxy-3-methylglutaryl CoA (HMGCoA) into mevalonic acid, suggesting an interference of lipophilic beta-adrenoceptor antagonists with HMHCoA-reductase, the feed-back regulated rate limiting step of cholesterol biosynthesis.

Clinical pharmacokinetics of beta-adrenoceptor antagonists. An update

The beta-adrenoceptor antagonists have been widely used clinically for over 20 years and their pharmacokinetics have been more thoroughly investigated than any other group of drugs. Their various lipid solubilities are associated with differences in absorption, distribution and excretion. All are adequately absorbed, and some like atenolol, sotalol and nadolol which are poorly lipid-soluble are excreted unchanged in the urine, accumulating in renal failure but cleared normally in liver disease. The more lipid-soluble drugs are subject to variable metabolism in the liver, which may be influenced by age, phenotype, environment, disease and other drugs, leading to more variable plasma concentrations. Their clearance is reduced in liver disease but is generally unchanged in renal dysfunction. All the beta-adrenoceptor antagonists reduce cardiac output and this may reduce hepatic clearance of highly extracted drugs. In addition, the metabolised drugs compete with other drugs for enzymatic biotransformation and the potential for interaction is great, but because of the high therapeutic index of beta-adrenoceptor antagonists, any unexpected clinical effects are more likely to be due to changes in the kinetics of the other drug. Because satisfactory plasma concentration effect relationships have been difficult to establish for most clinical indications, and little dose-related toxicity is seen, plasma beta-adrenoceptor antagonist concentration measurement is usually unnecessary. The investigation of the clinical pharmacokinetics of the beta-adrenoceptor antagonists has added greatly to our theoretical and practical knowledge of pharmacokinetics and made some contribution to their better clinical use.

The binding of agonists and antagonists to rat lung beta-adrenergic receptors as investigated by thermodynamics and structure-activity relationships

Interaction of several agonists and antagonists with the relevant beta receptor were studied in vitro using rat lung membranes as beta-adrenergic receptor source. The influence of temperature, between 4 degrees C and 37 degrees C, on the ability of beta-adrenergic agonists and antagonists to displace (-)-[125I]iodocyanopindolol from beta-adrenergic receptors was checked. Thermodynamic parameters were calculated from the Kj values thus obtained. Lipophilicity of the twenty molecules was also measured using a RP-HPLC method. The results obtained show: the density of receptors was not affected by the temperature but their affinity for the twenty agonists and antagonists increased with decreasing temperature; the binding of agonists is enthalpy driven while that of antagonists is entropy driven, with the exception of the lipophilic agonist dobutamine; a single relationship between entropy and lipophilicity exists for all twenty compounds and would suggest that molecular structure and physicochemical properties account for the thermodynamics of binding.

Liquid chromatographic retention of beta-adrenoceptor antagonists: an index of lipid solubility

The in-vitro lipophilicity of nine beta-adrenoceptor antagonists was evaluated based on retention on a reverse-phase C-18 high-pressure liquid chromatographic (hplc) system at physiologic pH. Propranolol was by far the most lipophilic drug, while atenolol and sotolol were the least. Hplc retention was highly correlated (r = 0.92) with octanol: buffer partition coefficient. Thus hplc retention is a rapid and replicable approach to the determination of in-vitro lipophilicity that does not require radioactive drug.

Quantitative relationships between structure and pharmacokinetics of beta-adrenoceptor blocking agents in man

The structure and pharmacokinetics relationship of 14-beta-adrenoceptor antagonists was investigated in humans. Statistically significant linear and parabolic correlations were found to exist between standard and derived mean pharmacokinetic parameters and the apparent octanol/buffer (pH 7.4) partition coefficient of the compounds. The lipophilic/hydrophilic properties were the primary determinants for the pharmacokinetic behavior of the compounds. Most of the pharmacokinetic parameters were also significantly correlated with the plasma protein/plasma water partition coefficient for the compounds. When the values of the pharmacokinetic parameters of the individual compounds were predicted from the regressions on the apparent partition coefficients in octanol/buffer (pH 7.4) and in plasma protein/plasma water, the error was on average 60%.

Comparison of efficacy of a single daily dose of 400 mg acebutolol and 100 mg atenolol in the treatment of arterial hypertension

A double-blind, crossover comparison of once-daily acebutolol (400 mg) and once-daily atenolol (100 mg) was carried out in 33 patients with mild to moderate essential hypertension. After 8-weeks' treatment with acebutolol, the reduction (12%) in mean arterial pressure was, on average, slightly greater than that on atenolol (11%), but the difference was not significant. The mean 22-hour serum concentration of acebutolol plus its acetyl metabolite was 24% of the peak level, whereas that of atenolol was 12%. Routine laboratory monitoring showed no important changes. Acebutolol gave a statistically significant lesser reduction in pulse rate than atenolol, and a lesser incidence, approaching statistical significance (p less than 0.1), of side-effects.

Influence of lipid solubility on the sperm immobilizing effect of beta-adrenoceptor blocking drugs

1 Sperm immobilizing effects of five beta-adrenoceptor blockers were measured on six human semen samples with a trans-membrane migration method. 2 The concentrations which decreased sperm motility to 50% of control (ED50) were 0.8, 4.2, 6.2, 11.0 and 33.0 mM for propranolol, oxprenolol, metoprolol, acebutolol and sotalol respectively. 3 There was a linear correlation between the log values of ED50 and the log partition coefficients of these beta-adrenoceptor blockers. 4 Because the lipid solubility determines the sperm immobilizing potencies of beta-adrenoceptor blockers, we support the theory that beta-adrenoceptor blockers inhibit sperm motility by stabilizing the cellular membrane. 5 The clinical implication of this finding is that more lipid soluble drugs are likely to have more interference on sperm function, at least when they are used locally.