Esko Iisalo

Comparison of radioreceptor assay and radioimmunoassay for atropine: pharmacokinetic application

SummaryA membrane suspension prepared from rat brain was able to bind the potent muscarinic antagonist quinuclidinyl benzilate (QNB). The KD for binding was 0.48 nM and Bmax was 1.42 pmol/mg protein. Atropine competitively inhibited the binding of tritiated QNB to muscarinic receptors. This new radioreceptor assay (RRA) for atropine has been compared with a radioimmunoassay (RIA) for atropine. The RRA measures only the active component of atropine, 1-hyscyamine and in this respect it differs from the RIA. As little atropine as 1.25 ng/ml (4.33 nmol/l) in a 25 µl serum sample could be reliably assayed by the RRA. Using both assay techniques the pharmacokinetics of atropine was studied after a single 0.02 mg/kg i.v. dose given to 8 anaesthetized patients. The half-life calculated by the RRA was 3.7±2.3 h (m ± SD) and by the RIA 4.3±1.7 h. Both the volume of distribution and the total clearance were higher according to the RRA than the RIA: 3.9±1.5 vs 1.7±0.71/kg and 15.4±10.3 vs 5.9±3.6 ml/min/kg, respectively. The AUC measured by the RRA and RIA was 29.8±18.9 and 103.9±110.7 µg·h/l, respectively. The differences in the pharmacokinetics according to the 2 methods are presumably due to preferential tissue uptake of the l-form.

Plasma and aqueous humour concentrations and systemic effects of topical betaxolol and timolol in man.

Abstract Plasma and aqueous humour concentrations and systemic effects of timolol and betaxolol were studied after ocular administration in 45 patients scheduled for extracapsular cataract extraction and intraocular lens implantation. The patients were divided randomly into three groups and received 40 μl of either 0.5% betaxolol, 0.25% timolol or placebo into the lower cul‐de‐sacs of both eyes. Blood samples were collected over a period of 4 h and blood pressure and heart rate were monitored during the study. Aqueous humour samples were aspirated at the beginning of the operation. Plasma and aqueous humour concentrations of timolol and betaxolol were analyzed using a sensitive radioreceptor assay. The mean plasma concentrations of betaxolol were lower than those of timolol. The concentration of betaxolol in the aqueous humour was twice as high as the concentration of timolol. Both drugs produced a significant decrease in heart rate. In the timolol group a decrease in heart rate was found 15 min after drug administration, and in the betaxolol group after one hour.

Selectivity of Acebutolol, Atenolol, and Metoprolol in Healthy Volunteers Estimated by the Extent the Drugs Occupy β2‐Receptors in the Circulating Plasma

The selectivity of acebutolol, atenolol, and metoprolol in healthy volunteers was estimated by determining the extent to which the drugs occupied β1‐receptors of rabbit lung and β2‐receptors of rat reticulocytes in the circulating plasma after drug intake. This ex vivo method had the advantage of including all drug components contributing to the drug‐receptor equilibrium in vivo and of excluding the factors regulating organ sensitivity to catecholamine stimulation. The oral doses of 400 mg acebutolol 100 mg atenolol, and 100 mg metoprolol were administered to six healthy male volunteers using a double‐blind, randomized, and cross‐over study design. The three drugs occupied β1‐receptors to a similar extent at 2 hours after drug intake. The receptor fraction occupied by metoprolol at 3 to 8 hours after drug intake was usually smaller, however (analysis of variance for repeated measures, P < .05) than that of the other drugs. Acebutolol occupied significantly larger fractions of β2‐receptors (analysis of variance for repeated measures, P < .05) than did atenolol and metoprolol. Therefore, at an identical β1‐receptor occupancy, the β2‐receptor occupancy of acebutolol was larger than that of the other agents. Apparently, active metabolites decreased markedly the selectivity of acebutolol, but not that of metoprolol. The receptor occupancy of the agents was well in agreement with the literature concerning the selectivity, intensity, and time‐course of drug actions after identical doses. Concluding, atenolol is slightly more selective than metoprolol and metoprolol is markedly more selective than acebutolol in the doses studied when estimated by the extent to which the drugs occupy β1‐ and β2‐receptors in the circulating plasma.

Measuring the bronchial effect of bronchodilating drugs in healthy subjects after methacholine provocation

SummaryTo study whether it would be possible to assess bronchodilating drugs in healthy subjects with methacholine — induced bronchoconstriction, salbutamol 100, 200 and 300 μg was inhaled in random order by 12 healthy volunteers in a double-blind, placebo-controlled study. Dose response “slope” (DRS = maximum percentage fall in pulmonary function / maximal noncumulative methacholine dose (μmol)) was used as an index of bronchial reactivity, and was calculated for forced expiratory flow volume in 1 s (DRSFEV1) and area under the flow-volume curve (DRSAEFV).Bronchial reactivity and its reproducibility were first tested by a standard methacholine provocation method. An abbreviated, single-dose method was used to measure the effect of salbutamol. The reproducibility of methacholine provocation was good, and the single-dose and standard methods gave comparable results. The DRS-values of all the doses of salbutamol differed significantly from placebo and from each other. AEFV did not show any advantage over the FEV1 in this context. A significant negative association between the dose of salbutamol (μg/kg) and airway reactivity was observed.In conclusion, use of the DRS showed it possible to evaluate the protective efficacy of β2-adrenergic agonists against induced bronchoconstriction in healthy subjects.

Extent of beta1- and beta2-receptor occupancy in plasma assesses the antagonist activity of metoprolol, pindolol, and propranolol in the elderly

SummaryWe estimated antagonist activity of metoprolol, pindolol, and propranolol in elderly cardiovascular patients by determining the extent to which the drugs occupied rabbit lung beta1- and rat reticulocyte beta2-adrenoceptors in plasma samples during drug treatment. The randomized, double-blind, crossover study was carried out by administering twice daily 100 mg metoprolol, 5 mg pindolol, and 80 mg propranolol for 7 days to 20 hypertensive subjects with a mean age of about 70 years. A 2-week interval was kept between administration of the different regimens. Receptor occupancy was measured at 1 hour before and 2 hours after administration of the last dose of each regimen by adding rabbit lung beta1- and rat reticulocyte beta2-receptors to plasma samples and by labeling the receptors with a radiolabeled beta-antagonist, (−)-[3H]CGP-12177. The results and conclusions were the following: (a) The extent to which metoprolol, pindolol, and propranolol occupied rabbit lung beta1-and rat reticulocyte beta2-adrenoceptors in plasma samples estimated accurately the intensity of beta-receptor antagonism in the patients who did not tolerate physiological and pharmacological tests measuring the degree of beta1- and beta2-adrenoceptor blockade. (b) The mean beta1- and beta2-receptor occupancy of pindolol and propranolol varied between 76% and 99% during the treatments. The mean beta1-receptor occupancy of the metoprolol regimen varied between 54% and 92%, and its beta2-receptor occupancy varied between 6% and 38%. Thus the antagonist activity of the metoprolol regimen differed significantly from that of the other regimens (ANOVA for repeated measures, p<0.05 and 0.001, for the beta1- and beta2-occupancy, respectively). (c) The extent of beta1- and beta2-receptor occupancy in plasma samples was in conformity with the literature on the intensity, selectivity, and duration of beta-blockade after similar drug doses. (d) The data on the receptor occupancy of beta-blocking drugs in plasma samples appear to be valuable in analyzing their effects, and it may be a method for optimizing drug therapy for aged cardiovascular patients.

Prevalence of debrisoquine oxidation phenotypes in glaucoma patients

The oxidation of debrisoquine, a sympatholytic antihypertensive agent, exhibits genetic polymorphism. The debrisoquine/4-OH-debrisoquine metabolic ratio (MR) separates the population to poor (PM, MR>12.6) and extensive (EM, MR<12.6) metabolizers. 5–10% of the caucasians belong to the PM phenotype. The oxidation of many other drugs, like timolol, correlates with the debrisoquine phenotype.We determined the debrisoquine phenotype in 102 glaucoma patients. The majority of the patients was treated with ophthalmic timolol. Five patients were classified as PMs. However, two of them were on quinidine, a well known inhibitor of debrisoquine oxidation.The prevalence of debrisoquine PM phenotype in glaucoma patients was 2.9% (excluding patients on quinidine) or 4.9% (with patients on quinidine). The figures are slightly lower than the mean value reported for the normal Finnish population. However, both figures lay within the 95% confidence limits of the prevalence of PM phenotype in the normal Finnish population. The beta-blocking activity of oral timolol is increased in PMs. The significance of timolol oxidation phenotype during ocular timolol therapy warrants further investigation.

Inotropic Action and Myocardial Uptake of Digoxin and Betamethyldigoxin in Isolated Guinea Pig Atria

The chronotropic and inotropic effects as well as the tissue levels of digoxin and methyldigoxin were studied in isolated guinea pig atria. Higher concentrations of methyldigoxin than of digoxin were required to cause arrhythmias. The inotropic potencies of the two glycosides did not differ from each another in equimolar concentrations in the organ bath. The increase in contractility correlated with the level of the glycoside in the organ bath and in the tissue. Digoxin was more effectively taken up by the heart tissue than methyldigoxin. Thus, when the tissue levels of the glycosides were the same, the contractility was greater after methyldigoxin than after digoxin.

SINGLE DOSE OF PINDOLOL IN NORMOTENSIVE PREGNANCY: THE LACK OF CHANGES IN HEMODYNAMICS IN SPITE OF EFFECTIVE BETA-RECEPTOR BINDING

Objective: To evaluate the absorption, placental transfer, receptor occupancy, and acute hemodynamic effects of a single peroral dose of pindolol in healthy term pregnancy.Methods: Ten pregnant healthy women admitted for cesarean section at term after uneventful pregnancy received each 5 mg of pindolol perorally before operation. The acute hemodynamic responses were measured, and maternal and fetal circulations were assessed with color Doppler ultrasonography. Pindolol concentrations were measured with a sensitive radioreceptor binding assay, and the corresponding beta2-receptor occupancy was determined.Results: Mean maternal concentration of pindolol 1 h after ingestion was 9.4 ng/ml, and it rose to 14.6 ng/ml at the time of cesarean section; respective beta2-receptor occupancies were 91.8% and 98.8%. In the umbilical artery, pindolol concentration was 50% of the maternal concentration. A plasma concentration of 2.5 ng/ml yielded 90% beta2-receptor occupancy, which reflects the high potency of pindolol.C...