Kinetics and cardiac effects of propranolol in humans

Drug Disposition in Subjects with Obesity: The Research Work of Darrell R. Abernethy

In 1979, the late Dr. Darrell R. Abernethy and colleagues began a series of clinical studies aimed at understanding the pertinent determinants of drug distribution, elimination, and clearance in obesity, and how those variables are interconnected. The studies confirmed that volume of distribution (Vd) and clearance are the principal independent biological variables, which conjointly determine elimination half-life as a dependent variable. For drugs distributed by passive diffusion, their pharmacokinetic Vd - after correcting for plasma protein binding - was increased in obesity, depending in part on the physicochemical lipophilicity of the individual drugs, and the quantitative extent of obesity in overweight individuals. Across all studies, the ratio of mean clearance in obese divided by control groups had an overall median value of 1.21 (range: 0.75 to 3.11), indicating a small and variable effect of obesity on clearance, without clear directionality. Since drug clearance was not clearly related to lipophilicity or degree of obesity, the prolonged half-life of lipophilic drugs in obese patients was largely explained by the increased Vd. Dr. Abernethy further identified delayed attainment of steady-state after initiation of multiple-dose treatment, and delayed washout after termination of dosage, as potential clinical consequences of the extended half-life in obese persons. These consequences for specific drugs have been recently emphasized in contemporary studies of chronic dosage in subjects with obesity. Without data identifying an obesity-related change in clearance for a specific drug, maintenance doses (in milligrams) should be based on ideal weight rather than adjusted upward based on total weight. This article is protected by copyright. All rights reserved.

Effect of lipophilicity on drug distribution and elimination: Influence of obesity

AIMS

For a given passively-distributed lipophilic drug, the extent of in vivo distribution (pharmacokinetic volume of distribution, V_d ) in obese individuals increases in relation to the degree of obesity. The present study had the objective of evaluating drug distribution in relation to in vitro lipophilicity, and the relative increase in V_d associated with obesity across a series of drugs.

METHODS

Cohorts of normal-weight control and obese subjects received single doses of drugs ranging from hydrophilic (acetaminophen, salicylate) to lipophilic (imipramine, verapamil). Lipid solubility was measured by the log-transformed values of the high-pressure liquid chromatographic (HPLC) retention index (Log₁₀ (HPLC)), and the octanol-water partition coefficient (LogP).

RESULTS

Among normal-weight controls, V_d normalized for protein binding was highly correlated with Log₁₀ (HPLC) (R² = .65) and with LogP (R² = .78). V_d of all drugs was increased in the obese cohort, but the relative increase (compared to controls) for individual drugs was disproportionately greater as lipid solubility increased. Since clearance was unrelated to lipophilicity, the increased V_d produced a parallel disproportionate increase in elimination half-life in the obese cohort that was associated with Log₁₀ (HPLC) (R² = .62).

CONCLUSION

Lipophilicity is a principal correlate of in vivo V_d , as well as the increased V_d of drugs in obese patients. The consequent prolongation of half-life in obesity has clinical safety implications in terms of delayed drug accumulation and washout during and after chronic dosage. The magnitude and importance of this effect for a given drug depends on the degree of obesity, as well as the lipid-solubility of the specific drug.

Pharmacodynamic comparison of L-bunolol with propranolol, metoprolol, and placebo

Twelve healthy volunteers received single oral doses of propranolol (80 mg), metoprolol (100 mg), L-bunolol (2 mg), and placebo in a four-way crossover study. Blood pressure, ventricular rate, and echocardiographically determined ejection fraction, ejection time, and mean rate of circumferential fiber shortening (mVcf) were measured before dosing and at multiple time points during 10 hours after each dose, with subjects maintained in the supine position. Reductions in systolic and diastolic blood pressure following administration of each of the beta blockers were greater than those observed with placebo, but differences among the four treatments were not significant. Heart rate reductions with the beta blockers differed significantly from placebo (P less than .001), but differences among the three beta blockers were not significant. Differences among the four treatments in mVcf decrement did not attain significance at the 5% level (.05 less than P less than .1), and there were no significant differences in ejection-time prolongation or ejection-fraction reduction. Thus, reduced blood pressure, heart rate slowing, and reduced cardiac contractility may be associated with placebo treatment and may indicate the need for placebo controls in studies of the cardiovascular effects of beta blockers. Despite differing secondary pharmacologic properties, the three beta blockers reduced heart rate to a similar extent. Other effects of the beta blockers on blood pressure and cardiac contractility could not be consistently distinguished from those associated with placebo.

Increased occurrence of left ventricular thrombi during early treatment with timolol in patients with acute myocardial infarction

To examine whether early intervention with timolol influences the occurrence of left ventricular thrombi in acute anterior myocardial infarction, 40 patients with acute anterior myocardial infarction admitted to hospital within 6 hr of onset of symptoms were randomly assigned to receive intravenous followed by oral timolol maleate or placebo. Five (25%) of 20 patients in the placebo group and 14 (73.7%) of 19 patients with confirmed infarction in the timolol group developed a left ventricular apical thrombus as detected by two-dimensional echocardiography from 2 to 10 days after inclusion (p less than .005). Patients received anticoagulants only after a left ventricular thrombus had been diagnosed. Only one patient with thrombus suffered peripheral embolization (timolol group). The treatment groups were comparable with respect to location of regional left ventricular dysfunction, electrocardiographic changes, and infarct size estimated by creatine kinase release. However, computer-assisted regional wall motion analysis demonstrated significantly reduced apical wall motion in the timolol group compared with the placebo group (p less than .01). Also, the mean heart rate during the first 10 days after the acute infarction was reduced by 13% in the timolol group (p less than .001). The reduction in heart rate and left ventricular apical wall motion caused by timolol in patients with acute anterior myocardial infarction may increase the occurrence of left ventricular thrombi.

Pharmacokinetics and dynamics of penbutolol in humans: evidence for pathway-specific stereoselective clearance

The pharmacokinetics and dynamics of the D- and L-isomers of the beta-adrenergic blocking agent penbutolol were investigated in healthy human volunteers. In Study One, subjects received a single 40-mg oral dose of L-penbutolol (the pharmacologically active stereoisomer), and matching placebo on two occasions. A mean peak serum penbutolol concentration of 268 ng/ml was reached at 0.9 h after dosing. Elimination half-life averaged 1.6 h, and total clearance 16.6 ml/min per kg body weight. Changes in blood pressure, ventricular rate, and rate of circumferential fiber shortening (Vcf) did not differ between L-penbutolol and placebo. In Study Two, subjects received 40 mg D-penbutolol, L-penbutolol, and placebo on three occasions. Total clearance of D-penbutolol was higher than for the L-isomer (43.7 vs 15.9 ml/min/kg; P less than 0.01); this was reflected in correspondingly increased area under the serum concentration curve for conjugates of the oxidized metabolite 4-hydroxy penbutolol (2.25 vs 0.66 micrograms/ml X h; P less than 0.005). In contrast, direct conjugates of L-penbutolol achieved higher serum concentrations than conjugates of D-penbutolol. Alterations in blood pressure, ventricular rate, and Vcf for D-penbutolol, L-penbutolol, and placebo were quantitatively small. Thus the clearance of penbutolol after oral administration in humans is stereoselective, but the oxidative pathway is more stereosensitive than the parallel conjugative pathway. Penbutolol causes minimal alterations in parameters of cardiac function after single 40-mg doses in healthy humans.

Relationship between plasma propranolol concentration and dose in young, healthy volunteers

The relationship between dose and bioavailability of propranolol was determined in 12 healthy male subjects. The doses employed were 10, 40, 80 or 160 mg of propranolol HCl given three times daily for 4 days. Daily minimum plasma levels showed that there was no accumulation of the drug. In all subjects, non-linear relationships were observed between peak plasma level, or area under the plasma concentration-time curve, and dose. The between subject variation of these parameters was not dose related.