Thomas E. Gaffney

Stereoselective delivery and actions of beta receptor antagonists

These studies have revealed that the delivery and actions of beta receptor antagonist drugs are controlled by a cascade of stereoselective processes involving multiple enzymes, transport proteins and receptors. In essence, the free concentration of the pharmacologically active (-)-enantiomer species of these drugs presented to cell surface beta receptors appears to be a function of the stereoselective clearance by hepatic cytochrome P-450 isoenzymes, enantiomer selective binding to alpha 1-acid glycoprotein and albumin and perhaps predominantly by stereoselective sequestration (and release) by the vesicular amine transport protein within adrenergic neurons. Stereoselectivity in the clearance of beta blocking drugs, which can favor either the (+)- or (-)-enantiomer, only appears to be important for the lipophilic drugs which are cleared by hepatic metabolism. Such stereoselectivity is due to differential stereochemical substrate requirements of individual hepatic cytochrome P-450 isoenzymes. Interindividual variations in the stereoselectivity can occur as a result of differences in the amount and expression of cytochrome P-450 isoenzymes due to genetic predisposition or other factors. In the same context, we have observed a significant correlation between the extent and stereoselectivity of binding of beta blocking drugs to plasma proteins. This is another finding which suggests that variability in the expression of individual proteins involved in the beta blocking drug-protein cascade determines the free concentration of the pharmacologically active enantiomer. However, since most observations have been made in young normal subjects, the extent of stereoselectivity in metabolism, binding and other processes is unknown in the general population where steady-state plasma concentrations can vary widely due to multiple biological factors. The observations from neural studies support the concept that adrenergic nerve endings provide a depot for the stereoselective storage and release of the active enantiomer of beta receptor antagonists. The mechanism of this release appears to involve exocytotic secretion of drug that has been stereoselectively accumulated by the neurotransmitter storage vesicles. In terms of this idea, beta receptor antagonists released during nerve stimulation may achieve concentrations of the (-)-enantiomer within the adrenergic synapse greatly in excess of those found in plasma. Such a mechanism could significantly influence both the intensity and duration of beta receptor blockade in the heart, blood vessels, brain and other target tissues.(ABSTRACT TRUNCATED AT 400 WORDS)

Postprandial alterations in hemodynamics and blood pressure in normal subjects

The effects of a standardized mixed meal, a self-selected meal and a sham meal on heart rate, arterial pressure, cardiac output, total systemic resistance and echocardiographic indexes of left ventricular performance were examined in normal volunteers. Supine heart rate and cardiac output increased after the meals (p less than 0.07 to 0.001), but not after the sham meal. Supine diastolic blood pressure and total systemic resistance decreased after the meals but not after the sham meal (p less than 0.05 to 0.001). Ejection fraction and mean velocity of circumferential fiber shortening increased after the standard meal (p less than 0.01) and tended to increase after the self-selected meal, but did not increase after the sham meal. Meals of normal size may induce splanchnic vasodilation and a decrease in total systemic resistance. Ingestion of food also significantly affects heart rate, blood pressure, cardiac output and echocardiographic indexes of left ventricular performance. Patients should not eat during short-term evaluation of cardiovascular interventions because the cardiovascular effects of a meal may compromise interpretation of the cardiovascular effects of the primary intervention. The hemodynamic effects of food may also interact with the effects of cardiovascular disease processes.

The predictable relationship between plasma levels and dose during chronic propranolol therapy.

The present study has examined the relationship between plasma propranolol concentrations and dose during chronic propranolol therapy and the between‐patient variation in this relationship under rigorously controlled conditions. Peak (2 hr) and trough (6 hr) plasma concentrations were measured at carefully established steady‐state conditions in 46 patients with hypertension or coronary artery disease. All patients were hospitalized in a clinical research unit. Propranolol doses ranged from 40 to 960 mg/day (every 6 hr). Propranolol was measured by gas chromatography‐mass spectrometry using a stable isotope‐labeled internal standard. Peak plasma propranolol (ng/ml) was linearly related to dose over the range 160 to 960 mg (y = 1.11x − 111; correlation coefficient = 0.96); a non linear relationship exists over the range 40 to 160 mg. Trough plasma concentrations were 51 ± 9% (mean ± SD) of the peak concentrations over the entire dose range. Between‐patient variation in plasma propranolol was much smaller than has previously been reported in spite of the fact that the patient population studied was quite heterogeneous and that numerous other drugs were concomitantly used with propranolol. A maximum, 3‐fold, variation was observed at the 40‐mg dose level and decreased linearly with dose to an only 1.3‐fold variation at doses exceeding 600 mg/day. The fact that the oral dose of propranolol can be used to predict a very narrow plasma concentration range indicates a very uniform pattern in the way patients handle propranolol, a pattern that could prove useful as an aid in dose selection as well as a basis for an evaluation of patient compliance.

4‐Hydroxypropranolol and its glucuronide after single and long‐term doses of propranolol

The disposition of the pharmacologically active 4‐hydroxypropranolol (HO‐P), its glucuronic acid conjugate (HO‐P‐G), and propranolol were compared after single intravenous and oral doses of propranolol in 6 normal subjects and after long‐term therapy in 32 patients with hypertension or coronary artery disease. The areas under the plasma concentration/time curves (AUC∞, ng · hr/ml) after 4‐mg intravenous doses of propranolol were 6.6 ± 2.2 (mean ± SEM) for HO‐P and 55 ± 11 for propranolol. After 20‐ and 80‐mg oral doses the AUC∞ for HO‐P were 59 ± 9 and 162 ± 21 and for propranolol were 72 ± 9 and 306 ± 46. Peak HO‐P concentrations were reached at 1 to 1.5 hr after the oral doses. Although there was a rapid decline in plasma HO‐P between 1.5 and 3 hr when HO‐P‐G was still rising to levels above HO‐P levels 3.5‐ to 5‐fold, the apparent half‐lifes (t½s) after 3 hr were in the same range for HO‐P, HO‐P‐G, and propranolol (3.0 to 4.2 hr). While during long‐term therapy plasma HO‐P rose over the whole dose range (40 to 960 mg daily) in an apparently linear fashion, the HO‐P/propranolol plasma level ratio fell from 1.07 ± 0.13 at 40 mg daily to only 0.09 ± 0.01 at 640 mg daily. Plasma HO‐P‐G rose exponentially with dose and demonstrated significant cumulation. HO‐P and HO‐P‐G in urine accounted for about 9% of long‐term propranolol doses. This study suggests a significant contribution of HO‐P to pharmacologic effects, in particular at low single and long‐term oral doses of propranolol and saturation of naphthalene ring oxidation as a main determinant of propranolol bioavailability.

Stereoselective clearance and distribution of intravenous propranolol

Our objective was to determine the kinetics of (+)‐ and (–)‐propranolol after intravenous doses of racemic drug. Five normal subjects received 0.1 mg/kg of a pseudoracemate of propranolol that consisted of deuterium‐labeled (+)‐propranolol and unlabeled (–)‐propranolol. Plasma concentrations of (+)‐ and (–)‐propranolol as measured by gas chromatography‐mass spectrometry demonstrated enantiomeric differences in systemic clearance (Cls) [(+)‐propranolol, 1.21 ± 0.15 l/min; (–)‐propranolol, 1.03 ± 0.12 l/min; P < 0.01] and apparent volume of distribution (Vd) [(+)‐propranolol, 4.82 ± 0.34 l/kg; (‐)‐propranolol, 4.08 ± 0.33 l/kg; P < 0.001], but no difference in distribution or elimination t½s (t½β 3.5 hr). The higher Cls of (+)‐propranolol suggests stereoselective hepatic elimination. The higher apparent Vd of (+)‐propranolol is mainly related to its lower plasma binding [(–)‐propranolol, 20.3 ± 0.8% unbound; (–)‐propranolol, 17.6 ± 0.7% unbound; P < 0.001]. There was no stereoselective uptake by red blood cells. These findings demonstrate that multiple stereoselective mechanisms are involved in the disposition of propranolol and determine the access of the drug to active sites.

Presystemic and systemic glucuronidation of propranolol.

The relative importance of presystemic and systemic glucuronidation of propranolol was examined in normal subjects given single oral and intravenous doses of propranolol. The areas under the plasma concentration‐time curves (AUCs) of propranolol glucuronide (PG), 41 ± 15 ng · hr/ml, and propranolol, 48 ± 15 ng · hr/ml, were of the same order after the intravenous dose (0.05 mglkg). After oral doses of 20 and 80 mg, the AUCs of PG were 302 ± 105 and 1,398 ± 409 ng · hr/ml; these were 7 times the AUCs of propranolol, 44 ± 15 and 220 ± 38 ng · hr/ml. The time lapse to peak concentration, 1.5 to 3.0 hr, and the plasma half‐life, 3.2 to 3.7 hr, were the same for PG and propranolol. These results demonstrate glucuronidation as an important determinant of propranolol bioavailability.

Increased clearance of propranolol and theophylline by high-protein compared with high-carbohydrate diet

The objective of this study was to determine whether changes in dietary protein and carbohydrate influence the oral clearance of propranolol, a high‐clearance drug, and theophylline, a low‐clearance drug. Six normal subjects studied in a clinical research center each received a single oral dose of propranolol, 80 mg, and theophylline, 5 mg/kg, after having been on each of two well‐defined diets for a period of 10 days. When the diet was altered from high carbohydrate/low protein to low carbohydrate/high protein, the oral clearance of propranolol increased by 74% ± 20% (mean ± SE; range 9% to 156%; P < 0.01) with no change in plasma half‐life or plasma binding. This dietary change resulted in an increase in theophylline clearance of 32% ± 6% (range 18% to 50%; P < 0.02) and a corresponding decrease in plasma half‐life of 26% ± 6% (range 6% to 42%; P < 0.05) with no alteration in the apparent volume of distribution. These observations reemphasize the importance of diet in drug disposition and suggest that the clearance of high‐clearance drugs like propranolol is more susceptible than the clearance of low‐clearance drugs to dietary manipulations, effects that may have to be considered in drug therapy.

Food‐induced increase in propranolol bioavailability—Relationship to protein and effects on metabolites

The influence of a meal on the disposition and metabolism of oral propranolol was examined in six normal subjects. The meal induced a mean 53% increase in propranolol bioavailability (range, 2% to 92%; P < 0.01) without affecting time to maximum concentration, half‐life, or the amount of unchanged drug in urine. There was no effect on the plasma concentrations of 4‐hydroxypropranolol or four other metabolites. The increased bioavailability was linearly related to the protein content of the meal (r = 0.884, P < 0.02) above a threshold content of about 7 gm.

Propranolol glucuronide cumulation during long-term propranolol therapy: a proposed storage mechanism for propranolol.

The comparative disposition of propranolol glucuronide (PG) and propranolol was determined in 35 patients with hypertension or coronary artery disease during initiation of propranolol therapy, during steady‐state conditions, and after discontinuation of propranolol (dose range, 40 to 960 mg daily, every 6 hr). The 2.3‐fold PG cumulation in plasma was identical to propranolol cumulation. PG plasma levels were about 4 times as high as propranolol levels over the whole dose range. Unexpectedly slow terminal elimination rate of propranolol (Ph approximately 16 to 24 hr) on discontinuation of propranolol appeared to be related to equally slow PG elimination. PG and propranolol could be detected in plasma and urine up to 3 to 5 days after propranolol discontinuation. The PG formed in man was deconjugated to propranolol in the dog after intravenous administration, suggesting that PG may serve as a storage pool for propranolol. Observations consistent with systemic and enteric deconjugation of PG, including enterohepatic recirculation, may, at least in part, explain the observed propranolol cumulation as well as the slow elimination of propranolol after its discontinuation. PG renal clearance (29 to 70 mi/min) and PG plasma levels were highly dependent on glomerular filtration rate, suggesting that PG may cumulate abnormally in patients with severe renal disease.

Depolarization‐induced release of propranolol and atenolol from rat cortical synaptosomes

1 The accumulation and release of [3H]‐propranolol and [3H]‐atenolol were examined in synaptosomes from rat cerebral cortex. 2 Synaptosomes accumulated 20 pmol propranolol and 0.6 pmol atenolol mg−1 protein when incubated at 30°C with radiolabeled drugs (0.1 μm). 3 Exposure of propranolol‐loaded synaptosomes to elevated K+, Rb+ or Cs+ evoked a concentration‐dependent increase in propranolol efflux. The action of these ions in releasing propranolol was highly correlated with their ability to produce synaptosomal membrane depolarization, as estimated with the voltage‐sensitive dye diS‐C3‐(5). 4 Elevated K+ also promoted atenolol release from synaptosomes in a concentration‐dependent manner. 5 Veratridine (10 μm) released propranolol and atenolol from synaptosomes and these effects were antagonized by tetrodotoxin (1 μm). 6 Under Ca2+‐free conditions, K+‐induced release of propranolol was reduced by 37% and atenolol release was diminished by 68%. 7 The results support the concept that both polar and non‐polar β‐adrenoceptor blocking drugs may be accumulated by nerve endings for release by membrane depolarization and suggest that neural storage and release of these molecules may influence their concentrations at localized sites of action.