Plasma protein binding and blood cell distribution of propranolol enantiomers in rats

Determination of unbound warfarin enantiomers in human plasma and 7-hydroxywarfarin in human urine by chiral stationary-phase liquid chromatography with ultraviolet or fluorescence and on-line circular dichroism detection

Enantiomers of warfarin and 7-hydroxywarfarin in human plasma and urine, respectively, were determined by high-performance liquid chromatography using a cellulose-derivative column with UV or fluorescent detection, and their absolute configuration was determined simultaneously by a circular dichroism spectropolarimeter connected in series. Enantiomers of warfarin and its major metabolites [i.e., (R)-6-hydroxywarfarin, (S)-7-hydroxywarfarin and (RS)-warfarin alcohol] were well resolved. The method was precise and sensitive: within- and between-day coefficients of variation were <9.6% for warfarin enantiomers in plasma and <7.1% for 7-hydroxywarfarin enantiomers in urine, respectively, and the lower detection limits were 20 ng/ml for (R)-warfarin, 40 ng/ml for (S)-warfarin, 2.5 ng/ml for (R)-7-hydroxywarfarin and 4.5 ng/ml for (S)-7-hydroxywarfarin in 0.5 ml of both plasma and urine. The ultrafiltration technique was used for determining unbound concentrations of warfarin enantiomers in plasma using [14C]warfarin enantiomers resolved by the present HPLC system. Clinical applicability of the method was evaluated by determining unbound concentrations of warfarin enantiomers in five consecutive plasma samples obtained from a patient exhibiting an unstable anticoagulant response to warfarin (4 mg/day, p.o.). Results indicated that the present method would be useful in clarifying factors responsible for a large intra- and inter-patient variability in warfarin effects with regard to unbound plasma enantiomer pharmacokinetics.

An investigation of the use of nile red as a long-wavelength fluorescent probe for the study of alpha 1-acid glycoprotein-drug interactions

Spectrofluorimetry in the long-wavelength region of the electromagnetic spectrum (600-1000 nm) is a fairly recent development in photoluminescence spectroscopy, which has numerous advantages over measurements in the more conventional ultraviolet and visible spectral region. 9-Diethylamino-5H-benzophenoxazine-5-one (Nile Red) is an unchanged, hydrophobic molecule, and long-wavelength fluorescence of which is strongly influenced by the polarity of its environment. When Nile Red was added to solutions of alpha 1-acid glycoprotein (Orosomucoid. OMD), it showed an enhancement in fluorescence intensity and a shift to blue in emission wavelength, suggesting it was binding hydrophobically to a non-polar site on the protein. The association constant (12,261,000 +/- 900,000 M-1) and number of binding sites (0.746 +/- 0.044) were calculated for the probe. Upon addition of both acidic and basic drugs, the Nile Red fluorescence reverted to its unbound form, indicating that OMD probably has one high-affinity, wide and flexible binding area for such drugs. Possible enantiomeric selectivity was shown with ephedrine, and the association constant determined for a racemic mixture of propranolol was found to be comparable to other values obtained with alternative, more conventional techniques.

Stereoselective binding of beta-blockers to purified rat alpha 1-acid glycoprotein

The stereoselectivity of binding of four beta-blockers, pindolol, propranolol, oxprenolol and acebutolol, to purified rat alpha 1-acid glycoprotein (AAG) was examined using equilibrium dialysis. Pindolol and propranolol were bound stereoselectively to AAG, whereas binding of oxprenolol was non-stereospecific. Neither of the enantiomers of acebutolol bound to either AAG or any other plasma protein. The affinity of (+)-pindolol was 25 times that of (-)-pindolol, as determined in a single enantiomer experiment. Both enantiomers of propranolol demonstrated two classes of binding sites in AAG, the total binding for the high affinity site for (+)-propranolol being double that of (-)-propranolol, which could explain the higher binding of the (+)-enantiomer in racemate experiments. These results further showed that stereoselective binding to a rat AAG is not a property common to all beta-blockers.

Selective effect of adjuvant arthritis on the disposition of propranolol enantiomers in rats detected using a stereospecific HPLC assay

Nonstereospecific studies have indicated that the pharmacokinetics of propranolol (PR) are altered in inflammatory conditions such as arthritis. However, as the kinetics and dynamics of PR are stereoselective, we examined the effect of adjuvant arthritis (AA) on the disposition of the individual enantiomers. A novel normal-phase stereospecific HPLC assay for PR was developed involving chiral derivatization with S-(naphthyl)ethyl isocyanate and fluorescence detection. Oral and iv doses of racemic PR were administered to control and AA rats (n = 6). AA had no significant effect on either clearance or S:R ratio after iv doses. On the other hand, after oral doses, clearance was significantly decreased in AA. Although significant for both enantiomers, this effect was more pronounced on the less active R-enantiomer. The AUC R:S ratio was, therefore, significantly altered (AA, 14 +/- 3.0; control, 4.3 +/- 1.2). Increased total (S+R) plasma concentrations of PR in AA, possibly due to a reduced intrinsic clearance, therefore, reflect mainly increased concentrations of the less active R-enantiomer.

The influence of aging on the stereoselective pharmacokinetics of propranolol in the rat

The influence of aging on the pharmacokinetics and the tissue distribution of (R)- and of (S)-propranolol was studied in 3-, 12-, and 24-month-old rats. After both iv and oral administration of rac-propranolol, the plasma concentrations were higher for the (R)- than for the (S)-enantiomer. For the tissue concentrations, the reverse was true. The free fraction of (S)-propranolol in plasma was about 4 times larger than that of (R)-propranolol, and this is the main factor responsible for the differences in kinetics between the two enantiomers. There was a suggestion for a difference in tissue binding between the two enantiomers. With aging, the plasma and tissue concentrations of both enantiomers increase, probably due to a decrease in blood clearance. Tissue binding did not change much with aging. Notwithstanding the marked differences between the kinetics of the propranolol enantiomers, the changes which occur with aging affect both enantiomers to the same degree.

Apparent stereoselectivity in propafenone uptake by human and rat erythrocytes

The distribution of propafenone (PPF) enantiomers between the plasma and red blood cells (RBCs) was investigated using human and rat blood. In separate experiments, effects of incubation time (15-60 min), blood concentration (100-5000 ng ml-1), and plasma proteins on the RBC uptake of the enantiomers were studied. In both humans and rats, the distribution of propafenone enantiomers into RBCs was rapid, extensive, and stereoselective. However, the extent of RBC uptake and the direction of stereoselectivity were different in these two species. In humans, preferential distribution of (-)-PPF into RBCs resulted in lower plasma concentrations for this enantiomer, whereas in rat plasma, (-)-PPF was the dominant enantiomer. When the plasma was replaced with buffer, the stereoselectivity in the RBC uptake of the enantiomers was abolished. This suggested that stereoselective protein binding may be responsible for this phenomenon. A direct measurement of the extent of binding of PPF enantiomers to rat and human plasma proteins further confirmed this. Moreover, the distribution of the enantiomers in RBCs was not affected by low temperatures or addition of ouabain, suggesting passive diffusion as the underlying mechanism. These results suggest that stereoselective red blood cell uptake may be responsible, at least in part, for the differences in the plasma pharmacokinetics of PPF enantiomers observed after the drug administration to humans and rats.