Preparation of phenylephrine 3-O-sulfate as the major in vivo metabolite of phenylephrine to facilitate its pharmacokinetic and metabolism studies.

INTRODUCTION: The in vivo disposition and metabolism of phenylephrine have not been establishedby previous analytical methods and there is a lack of available standards for quantitating the metabolites. METHODS: We pursued and compared the preparation of sulfation metabolites of phenylephrine and its ethyl analog etilefrine via chemical and bio-synthesis. RESULTS: Both sulfates were obtained in higher yield and purity through chemical syntheses compared to biosynthesis. DISCUSSION: A facile method for the production of phenylephrine 3-O-sulfate and etilefrine 3-O-sulfate was established. These compounds will be useful in the development of analytical assays for studying the pharmacokinetics of phenylephrine and its main route of metabolism in the presence of formulation changes and pharmacogenetic variation.

[Bioavailability of etilefrine from Thomasin and Thomasin sustained-release tablets]. / Zur Bioverfügbarkeit von Etilefrin aus Thomasin- und Thomasin retard-Tabletten.

In 6 healthy volunteers absorption and elimination of etilefrine were studied in cross-over after intake of 20 mg each of a solution (A) and tablet (B) (Thomasin) or after intake of 25 mg as a sustained-release tablet (C; Thomasin retard). Etilefrine and its sulfoconjugate were measured by the GC technique in plasma and urine. From the data obtained the AUC-(plasma) and CUE-values (urine) were calculated. Peak plasma levels of 10 to 25 (A) and 6 to 13 ng . ml-1 were observed 30 min after intake. The concentrations decreased to the lower detection limit (less than 2 ng . ml-1) 2 h after intake. Plasma peak levels of 5 ng . ml-1 were measured 1-2 h after intake of the sustained-release form (C). The etilefrine plasma level decreased more slightly (C) than after intake of the other formulations (A, B). The etilefrine conjugate reached plasma peak concentrations of 600 ng . ml-1 1 h (A, B) or 2 h (C) after intake. A mean bioavailability of 70 and 58 per cent (Thomasin) or of 78 and 108 per cent (Thomasin retard) was calculated by comparison of the corresponding AUC- and CUE-values of the total etilefrine.

Dihydroergotamine increases the bioavailability of orally administered etilefrine.

Etilefrine undergoes considerable first-pass metabolism through conjugation in the gut wall. In six volunteers bioavailability was reduced to 35% for a fast release tablet and to 17% for a sustained release preparation. The addition of dihydroergotamine (DHE) to the sustained release preparation surprisingly increased bioavailability to 61%. The plasma levels of the main metabolite formed during the passage through the gut wall indicated an increase in the rate of enteric absorption and therefore also in bioavailability by DHE. This might be due to the influence of DHE upon the vascular resistance of the vessels in the gut wall or a change in gastro-intestinal motility with a prolongation of drug contact time within the absorbing gut segment.

Prodrugs of etilefrine: synthesis and evaluation of 3'-(O-acyl) derivatives.

A series of 3'-(O-acyl) derivatives of etilefrine (alpha-[(ethylamino)methyl]-3'-hydroxybenzyl alcohol) was synthesized. Correlations between structure and solubility, pKa value, lipophilicity, and esterase-catalyzed hydrolysis were demonstrated. Of special interest is the 3'-(O-pivaloyl) derivative, which shows, in addition to favorable solubility and improved lipophilicity, marked stability against enzymatic cleavage in blood along with a high rate of hydrolysis in the liver.

Pharmacokinetic comparison of etilefrine to its prodrug, the stearic acid ester of etilefrine.

The relative bioavailability of a prodrug of etilefrine, its stearic acid ester, was determined by means of plasma levels and renal excretion. The comparison of the plasma levels and renal excretion was carried out in a cross-over design in six subjects. 3H-etilefrine (20 mg) and 3H-2-etylamino-1-(3-stearoylphenyl)ethanol hydrochloride (44.42 mg) were administered orally in equimolecular amounts. The stearic acid ester of etilefrine does not appear in the blood; the ester is split even during absorption. The relative bioavailability of the stearic acid ester of etilefrine, which was determined from the comparison of the areas under the plasma level and the renal excretion, amounts to 51% related to etilefrine. The investigation of the renal excretory products after administration of etilefrine and its prodrug showed the same metabolic pattern. The sulfuric acid ester of etilefrine is the main metabolite. In addition to etilefrine, two basic metabolites are excreted. According to mass- and NMR-spectrometric findings, these two metabolites are two isomeric tetrahydroisoquinolines which formally developed by condensation of etilefrine with formaldehyde. These tetrahydroisoquinolines are excreted free and conjugated with sulfuric acid.

The physiological disposition of etilefrine in man.

Pharmacokinetic and metabolic studies with 3H-etilefrine were performed to assess the importance of a first-pass effect on the pharmacodynamic action of this sympathomimetic amine. Identical amounts of 3H-activity, ca. 80% of the dose, were excreted in the urine after intravenous or oral administration, which indicates complete enteral absorption of the drug. Comparison of the areas under the plasma curves of unchanged etilefrine after both routes of administration resulted in a bioavailability factor of 0.55, which can be explained by an extensive first-pass effect. The time curve of plasma levels of etilefrine was compatible with an open 2-compartment model characterized by a rather large volume of distribution (Vd, beta) of 160 1, and a predominant half life of 2 hours. The pharmacodynamic action corresponded to the amount of drug in the central compartment. The major pathway of metabolism of etilefrine was conjugation to form the phenolic sulphate, and a very minor proportion of the drug was excreted as the corresponding hydroxymandelic acid. This metabolic pattern seems to confirm our hypothesis that phenylalkylamines with hydroxyl group in the m-position of the benzene ring are predominantly conjugated in contrast to p-hydroxylated compounds which are mainly deaminated.