Mechanism of transcorneal permeation of pilocarpine.

The mechanism of transcorneal permeation of pilocarpine has been investigated in relation to the physicochemical properties of the permeating species and its interaction with the membrane biophase. In vitro corneal transport experiments suggested the transport of un-ionized as well as ionized pilocarpine species across the corneal membrane. However, the permeability of the ionized pilocarpine species was 4.818 x 10(-6) cm s-1, a value only one-half of that obtained for the un-ionized pilocarpine species (9.744 x 10(-6) cm s-1). Further evidence of ion transport across the cornea was obtained by examining the transport of the quaternized pilocarpine compound (i.e., pilocarpinium methyl iodide). The quaternized compound had a corneal permeability of 4.66 x 10(-6) cm s-1, similar to that obtained for the ionized pilocarpine species. The lipoidal epithelial layer of the corneal membrane appears to be the predominant barrier to the transport of polar species. Therefore, the transport of pilocarpinium cations across the lipoidal epithelium might have occurred as tightly bound ion pairs with dihydrogen phosphate and/or nitrate counter ions. Excellent linear correlation has been obtained between pilocarpine corneal permeability and the 1-octanol-water partition coefficient as a function of the state of ionization of pilocarpine. The ratio of un-ionized to ionized drug permeability across the cornea is expected to be much higher for drugs with higher 1-octanol-water partition coefficients.

Single-dose pharmacokinetics of clomiphene citrate in normal volunteers.

Twenty-four healthy adult female volunteers participated in a randomized, three-phase double-blind crossover trial comparing the single-dose (50 mg) pharmacokinetics of three formulations of clomiphene citrate (CC). Plasma levels of both the Z(cis) and E(trans) isomers of CC, as well as principal metabolites, were determined at periodic intervals; and no differences between formulations were observed. The active Z isomer attained peak blood levels later than the inactive E isomer and was eliminated much more slowly, significant plasma concentrations still being detected up to 1 month after treatment. The results of this study demonstrate that three commercially available formulations of CC are bioequivalent.

Pilocarpine prodrugs. II. Synthesis, stability, bioconversion, and physicochemical properties of sequentially labile pilocarpine acid diesters.

Various novel diesters of pilocarpic acid were synthesized and evaluated as prodrug forms for pilocarpine with the aim of improving the ocular delivery characteristics of the drug. The pilocarpic acid monoesters previously studied cyclized spontaneously to pilocarpine in aqueous solution and although they showed enhanced corneal permeability when compared with pilocarpine these monoesters suffered from poor solution stability. The present study shows that this problem can be totally overcome by blocking the free hydroxyl group in the monoesters. Diesters of pilocarpic acid were obtained by esterification of this group. Such compounds were found to possess a high stability in aqueous solution (shelf lives of more than 5 years at 20 degrees C were estimated) but at the same time were readily converted to pilocarpine under conditions simulating those occurring in vivo through a sequential process involving enzymatic hydrolysis of the O-acyl bond followed by spontaneous lactonization of the intermediate pilocarpic acid monoester. Rate data are given for the conversion of the diesters in human plasma and in various rabbit eye homogenates. The pH-solubility profile was derived for a diester and lipophilicity parameters were determined for the compounds. All diesters were markedly more lipophilic than pilocarpine and the corresponding pilocarpic acid monoesters. The results suggest that pilocarpic acid diesters may be potentially useful pilocarpine prodrugs as they combine a high solution stability with an adequate rate of conversion to pilocarpine under in vivo conditions.

Pilocarpine prodrugs I. Synthesis, physicochemical properties and kinetics of lactonization of pilocarpic acid esters.

Various alkyl and aralkyl esters of pilocarpic acid were synthesized and evaluated as prodrug forms for pilocarpine with the purpose of improving the ocular bioavailability of pilocarpine through increased corneal membrane permeability. The esters were found to undergo a quantitative cyclization to pilocarpine in aqueous solution of pH 3.5-10, the rate of cyclization being a function of the polar and steric effects within the alcohol portion of the esters. The rates of lactonization increased proportionally with the hydroxide ion activity over the pH range studied which is in accord with a reaction mechanism involving intramolecular nucleophilic attack of alkoxide ion on the ester carbonyl moiety. At pH 7.4 and 37 degrees C, half-times of lactonization ranging from 30 min (p-chlorobenzyl ester) to 1105 min (n-hexyl ester) were observed for the various esters. The esters are markedly more lipophilic than pilocarpine. The results suggested that the pilocarpic acid esters may be potentially useful prodrugs, especially when further derivatized to give in vitro stable pilocarpic acid diesters.

Analysis of clomiphene isomers in human plasma and detection of metabolites using reversed-phase chromatography and fluorescence detection.

A method is described for the quantitative clinical analysis of plasma concentrations of the E and Z isomers of clomiphene, which is used in the induction of ovulation. The isomers of clomiphene, in addition to metabolites, are extracted from plasma with tert-butyl methyl ether (MTB). The MTB layer is dried, reconstituted and an aliquot subjected to chromatography. The drug and metabolites are separated by reversed-phase high-performance liquid chromatography. The eluent is fed into a knitted or braided, cylindrical reaction coil made of Teflon, into which is inserted a low-energy mercury lamp. This results in a photoinduced stilbene-to-phenanthrene oxidation yielding highly fluorescent analytes; this provides excellent sensitivity for the quantitation of the intact drug isomers and the detection of presently uncharacterized metabolites. Use of the reversed-phase chromatographic mode results in elution of the polar metabolites prior to the intact drug isomers. A combination of reversed-phase chromatography and an in-line post-column reaction coil results in a sensitive method that is more reliable and rapid than those previously reported and is applicable to the routine analysis of clinical samples. The method has been applied to individual isomers of clomiphene in plasma at concentrations of 0.06-600 ng/ml.

High-performance liquid chromatographic determination of pilocarpine in aqueous humor: derivatization by quaternization of methylimidazole tertiary amine group.

A derivatization procedure and a high-performance liquid chromatographic (HPLC) method of analysis for pilocarpine are described. The method is based on the quaternization of the 3'-tertiary amino group of the methylimidazole ring of pilocarpine with p-nitrobenzyl bromide. The HPLC system employs and RP-ODS column with a methanol-water mobile phase containing octanesulfonate as an ion-pairing agent. The sensitivity of the method permits the analysis of pilocarpine in biological fluids such as aqueous humor. The method is selective for pilocarpine in the presence of isopilocarpine. Its applicability to the analysis of aromatic heterocyclic and alkyl tertiary amines is demonstrated.

Effect of self-association of phenol on its transport across polyethylene film.

Phenol diffusion across a high density polyethylene film from isooctane, in which phenol self-associates, demonstrated nonideal behavior. The steady-state flux of phenol across the film was not directly proportional to its concentration in the donor phase. At higher donor phase concentrations, negative steady-state flux deviations were observed. These negative deviations were due to phenol self-association in the donor phase and the resulting decreases in thermodynamic activity. By using a monomer--pentamer model for phenol self-association in isooctane, the steady-state flux was shown to be directly proportional to the phenol monomer concentration in the donor phase. Although steady-state flux concentration deviations were observed, the diffusion time lag was independent of the permeant concentration and reflected the intrinsic diffusivity of the film to phenol.