Comparison of enzymatic hydrolysis of pilocarpine prodrugs in human plasma, rabbit cornea, and butyrylcholinesterase solutions

Drug and pro-drug substrates and pseudo-substrates of human butyrylcholinesterase

Butyrylcholinesterase (BChE) is present in plasma and numerous cells and organs. Its physiological function(s) is(are) still unclear. However, this enzyme is of pharmacological and toxicological importance. It displays a broad specificity and is capable of hydrolyzing a wide range of substrates with turnovers differing by several orders of magnitude. Nowaday, these substrates include more than two dozen carboxyl-ester drugs, numerous acetylated prodrugs, and transition state analogues of acetylcholine. In addition, BChE displays a promiscuous hydrolytic activity toward amide bonds of arylacylamides, and slowly hydrolyzes carbamyl- and phosphoryl-esters. Certain pseudo-substrates like carbamates and organophosphates are major drugs of potential medical interest. The existence of a large genetic poly-allelism, affecting the catalytic properties of BChE is at the origin of clinical complications in the use of certain drugs catabolized by BChE. The number of drugs and prodrugs hydrolyzed by BChE is expected to increase in the future. However, very few quantitative data (Km, kcat) are available for most marketed drugs, and except for myorelaxants like succinylcholine and mivacurium, the impact of BChE genetic mutations on catalytic parameters has not been evaluated for most of these drugs.

Ocular prodrugs: Attributes and challenges

Ocular drug delivery is one of the most attention-grabbing and challenging endeavors among the numerous existing drug delivery systems. From a drug delivery point of view, eye is an intricate organ to investigate and explore. In spite of many limitations, advancements have been made with the intention of improving the residence time or permeation of the drug in the ocular region. Poor bioavailability of topically administered drugs is the major issue pertaining to ocular drug delivery. Several efforts have been made towards improving precorneal residence time and corneal penetration, e.g. iontophoresis, prodrugs and ion-pairing, etc. Prodrug approach (chemical approach) has been explored by the formulation scientists to optimize the physicochemical and biochemical properties of drug molecules for improving ocular bioavailability. Formulation of ocular prodrugs is a challenging task as they should exhibit optimum chemical stability as well as enzymatic liability so that they are converted into parent drug after administration at the desired pace. This review will encompass the concept of derivatization and recent academic and industrial advancements in the field of ocular prodrugs. The progression in prodrug designing holds a potential future for ophthalmic drug delivery.

Interspecies Differences in the Metabolism of a Multiester Prodrug by Carboxylesterases

The pentaethyl ester prodrug of the chelating agent diethylene triamine pentaacetic acid (DTPA) referred to as C2E5 is being developed as an orally bioavailable radionuclide decorporation agent. The predicted human efficacy obtained in these experimental animals is confounded by interspecies variations of metabolism. Therefore, in the present study, carboxylesterase-mediated metabolism of [(14)C]-C2E5 was compared in the S9 intestinal and hepatic fractions of human, dog, and rat and their respective plasma. Intestinal hydrolysis of C2E5, resulting in the formation of the tetraethyl ester of DTPA (C2E4), was only detected in human and rat. The primary metabolite in human and dog hepatic fractions was C2E4, whereas the predominant species identified in rat hepatic fractions was the triethyl ester (C2E3). Hepatic hydrolysis of C2E5 causes the formation of C2E4 in human, dog, and rat and C2E3 in rat only. Minimal C2E5 hydrolysis was observed in human and dog plasma, whereas in rat plasma C2E5 converted to C2E3 rapidly, followed by slower further metabolism. Both recombinant CES1 and CES2 play roles in C2E5 metabolism. Together, these data suggest that dogs may be the most appropriate species for predicting human C2E5 metabolism, whereas rats might be useful for clarifying the potential toxicity of C2E5 metabolites.

Enzymes involved in the bioconversion of ester-based prodrugs

Enzymes are essential for the activation of many prodrugs. In this review, the most important enzymes (e.g., paraoxonase, carboxylesterase, acetylcholinesterase, cholinesterase) involved in the bioconversion of ester-based prodrugs will be discussed in terms of their biology and biochemistry. Most of these enzymes fall into the category of hydrolytic enzymes. However, nonhydrolytic enzymes, including cytochrome P450s, can also catalyze the bioconversion of ester prodrugs and thus will be discussed here. Other factors influencing the ability of these enzymes to catalyze the bioconversion of ester-based prodrugs, particularly species and interindividual differences and stereochemical and structural features of the prodrugs, will be discussed.

Conversion of cyclosporine A prodrugs in human tears vs rabbits tears

The aim of this study was to evaluate the rate and mechanism of conversion of two water-soluble prodrugs of cyclosporine A (CsA) intended for topical delivery to the eye. The new molecules were designed according to the double prodrug concept: a solubilizing moiety was grafted onto CsA via an ester function, which could be hydrolysed via a two-step process (enzymatic and chemical). Prodrug solutions were prepared extemporaneously in an isotonic and neutral aqueous medium compatible with ophthalmic use. The rates of conversion into the parent molecule were determined by incubating the prodrugs in fresh rabbit or human tears or in a phosphate buffer solution (PBS) at pH 7.4. Both prodrugs were converted into CsA within the first minute in the presence of rabbit tears with rate constants of k=5.9x10(-3)min(-1) and k=3.8x10(-3)min(-1), respectively, for UNIL088 and UNIL089, whereas chemical conversion in PBS was negligible (k=0.5x10(-3)min(-1) for both molecules). Incubation of UNIL088 in human tears showed a significantly high conversion rate. It is concluded that the developed double prodrugs underwent a bioconversion in physiological media and thus represent promising candidates for topical delivery of CsA to the eye.

Synthesis and in vitro chemical and enzymatic stability of glycosyl 3'-azido-3'-deoxythymidine derivatives as potential anti-HIV agents

New glycosyl derivatives of 3'-azido-3'-deoxythymidine (AZT) (1 and 2) were synthesized in order to improve AZT retention in the blood and to guarantee its sustained release, overcoming the necessity of multiple drug administrations. The esters synthesized (1 and 2) link AZT, by a succinyl linker, to the C-3 position of glucose and to C-6 of galactose. Furthermore, the chemical and enzymatic stabilities of esters 1 and 2 were evaluated in order to determine both their stability in aqueous medium and their feasibility to undergo enzymatic cleavage by esterase to regenerate the original drug. The pharmacokinetic profiles of esters 1 and 2, obtained after systemic administration, showed an interesting controlled release, in particular for ester 2, compared to the pharmacokinetic profile of AZT.

New oligoethylene ester derivatives of 5-iodo-2'-deoxyuridine as dermal prodrugs: synthesis, physicochemical properties, and skin permeation studies

Five new oligoethylene ester derivatives (9-13) of 5-iodo-2'-deoxyuridine (IDU) were synthesized and assayed to determine their lipophilicity by both experimental lipophilicity indices (log K') and calculated partition coefficients (CLOGP). In vitro experiments were carried out to evaluate the chemical and enzymatic stability and fluxes through excised human skin of these new IDU derivatives. Esters 9-13 showed increased lipophilicity compared with the parent drug (IDU), had good stability in phosphate buffer (pH 7.4), and were readily hydrolyzed by porcine esterase. No correlation between lipophilicity and skin permeation fluxes of synthesized esters 9-13 was observed. Results from in vitro percutaneous absorption studies showed that, among all esters synthesized, only esters 9 and 10 significantly increased the cumulative amount of IDU that penetrated through excised human skin compared with the parent drug (IDU).

Synthesis and in vitro evaluation of aminoacyloxyalkyl esters of 2-(6-methoxy-2-naphthyl)propionic acid as novel naproxen prodrugs for dermal drug delivery

PURPOSE

To synthesize and evaluate various novel aminoacyloxyalkyl esters of naproxen (3a-i) and naproxenoxyalkyl diesters of glutamic and aspartic acids (3j-m) as potential dermal prodrugs of naproxen.

METHODS

The prodrugs 3a-m were synthesized, and their aqueous solubilities, lipophilicities and hydrolysis rates were determined in a buffered solution and in human serum. The permeation of selected prodrugs across excised postmortem human skin was studied in vitro.

RESULTS

The aminoacyloxyalkyl prodrugs showed higher aqueous solubilities and similar lipid solubilities, in terms of octanol-buffer partition coefficients (log Papp) at pH 5.0, when compared with naproxen. At pH 7.4 the prodrugs were significantly more lipophilic than naproxen. Prodrugs 3a-i showed moderate chemical stability in aqueous solutions at pH 5.0 and were rapidly converted to naproxen in human serum (t1/2 = 4-19 min). The selected aminoacyloxyalkyl prodrugs possessed a higher flux across the skin than naproxen, with a maximum enhancement of 3-fold compared to naproxen. Prodrugs 3j-m showed poor aqueous solubility and permeation across the skin.

CONCLUSIONS

Combinations of adequate aqueous solubility and lipophilicity of naproxen aminoacyloxyalkyl prodrugs having fast rates of enzymatic hydrolysis resulted in improved dermal delivery of naproxen.

Miotic effect and irritation potential of pilocarpine prodrug incorporated into a submicron emulsion vehicle

Pilocarpine prodrug, O,O'-dipivaloyl(1,2-ethylene) bispilocarpic acid diester, was introduced to a submicron emulsion vehicle in a dose equivalent to 0.5% pilocarpine base, and the formulation was studied in albino rabbits using miotic assay. Compared with pilocarpine HCl 0.5% solution delayed and prolonged miosis was observed after application of the prodrug emulsion. AUC(0-6 h) values for the prodrug emulsion and pilocarpine solution were 9252+/-1345 and 6845+/-1967%xmin, respectively. The prodrug was also administered twice daily for 5 days in the form of aqueous solution or submicron emulsion in order to study ocular irritation. Irritation potential of the prodrug was significantly reduced when submicron emulsion was used as a vehicle.

In vitro evaluation of acyloxyalkyl esters as dermal prodrugs of ketoprofen and naproxen

A series of acyloxyalkyl esters of ketoprofen and naproxen were synthesized and investigated as topical prodrugs with the aim of improving the dermal delivery of the drugs. In addition, some hydroxyalkyl esters of ketoprofen and naproxen were synthesized as possible intermediates of acyloxyalkyl prodrugs. All of the prodrugs were more lipophilic than their parent molecules, as evaluated by drug partitioning between 1-octanol and phosphate buffer at pH 7.4 (log Papp). However, their solubilities in aqueous solutions decreased markedly compared with the parent molecules. The prodrugs were stable toward chemical hydrolysis in aqueous solutions (pH 7.4), but were hydrolyzed to the parent drug both in 80% human serum and in human skin homogenate, with half-lives ranging from 4 to 137 min and from 13 to 403 min, respectively. The abilities of the selected naproxen acyloxyalkyl prodrugs to deliver naproxen through excised human skin were evaluated. Generally, the prodrugs showed similar dermal delivery as the parent drug through cadaver skin. In the present series of lipophilic prodrugs of naproxen, the prodrug with the highest aqueous solubility was the most effective prodrug to deliver naproxen through the skin.

Modified beta-cyclodextrin (SBE7-beta-CyD) with viscous vehicle improves the ocular delivery and tolerability of pilocarpine prodrug in rabbits

The complexation of pilocarpine prodrug, O,O'-dipropionyl-(1,4-xylylene) bispilocarpate, with various beta-cyclodextrin (beta-CyD) derivatives was studied by the phase solubility method. The effects of coadministered sulphobutyl ether beta-CyD (SBE7-beta-CyD) with and without poly(vinyl alcohol) (PVA) on the miotic response and eye irritation of the prodrug were investigated in pigmented rabbits. The pilocarpine prodrug formed 1:1 inclusion complexes with variably substituted sulphobutyl ether derivatives of beta-CyD (SBE4-beta-CyD and SBE7-beta-CyD), and 1:1 and 1:2 complexes with hydroxypropyl-beta-CyD (HP-beta-CyD) at pH 7-4. Coadministered SBE7-beta-CyD eliminated the eye irritation due to the pilocarpine prodrug, but also decreased the miotic response. Ocular absorption of the prodrug was improved by increasing the viscosity of prodrug/SBE7-beta-CyD solution with PVA without inducing any eye irritation. Eye irritation due to viscous prodrug/SBE7-beta-CyD solutions was comparable with isotonic NaCl solution. We conclude that administration of pilocarpine prodrug in viscous SBE7-beta-CyD solution decreases substantially eye irritation while ocular absorption is not affected.