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Purpose: To investigate mechanisms of compound-corneocyte interactions in a combined experimental and theoretical approach., Materials and Methods: Experimental methods are presented to investigate compound-corneocyte interactions in terms of dissolution within water of hydration and protein binding and to quantify the extent of the concurrent mechanisms. Results are presented for three compounds: caffeine, flufenamic acid, and testosterone. Two compartmental stratum corneum models M1 and M2 are formulated based on experimentally determined input parameters describing the affinity to lipid, proteins and water. M1 features a homogeneous protein compartment and considers protein interactions only via intra-corneocyte water. In M2 the protein compartment is sub-divided into a cornified envelope compartment interacting with inter-cellular lipids and a keratin compartment interacting with water., Results: For the non-protein binding caffeine the impact of the aqueous compartment on stratum corneum partitioning is overestimated but is successfully modeled after introducing a bound water fraction that is non-accessible for compound dissolution. For lipophilic, keratin binding compounds (flufenamic acid, testosterone) only M2 correctly predicts a concentration dependence of stratum corneum partition coefficients., Conclusions: Lipophilic and hydrophilic compounds interact with corneocytes. Interactions of lipophilic compounds are probably confined to the corneocyte surface. Interactions with intracellular keratin may be limited by their low aqueous solubility.

1-O-acyl glucuronides are reactive Phase II metabolites which can alkylate chemical nucleophiles. Industrial sulfate ester mixed anhydrides have been reported to be active acylating agents. This study was undertaken in order to establish that sulfate ester mixed anhydrides of clinically useful drugs could be synthesized, purified, and characterized as reactive chemical species. Their ability to alkylate 4-(p-nitrobenzyl)pyridine (NBP) and their stability in aqueous solution was compared with 1-O-acyl glucuronide conjugates of the same drugs. Synthesis of the 1-O-acyl glucuronides of the hypolipidemic agent, clofibric acid, and the nonsteroidal antiinflammatory drugs flufenamic acid and indomethacin were catalyzed by immobilized microsomal rabbit liver UDP-glucuronyltransferase. Potassium salts of the sulfate ester mixed anhydrides of these drugs were synthesized chemically by temperature-controlled reaction with chlorosulfonic acid in anhydrous pyridine. Half-lives at pH 2.0, 6.0, 7.4, and 10.0 were determined for each compound. The reactivity of the acyl glucuronides and sulfate ester mixed anhydrides towards the standard chemical nucleophile, 4-(p-nitrobenzyl pyridine (NBP), was measured using a spectrophotometric assay at several substrate concentrations. Acyl sulfate ester mixed anhydrides were shown to be 3-20 times more reactive towards NBP than their corresponding 1-O-acyl glucuronides. For both glucuronides and sulfate esters, relative reactivity towards NBP was: clofibric acid greater than indomethacin greater than flufenamic acid. This behavior paralleled the hydrolytic instability of the compounds.