Improved delivery through biological membranes VIII: Design, synthesis, and in vivo testing of true prodrugs of aspirin

Autocatalytic methylthiomethylation of carboxylic acid/phenol involving the formation of DMSO enolate: convenient synthesis of methylthiomethyl ester/ether

This work reported a simple and practical protocol for the preparation of methylthiomethyl (MTM) esters/ethers directly from carboxylic acid/phenol and dimethylsulfoxide (DMSO) as solvent and methylthiomethyl source. With different types of carboxylic acids/phenols the reactions underwent smooth transformation to afford the corresponding MTM esters/ethers in moderate to excellent yields. This method features catalyst-free, easy to operate, broad substrate scope, good functional group tolerance and involvement of the formation of DMSO enolate.

This work reported a simple and practical protocol for the preparation of methylthiomethyl (MTM) esters/ethers directly from carboxylic acid/phenol and dimethylsulfoxide (DMSO) as solvent and methylthiomethyl source.

Natural COX-2 Inhibitors as Promising Anti-inflammatory Agents: An Update

COX-2, a key enzyme that catalyzed the rate-limiting steps in the conversion of arachidonic acid to prostaglandins, played a pivotal role in the inflammatory process. Different from other family members, COX-2 was barely detectable in normal physiological conditions and highly inducible during the acute inflammatory response of human bodies to injuries or infections. Therefore, the therapeutic utilization of selective COX-2 inhibitors has already been considered as an effective approach for the treatment of inflammation with diminished side effects. Currently, both traditional and newer NSAIDs are the commonly prescribed medications that treat inflammatory diseases by targeting COX-2. However, due to the cardiovascular side-effects of the NSAIDs, finding reasonable alternatives for these frequently prescribed medicines are a hot spot in medicinal chemistry research. Naturallyoccurring compounds have been reported to inhibit COX-2, thereby possessing beneficial effects against inflammation and certain cell injury. The review mainly concentrated on recently identified natural products and derivatives as COX-2 inhibitors, the characteristics of their structural core scaffolds, their anti-inflammatory effects, molecular mechanisms for enzymatic inhibition, and related structure-activity relationships. According to the structural features, the natural COX-2 inhibitors were mainly divided into the following categories: natural phenols, flavonoids, stilbenes, terpenoids, quinones, and alkaloids. Apart from the anti-inflammatory activities, a few dietary COX-2 inhibitors from nature origin also exhibited chemopreventive effects by targeting COX-2-mediated carcinogenesis. The utilization of these natural remedies in future cancer prevention was also discussed. In all, the survey on the characterized COX-2 inhibitors from natural sources paves the way for the further development of more potent and selective COX-2 inhibitors in the future.

Skin structure and metabolism: relevance to the design of cutaneous therapeutics

The outer layer of the epidermis or stratum corneum is the major barrier to percutaneous absorption. It has been shown that there are numerous enzyme systems beneath the stratum corneum in the viable epidermis capable of metabolizing drugs. A number of prodrug and soft drug topical therapeutic agents have been designed. After these agents penetrate the stratum corneum, they are metabolized by the cutaneous esterase systems to the desired metabolites.

Single oral dose study of two isosorbide-based aspirin prodrugs in the dog

The objective of this study was to compare two aspirin prodrugs, isosorbide diaspirinate (ISDA) and a nitroaspirin (ISMNA), with aspirin in terms of effects on dog platelet function after administration of a single oral dose. Groups of six dogs were administered ISDA (2 mg kg−1), ISMNA (4 mg kg−1) or aspirin (2 mg kg−1). Blood was sampled at 1, 2, 4, 8, 12 and 24 h post-dosing and evaluated for capacity to generate post-clotting thromboxane (TX)B2. The aggregation response to arachidonic acid (AA) (100 μM), ADP (30 μM) or collagen (10 μg mL−1) was estimated at each time-point using the whole blood impedance method. Plasma ISMN following oral administration of ISMNA was also measured and compared with plasma ISMN following administration of a physical mixture of ISMN and aspirin. ISDA administration (2 mg kg−1) was associated with a significant reduction (P< 0.05) in serum TXB2 at 12 and 24 h (>90%) post-dosing and persistent inhibition of AA-induced platelet aggregation. ISDA administration caused a more marked depression of post-clotting TXB2 levels than aspirin in this study, although its ability to inhibit platelet aggregation was less consistent than that of aspirin. The nitroaspirin ISMNA was least effective at inhibiting platelet aggregation response or TXB2 production. The ISMN AUC0–24h for the ISMNA-treated dogs was 77% of that for the physical mix-treated dogs and the tmax was delayed. This study indicates that the two aspirin esters cause aspirin-like effects on platelet function, probably through aspirin release, when administered orally to dogs.

Isosorbide-based aspirin prodrugs. II. Hydrolysis kinetics of isosorbide diaspirinate

Aspirin prodrugs have been intensively investigated in an effort to produce compounds with lower gastric toxicity, greater stability or enhanced percutaneous absorption, relative to aspirin. This report describes the hydrolysis kinetics and aspirin release characteristics of isosorbide diaspirinate (ISDA), the aspirin diester of isosorbide. ISDA underwent rapid hydrolysis when incubated in phosphate buffered human plasma solutions (pH 7.4) at 37 degrees C, producing appreciable quantities of aspirin. In 30% human plasma solution the half-life was 1.1 min and 61% aspirin was liberated relative to the initial ester concentration. The hydrolysis kinetics of ISDA were monitored in aqueous solution at 37 degrees C over the pH range 1.03-9.4. The aqueous hydrolysis followed pseudo-first-order kinetics over several half-lives at all pH values, resulting in a U-shaped pH rate profile. Salicylate esters and salicylic acid were formed during these processes. The hydrolysis characteristics of ISDA were also investigated in pH 7.4 phosphate buffered solutions containing alpha-chymotrypsin [EC 3.1.1.1] (t(1/2)=200.9 min), carboxyl esterase [EC 3.1.1.1] (t(1/2)=31.5 min), human serum albumin (t(1/2)=603 min), purified human serum butyrylcholinesterase [EC 3.1.1.8] (80 micro g/ml; t(1/2)=9.4 min; 55% aspirin), purified horse serum butyrylcholinesterase (100 micro g/ml; t(1/2)=1.85 min;11% aspirin) and in 10% human plasma solution in the presence of physostigmine (3 micro M). The results indicate that a specific enzyme present in human plasma, probably human butyrylcholinesterase, catalyses aspirin release from isosorbide diaspirinate.

Soft drugs. XX. Design, synthesis, and evaluation of ultra-short acting beta-blockers

A new type of ultra-short acting beta-blocker which might prove advantageous in treating acute arrhythmias was designed, synthesized and investigated. Based on the soft drug "inactive metabolite approach," the inactive phenylacetic acid metabolite of both metoprolol and atenolol was reactivated by esterification with sulfur-containing aliphatic alcohols. Since the sulfur-containing moieties are labile to the ubiquitous esterases, the new compounds should be inactivated by a one step enzymatic cleavage back to the inactive phenylacetic acid derivative. Pharmacological and pharmacokinetic profiles of the new compounds were evaluated in rats and rabbits. Isoproterenol-induced tachycardia was inhibited with short-term infusion of each compound. This tachycardia blocking effect rapidly disappeared upon termination of infusion, while beta-blocking activity was 2-4-fold longer after comparable doses of the short-acting beta-blocker, esmolol. The rapid recovery from the beta-receptor blockade is believed due to fast hydrolysis of the soft drugs in the body. This is supported from in vitro results showing the t1/2 of esmolol is about 10-fold longer than the new soft drugs in rat, rabbit, dog and human blood. Hydrolysis studies in phosphate buffered solutions indicated that the esters are labile to base-catalyzed hydrolysis. However, the relative t1/2 values measured in biological media compared to phosphate buffered solution clearly support rapid enzymatic cleavage of the soft drugs. Interestingly, one of the soft beta-blockers, the sulfonyl ester derivative, showed a unique property of exhibiting good beta-receptor blocking activity without significant hypotensive action.

The metabolism of a series of ester pro-drugs by NCTC 2544 cells, skin homogenate and LDE testskin

The metabolism of a series of substituted pyrazolopyridine ester pro-drugs was investigated using NCTC 2544 cells, human skin homogenate and LDE Testskin as model systems. The compounds were incubated in each system and the disappearance of drug and the production of the major hydrolysis product was observed with time and quantitated using HPLC. The toxicity of the ester pro-drugs and the metabolites was examined in NCTC 2544 cells using a cell viability assay procedure. Hydrolytic activity was slightly higher in the cell culture model than in skin homogenate solution but the rank order of activity for each pro-drug was similar. The metabolic activity of LDE Testskin was much reduced compared with the other systems, but again the overall pattern of metabolism was not dissimilar. These findings indicate that NCTC 2544 cells provide a reasonable model for human skin ester hydrolysis both in terms of rate and in terms of substrate specificity.

Pharmacokinetics and toxicity testing

The pharmacokinetic basis for the design of toxicity tests is discussed with reference to the absorption and clearance of drugs. The absorption and clearance of a wide range of drugs by laboratory animals and man has been examined and reviewed to provide a firm basis against which new drugs can be compared. Some pitfalls in either the empirical approach to toxicology or the incorrect interpretation of kinetic data are highlighted. An approach is outlined for the rational application of animal pharmacokinetic data in the assessment of the safety in man of a new therapeutic agent.

Improved delivery through biological membranes IX: Kinetics and mechanism of hydrolysis of methylsulfinymethyl 2-acetoxybenzoate and related aspirin prodrugs

The complex kinetics and mechanism of the hydrolysis of the methylthiomethyl, methylsulfinylmethyl, and methylsulfonylmethyl 2-acetoxybenzoates, novel aspirin prodrugs, were studied. The pH profiles for the related salicylates and benzoates also were determined. Based on the activation parameters, isotope effects, and other data, it was established that the methylthiomethyl esters hydrolyze via a unimolecular alkyl-oxygen cleavage. The methylsulfinylmethyl and methylsulfonylmethyl 2-acetoxybenzoates undergo neutral hydrolysis of esters at pH greater than 4 to form aspirin, while water acts as a general base; but at lower pH, a different mechanism takes place and the o-acetyl group cleaves first, releasing the corresponding salicylates.