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Biological thiol-dependent enzymes have recently received extensive attention in the literature because of their involvement in a variety of physiopathological conditions. The active thiol groups of these enzymes are derived from the cysteine residues present. Hence, in a biological system, the selective reversible or irreversible inhibition of the activity of these enzymes by modification of the thiol moiety may potentially lead to the development of a chemotherapeutic treatment. Despite all the research efforts involved in the attempt to develop potential chemotherapeutic treatments for the major diseases involving cysteine proteases, there are in fact no such treatments available yet. However, AG7088 (1) an inhibitor of rhinovirus-3C is in phase II/III clinical trial for the treatment of common cold and VX-740 (2, pralnacasan) an inhibitor of caspase-1 is in phase II clinical trial as an anti-inflammatory agent for rheumatoid arthritis. Several other cysteine protease inhibitors (i.e., cathepsin K, and S) are in pre-clinical evaluation or pre-clinical development. Structure-based drug design approaches have been instrumental in the development of these inhibitors. Intensive biochemical studies on the cysteine proteases have shed some light on some potential targets for therapeutic development. In addition, new techniques and new ideas are constantly emerging. As such, an up-to-date review of the literature on thiol-dependent enzymes as potential targets and their inhibitors designed from peptidic, modified peptidomimetic scaffolds and from small heterocyclic molecules is presented.

The occurrence of in vivo iron toxicity in the human body can be categorized into iron overload and non-iron overload conditions. Iron overload conditions are common in beta-thalassemia and hereditary hemochromatosis patients, and anthracycline mediated cardiotoxicity is an example of a non-iron overload condition in cancer patients, in which the toxicity is iron-dependent. While hundreds of iron chelators have been evaluated in animal studies, only a few have been studied in humans. Examples of iron chelator drugs are desferrioxamine (DFO), deferiprone (L1), and dexrazoxane (ICRF 187). The compound ICL670 has completed phase II clinical trials and a phase III trial is planned in 2003. Triapine is currently in phase II clinical trial as an anticancer agent. CP502, GT56-252, NaHBED, and MPB0201 are examples of new chelators in preclinical/clinical development. In the past decade, many new viable utilities for iron chelators have been reported. This includes the use of iron chelators as antiviral, photoprotective, antiproliferative, and antifibrotic agents. This review will focus on the status of drug development for the treatment of iron overload in patients with beta-thalassemia and the potential use of iron chelators in the prevention and treatment of other diseases.

A better understanding of the biological roles and the pathological consequences of thiol-dependent enzymes has emerged in recent years, and hence considerable progress has been made in identifying and delineating cysteine proteases that can be considered promising drug targets from those involved in housekeeping functions. Cysteine proteases have been implicated in a wide variety of disease processes ranging from cardiovascular, inflammatory, viral and immunological disorders to cancer. The first milestone in drug development of cysteine protease inhibitors has probably been reached, as IDN-6556 (a broad spectrum caspase inhibitor) has recently received Orphan Drug label by the U.S. Food and Drug Administration for use in the treatment of the patients undergoing liver transplantation and other solid organ transplantation. IDN-6556, which blocks apoptosis, is in Phase II human clinical trial in patients undergoing liver transplantation. In addition, more than ten cysteine protease inhibitors are presently at various phases of clinical development/trials for diverse diseases. This review emphasises on the new development from the literature reports since the year 2000 in the exploration of potential cysteine proteases as prospective drug targets, and the investigation of promising inhibitors that can potentially be developed for the treatment of human diseases. Transglutaminases, another class of thiol-dependent enzymes, are not discussed here.

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Histochemical methods of detecting iron in the rodent brain result mainly in the labeling of oligodendrocytes, but as all cells utilize iron, this observation suggests that much of the iron in the central nervous system goes undetected. Paraffin embedding of tissue is a standard procedure that is used to prepare sections for microscopic analysis. In the present study, we questioned whether we could modify the iron histochemical procedure to enable a greater detection of iron in paraffin sections. Indeed, various modifications led to the widespread labeling of iron in mouse brain tissue (for instance, labeling of neurons and neuropil). Sites of focal concentrations, such as cytoplasmic punctate or nucleolar staining, were also observed. The modified procedures were applied to paraffin sections of a mouse model (APP/PS1) of Alzheimer's disease. Iron was revealed in the plaque core and rim. The plaque rim had a fibrillary or granular appearance, and it frequently contained iron-labeled cells. Further analysis indicated that the iron was tightly associated with the core of the plaque, but less so with the rim. In conclusion, modifications to the histochemical staining revealed new insights into the deposition of iron in the central nervous system. In theory, the approach should be transferrable to organs besides the brain and to other species, and the underlying principles should be incorporable into a variety of staining methods.

Two related synthetic strategies were devised to convert the Diels-Alder adduct 3c of Boc-pyrrole and p-toluenesulfonylacetylene into various racemic and optically pure conduramines. One process consists of the regio-and stereoselective hydroxylation of 3c to the tri-and dihydroxylated azabicyclo-[2.2.1]heptane derivatives 10b (Scheme 2) and the exo-endo mixture 13-14 (Scheme 3). Anionic fragmentation of 10b (methylmagnesium bromide) and of the 13-14 sulfone mixture (lithium bis-(trimethylsilyl)amide) generated the corresponding tri-and dihydroxylated aminocyclohexenes 17 and 16 (Scheme 3). Compound 17 is an aminocyclitol with a stereochemistry and partial aminotriol sequence identical to that found in neoinosamine. Compound 16 served as a source of the protected and free aminodiols 35b and 35a (Scheme 6), which were stereospecifically epoxidized to 36 (Scheme 6) and 40 (Scheme 7). Phenylselenide cleavage of these epoxides provided 37 (Scheme 6) and 41a (Scheme 7), which after selenoxide cycloelimination and protecting group manipulation were converted into (()-conduramine C-1 (39a, Scheme 6) and the previously unreported, all-cisconduramine D-1 (43a, Scheme 7). In a second process, anionic fragmentaion of the bicyclic system is effected prior to introduction of the hydroxyl groups, as exemplified by the high-yielding conversion of the exo-endo mixture of azabicycloheptenes 11 and 12 into the aminocyclohexadiene 15 (Scheme 3). Osmate catalyzed cis-dihydroxylation of the derived bis-Boc derivative 20 (Scheme 4) led stereospecifically to the R-cis-diol 21 which was transformed into (()-conduramine A-1 (27a) and its tetraacetyl derivative 27b via the epoxy compound 24. On the other hand, peracid oxidation of the diene 15 gave the β-epoxide 28 (Scheme 5) which was cleaved to the trans-diol 29 with aqueous sulfuric acid. This diol was converted into (()-conduramine F-1 (34a) and its tetraacetyl derivative (34b) by a reaction sequence similar to that used for the other conduramine syntheses. Fractional crystallization of the diastereomeric mixture of Michael adducts obtained from (()-3c and (-)methyl lactate gave (-)-44a and (-)-45a both in g47% yield (Scheme 8). Both the carboxylic acid (+)-44b and the primary alcohol (+)-46 derived from (-)-44a were converted into (-)-3c with excess methylmagnesium bromide (ca. 40% overall yield). In the same way (-)-45a was transformed into optically pure (+)-3c. (-)-3c and (+)-3c were then converted into (-)-conduramine C-1 [(-)-39a] and (+)-conduramine D-1 [(+)-43a] by procedures identical to those used for the racemic compounds.

1,2,4-Thiadiazole is a distinctive class of small heterocyclic thiol trapping agents that serve as an interesting pharmacophore in the design of inhibitors targeting the cysteine residues of proteins. X-Ray crystal structures of enzyme-inhibitor complex indicate that the cysteine thiol reacts with the N-S bond of the thiadiazole moiety to form a disulfide bond resulting in the inactivation of the enzymes. This review addresses the medicinal chemistry and various properties of 1,2,4-thiadiazoles in their potential as new electrophilic "warheads" for targeting the cysteine residues of biomolecules (e.g, H+/K+ ATPase), and cysteine-dependent enzymes (e.g., cathepsin B and transglutaminase).