Thiol proteases: inhibitors and potential therapeutic targets.

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.

Synthesis of 3-substituted bicyclic imidazo[1,2-d][1,2,4]thiadiazoles and tricyclic benzo[4,5]imidazo[1,2-d][1,2,4]thiadiazoles.

A versatile synthetic route to potentially useful fused-ring [1,2,4]thiadiazole scaffolds (e.g., 7a and 10b) via exchange reactions of the precursor [1,2,4]thiadiazol-3-(2H)one derivatives (e.g., 6 and 9) with appropriately substituted nitriles (e.g., cyanogen bromide or p-toluenesulfonyl cyanide) under mild conditions is described. For example, the tricyclic 3-bromo [1,2,4]THD derivative (7a) underwent S(N)Ar substitution with a variety of nucleophiles, which included amines, malonate esters and alcohols. Likewise, the bicyclic 3-p-tosyl [1,2,4]THD (10b) was employed as a template in reaction with diamines, and the resulting substituted diamines (e.g., 12a or 12e) were further selectively derivatized at the N1 and/or N2 positions in a linear fashion. The X-ray crystal structure of the 3-methyl bicyclic [1,2,4]THD (21) was obtained, and selective methylation at the N1 position via a protection-alkylation-deprotection protocol, as illustrated in Scheme 6, was confirmed. Alternatively, a short convergent synthesis of N1-functionalized derivatives from the reaction of 10b with appropriately substituted secondary amines was also developed. Hence, these synthetic strategies were advantageously exploited to provide access to a variety of diversely derivatized 3-substituted fused-ring [1,2,4]thiadiazole derivatives.

Medicinal chemistry and properties of 1,2,4-thiadiazoles.

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).

1,2,4-thiadiazole: a novel Cathepsin B inhibitor.

A novel class of Cathepsin B inhibitors has been developed with a 1,2,4-thiadiazole heterocycle as the thiol trapping pharmacophore. Several compounds with different dipeptide recognition sequence (i.e., P1'-P2'=Leu-Pro-OH or P2-P1=Cbz-Phe-Ala) at the C5 position and with different substituents (i.e., OMe, Ph, or COOH) at the C3 position of the 1,2,4-thiadiazole ring have been synthesized and tested for their inhibitory activities. The substituted thiadiazoles 3a-h inhibit Cat B in a time dependent, irreversible manner. A mechanism based on active-site directed inactivation of the enzyme by disulfide bond formation between the active site cysteine thiol and the sulfur atom of the heterocycle is proposed. Compound 3a (K(i)=2.6 microM, k(i)K(i)=5630 M(-1)s(-1)) with a C3 methoxy moiety and a Leu-Pro-OH dipeptide recognition sequence, is found to be the most potent inhibitor in this series. The enhanced inhibitory potency of 3a is a consequence of its increased enzyme binding affinity (lower K(i)) rather than its increased intrinsic reactivity (higher k(i)). In addition, 3a is inactive against Cathepsin S, is a poor inhibitor of Cathepsin H and is >100-fold more selective for Cat B over papain.

Iron chelator research: past, present, and future.

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.

Thiol-dependent enzymes and their inhibitors: a review.

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.