Current Medicinal Chemistry - Volume 7, Issue 12, 2000

Calpain is unique among the cysteine protease family of enzymes in that it combines thiol protease activity with calmodulin-like activity. Despite its wide spread distribution the exact physiological function(s) of calpain is yet to be deciphered. The enzyme is however, implicated in a number of pathophysiological conditions. Due to the potential of calpain as a therapeutic target a number of inhibitors have been described for the enzyme. In this article we have grouped calpain inhibitors into those derived from natural sources, and those derived from chemical synthesis. Additionally, an overview of functional groups that have been used as “warheads” of calpain inhibitors is presented along with a discussion of the structure activity relationship studies of the address region of peptidyl calpain inhibitors. Recent work in this area has led to a better understanding of the structural requirements for tight binding of inhibitors to the active site of calpain. A discussion of peptidomimetic calpain inhibitors, nonpeptide calpain inhibitors, and selectivity of some calpain inhibitors are also presented. The recent disclosure of the crystal structure of a nonpeptide calpain inhibitor bound to a hydrophobic pocket on the calcium-binding domain of calpain has opened the door to future development of potent cell permeable nonpeptide calpain inhibitors.

DNA topoisomerases are essential for the survival of prokaryotic and eukaryotic organisms. Topoisomerases inhibitors, due to their capacity to induce DNA breaking, can exhibit interesting antitumor properties. While there are many potent antitumor agents which target topoisomerase II, relatively few families of specific topoisomerase I inhibitors have been identified.The present review describes a new family of topoisomerase I inhibitors, analogues of the bacterial metabolite rebeccamycin. These compounds possess an indolocarbazole chromophore onto which is attached a sugar residue. Important structure-activity relationships studies in this series have helped to understand the role of the carbohydrate moiety which is absolutely necessary for topoisomerase I poisoning, the influence of the stereochemistry (α or β) of its linkage to indole, the influence of the functionalities and substitutions on the sugar moiety and on the aromatic framework have been investigated. In addition to their action on DNA, rebeccamycin analogues may inhibit the SR kinase activity of topoisomerase I and therefore constitute a unique family of topoisomerase I poisons quite different from the well known camptothecins.

Initiation, progression, and completion of the cell cycle are regulated by various cyclin-dependent kinases (CDKs), which are thus critical for cell growth. Tumour development is closely associated with genetic alteration and deregulation of CDKs and their regulators, suggesting that inhibitors of CDKs may be useful anti-cancer therapeutics. Indeed, early results suggest that transformed and normal cells differ in their requirement for e.g. cyclin/CDK2 and that it may be possible to develop novel antineoplastic agents devoid of the general host toxicity observed with conventional cystostatic drugs. Numerous active-site inhibitors of CDKs have been studied; the main limitation with these ATP antagonists is kinase specificity for CDKs. However, screening of compound collections, as well as rational design based on enzyme-ligand complex crystal structures, are now yielding pre-clinical candidates, particularly certain purine and flavonoid analogues, with impressive potency and selectivity. Natural CDK inhibitors (CKIs), e.g. the tumour suppressor gene products p16INK4, p21WAF1, and p27KIP1, form the starting point for the design of mechanism-based CDK inhibitors. A number of these small proteins have been dissected and inhibitory lead peptides amenable to peptidomimetic development have been identified. Conversion of these peptides into pharmaceutically useful molecules is greatly aided by the recent elucidation of CKI/CDK crystal and solution structures. Additional interaction sites on CDKs being exploited for the purposes of inhibitor design include: phosphorylation/dephosphorylation sites, macromolecular substrate binding site, CKS regulatory subunit binding sites, cyclin-binding site, cellular localisation domain, and destruction box. Finally, progress has recently been made in the application of antisense technology in order to target CDK activity.

Crisis periods in development are critical periods of cell death that have long been suggested as “epigenetic crises” which are central to normal and abnormal embryogenesis. Under in-vitro culture conditions, there are similar crisis periods or Hayflick limits of culture senescence. Epigenetic modulations from CpG methylation coupled to DNA replication provide an alternate timing mechanism to the telomere/telomerase biological clock. Physiological cell death in both development and in in-vitro isolates is primarily apoptotic. Arguments of divergent apoptotic death commitments as caspase-dependent and -independent pathways seem to suggest that there is no possibility of a global life-and-death signal. However recent reports implicating CpG-specific cleavage in apoptosis implies that the powerful imprint mark that silences genes, protects genes from nuclease restriction, and modulates chromatin conformations, could provide a common commitment pathway of convergence in the death cascade. If the imprint mark were central to the apoptotic commitment, then apoptosis is Lamarckian not Darwinian.

Radical induced oxidative damage is extremely harmful to tissues and organs due to molecular modifications brought to polyunsaturated membrane lipids, proteins and nucleic acids. Oxidative stress is believed to be one of the pathophysiological mechanisms that operate in neurodegenerative disorders such as cerebral ischemias, amyotrophic lateral sclerosis, Parkinson's and Alzheimer's diseases.Nitrones oppose oxidative challenges by virtue of their ability to trap very rapidly oxygen or carbon centered radicals thus generating nitroxide radical species which are more stable and biochemically less harmful than the original radical. However the operational mechanism of nitrones might also go beyond direct scavenging of radicalsThe chemical and pharmacological properties of nitrones depend strongly on the connectivity as well as on the type and position of the substituents in the compound's architecture.Heteroaryl-nitrones are known, but except for a few cases (for example pyridyl-nitrones) no particular attention has been given to this class of molecules. The following review is a survey of the literature reports on this subject from 1980 to 1999. The structures were classified according to the heterocyclic substituent on the nitrone double bond, and documented pharmaceutical features were emphasized. Whenever possible heteroaromatic and related aromatic nitrones were compared.

Developments in the field of adenosine receptor (AR) agonists of the past years are presented and discussed. Four different AR subtypes, A1, A2A, A2B, and A3, have been cloned from different species including the human receptors. Recombinant ARs expressed in permanent mammalian cell lines have found wide application in the screening of new ligands. Considerable differences are observed among data from different laboratories, using recombinant receptors for the assays. Reevaluation of compounds at all four receptor subtypes has shown that agonists that were believed to be selective for either A1 or A2A ARs may be potent A3 agonists and thus, non-selective. Potent and selective agonists for two of the AR subtypes, A1 and A3, have been developed. Truly selective A2A AR agonists, however, are presently not available. Potent or selective A2B agonists are still lacking. Since the treatment with AR agonists may lead to fast desensitization of the receptors, partial agonists, and indirect AR agonists, such as adenosine kinase inhibitors, or allosteric enhancers of adenosine binding, are being developed as site- and event-specific agents.

The metallocene dihalides are a relatively new class of small, hydrophobic organometallic anticancer agents that exhibit antitumour properties against numerous cell lines including leukemias P388 and L1210, colon 38 and Lewis lung carcinomas, B16 melanoma, solid and fluid Ehrlich ascites tumours and several human colon and lung carcinomas transplanted into athymic mice. Titanocene dichloride 1 has been the most widely studied metallocene and the drug is currently in phase II clinical trials. Formation of metallocene-DNA complexes has been implicated in the mechanism of antitumour properties of the metallocenes, as both titanocene dichloride 1 and vanadocene dichloride 2 inhibit DNA and RNA synthesis, and titanium and vanadium accumulate in nucleic acid-rich regions of tumour cells. However, in contrast to the well characterized platinum-based anticancer drugs, the active species responsible for antitumour activity in vivo has not been identified and the mechanism whereby irreparable DNA damage and/or structural modification of DNA or other cellular targets occurs is poorly understood. This review will focus on recent studies that have been carried out in order to identify the biologically active species and more fully understand the molecular level mechanism of action of the metallocene dihalides. Studies with nucleotides, oligonucleotides, DNA and proteins including topoisomerases, protein kinase C and transferrin have provided important insight into potential cellular transport mechanisms and the interaction of metallocenes with biomolecular targets. New structure activity studies including the design of hydrolytically stable metallocenes and the preparation of highly water soluble amino acid analogues have not led to improved anticancer activity of titanocene dichloride 1. The vastly different chemical and hydrolytic stability of each of the metallocenes points to a unique mechanism of action of each metallocene in vivo

20-Hydroxyecdysone (20E) is an insect molting hormone that is also widely spread in various plants. Many chromatographic methods can be used to identify and/or determine 20E content in samples of biological origin and various spectroscopic methods serve to identify its structural elements. We have utilized X-ray crystallography to reveal the stereostructures of 20E. Our data demonstrates that 20E exists in two different crystalline forms that are both orthorhombic modifications. One form is homo-molecular, with a limited freedom of internal rotation of the side chain around the C23-C24 bond and the other, which is a clathrate formed with methanol and water, which minimize the conformational freedom of the side chain.