Isaac O. Donkor

A Survey of Calpain Inhibitors

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.

Calpain inhibitors: a survey of compounds reported in the patent and scientific literature

Introduction: Calpain is a cysteine protease that participates in normal signal transduction events, but deregulation of its action may result in pathologies such as neurological disorders, muscular dystrophies, cataract, cancer and diabetes. Inhibition of calpain is demonstrably beneficial in animal models of these diseases; hence, the enzyme has been proposed as a potential drug target. Areas covered: A comprehensive review of calpain inhibitors reported in the patent and scientific literature over the past decade (2001 – 2010) is presented in this paper. Peptides, peptidomimetics and nonpeptide inhibitors with and without reactive warheads are discussed. Advances made in enhancing the cellular uptake of peptide calpain inhibitors, improving the pharmacokinetic properties of the inhibitors and site specific targeting of calpain inhibitors are also discussed. Expert opinion: Calpain inhibitors have demonstrated efficacy in animal models of calpain related diseases, but progression of the inhibitors into clinical trials has been hampered partly due to lack of calpain isoform selectivity and the general reactivity of the inhibitors. Therefore, efforts should be directed towards the discovery of compounds devoid of these problems. Exploration of compounds that bind to allosteric sites of the enzyme may circumvent these problems and afford new drug leads.

Synthesis and radioprotective effects of adamantyl substituted 1,4-dihydropyridine derivatives

A series of 2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylic acid diesters substituted at the N-1 and/or C-4 positions of the dihydropyridine ring was synthesized. The in vitro cytotoxicity and in vitro and in vivo radioprotective efficacy of these agents were evaluated in Chinese hamster (V-79) cells and CD2F1 male mice, respectively. Compounds with at least one adamantyl substituent afforded better radioprotection than those without this substituent. Substitution of an aromatic ring at the C-4 position of the dihydropyridine ring did not enhance the radioprotectant action of the compounds.

Synthesis and DHFR inhibitory activity of a series of 6-substituted-2,4-diaminothieno[2,3-d]pyrimidines

A series of 6-aralkyl substituted 2,4-diaminothieno[2,3-d]pyrimidines in which the 6-aryl group is separated from the thieno[2,3-d]pyrimidine ring by two to five methylene groups were synthesized and studied as inhibitors of dihydrofolate reductase from Pneumocystis carinii, Toxoplasma gondii, Mycobacterium avium, and rat liver. Compounds in which the thieno[2,3-d]pyrimidine ring is separated from the 6-aryl substituent by three methylene groups were the most potent inhibitors of the series (with IC(50) values ranging from 0.24 and 11.0 microM) but those with two methylene groups between the aromatic rings were the most selective agents.

Synthesis and anti-Pneumocystis carinii activity of conformationally restricted analogues of pentamidine

Abstract Aseries of conformationally restricted analogues of pentamidine in which the flexible central bridge has been replaced by trans-cyclopropyl, phenyl, pyridinyl, piperazinyl or homopiperazinyl groups as conformationally restricted linkers have been synthesized. The anti-Pneumocystis carinii activity of these compounds was evaluated in a cell culture model and the DNA binding affinity was determined by thermal denaturation measurements. At 1 μM, compounds 2, 3, 5, 7, 9 and pentamidine were highly effective and caused total inhibition of P. carinii growth in culture. At 0.1 μM, compounds 2, 5, 7 and 10 were more active than pentamidine with N, N′-bis(4-amidinophenyl)piperazine 7 being approximately 15-fold more effective than pentamidine. The most active compounds, 7 and 10, showed strong binding affinities for calf thymus DNA and poly(dA-dT); however, a clear correlation between DNA binding affinity and the in vitro anti-P. carinii activity of these compounds was not observed. The results suggest that the nature of the central linker influences the biological actions of these compounds.

In vitro antimicrobial activity of aromatic diamidines and diimidazolines related to pentamidine

Geometric isomers of pentamidine analogues were screened for antimicrobial activity in vitro. cis isomers demonstrated good antibacterial activity compared to their trans counterparts. Both isomers were moderately active against opportunistic pathogens that afflict AIDS patients with minimum inhibitory concentrations in the range of 3.12-12.5 microg/mL.

An updated patent review of calpain inhibitors (2012 – 2014)

Introduction: Calpain is a family of cysteine proteases found in eukaryotes and a few bacteria. There is considerable interest in the search for calpain inhibitors because the enzyme has been implicated in several diseases including ocular disorders, neurodegenerative disorders, metabolic disorders and cancer. Areas covered: An overview of calpain inhibitors disclosed between 2012 and 2014 is presented. Among these are epoxysuccinates, dipeptide imaging agents, macrocyclic inhibitors, α-helical peptidomimetic inhibitors, carboxamides, 5-azolones and α-mercaptoacrylates. Additionally, preclinical studies of calpain inhibitors in pathologies such blood disorders, ocular disorders, neurological disorders and muscle disorders are discussed. Expert opinion: Major advances made in calpain inhibitor research between 2012 and 2014 include: i) the discovery of cytosolic-stable carboxamide calpain inhibitors; ii) synthesis of epoxysuccinates with excellent bioavailability; iii) disclosure of the X-ray crystal structures of novel α-mercaptoacrylates bound to the pentaEF hand region from human calpain; and iv) disclosure of calpain inhibitors as anti-sickling agents. Several calpain inhibitors were reported but limited effort was directed towards the discovery of calpain isoform selective agents, which continues to dampen the therapeutic potential of calpain inhibitors.

Trypanocidal activity of dicationic compounds related to pentamidine

Eight dicationic compounds related to pentamidine were studied for trypanocidal activity in seven trypanosome isolates. In vitro studies revealed that diamidines are more potent than diimidazolines. For example, 2 (a diamidine) and 4 (a diimidazoline) inhibited the growth of KETRI 243 with IC50 values of 2.3 and 900 nM, respectively. Introduction of polar groups into the linker decreased the effectiveness of the compounds against drug-resistant trypanosomes. In compounds with a 2-butene linker between the cationic groups, trans-isomers were more potent than cis-isomers. The cis- and trans-buteneamidines cured infection caused by Trypanosoma brucei brucei (EATRO Lab 110) and protected mice against infection by Trypanosoma brucei rhodesiense isolates, some of which are resistant to diamidines and melarsoprol.

Significance of Hydrogen Bonding at the S1′ Subsite of Calpain I

alpha-Ketohydroxamates were synthesized as bioisosteres of alpha-ketoamides. The alpha-ketohydroxamates were generally more potent than the corresponding alpha-ketoamides. The potency of the compounds suggests that hydrogen bonding and steric bulk of substituents on the nitrogen atom of the ketoamide moiety influence calpain inhibition.

Peptidyl aldehyde inhibitors of calpain incorporating P2-Proline mimetics

Four new peptidyl aldehydes bearing proline mimetics at the P(2)-position were synthesized and studied as inhibitors of calpain I, cathepsin B, and selected serine proteases. The ring size of the P(2)-constraining residue influenced the inhibitory potency and selectivity of the compounds for calpain I compared to the other proteases.