Hans-Georg Häcker

Allosteric modulation of caspases

Caspases are proteolytic enzymes mainly involved in the induction and execution phases of apoptosis. This type of programmed cell death is an essential regulatory process required to maintain the integrity and homeostasis of multicellular organisms. Inappropriate apoptosis is attributed a key role in many human diseases, including neurodegenerative disorders, ischemic damage, autoimmune diseases and cancer. Allosteric modulation of the function of a protein occurs when the regulatory trigger, such as the binding of a small effector or inhibitor molecule, takes place some distance from the proteins active site. In recent years, several caspases have been identified that possess allosteric sites and binding of small molecule to these sites resulted in the modulation of enzyme activities. Regulation of caspase activity by small molecule allosteric modulators is believed to be of great therapeutic importance. In this review we give brief highlights on recent developments in identifying and characterizing natural and synthetic allosteric inhibitors as well as activators of caspases and discuss their potential in drug discovery and protein engineering.

Aromatic 2-(thio)ureidocarboxylic acids as a new family of modulators of multidrug resistance-associated protein 1: synthesis, biological evaluation, and structure-activity relationships.

Four series of aromatic carboxylic acids were prepared with a urea or thiourea moiety at the neighboring position to the carboxyl group and benzene or thiophene as aromatic scaffold. Using a calcein AM assay, these compounds were evaluated as inhibitors of multidrug resistance-associated protein 1 (MRP1) and selected compounds were examined toward P-glycoprotein (P-gp) as well as breast cancer resistance protein (BCRP) to assess selectivity for MRP1. Two 2-thioureidobenzo[b]thiophene-3-carboxylic acids (48, 49) were identified as particularly potent inhibitors of MRP1, with IC50 values of around 1 microM. The structural features of this new family of nontoxic MRP1 inhibitors include a (thio)urea disubstituted with preferentially two alkyl groups at the terminal nitrogen and an additional fused aromatic ring.

A 4-aminobenzoic acid derivative as novel lead for selective inhibitors of multidrug resistance-associated proteins.

We present a novel lead for inhibitors of multidrug resistance-associated proteins (MRPs). Compound 1 (4-[(5,6,7,8-tetrahydro-4-oxo-4H-[1]benzothieno[2,3-d][1,3]thiazin-2-yl)amino]benzoic acid) was about six times more potent than the known inhibitor MK571 at MRP1, while at MRP2 its effect was similar to that of MK571. Structural analogs were also evaluated. Among them, compound 2, sharing the 4-aminobenzoic acid substructure with 1, also inhibited MRP1. Both derivatives were inactive against P-gp. It can be concluded that their carboxyl group is needed for inhibition of MRPs and accounts for the selectivity of these compounds.

Analogs of a 4-aminothieno[2,3-d]pyrimidine lead (QB13) as modulators of P-glycoprotein substrate specificity

P-glycoprotein (P-gp) is an important factor in the development of multidrug resistance (MDR) in cancer cells. In literature reports, a thieno[2,3-d]pyrimidine (QB13) was described as P-gp modulator and opposed effects on the cell accumulation of distinct P-gp substrates were postulated. On the basis of this lead structure, a series of 2-alkylthio-4-aminothieno[2,3-d]pyrimidines was prepared and tested in a daunorubicin accumulation assay. Modulation of substrate specificity was shown for selected compounds in cytotoxicity (MTT) assays.

Direct formation of ring-fused 1,3-thiazine-2,4-dithiones from aromatic o-amino carboxylic acids: observation of a carbon disulfide mediated thionation.

A facile synthesis of 2H-3,1-benzothiazine-2,4(1H)-dithiones (trithioisatoic anhydrides) or 2H-naphtho[2,3-d][1,3]thiazine-2,4(1H)-dithione solely from anthranilic acids or 3-amino-2-naphthoic acid and carbon disulfide, performed at room temperature in 1,4-dioxane in the presence of Et(3)N, is reported. Corresponding 2-alkylsulfanyl derivatives were obtained in one-pot reactions under the same conditions after addition of alkyl halides. The mechanism of the thiazine cyclization was investigated using (13)C-labeled carbon disulfide to reveal that carbon disulfide was incorporated into the heterocycle and additionally acted as a thionation reagent.

2-Amino- and 2-Alkylthio-4H-3,1-benzothiazin-4-ones: Synthesis, Interconversion and Enzyme Inhibitory Activities

The synthetic access to 2-sec-amino-4H-3,1-benzothiazin-4-ones 2 was explored. Compounds 2 were available from methyl 2-thioureidobenzoates 1, 2-thioureidobenzoic acids 3, and novel 2-thioureidobenzamides 6, respectively, under different conditions. 2-Alkylthio-4H-3,1-benzothiazin-4-ones 5 have been prepared from anthranilic acid following a two step route. Both, benzothiazinones 2 and 5 underwent ring cleavage reactions to produce thioureas 1 and 6, respectively. Twelve benzothiazinones were evaluated as inhibitors against a panel of eight proteases and esterases to identify one selective inhibitor of human cathepsin L, 2b, and one selective inhibitor of human leukocyte elastase, 5i.