Biological Effects of Modifications of the Englerin A Glycolate

Modifications at the glycolate moiety of englerin A were made to explore variations at the most sensitive site on the molecule for activity in the NCI 60 screen, wherein englerin A is highly potent and selective for renal cancer cells. Replacement of the glycolate by other functionalities as well as esterification of the glycolate hydroxyl yielded compounds which displayed excellent selectivity and potency compared with the natural product. TRPC4/5 ion channel experiments with five compounds showed delayed or reduced agonism with TRPC5, at much higher concentrations than englerin A. With TRPC4, these compounds all had no effect at 10 μM. The same compounds were not detectable in mouse serum after a single oral dose of 12.5 mg/kg. At 100 mg/kg p.o., no toxicity was observed, and blood levels were barely detectable. Intravenous administration led to toxicity but at substantially lower doses than for englerin A.

P6981, an arylstibonic acid, is a novel low nanomolar inhibitor of cAMP response element-binding protein binding to DNA

Several basic leucine zipper (B-ZIP) transcription factors have been implicated in cancer, substance abuse, and other pathological conditions. We previously identified arylstibonic acids that bind to B-ZIP proteins and inhibit their interaction with DNA. In this study, we used electrophoretic mobility shift assay to analyze 46 arylstibonic acids for their activity to disrupt the DNA binding of three B-ZIP [CCAAT/enhancer-binding protein α, cyclic AMP-response element-binding protein (CREB), and vitellogenin gene-binding protein (VBP)] and two basic helix-loop-helix leucine zipper (B-HLH-ZIP) [USF (upstream stimulating factor) and Mitf] proteins. Twenty-five arylstibonic acids showed activity at micromolar concentrations. The most active compound, P6981 [2-(3-stibonophenyl)malonic acid], had half-maximal inhibition at ~5 nM for CREB. Circular dichroism thermal denaturation studies indicated that P6981 binds both the B-ZIP domain and the leucine zipper. The crystal structure of an arylstibonic acid, NSC13778, bound to the VBP leucine zipper identified electrostatic interactions between both the stibonic and carboxylic acid groups of NSC13778 [(E)-3-(3-stibonophenyl)acrylic acid] and arginine side chains of VBP, which is also involved in interhelical salt bridges in the leucine zipper. P6981 induced GFP-B-ZIP chimeric proteins to partially localize to the cytoplasm, demonstrating that it is active in cells. P6981 inhibited the growth of a patient-derived clear cell sarcoma cell line whose oncogenic potential is driven by a chimeric protein EWS-ATF1 (Ewing's sarcoma protein-activating transcription factor 1), which contains the DNA binding domain of ATF1, a B-ZIP protein. NSC13778 inhibited the growth of xenografted clear cell sarcoma, and no toxicity was observed. These experiments suggest that antimony containing arylstibonic acids are promising leads for suppression of DNA binding activities of B-ZIP and B-HLH-ZIP transcription factors.

Arylstibonic acids: novel inhibitors and activators of human topoisomerase IB

Human topoisomerase IB (hTopo) forms a covalent phosphotyrosyl linkage with the DNA backbone, and controls genomic DNA topology by relaxing DNA supercoils during the processes of DNA replication, transcription, chromosome condensation and decondensation. The essential role of hTopo in these processes has made it a preeminent anticancer drug target. We have screened a small library of arylstibonic acids for their effects on plasmid supercoil relaxation catalyzed by hTopo. Despite the similar structures of the library compounds, some compounds were found to be effective competitive inhibitors, and others, nonessential activators. Some arylstibonic acids show selectivity in their action against hTopo and the related enzyme from poxvirus (vTopo). Structure-activity relationships and structural modeling suggest that competitive inhibition may result from positioning of the negatively charged stibonic acid and carboxylate groups of the inhibitors into DNA phosphate binding pockets on hTopo. The hTopo activators act by a surprising allosteric mechanism without interfering with DNA binding or binding of the widely used hTopo poison camptothecin. Arylstibonic acid competitive inhibitors may become useful small molecules for elucidating the cellular functions of hTopo.

Potent inhibition of human apurinic/apyrimidinic endonuclease 1 by arylstibonic acids

Human apurinic/apyrimidinic endonuclease (Ape1) plays an important role by processing the >10,000 highly toxic abasic sites generated in the genome of each cell every day. Ape1 has recently emerged as a target for inhibition, in that its overexpression in tumors has been linked with poor response to both radiation and chemotherapy and lower overall patient survival. Inhibition of Ape1 using siRNA or the expression of a dominant-negative form of the protein has been shown to sensitize cells to DNA-damaging agents, including various chemotherapeutic agents. However, potent small-molecule inhibitors of Ape1 remain to be found. To this end, we screened Ape1 against the NCI Diversity Set of small molecules and discovered aromatic nitroso, carboxylate, sulfonamide, and arylstibonic acid compounds with micromolar affinities for the protein. A further screen of a 37-compound arylstibonic acid sublibrary identified ligands with IC(50) values in the range of 4 to 300 nM. The negatively charged stibonic acids act by a partial-mixed mode and probably serve as DNA phosphate mimics. These compounds provide a useful scaffold for development of chemotherapeutic agents against Ape1.