Poly(ADP-ribose) polymerases (PARP1, -2, and -3) play important roles in DNA damage repair. As such, a number of PARP inhibitors are undergoing clinical development as anticancer therapies, particularly in tumors with DNA repair deficits and in combination with DNA-damaging agents. Preclinical evidence indicates that PARP inhibitors potentiate the cytotoxicity of DNA alkylating agents. It has been proposed that a major mechanism underlying this activity is the allosteric trapping of PARP1 at DNA single-strand breaks during base excision repair; however, direct evidence of allostery has not been reported. Here the data reveal that veliparib, olaparib, niraparib, and talazoparib (BMN-673) potentiate the cytotoxicity of alkylating agents. Consistent with this, all four drugs possess PARP1 trapping activity. Using biochemical and cellular approaches, we directly probe the trapping mechanism for an allosteric component. These studies indicate that trapping is due to catalytic inhibition and not allostery. The potency of PARP inhibitors with respect to trapping and catalytic inhibition is linearly correlated in biochemical systems but is nonlinear in cells. High-content imaging of gH2Ax levels suggests that this is attributable to differential potentiation of DNA damage in cells. Trapping potency is inversely correlated with tolerability when PARP inhibitors are combined with temozolomide in mouse xenograft studies. As a result, PARP inhibitors with dramatically different trapping potencies elicit comparable in vivo efficacy at maximum tolerated doses. Finally, the impact of trapping on tolerability and efficacy is likely to be context specific.Implications: Understanding the context-specific relationships of trapping and catalytic inhibition with both tolerability and efficacy will aid in determining the suitability of a PARP inhibitor for inclusion in a particular clinical regimen.

Polycomb repressive complex 2 (PRC2) is a regulator of epigenetic states required for development and homeostasis. PRC2 trimethylates histone H3 at lysine 27 (H3K27me3), which leads to gene silencing, and is dysregulated in many cancers. The embryonic ectoderm development (EED) protein is an essential subunit of PRC2 that has both a scaffolding function and an H3K27me3-binding function. Here we report the identification of A-395, a potent antagonist of the H3K27me3 binding functions of EED. Structural studies demonstrate that A-395 binds to EED in the H3K27me3-binding pocket, thereby preventing allosteric activation of the catalytic activity of PRC2. Phenotypic effects observed in vitro and in vivo are similar to those of known PRC2 enzymatic inhibitors; however, A-395 retains potent activity against cell lines resistant to the catalytic inhibitors. A-395 represents a first-in-class antagonist of PRC2 protein-protein interactions (PPI) for use as a chemical probe to investigate the roles of EED-containing protein complexes.

We have developed a series of cyclic amine-containing benzimidazole carboxamide PARP inhibitors with a methyl-substituted quaternary center at the point of attachment to the benzimidazole ring system. These compounds exhibit excellent PARP enzyme potency as well as single-digit nanomolar cellular potency. These efforts led to the identification of 3a (2-[(R)-2-methylpyrrolidin-2-yl]-1H-benzimidazole-4-carboxamide, ABT-888), currently in human phase I clinical trials. Compound 3a displayed excellent potency against both the PARP-1 and PARP-2 enzymes with a K(i) of 5 nM and in a C41 whole cell assay with an EC(50) of 2 nM. In addition, 3a is aqueous soluble, orally bioavailable across multiple species, and demonstrated good in vivo efficacy in a B16F10 subcutaneous murine melanoma model in combination with temozolomide (TMZ) and in an MX-1 breast cancer xenograft model in combination with either carboplatin or cyclophosphamide.

In our continued efforts to search for potent and novel receptor tyrosine kinase (RTK) inhibitors as potential anticancer agents, we discovered, through a structure-based design, that 3-aminoindazole could serve as an efficient hinge-binding template for kinase inhibitors. By incorporating an N,N'-diaryl urea moiety at the C4-position of 3-aminodazole, a series of RTK inhibitors were generated, which potently inhibited the tyrosine kinase activity of the vascular endothelial growth factor receptor and the platelet-derived growth factor receptor families. A number of compounds with potent oral activity were identified by utilizing an estradiol-induced mouse uterine edema model and an HT1080 human fibrosarcoma xenograft tumor model. In particular, compound 17p (ABT-869) was found to possess favorable pharmacokinetic profiles across different species and display significant tumor growth inhibition in multiple preclinical animal models.

No abstract