Brian E. Cathers

Optimization of a Series of Triazole Containing Mammalian Target of Rapamycin (mTOR) Kinase Inhibitors and the Discovery of CC-115

We report here the synthesis and structure-activity relationship (SAR) of a novel series of triazole containing mammalian target of rapamycin (mTOR) kinase inhibitors. SAR studies examining the potency, selectivity, and PK parameters for a series of triazole containing 4,6- or 1,7-disubstituted-3,4-dihydropyrazino[2,3-b]pyrazine-2(1H)-ones resulted in the identification of triazole containing mTOR kinase inhibitors with improved PK properties. Potent compounds from this series were found to block both mTORC1(pS6) and mTORC2(pAktS473) signaling in PC-3 cancer cells, in vitro and in vivo. When assessed in efficacy models, analogs exhibited dose-dependent efficacy in tumor xenograft models. This work resulted in the selection of CC-115 for clinical development.

A Cereblon Modulator (CC-220) with Improved Degradation of Ikaros and Aiolos

The drugs lenalidomide and pomalidomide bind to the protein cereblon, directing the CRL4-CRBN E3 ligase toward the transcription factors Ikaros and Aiolos to cause their ubiquitination and degradation. Here we describe CC-220 (compound 6), a cereblon modulator in clinical development for systemic lupus erythematosis and relapsed/refractory multiple myeloma. Compound 6 binds cereblon with a higher affinity than lenalidomide or pomalidomide. Consistent with this, the cellular degradation of Ikaros and Aiolos is more potent and the extent of substrate depletion is greater. The crystal structure of cereblon in complex with DDB1 and compound 6 reveals that the increase in potency correlates with increased contacts between compound 6 and cereblon away from the modeled binding site for Ikaros/Aiolos. These results describe a new cereblon modulator which achieves greater substrate degradation via tighter binding to the cereblon E3 ligase and provides an example of the effect of E3 ligase binding affinity with relevance to other drug discovery efforts in targeted protein degradation.

pH dependence of inhibitors targeting the occluding loop of cathepsin B

Potent and selective cathepsin B inhibitors have previously been synthesized based upon the natural product cysteine protease inhibitor E-64. X-ray crystal data indicates that these compounds interact through their free carboxylate with the positively charged histidine residues located on the prime-side of the active site within the occluding loop of cathepsin B. Herein, we examine the pH dependence of two prime-side-binding compounds. In each case there is a dramatic decrease in k(inact)/K(I) as the pH is raised from 4 to 7.8 corresponding to a single ionization of pK(a) 4.4. These results suggest that targeting of the occluding loop of cathepsin B may be a poor inhibitor design strategy if the enzyme environment has a pH greater than 5.5. However, this type of inhibitor may be a useful tool to help elucidate the role and the environment of cathepsin B in invading tumors.

Discovery of Mammalian Target of Rapamycin (mTOR) Kinase Inhibitor CC-223

We report here the synthesis and structure-activity relationship (SAR) of a novel series of mammalian target of rapamycin (mTOR) kinase inhibitors. A series of 4,6- or 1,7-disubstituted-3,4-dihydropyrazino[2,3-b]pyrazine-2(1H)-ones were optimized for in vivo efficacy. These efforts resulted in the identification of compounds with excellent mTOR kinase inhibitory potency, with exquisite kinase selectivity over the related lipid kinase PI3K. The improved PK properties of this series allowed for exploration of in vivo efficacy and ultimately the selection of CC-223 for clinical development.

Abstract SY37-02: Ligand-directed degradation of GSPT1 by a novel cereblon modulator drives potent antitumor effects

The protein cereblon is part of the CRL4-CRBN E3 ubiquitin ligase complex, and has been shown to be the molecular target for the drugs lenalidomide and pomalidomide. These drugs bind to the surface of cereblon, triggering the recruitment of substrate proteins to the ligase complex where they can be ubiquitinated and subsequently degraded by the proteosome. By this mechanism, lenalidomide and pomalidomide cause the degradation of the zinc finger transcription factors Ikaros and Aiolos, which mediate the antimyeloma activity of these compounds. Here we describe the discovery of CC-885, a novel cereblon modulator with potent and broad-spectrum antiproliferative activity against a panel of tumor cell lines. Acute myeloid leukemia (AML) cell lines and patient-derived AML cells show particular sensitivity to CC-885 treatment. CC-885 achieves this activity by causing the degradation of G1 to S phase transition 1 (GSPT1), a translation termination factor required for the release of nascent peptides from the ribosome. A crystal structure of cereblon in complex with the ligase adapter protein DDB1, as well as CC-885 and GSPT1, reveals that GSPT1 interacts with both CC-885 and the surface of cereblon. The principal molecular feature on GSPT1 that binds to cereblon is a beta-hairpin incorporating a glycine residue that docks against CC-885. Surprisingly, we found evidence that a similar molecular feature mediates Ikaros recruitment, even though there is no common structural fold or sequence homology other than the key glycine residue. We thereby define the common molecular feature, or degron, shared by the known cereblon neomorphic substrates. We further describe a novel therapeutic target, GSPT1, with promise in cancer such as AML. These results further show that cereblon-mediated protein degradation can be directed against new proteins and that this mechanism enables the targeting of multiple protein classes that may be considered undruggable with conventional approaches. Citation Format: Mary Matyskiela, Gang Lu, Takumi Ito, Barbra Pagarigan, Chin-Chu Lu, Karen Miller, Wei Fang, Nai-Yu Wang, Derek Nguyen, Jack Houston, Gilles Carmel, Tam Tran, Mariko Riley, Lyn9Al Nosaka, Gabriel Lander, Svetlana Gaidarova, Shuichan Xu, Alexander Ruchelman, Hiroshi Handa, James Carmichale, Thomas O. Daniel, Brian E. Cathers, Antonia Lopez-Girona, Philip Chamberlain. Ligand-directed degradation of GSPT1 by a novel cereblon modulator drives potent antitumor effects [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr SY37-02. doi:10.1158/1538-7445.AM2017-SY37-02