A selective and orally bioavailable VHL-recruiting PROTAC achieves SMARCA2 degradation in vivo

Targeted protein degradation offers an alternative modality to classical inhibition and holds the promise of addressing previously undruggable targets to provide novel therapeutic options for patients. Heterobifunctional molecules co-recruit a target protein and an E3 ligase, resulting in ubiquitylation and proteosome-dependent degradation of the target. In the clinic, the oral route of administration is the option of choice but has only been achieved so far by CRBN- recruiting bifunctional degrader molecules. We aimed to achieve orally bioavailable molecules that selectively degrade the BAF Chromatin Remodelling complex ATPase SMARCA2 over its closely related paralogue SMARCA4, to allow in vivo evaluation of the synthetic lethality concept of SMARCA2 dependency in SMARCA4-deficient cancers. Here we outline structure- and property-guided approaches that led to orally bioavailable VHL-recruiting degraders. Our tool compound, ACBI2, shows selective degradation of SMARCA2 over SMARCA4 in ex vivo human whole blood assays and in vivo efficacy in SMARCA4-deficient cancer models. This study demonstrates the feasibility for broadening the E3 ligase and physicochemical space that can be utilised for achieving oral efficacy with bifunctional molecules.

Selective Tumor Cell Apoptosis and Tumor Regression in CDH17-Positive Colorectal Cancer Models using BI 905711, a Novel Liver-Sparing TRAILR2 Agonist

Activation of TRAILR2 has emerged as an important therapeutic concept in cancer treatment. TRAILR2 agonistic molecules have only had limited clinical success, to date, due either to lack of efficacy or hepatotoxicity. BI 905711 is a novel tetravalent bispecific antibody targeting both TRAILR2 and CDH17 and represents a novel liver-sparing TRAILR2 agonist specifically designed to overcome the disadvantages of previous strategies. Here, we show that BI 905711 effectively triggered apoptosis in a broad panel of CDH17-positive colorectal cancer tumor cells in vitro. Efficient induction of apoptosis was dependent on the presence of CDH17, as exemplified by the greater than 1,000-fold drop in potency in CDH17-negative cells. BI 905711 demonstrated single-agent tumor regressions in CDH17-positive colorectal cancer xenografts, an effect that was further enhanced upon combination with irinotecan. Antitumor efficacy correlated with induction of caspase activation, as measured in both the tumor and plasma. Effective tumor growth inhibition was further demonstrated across a series of different colorectal cancer PDX models. BI 905711 induced apoptosis in both a cis (same cell) as well as trans (adjacent cell) fashion, translating into significant antitumor activity even in xenograft models with heterogeneous CDH17 expression. In summary, we demonstrate that BI 905711 has potent and selective antitumor activity in CDH17-positive colorectal cancer models both in vitro and in vivo. The high prevalence of over 95% CDH17-positive tumors in patients with colorectal cancer, the molecule preclinical efficacy together with its potential for a favorable safety profile, support the ongoing BI 905711 phase I trial in colorectal cancer and additional CDH17-positive cancer types (NCT04137289).

Abstract 2051: BI 905711, a novel CDH17-targeting TRAILR2 agonist, effectively triggers tumor cell apoptosis and tumor regressions selectively in CDH17-positive colorectal cancer models

The tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) has the capability to induce apoptosis in a wide variety of tumor cells and has emerged as an important therapeutic concept for cancer treatment. To date however, TRAILR2 agonistic antibodies, designed to trigger tumor cell apoptosis, have only had limited clinical success due either to lack of efficacy or liver toxicity. BI 905711, a tetravalent bispecific antibody targeting both TRAILR2 and CDH17, was designed to overcome the disadvantages of current TRAILR2 antibodies. CDH17-dependent clustering of TRAILR2 permits BI 905711 to selectively induce apoptosis in CDH17-expressing tumor cells. CDH17 was selected as the anchor target due to its restricted expression, in particular a lack of expression in normal liver tissue, thereby avoiding the clinical hepatotoxicity associated with TRAILR2 agonism. Here, we report the preclinical activity of BI 905711 using colorectal cancer (CRC)-derived cell lines and patient-derived xenograft (PDX) models. We demonstrated that BI 905711 effectively triggered apoptosis in a broad range of CDH17-positive CRC tumor cells in vitro. Furthermore, BI 905711 potently initiated the apoptosis cascade as evidenced by a strong post-treatment increase of both caspase-8 and caspase-3/7. Importantly, induction of extrinsic apoptosis signaling by BI 905711 was strictly CDH17-dependent, as further demonstrated using a pair of CDH17-positive and -negative clones of the CRC cell line GP2d. When comparing the CDH17-negative clone with the parental cell line, the absence of CDH17 translated into a more than 1000-fold drop in BI 905711-dependent efficacy. Moreover, BI 905711 demonstrated single-agent tumor regressions in the GP2d colorectal cancer xenograft model. The antitumor efficacy of BI 905711 was further investigated in vivo in different CRC PDX models, showing effective tumor growth inhibition in a q14d dosing schedule. In summary, we demonstrated that BI 905711 potently triggers the extrinsic apoptosis pathway specifically in CDH17-positive tumor cells, which translates into significant tumor growth inhibition in CRC patient-derived xenograft tumors. BI 905711 is a first-in-class molecule that shows superiority to existing TRAILR2 agonists, represents a targeted strategy for the treatment of CRC and additional CDH17-positive cancer types. Together with its potential for a favorable safety profile, these data support our plan for a phase I trial of BI 905711 in these patient populations.

Citation Format: Juan Manuel Garcia-Martinez, Andreas Wernitznig, Joerg Rinnenthal, Maria Antonietta Impagnatiello, Frank Hilberg, Craig Giragossian, Norbert Kraut, Mark Pearson, Klaus-Peter Kuenkele. BI 905711, a novel CDH17-targeting TRAILR2 agonist, effectively triggers tumor cell apoptosis and tumor regressions selectively in CDH17-positive colorectal cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2051.

Abstract 3849: Structure-based PROTAC design demonstrates BAF complex ATPase vulnerabilities in cancer

Targeting subunits of BAF/PBAF complexes has been proposed as an approach to exploit cancer vulnerabilities, yet small molecules developed to date have remained largely ineffectual. Here we develop PROTAC degraders of the BAF ATPase subunits SMARCA2 and SMARCA4 using a bromodomain ligand and recruitment of the E3 ubiquitin ligase VHL. High-resolution ternary complex crystal structures and biophysical investigation guided the rational optimization of ACBI1, a potent cooperative SMARCA2/4 degrader. ACBI1 induced antiproliferative effects and cell death caused by SMARCA2 depletion in SMARCA4 mutant cancer cells, and in acute myeloid leukemia cells dependent on SMARCA4 ATPase activity. These findings exemplify a successful biophysics and structure driven campaign to degrade targets previously considered undruggable, and pave the way towards new therapeutics for the treatment of tumors sensitive to the loss of BAF complex ATPases.

Citation Format: Manfred Koegl, William Farnaby, Claire Whitworth, Michael Roy, Emelyne Diers, Nicole Trainor, Steffen Steurer, Carina Riedmueller, Teresa Gmaschitz, Johannes Wachter, Christian Dank, Michael Gallant, Bernadette Sharps, Klaus Rumpel, Elisabeth Traxler, Romario Lorenz, Oliver Petermann, Peter Greb, Harald Weinstabl, Gerd Bader, Andreas Zoephel, Alexander Weiss-Puxbaum, Joerg Rinnenthal, Heribert Arnnhof, Nicola Wiechens, Meng-Ying Wu, Tom Owen-Hughes, Peter Ettmayer, Mark Pearson, Darryl B. McConnell, Alessio Ciulli. Structure-based PROTAC design demonstrates BAF complex ATPase vulnerabilities in cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3849.

BAF complex vulnerabilities in cancer demonstrated via structure-based PROTAC design

Targeting subunits of BAF/PBAF chromatin remodeling complexes has been proposed as an approach to exploit cancer vulnerabilities. Here we develop PROTAC degraders of the BAF ATPase subunits SMARCA2 and SMARCA4 using a bromodomain ligand and recruitment of the E3 ubiquitin ligase VHL. High-resolution ternary complex crystal structures and biophysical investigation guided rational and efficient optimization towards ACBI1, a potent and cooperative degrader of SMARCA2, SMARCA4 and PBRM1. ACBI1 induced antiproliferative effects and cell death caused by SMARCA2 depletion in SMARCA4 mutant cancer cells, and in acute myeloid leukemia cells dependent on SMARCA4 ATPase activity. These findings exemplify a successful biophysics- and structure-based PROTAC design approach to degrade high profile drug targets and pave the way towards new therapeutics for the treatment of tumors sensitive to the loss of BAF complex ATPases.