Cereblon-based Bifunctional Degrader of SOS1, BTX-6654, Targets Multiple KRAS Mutations and Inhibits Tumor Growth

Mutations within the oncogene KRAS drive an estimated 25% of all cancers. Only allele-specific KRAS G12C inhibitors are currently available and are associated with the emergence of acquired resistance, partly due to upstream pathway reactivation. Given its upstream role in the activation of KRAS, son of sevenless homolog 1 (SOS1), has emerged as an attractive therapeutic target. Agents that target SOS1 for degradation could represent a potential pan-KRAS modality that may be capable of circumventing certain acquired resistance mechanisms. Here, we report the development of two SOS1 cereblon-based bifunctional degraders, BTX-6654 and BTX-7312, cereblon-based bifunctional SOS1 degraders. Both compounds exhibited potent target-dependent and -specific SOS1 degradation. BTX-6654 and BTX-7312 reduced downstream signaling markers, pERK and pS6, and displayed antiproliferative activity in cells harboring various KRAS mutations. In two KRAS G12C xenograft models, BTX-6654 degraded SOS1 in a dose-dependent manner correlating with tumor growth inhibition, additionally exhibiting synergy with KRAS and MEK inhibitors. Altogether, BTX-6654 provided preclinical proof of concept for single-agent and combination use of bifunctional SOS1 degraders in KRAS-driven cancers.

Abstract 1963: Targeting hematological malignancies with two functionally and mechanistically distinct classes of cereblon mediated protein homeostatic modulators

BioTheryX's novel and unique Protein Homeostatic Modulators (PHMsTM) are small molecule “molecular glues” that promote ubiquitination of substrate proteins by the CRL4CRBN (CUL4-DDB1-RBX1-CRBN) ubiquitin ligase, leading to their subsequent degradation by the 26S proteasome. The therapeutic benefit of targeted protein degradation via cereblon (CRBN) has been clinically validated by the immunomodulatory drugs lenalidomide and pomalidomide in 5q-deletion-associated myelodysplastic syndrome and multiple myeloma, respectively. We present PHM® therapeutic candidates with novel targeted degradation profiles of disease-relevant proteins including GSPT1, Ikaros, and CK1α via CRBN with significant clinical potential in the treatment of hematological malignancies as well as solid tumors.

Phenotypic screens of our proprietary PHM® library has led to the discovery of compounds that exhibit significant cytotoxicity in human AML and lymphoma cell lines, unlike classical IMiDs such as lenalidomide and pomalidomide. Cell viability assays revealed that these PHMsTM have IC50s in the low-nanomolar range. Importantly, they exhibit a large in vitro safety window with higher IC50s in normal human liver epithelial cells. The compounds are highly potent and can achieve significant AML and lymphoma selective cytotoxicity with short-term exposure of only 8 hours suggesting a highly rapid mechanism of action. Immunoblot analysis shows that the PHMsTM degrade CRBN substrates such as GSPT1, Ikaros and CK-1α and concomitantly activate the apoptosis machinery within 6 hours of treatment. The substrate degradation profile of PHMsTM combined with the cytokine profile divides the PHMsTM into two distinct mechanistic and functional classes of molecules - (1) purely cytotoxic, and (2) cytotoxic and immune-modulatory PHMsTM. Human PBMCs activated with lipopolysaccharide or α-CD3 followed by DMSO or compound treatment for 24 hours revealed that PHMsTM belonging to the latter class inhibit proinflammatory cytokines such as IL-1β, IL-6 and TNF-α as well as induce IL-2, an indicator of T cell activation, and are a 100-fold more potent than pomalidomide.

In vivo efficacy of BTX-PHMsTM was evaluated using the MV-4-11 human AML xenograft model in athymic nude mice. The study shows a significant reduction in tumor volume with daily dosing, further establishing the PHMsTM as clinical candidates for AML. Of significance, is the effect of BTX1188 that completely obliterates the tumors within 30 days of dosing and maintains tumor free animals until the end of the study. The effective short-term exposure in vitro is also reflected in vivo where intermittent dosing with the PHM results in significant reduction in tumor volume. Finally, BTX-PHMsTM exhibit significant oral bioavailability, thus making them promising clinical candidates for treatment of hematological malignancies as well as early discovery candidates for solid tumors.

Citation Format: Aparajita Hoskote Chourasia, Leah Fung, Angela Pasis, Brooke McElwee, Angela Schoolmeesters, Normand Richard, Imelda Lam, Eduardo Torres, Paul Erdman, Robert Sullivan, David Hecht, Kyle W. Chan, Frank Mercurio, David I. Stirling. Targeting hematological malignancies with two functionally and mechanistically distinct classes of cereblon mediated protein homeostatic modulators [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1963.

Kinome-wide functional genomics screen reveals a novel mechanism of TNFα-induced nuclear accumulation of the HIF-1α transcription factor in cancer cells

Hypoxia-inducible factor-1 (HIF-1) and its most important subunit, HIF-1α, plays a central role in tumor progression by regulating genes involved in cancer cell survival, proliferation and metastasis. HIF-1α activity is associated with nuclear accumulation of the transcription factor and regulated by several mechanisms including modulation of protein stability and degradation. Among recent advances are the discoveries that inflammation-induced cytokines and growth factors affect protein accumulation of HIF-1α under normoxia conditions. TNFα, a major pro-inflammatory cytokine that promotes tumorigenesis is known as a stimulator of HIF-1α activity. To improve our understanding of TNFα-mediated regulation of HIF-1α nuclear accumulation we screened a kinase-specific siRNA library using a cell imaging–based HIF-1α-eGFP chimera reporter assay. Interestingly, this systematic analysis determined that depletion of kinases involved in conventional TNFα signaling (IKK/NFκB and JNK pathways) has no detrimental effect on HIF-1α accumulation. On the other hand, depletion of PRKAR2B, ADCK2, TRPM7, and TRIB2 significantly decreases the effect of TNFα on HIF-1α stability in osteosarcoma and prostate cancer cell lines. These newly discovered regulators conveyed their activity through a non-conventional RELB-depended NFκB signaling pathway and regulation of superoxide activity. Taken together our data allow us to conclude that TNFα uses a distinct and complex signaling mechanism to induce accumulation of HIF-1α in cancer cells. In summary, our results illuminate a novel mechanism through which cancer initiation and progression may be promoted by inflammatory cytokines, highlighting new potential avenues for fighting this disease.