Targeting GSPT1 by a novel cereblon E3 ligase modulator for the treatment of Acute Lymphoblastic Leukemia

Acute Lymphoblastic Leukemia (ALL), the most common childhood cancer and the second most common acute leukemia in adults, arises from clonal expansion of undifferentiated lymphoid precursor cells in bone marrow. Although the majority of childhood ALL cases harbor clonal genetic alterations of transcription factor (TF) gene mutations or rearrangements, TF alterations remain difficult therapeutic targets. Small molecule induced protein degradation is a novel strategy that can be applied to currently undruggable targets such as TF and fusion oncoproteins. In this paradigm, small molecule degraders (either PROTAC or Molecular Glue (MG)) redirect the cell’s endogenous ubiquitin proteasome system and induce ubiquitination of the target protein or non-native substrate of an E3 ligase (neosubstrate) and its subsequent proteasomal degradation. Recently, the CRBN E3 ligase modulator, CC90009 was reported to show potent antitumor activity in acute myeloid leukemia, leading to the identification of GSPT1 (G1 to S phase transition factor) as a CRBN neosubstrate. These findings suggest the potential of MGs to target unanticipated vulnerabilities in different malignancies. Here, using a structurally diverse and unique set of MGs (Molecular Glue Library (MGL)) with confirmed CRBN binding affinity, we sought to identify novel CRBN modulators through phenotypic and proteomic methods. Our screening of the MGL in a panel of representative acute leukemia cell lines, including the CRLF2-rearranged ALL cell line MHH-CALL-4 identified several active MGs with EC50 <5 μM. A lenalidomide competition assay and MHH-CALL-4 CRBN knock down cells confirmed the CRBN dependency of these MGs. Among these compounds, SJ6986, a thalidomide-driven sulfonamide showed potent cytotoxicity in over 10 ALL cell lines tested in vitro. TMT-MS proteomic analyses identified GSPT1/2 as the primary targets for this compound with high selectivity. We next tested SJ6986 activity in a panel of patient derived ALL xenografts (PDX) harboring rearrangement of IGH-CRLF2, EPOR, ATF7IP-JAK2 ex vivo. All the tested tumors were highly sensitive to SJ6986 with IC50s at nanomolar range. PK analyses in NSG mice indicated rapid absorption and over 80% oral bioavailability for SJ6986. PD studies in an IGH-CRLF2 PDX showed dose-dependent degradation of GSPT1 within 48 hours of treatment. Finally, we examined the antitumor activity of SJ6986 in 6 different PDX representative of high-risk subtypes of ALL including near haploid, low-hypodiploid, CRLF2-rearranged and EPOR-rearranged, in vivo for 28 days. SJ6986 was able to dramatically decrease the tumor burden at 1 mg/kg dose in most of the tumor models. Collectively, these results affirm that SJ6986 is a novel CRBN modulator and a potential therapeutic agent by targeting GSPT1 protein with high selectivity and potency for the treatment of ALL. Citation Format: Fatemeh Keramatnia, Yunchao Chang, Gisele Nishiguchi, Jaeki Min, Charles Mullighan, Marcus Fischer, Zoran Rankovic, Fatemeh Keramatnia. Targeting GSPT1 by a novel cereblon E3 ligase modulator for the treatment of Acute Lymphoblastic Leukemia [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr LBA002.