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864 Identification of a novel allosteric oral Cbl-b inhibitor that augmented T cell response and enhanced NK cell killing in vitro and in vivo
  1. Jun Kuai,
  2. Yingzhi Bi,
  3. Yilin Qi,
  4. Deborah Conrady,
  5. Rajiv Govindaraj,
  6. Graham Hone,
  7. R Aldrin Denny,
  8. Ken Carson,
  9. Geraldine Harriman and
  10. Fang Wang
  1. HoSpot Therapeutics, Boston, MA, USA


Background Immunotherapies aiming to boost anti-tumor cell responses in cancer patients has been proven successful by checkpoint inhibitors targeting PD1 or CTLA-4, but the majority of cancer patients do not garner durable benefit. Co-stimulation through the CD28 pathway is one potential approach to maximize the benefits of immunotherapies. The E3 ubiquitin ligase Cbl-b (casitas b-lineage lymphoma proto-oncogene b) has been established as a master negative regulator of T-cells and NK cells and plays an important role in immune suppression. Genetic ablation of Cbl-b or functional inactivation of its E3 ligase activity in mice resulted in CD8 T-cell-mediated rejection of primary tumors in several mouse models. Based on the overwhelming evidence supporting the role of Cbl-b in immune suppression, targeting Cbl-b with small molecule inhibitors is attractive for cancer immunotherapy.

Methods Cbl-b is activated by tyrosine kinases and undergoes a large conformational change from closed inactive form to open active form. Historically, it had been difficult to identify inhibitors of Cbl-b. Through the utilization of our proprietary SpotFinder platform, a druggable phosphoregulatory pocket was identified in the inactive form of Cbl-b. Learnings from the platform allowed for the development of screening assays utilizing specifically designed protein constructs. Assays were developed to identify inhibitors that bind to the hotspot and lock Cbl-b in its inactive form.

Results Here we report on a member of our lead series of inhibitors, a low nanomolar potent inhibitor identified via application of our SpotFinder platform. This inhibitor binds to the inactive form of Cbl-b, its binding mode in the identified hotspot confirmed by co-crystal structures. It inhibits the phosphorylation of Cbl-b by kinases, inhibits the E3 ligase activity of Cbl-b, promotes cytokine release and enhances T cell proliferation well as NK cell activation and killing. In vivo, our CBL-B inhibitors efficaciously augmented the T cell response in anti-CD3 treated mice.

Conclusions We herein demonstrated the validation of our proprietary SpotFinder platform via the prediction and drugging of a regulatory hotspot on an important immune oncology target that has to date been very difficult to drug.

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