The efficacy of CB-103, a first-in-class transcriptional Notch inhibitor, in preclinical models of breast cancer

Background Notch signaling has been shown to mediate treatment resistance and support cancer stem cells (CSCs) in endocrine-resistant estrogen receptor positive (ER+) and triple negative breast cancers (TNBCs). The clinical development of GSIs, first generation Notch inhibitors, has been hindered by lack of Notch specificity and dose-limiting toxicity. Here we describe the safety and efficacy of a first-in-class, clinical stage, orally available small molecule pan-Notch inhibitor, CB-103. Due to its unique mode of action, CB-103 doesn’t induce GI toxicities noted with GSIs. There is a critical need for effective, safe, targeted therapies for patients with endocrine-refractory metastatic breast cancer. Recently approved targeted therapies for TNBC are only effective for a subset of patients. Moreover, GSI-resistant, constitutively activating Notch1 or Notch2 mutations are observed in ∼10% of TNBC. Our study elucidating the synergy of CB-103 with fulvestrant and paclitaxel in preclinical models of both hormone-refractory ER+ BC and TNBC respectively provides a novel and unique opportunity to address major unmet therapeutic needs. Methods CB-103 was screened in combination with a panel of anti-neoplastic drugs. We evaluated the anti-tumor activity of CB-103 with fulvestrant in ESR1-mutant (Y537S), endocrine-resistant BC xenografts. In the same model, we examined anti-CSC activity in mammosphere formation assays for CB-103 alone or in combination with fulvestrant or palbociclib. We also evaluated the effect of CB-103 plus paclitaxel on primary tumors and CSC in GSI-resistant TNBC model HCC1187. Comparisons between groups were performed with two-sided unpaired Student’s t -test. One-way or two-way ANOVA followed by Tukey’s post analysis was performed to analyze in vivo efficacy study results. Results CB-103 showed synergism with fulvestrant in ER+ cells and with paclitaxel in TNBC cells. CB-103 combined with fulvestrant or paclitaxel potently inhibited mammosphere formation in both models. Combination of CB-103 and fulvestrant significantly reduced tumor volume in an ESR1-mutant, endocrine-resistant BC model. In a GSI-resistant TNBC model, CB-103 plus paclitaxel significantly delayed tumor growth compared to paclitaxel alone. Conclusions Our data indicate that CB-103 is an attractive candidate for clinical investigation in endocrine resistant, recurrent breast cancers with biomarker-confirmed Notch activity in combination with SERDs and/or CDKis and in TNBCs with biomarker-confirmed Notch activity in combination with taxane-containing chemotherapy regimens. ### Competing Interest Statement The authors have declared no competing interest. * ANOVA : Analysis of variance BC : Breast Cancer CDK : Cyclin-dependent kinase Cmax : Maximal plasma concentration CR : Complete response CSCs : Cancer Stem Cells DMSO : Dimethyl sulfoxide ER+ : Estrogen receptor positive ESR1 : Estrogen Receptor Gene α FCS : Fetal Calf Serum GSIs : gamma-secretase inhibitors H&E : Hematoxylin and Eosin LC-MS : Liquid chromatography–Mass Spectrometry MAPK : Mitogen-activated protein kinase MZB cells : Marginal Zone B cells PD : Pharmacodynamics PDX : Patient-derived xenograft PI3K : Phosphoinositide 3-kinase PK : Pharmacokinetics PR : Partial response PR+ : Progesterone receptor positive SD : Stable disease SERD : Selective Estrogen Receptor Disruptor TNBC : Triple Negative Breast Cancer TV : Tumor volume