AL101 mediated tumor inhibition in notch-altered TNBC PDX models.

1064 Background: The Notch pathway is activated during mammary gland development and has been implicated as a key driver in breast cancer. There is an urgent need to identify new therapeutic strategies for triple-negative breast cancer (TNBC), a sub-type associated with poor prognosis and no available targeted therapies. Notch gain of function (GOF) genetic alterations are potential tumor drivers found in ~10% of TNBC. This motivated the development of Notch inhibitors, including AL101 a pan-Notch, gamma secretase inhibitor (J Clin Oncol 36, 2018 abstract 2515). AL101 is currently being evaluated in Adenoid Cystic Carcinoma patients with activating Notch mutations (NCT03691207, ACCURACY trial). Here, we aim to test the activity of AL101 in TNBC patient derived xenograft (PDX) models with Notch activating genetic alterations. Methods: Gene expression cluster analysis was performed for 38 TNBC PDX tumors using a list of 21 Notch target genes. Seven tumors, bearing a “Notch-on” signature, were enriched with mutated/fusion (M/F) Notch genes and clustered separately from all other tumors. Of 9 models selected for study, 4 had a Notch-on signature and were expected to respond to AL101. Tumors were implanted into female athymic nude mice. Once tumors reached an average size of 150-300 mm 3 , mice (n = 5/group) were randomized to Vehicle or AL101 treatment arms (3 mg/kg, PO, 4on/3off) until tumors reached 1500 mm 3 or day 60. Results: As measured by tumor growth inhibition (TGI), AL101 was more potent in tumors with a putative Notch-on signature. Within these 4 models, M/F genes were present in Notch1-NRR GOF (103% TGI p = 0.0004); Notch2-fusion (62%TGI p = 0.036); Notch3-fusion (75% TGI p = 0.032); or Notch4-fusion (147% TGI p < 0.00001). Tumors lacking the Notch signature did not respond significantly to AL101: WT Notch (43% TGI p = 0.0104; 64% TGI p = 0.13); Notch1 with a predicted loss of function mutation (12% TGI p = 0.53), Notch1 Variant of Unknown Significance (VUS) (30% TGI p = 0.44), Notch2 VUS (41% TGI p = 0.44). Conclusions: We demonstrate that in TNBC PDX models, the presence of a Notch-on signature and Notch GOF mutations/fusions correlates with potent response to AL101. These data support the clinical development of AL101 as a targeted therapy for TNBC with Notch GOF alterations.