Characterization of activating mutations of NOTCH3 in T-cell acute lymphoblastic leukemia and anti-leukemic activity of NOTCH3 inhibitory antibodies

Notch receptors have been implicated as oncogenic drivers in several cancers, the most notable example being NOTCH1 in T-cell acute lymphoblastic leukemia (T-ALL). To characterize the role of activated NOTCH3 in cancer, we generated an antibody that detects the neo-epitope created upon gamma-secretase cleavage of NOTCH3 to release its intracellular domain (ICD3), and sequenced the negative regulatory region (NRR) and PEST domain coding regions of NOTCH3 in a panel of cell lines. We also characterize NOTCH3 tumor-associated mutations that result in activation of signaling and report new inhibitory antibodies. We determined the structural basis for receptor inhibition by obtaining the first co-crystal structure of a NOTCH3 antibody with the NRR protein and defined two distinct epitopes for NRR antibodies. The antibodies exhibit potent anti-leukemic activity in cell lines and tumor xenografts harboring NOTCH3 activating mutations. Screening of primary T-ALL samples reveals that two of 40 tumors examined show active NOTCH3 signaling. We also identified evidence of NOTCH3 activation in 12 of 24 patient-derived orthotopic xenograft models, two of which exhibit activation of NOTCH3 without activation of NOTCH1. Our studies provide additional insights into NOTCH3 activation and offer a path forward for identification of cancers that are likely to respond to therapy with NOTCH3 selective inhibitory antibodies.

Abstract P6-11-06: A phase Ib study of LCL161, an oral inhibitor of apoptosis (IAP) antagonist, in combination with weekly paclitaxel in patients with advanced solid tumors

Background: Impaired apoptosis is a common feature of cancer cells and may contribute to chemoresistance. LCL161 is an oral small molecule antagonist of Inhibitor of Apoptosis Proteins (IAPs) that sensitizes a subset of tumors from diverse lineages to treatment with cytotoxic therapies, including paclitaxel. Multiple breast cancer models are sensitive to LCL161 as a single agent and LCL161 acts synergistically with paclitaxel in these models. A phase I study established an LCL161 dose of 1800 mg once weekly as well tolerated, with strong evidence of pharmacodynamic activity at doses ≥320 mg. This ongoing phase Ib study defines the dose limiting toxicities (DLTs), maximum tolerated dose (MTD), safety, and pharmacokinetics (PK) of LCL161 in combination with weekly paclitaxel.

Methods: Patients with advanced/metastatic solid tumors were treated with paclitaxel 80 mg/m2 each week followed by escalating doses of LCL161 administered once weekly immediately following paclitaxel. PK and biomarker sampling was performed.

Results: Thirty-two patients have received LCL161 doses of 600 mg (n = 3), 1200 mg (n = 5), 1500 mg (n = 4), and 1800 mg (n = 20). The most frequent adverse events considered LCL161-related included diarrhea (n = 11; 1 Grade 3), nausea (n = 8), fatigue (n = 7; 2 Grade 3), peripheral neuropathy (n = 6; 1 Grade 3), vomiting (n = 6), decreased appetite (n = 5), alopecia (n = 4), and anemia (n = 4). The principal DLTs were neutropenia, fatigue, and neuropathy. Significant cytokine release syndrome, the DLT of single-agent LCL161, has not been observed likely due to the use of dexamethasone as a premedication. No PK interaction between LCL161 and paclitaxel was observed. RECIST partial responses have been observed in 4 patients with diverse tumor types, including breast cancer. Preliminary antitumor activity in the expansion cohort with breast cancer patients will be presented.

Discussion: LCL161 and paclitaxel combination therapy is well tolerated, with manageable toxicities and no evidence of a PK interaction that might interfere with the activity of either agent. Enrollment of additional patients with breast and ovarian cancer into an expansion cohort is ongoing, utilizing an approach to identify those more likely to respond to treatment with IAP antagonists.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P6-11-06.