Phase 1b trial of cirmtuzumab and paclitaxel for locally advanced, unresectable and metastatic breast cancer

Background: Cirmtuzumab is a humanized monoclonal antibody that targets the receptor tyrosine kinase like orphan receptor 1 (ROR1), which is expressed on poor prognosis breast cancer, ovarian cancer, other solid tumors, CLL and mantle cell lymphoma. One clinical study showed increased expression of ROR1 in breast cancers after neoadjuvant chemotherapy and a preclinical PDX breast cancer model showed cirmtuzumab and paclitaxel to have at least additive efficacy1. A recently completed Phase 1 trial of cirmtuzumab in CLL showed the antibody to be both safe and effective in inhibiting tumor cell ROR1 signaling in patients with CLL2. Methods: The primary aim of this trial was to determine the safety of cirmtuzumab and weekly paclitaxel in patients with advanced Her2 negative breast cancer based upon dose limiting toxicities (DLTs) in the first cycle of treatment. Secondary endpoints were clinical activity, pharmacokinetics and correlative biomarkers on tumor specimens. Eligible patients were those with locally advanced, unresectable or metastatic Her2 negative breast cancer who had not received paclitaxel in the metastatic setting, had not developed metastatic disease within 6 months of (neo)adjuvant paclitaxel, had ECOG performance status of 0-2, and had adequate laboratory parameters. Any number of prior lines of therapy were allowed. Study treatment included fixed dose 600 mg cirmtuzumab given days 1 and 15 of cycle 1 and then day 1 of each subsequent 28-day cycle. Paclitaxel was given weekly at a dose of 80mg/m2. Patients were evaluated in dose cohorts of 5 for DLTs with a target of 15 evaluable patients. Results: To date, 6 patients evaluable for safety and 5 patients evaluable for DLTs were treated. Age range was 30 to 59 years. Three of 6 safety-evaluable patients had triple negative breast cancer at study enrollment. Prior lines of chemotherapy in the metastatic setting ranged from 0-3. No discontinuations for toxicity and no DLTs have been observed to date. Adverse events (AEs) have been consistent with the known safety profile of paclitaxel, with 3 episodes of grade 3 neutropenia in 2 patients and 1 episode of grade 3 hyperglycemia. All other AEs were grade 1 or 2. Partial responses have been observed in 2/5 patients with one patient response ongoing with cirmtuzumab alone for at least 17 weeks after stopping paclitaxel. Pharmacokinetic analysis of serial plasma samples for free unbound antibody from two patients provided results similar to those observed in CLL patients treated with cirmtuzumabwith a projected half-life of 30 days2. No decline in antibody concentration over time was observed consistent with the absence of neutralizing antibodies. Conclusions: Preliminary information indicates that the combination of fixed dose cirmtuzumab plus paclitaxel is well-tolerated and safe. Responses to therapy have been observed and preliminary pharmacokinetic results are consistent with sustained potentially therapeutic levels. Updated safety, clinical activity, pharmacokinetics and biomarker analyses will be presented. 1Zhang S et al. Proc Natl Acad Sci USA. 116(4): 1370-1377. PMID 30622177. 2Choi MY et al. Cell Stem Cell 22(6): 951-959. PMID 29859176. Funding sources: CIRM UC San Diego Alpha Stem Cell Clinic and Sanford Stem Cell Clinical Center; Oncternal Therapeutics, Inc.; UC San Diego Moores Cancer Center Padres Pedal the Cause Grant; Gonick Breast Cancer Research Fund Citation Format: Rebecca A Shatsky, Richard B Schwab, Teresa L Helsten, Emily I Pittman, Ruifeng Chen, James B Breitmeyer, Catriona HM Jamieson, Thomas J Kipps, Barbara A Parker. Phase 1b trial of cirmtuzumab and paclitaxel for locally advanced, unresectable and metastatic breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-10-18.