CX-2029, a PROBODY drug conjugate targeting CD71 (transferrin receptor): Results from a first-in-human study (PROCLAIM-CX-2029) in patients (Pts) with advanced cancer.

3502 Background: CX-2029 is a PROBODY drug conjugate (PDC) of MMAE, a potent microtubule inhibitor, directed against CD71 (transferrin receptor 1). In addition to being an abundant tumor antigen, CD71 is highly expressed on normal cells, precluding targeting by a traditional antibody drug conjugate (ADC). PDCs are masked ADCs, unmasked predominantly by tumor-associated proteases, thereby restricting target engagement to tumors. Both a CD71 PDC and ADC displayed broad activity in multiple xenograft tumor models; in toxicology studies, the PDC was tolerable at doses consistent with efficacy in non-clinical tumor models while the ADC was not. Methods: In a phase 1/2 first-in-human study of PDC CX-2029 in advanced solid tumors (N CT03543813 ), pts with ECOG 0–1 and ≥1 prior systemic therapy were enrolled into escalating dose cohorts of the PDC CX-2029 given IV every 21 days. Endpoints included evaluation of MTD, safety, antitumor activity, and potential biomarkers; plasma and tissue samples were collected for PK/PD analyses. Preliminary results are reported. Results: As of 30 November 2019, 34 pts were enrolled (median age 59 y; 59% male; 71% ECOG 1; median [range] of 3 [1–16] prior therapies). Pts received a median of 3 (1–12) CX-2029 doses. Starting dose for escalation was 0.1 mg/kg. Following a single CX-2029 dose, median molar ratio of masked CX-2029 to total CX-2029 for AUC tau was 0.938 (0.864–0.942); the ratio of free MMAE to total CX-2029 was <0.03. Infusion-related reactions were the most common treatment-related AE (TRAE) of any grade (88%; primarily low grade and with first infusion), followed by anemia (56%), fatigue and nausea (24% each), neutropenia (21%), and leukopenia (12%). Grade 3+ TRAEs in ≥10% pts were anemia (35%) and neutropenia (18%). In 32 response-evaluable pts, 1 pt had a confirmed partial response (squamous NSCLC); 9 had stable disease including 1 pt with ocular melanoma treated for 36 weeks. Conclusions: The observed safety profile for CX-2029 effectively reduces on-target toxicity for this previously undruggable target, supporting the PROBODY platform. Evidence of anti-tumor activity was observed. Dose escalation continues. Clinical trial information: NCT03543813 .