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CytomX Therapeutics Shares Positive Phase 1a Dose Escalation Findings for Monotherapy CX-904 (EGFRxCD3 PROBODY® T-Cell Engager)

Thursday, May 09, 2024

CytomX Therapeutics, Inc. (Nasdaq: CTMX) announced positive initial findings from the ongoing CX-904 Phase 1a dose escalation clinical study. The study focuses on CX-904, a novel masked, conditionally activated PROBODY T-cell engager designed to target the epidermal growth factor receptor (EGFR) on cancer cells and the CD3 receptor on T cells within the tumor microenvironment.

CytomX, expressed excitement about the results, emphasizing CX-904's potential to transform patient care with safer, more effective therapies. The data, McCarthy noted, validate over a decade of innovation at CytomX and suggest broader applications for T-cell engagers across various cancer types and targets.

The Phase 1 study enrolled 35 patients with advanced metastatic solid tumors expressing EGFR, including pancreatic, colorectal (CRC), non-small cell lung cancer (NSCLC), head and neck squamous cell carcinoma (HNSCC), gastric, and esophageal cancers. Patients had undergone a median of four previous lines of therapy. The study included non-step dosing cohorts with target doses ranging from 0.007 mg to 6 mg and step-dosing cohorts with target doses ranging from 5 mg to 10 mg and tocilizumab prophylaxis. Enrollment for a cohort targeting 15 mg is ongoing.

CX-904 demonstrated a favorable safety profile, allowing for outpatient administration and monitoring. No cases of CRS of any grade were observed in step-dosing cohorts, and only Grade 1 CRS was reported in non-step dosing cohorts, mostly at the highest dose of 6 mg. Common treatment-related adverse events (TRAEs) included rash, arthralgia, arthritis, pruritus, and vomiting, mostly low grade.

Eight patients showed measurable tumor reduction at the data cutoff, including two of six efficacy-evaluable patients with pancreatic cancer achieving confirmed partial responses per RECIST 1.1. All efficacy-evaluable pancreatic cancer patients achieved disease control, with one patient experiencing an 83% tumor reduction and another a 51% reduction, maintaining study treatment. Additionally, a third pancreatic cancer patient exhibited stable disease with no evidence of tumor growth through 3.5 months of treatment.

Preliminary pharmacokinetic and pharmacodynamic data supported the PROBODY TCE mechanism of action, showing maintained masking in circulation and CD8+ T-cell margination and tumor infiltration.

The CX-904 Phase 1a dose escalation and optimization are ongoing, with future enrollment focused on establishing a recommended Phase 2 dose. The company expects to provide an additional Phase 1a dose escalation update by the end of 2024, informing discussions with partner Amgen regarding the initiation of Phase 1b expansion cohorts targeting specific EGFR-positive tumor types.



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