Phase 1 dose escalation of ZW25, a HER2-targeted bispecific antibody, in patients (pts) with HER2-expressing cancers.

1035 Background: The HER2 receptor is expressed across a range of cancers (ca) although expression and heterogeneity vary greatly within and between tumors. FDA approved HER2-targeted therapies (tx) have shown clinical benefit only in HER2 high (IHC3+ or IHC2+/FISH+) breast and gastric ca. ZW25, built on the Azymetric™ platform which utilizes unique Fc mutations to create IgG1-like bispecific antibodies, binds to the same extracellular domains of HER2 as trastuzumab (T) and pertuzumab (P) with increased binding and internalization compared to T alone. In preclinical studies ZW25 was well tolerated and active in models of HER2 low to high ca. Methods: Eligible pts (HER2 IHC 1-3+, progression after standard of care (SOC) tx, normal LVEF) were enrolled in a 3+3 dose escalation study of ZW25 (5, 10 or 15 mg/kg) IV weekly in 4 week cycles. Assessments included adverse events (AEs), LVEF, tumor response and PK. Results: 9 pts have received ZW25 at 5 (n = 3) or 10 mg/kg (n = 6); 15 mg/kg is ongoing. All pts were HER2 high (breast = 4; gastric/GEJ = 4; adnexal = 1). Prior tx included T in all pts; P and T-DM1 in 4 and lapatinib (L) in 3 of 4 breast pts. The most common AEs (all Gr 1/2) were infusion reaction (IR) (5/9), diarrhea (4/9), and fatigue (3/9), with no DLTs and one related Gr 3 AE of hypophosphatemia. The IRs occurred only with 1st dose and did not recur with prophylactic tx (primarily acetaminophen and diphenhydramine; steroids included for 2 pts). At 5 mg/kg, peak drug levels after C1D1 = ~100 ug/ml, accumulating ~50% higher by dose 4. Best response in 8 pts (1 too early): 2 PR (both breast ca with prior T, P, T-DM1, and L; 10 mg/kg, 55% and 33% target lesion decrease, respectively, after 2 cycles), 1 SD (breast ca, 5 mg/kg, ongoing after 4 cycles), and 5 PD. 5 pts currently active; includes 1 gastric pt (10 mg/kg) with PD (new nodal lesions) and clinical benefit with decrease in target lesions and CEA (33 to 1.5 ng/ml). Conclusions: ZW25 has been well tolerated with promising anti-tumor activity in pts with HER2-expressing ca progressing after SOC therapy. These early signs of activity support the therapeutic potential for bispecific antibodies using the Azymetric platform. Development of ZW25 is ongoing, including in lower HER2-expressing ca. Clinical trial information: NCT02892123.

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