A phase I and biodistribution study of ABT-806i, an 111indium-labeled conjugate of the tumor-specific anti-EGFR antibody ABT-806.

Abstract

2520 Background: ABT-806 is a humanized antibody targeting a conformationally exposed epitope only available when tumor Epidermal Growth Factor Receptor (EGFR) is overexpressed or (EGFRvIII) mutated. A prior trial treated 8 patients (pts) with a chimeric homologue (single-dose 5-40 mg/kg), with a minor response in one squamous cell carcinoma of skin pt. A prior phase 1A study of ABT-806 treated 26 pts (2-24 mg/kg IV q2w), with prolonged SD in one head and neck (H&N) cancer pt. No pt had a typical EGFR-inhibitor rash. The current study gathers further ABT-806 clinical data, assesses ABT-806i dosimetry in normal and malignant tissues and examines its relationship to clinical and PK/PD data. Methods: Pts with advanced tumors likely to express EGFR, ECOG 0-2, measurable disease and adequate organ function were enrolled. ABT-806i scans comprised ABT-806i 5-7mCi injection followed by whole body and regional SPECT scans over one week. Cohort (C) 1 pts (n=6) had ABT-806i scans alone. C2 pts (n=12) had ABT-806i scans at baseline and at week 6 (after 3 fortnightly doses of ABT-806; 6 pts at 18 mg/kg and 6 at 24 mg/kg). Subjects with PR/SD could receive ABT-806 on an extension study until progression. Results: 18 pts (M:F 11:7; median age 57 yrs) with tumors of H&N (6), colon (3), lung (2), brain (2), bladder (1), cervical (1) and other (3) were treated. An H&N pt had a confirmed PR whilst 5 pts had SD (lasting 37 and 24 wks in an adrenal carcinoma and H&N pt respectively). Two potential toxicities were seen at 24mg/kg on the extension study: equivocal rash (G1; three transient lesions on nose) and allergic reaction (Sweet’s syndrome, G2). Dosimetry in C1 pts confirmed safe radiation exposure levels to normal tissues. Many pts showed high, specific tumor uptake of ABT-806i, including 1 pt with an intracranial tumor. ABT-806i uptake was not significantly affected by concurrent ABT-806 treatment. Ongoing analyses of how ABT-806i uptake correlates with tumor EGFR and clinical response may inform the recommended phase 2 dose of ABT-806. Conclusions: The high therapeutic index and specificity of ABT-806 merits further investigation as monotherapy or an antibody-drug conjugate. ABT-806i may have utility as a bioimaging agent. Clinical trial information: NCT01472003.