Ado-trastuzumab emtansine in patients with HER2 amplified salivary gland cancers (SGCs): Results from a phase II basket trial.

Abstract

6001 Background: SGCs are rare tumors with no approved therapy for metastatic disease. HER2 amplification occurs in 8% among all SGC histologies, and 25-33% of the aggressive salivary duct carcinoma (SDC) histologic subtype. We hypothesized that ado-trastuzumab emtansine, a HER2 targeted antibody drug conjugate, may be clinically active in these patients. Methods: A cohort of patients with HER2 amplified SGCs were enrolled into a multi-histology basket trial of ado-trastuzumab emtansine, treated at 3.6mg/kg IV every 3 weeks. The primary endpoint was overall response rate (ORR) by RECIST v1.1 or PERCIST. A Simon two-stage optimal design was applied with type I error rate under 2.7%, power of 89%, H0 10%, H1 40%; H0 will be rejected if 6 or more responses are observed in 24 patients. Other endpoints include duration of response (DOR), progression-free survival (PFS), and toxicity. HER2 amplification was identified by next generation sequencing (NGS), and tumors were subsequently tested by fluorescence in situ hybridization (FISH) and immunohistochemistry (IHC). Fluorescence lifetime imaging microscopy - Förster resonance energy transfer (FLIM-FRET) assessed the propensity for HER2-HER3 heterodimerization, which leads to receptor internalization. Results: 10 patients with HER2 amplified SGCs were treated. The median age was 65 (range 36-90 years), 90% were male. The median lines of prior systemic therapy was 2 (range 0-3). ORR was 90% (9/10, 95% CI 56-100%) including 5 complete responses after prior trastuzumab, pertuzumab and anti-androgen therapy. After a median follow up period of 12 months (range 4-20 months), median DOR (range 2-19+) and median PFS (95% CI 4–22+ months) were not reached. Toxicities included grade 1 or 2 infusion reaction, thrombocytopenia and transaminitis; there were no treatment related deaths. HER2 amplification by NGS (fold change 2.8 to 22.8) correlated with HER2/CEP17≥2 by FISH (8/8 tested) or IHC3+ (10/10 tested). FLIM-FRET tested positive in 3/3. Conclusions: Ado-trastuzumab emtansine is highly efficacious in patients with HER2 amplified SGCs as identified by NGS. This study has met its primary endpoint, and cohort expansion is warranted to confirm these results. Clinical trial information: NCT02675829.