Background: Trastuzumab combined with chemotherapy is the standard of care for pts with HER2+ EG cancer. Resistance to trastuzumab clearly emerges in this population. Afatinib, a potent ErbB Family Blocker, induced tumor regression in MSKCC HER2+ patient derived xenografts (PDX). This study assesses safety and preliminary efficacy of afatinib in patients with trastuzumab refractory EG cancer. Methods: Pts with HER2+ (IHC 3+ or FISH>2.0) EG adenocarcinoma, after progression on trastuzumab, received oral afatinib 40 mg daily. Archival pre-trastuzumab tissue, tumor biopsy after progression on trastuzumab and after 1 week on afatinib is mandated on protocol for next generation sequencing (NGS), proteomics and establishment of PDX. The primary endpoint-overall clinical benefit at 4 months: stable disease (SD) or partial response (PR). Results: 20 pts treated with afatinib; median age 61, KPS 80; median 2 (1 to 4) prior trastuzumab regimens, 67% of tumors IHC3+; 33% IHC2+/FISH>2.0. Common adverse events included: rash or dry skin (Grade 1/2:80%), diarrhea (Grade 1/2:60%), nausea/vomiting (grade 1/2:40%) fatigue (grade1/2: 25%). To date, 19 pts evaluable for response, 2 PRs and 6 SD, 42% disease stabilization rate (PR+SD) at 4months (4 to 13 mos). PDXs established from biopsies of 7 pts. EGFR amplification was detected in the tumor of 2 pts with PRs and 1 of 6 pts with SD. Recurrent PIK3CA, ERBB3 and MTORmutations were observed. Conclusions: Afatinib shows clinical efficacy and is well tolerated in patients with trastuzumab refractory, heavily pretreated EG cancer. Enrollment has now begun in an afatinib + trastuzumab cohort. Efforts to elucidate the mechanisms of trastuzumab resistance including validation of potential drivers of trastuzumab resistance using HER2+ PDXs are ongoing. Updated molecular and clinical data will be presented. Clinical trial information: NCT01522768