Background: Heat shock protein 90 (HSP90) is a molecular chaperone involved in the maintenance and function of client proteins, many of which are integral to key oncogenic processes. AUY922 is a competitive inhibitor of HSP90. Preclinical evidence suggests potential synergy between HSP90 inhibition and fluorouracil. This phase I study was designed to determine the maximum tolerated dose (MTD) of AUY922 in combination with standard dose of capecitabine as treatment for patients with advanced solid tumors. Methods: Pts with refractory solid tumors received AUY922 with capecitabine in a standard 3+3 dose escalation. Dose levels were capecitabine 1000mg/m² PO BID d 1-14 of 21-day cycles, with escalating doses of AUY922 IV days 1, 8, and 15; the 6th dose level combined the MTD of AUY922 with capecitabine 1250mg/m². Dose-limiting toxicities (DLTs), safety, and efficacy were evaluated. Results: 23 pts were treated at 6 dose levels: 22mg/m² (n = 3); 28mg/m² (n = 3); 40mg/m² (n = 3); 55mg/m² (n = 5); 70mg/m² (n = 3); 70mg/m² with capecitabine 1250mg/m² (n= 6). No DLTs were observed until the 6th dose level (grade 3 diarrhea). Related adverse events (% grade 1/2; % grade 3/4) included: diarrhea (43%; 17%), fatigue (30%; 13%), nausea (39%; 0), hand-foot skin reaction (30%; 5%), anorexia (30%; 4%), vomiting (30%; 0), and darkening vision (26%; 0). Vision darkening, a class effect of HSP90 inhibitors, was reversible with drug hold and retreatment was possible. Two pts (9%) had hematologic G 3/4 events of neutropenia. Of the 19 pts evaluable for response, partial response was noted in 4 patients (colorectal, 2; breast, 1; stomach, 1); 2 had progressed on prior fluorouracil, and remained on treatment for 13-35 wks. Stable disease was noted in 8 pts (35% [colorectal, 5; pancreas, 2; breast, 1]) with a median duration of 25.5 wks (range: 11-44+). All 5 colorectal pts were refractory to 5-FU. Conclusions: The addition of AUY922 to standard dose capecitabine was well-tolerated at doses of up to 70mg/m². Preliminary efficacy is encouraging, particularly as seen in pts previously resistant to fluorouracil, and warrants further investigation of this regimen. Clinical trial information: NCT01226732.