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, with demonstrated activity in a variety of preclinical models. Further preclinical evidence suggests potential synergy between inhibition of HSP90 and fluorouracil treatment (Burkitt et al. 2007). 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: Patients with refractory solid tumors for which capecitabine was an appropriate therapy received AUY922 with capecitabine in a standard 3+3 dose escalation. Capecitabine 1000mg/m² was administered twice daily for days 1-14 of 21-day cycles, with escalating doses of AUY922 administered by intravenous (IV) infusion on 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 patients 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). There were no DLTs observed until the 6th dose level (grade 3 diarrhea). Common adverse events (all grades) included: diarrhea (61%), nausea (57%), fatigue (43%), hand-foot skin reaction (39%), anorexia (39%), vomiting (35%), rash (30%), and darkening vision (22%). Myelosuppression was uncommon, with no instances of grade ≥3 thrombocytopenia, and only 2 patients (9%) with grade 3/4 neutropenia (1 patient each). Of the 19 patients evaluable for response per RECIST 1.1, unconfirmed partial response (PR) was noted in 3 patients (13%; colorectal, 1; breast, 1; stomach, 1), with 1 additional confirmed PR (4%; colorectal); two of these had progressed on prior fluorouracil. Stable disease was noted in 8 patients (35%). Conclusions: The addition of AUY922 to standard dose capecitabine was well-tolerated at doses of up to 70mg/m². Preliminary efficacy is encouraging, and warrants further investigation of this regimen. Clinical trial information: NCT01226732.