3096 Background: In preclinical studies, combination of sirolimus with gemcitabine enhances apoptosis in vitro and increases anti-tumor efficacy in vivo. Methods: Patients with advanced solid tumors, age 18-70 years, no prior mTOR inhibitor or gemcitabine, ECOG PS 0-1, and adequate hematological, renal and hepatic function, were enrolled in this phase I study to assess safety, tolerability, pharmacokinetics (PK), and to identify the dose limiting toxicity (DLT), maximum tolerated dose (MTD) and recommended dose (RD) of the combination of sirolimus and gemcitabine. A 3+3 dose escalation design with cohorts of 3-6 patients was used. Sirolimus was given po continuously. Gemcitabine was given iv 10mg/m2/minute on days 1 and 8 every 3 weeks. Dose levels 1, 2 and 3 corresponded to sirolimus 2, 2 and 5mg/24h plus gemcitabine 800, 1000 and 1000mg/m2 respectively. After observing DLTs at higher dose level and poorer mTOR signaling inhibition at lower doses, a new cohort of sirolimus 5mg/24h plus gemcitabine 800 mg/m2 was added. Skin biopsies pre and post treatment were performed to assess the inhibition of mTOR pathway. Results: 19 patients were enrolled: median age 51 years (36-70); gender 12M, 7F. Median number of cycles was 4. Patients were treated at 4 dose levels, the MTD was reached at level 3 and the RD was: sirolimus 5mg/24h and gemcitabine 800mg/m2. 3 DLTs were observed, 1 at dose level 2 and 2 at dose level 3: transaminitis grade 3, thrombocytopenia grade 3 and thrombocytopenia grade 4. Other toxicities grade 1-2 included anemia, neutropenia, asthenia, mucositis and high cholesterol levels. 2 patients achieved partial response (1 uterine cervix cancer and 1 colon cancer). Immunohistochemistry of pS6 in skin biopsies showed significative inhibition of mTOR pathway at RD. PK parameters estimated were in agreement with those previously reported in the literature. No influence of sirolimus administration on gemcitabine clearance was found. Conclusions: Combination of sirolimus and gemcitabine is feasible and safe, allowing administration of active doses of both agents and achieving mTOR signaling inhibition. A phase II study to assess the activity of this combination in sarcomas is ongoing.