2549 Background: Like many pairs of chemotherapy agents, the combination of IRI and FLOX displays ratio-dependent activity in vitro. CPX-1, a liposome formulation of IRI:FLOX, was developed to maintain a synergistic 1:1 molar ratio in vivo, was highly active in preclinical models, and was evaluated in a phase 1 trial (CLTR0104–101). Methods: Doses were escalated from 30U/m2 (1U= 1 mg IRI + 0.36 mg FLOX) to 270 U/m2 given on day 1 and 15 of each 28 day cycle. Adult patients (pts) with advanced solid tumors, ECOG PS<2, adequate bone marrow, liver, and renal function were eligible; 4 pts per cohort. After defining the MTD, additional pts with CRC were enrolled (extension phase). IRI completed greater than 12 months prior to this trial was allowed in the absence of resistance to IRI. PK was done on day 1 and 15 of the 1st cycle. Results: Safety: The dose escalation phase enrolled 24 pts in 6 cohorts and added 2 pts in the 5th cohort (210U/m2; the MTD) after noting dose limiting diarrhea (3 pts) and neutropenia (1 pt) including one death from dehydration and renal failure due to prolonged diarrhea (gr3) & vomiting (gr2) at 270U/m2. An additional 7 pts with CRC received 210U/m2 in the extension phase. Grade 3/4 adverse events included diarrhea, nausea, vomiting, neutropenia and thrombocytopenia with most occurring at 270U/m2. No new toxicities were observed for this combination. Response: 30/33 pts were evaluable with 2 confirmed PRs (NSCLC and CRC), 21 SD and 7 PD. Median PFS was 5.4 mos. (0.3–11.8 mos.) in 15 pts w/CRC. PK: All pts maintained synergistic plasma IRI:FLOX ratios for 24h. IRI and FLOX AUCs (0-inf) were greater for CPX-1 than expected for conventional drugs. AUCs for SN-38 and 5FU at 210U/m2 were 0.8 ± 0.1 and 10 ± 8.7 μg-hr/mL, respectively, indicating bioavailability for both drugs. Conclusion: CPX-1 was well tolerated in the outpatient setting and evidence of anti-tumor activity was obtained. This is the first clinical evaluation of ratiometric dosing in which a synergistic drug ratio, pre-selected in vitro based on optimal anti-tumor activity, was maintained systemically to enhance therapeutic benefit. [Table: see text]