Background: CA19-9 has been shown to correlate with response to therapy and OS in patients with mPAC. NAPOLI-1, a randomized phase 3 study evaluated nal-IRI, a nanoliposomal formulation of irinotecan, with or without 5-FU/LV vs 5-FU/LV in patients with mPAC previously treated with gemcitabine-based therapy. Nal-IRI+5-FU/LV significantly improved OS (primary endpoint) vs 5-FU/LV (6.1 mo vs 4.2 mo; unstratified hazard ratio [HR] = 0.67; P = 0.012). CA19-9 response (≥50% decline from baseline) was superior with nal-IRI+5FU/LV compared with 5-FU/LV (29% vs 9%; P=0.0006). Nal-IRI alone did not show a statistical improvement in survival. Methods: Patients with a recorded baseline CA19-9 measurement were divided into quartiles to evaluate the treatment effect pattern of CA19-9 from nal-IRI+5-FU/LV and 5-FU/LV arms. Quartile ranges were based on 404 available CA19-9 values from randomized patients (N=417). Unstratified Cox proportional hazards regression was used to estimate HRs and corresponding 95% CIs. Effect of baseline CA19-9 on time to response, progression-free survival, and response will be presented. Results: Of patients randomized to receive nal-IRI+5-FU/LV (n = 117) or 5-FU/LV enrolled contemporaneously (n = 119), 218 received study drug and had a baseline CA19-9 measurement. Results show a greater treatment effect on OS with higher CA19-9 level relative to 5-FU/LV. Conclusions: In patients with mPAC previously treated with gemcitabine-based therapy, nal-IRI+5-FU/LV significantly improved OS supported by progression free survival and objective response rate. The CA19-9 serum level can provide important information with regards to overall survival. Clinical trial information: NCT01494506