711 Background: The long circulating liposomal irinotecan nal-IRI (MM-398/PEP02, Onivyde) is approved for treatment of metastatic pancreatic cancer after disease progression with gemcitabine-based therapy. Besides their direct cytotoxic activity, camptothecins are thought to inhibit tumor angiogenesis via downregulating hypoxia-inducible factor 1 leading to attenuation of VEGF expression. Due to different deposition kinetics, irinotecan HCl and nal-IRI are likely to show different activities in vivo suggesting that a combination of the two agents may optimize intratumoral exposure and improve treatment efficacy. Methods: The activities of nal-IRI, irinotecan and their combination were compared in three human CRC xenograft models with different sensitivity to SN-38, the active metabolite of irinotecan, in vitro. Nal-IRI was dosed at 5 mg/kg q7d, while irinotecan HCl was dosed at 25 mg/kg at days 1 and 2 q7d. The activity of different regimens on tumor cell viability, hypoxia markers and the microvascular density was determined by quantitative biomarker analysis. Results: The relative antitumor activity of nal-IRI was most pronounced in tumor models with natural or acquired irinotecan resistance. Combinations of nal-IRI with irinotecan HCl was significantly better than irinotecan HCl alone in all tumor models although nal-IRI only provided 10% additional irinotecan. The antitumor activity of nal-IRI and Irintecan HCl in combination was accompanied by up to two times more tumor cell death and a marked 3-7 fold reduction of the microvessel density. Despite the strong antiangiogenic effect resulting in tumor hypoxia, the increase in HIF1α and, to lesser degree, HIF2α, was relatively modest and VEGF signal intensity remained at 85-115% of control values. Conclusions: Our results suggest that both irinotecan HCl and nal-IRI can counteract the hypoxia-mediated increase of HIF1α in vivo as previously reported in vitro. Furthermore, the combination of the two formulations demonstrated significant efficacy benefits. A combination of irinotecan HCl and nal-IRI merits further clinical investigation.