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Tumor hypoxia is strongly linked to aggressive disease progression and resistance to therapy. Positron emission tomography (PET) imaging with hypoxia tracers such as [18F]fluoroazomycin arabinoside (FAZA) allows for non-invasive quantification of tumor hypoxia during treatment. We and others have previously shown by immunohistochemical methods that treatments with longer lasting camptothecin formulations reduce tumor hypoxia after either single or multiple treatment cycles. Here we evaluated the kinetics and magnitude of hypoxia changes in tumors after treatment with irinotecan sucrosofate liposome injection (MM-398) which has shown an extended plasma half-life and higher intratumoral deposition in animal models relative to free pro-drug and compare it to the effects of free irinotecan at equivalent exposure levels. FAZA-PET/CT was used for longitudinal monitoring of tumor hypoxia changes in the HT29 mouse colon cancer xenograft model over a 21-day period following weekly chemotherapy administrations of either MM-398 (5 & 10mg/kg) or free irinotecan (50mg/kg). These dosages were predicted to result in comparable SN-38 exposure in either plasma or tumor based on a mechanistic pharmacokinetic model of MM-398 and free irinotecan that was developed using a systems pharmacology approach. Baseline levels of FAZA uptake in tumors were similar across treatment groups. Significant differences in tumor FAZA uptake were observed between these groups as early as Day 7 post treatment initiation, with increased FAZA uptake seen in tumors treated with free irinotecan. In contrast, differences in tumor volume only became statistically significant on Day 16. MM-398 at 10mg/kg was the most effective treatment for control of tumor volume and also minimized changes in FAZA uptake at all time-points. Background FAZA levels in the muscle were consistent over time across all treatment groups (0.78±0.18 %ID/g, 0.81±0.11 %ID/g and 0.71±0.20 %ID/g). However, normalization with muscle signal did not improve quantification of FAZA uptake differences in tumors. Tumor-specific hypoxia status at the study end point was confirmed by co-staining for CA9 and EF5 levels, which were, as expected, highly correlated. Average EF5 intensity/tumor area was lowest in the MM-398 (10mg/kg) treatment group, while being highest in the irinotecan (50mg/kg) treatment group. This study demonstrated the feasibility of performing longitudinal and repeated tumor hypoxia assessment using FAZA-PET imaging. Treatment with MM-398, but not free irinotecan, led to significant changes in the tumor microenvironment as measured by reduced hypoxia levels that occurred far earlier than anatomical changes assessed by tumor volume. Imaging of hypoxia levels after anti-cancer therapy with MM-398 has the potential to allow early assessment of treatment activity. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C293. Citation Format: Stephan G. Klinz, Jinzi Zheng, Raquel De Souza, Michael Dunne, Jason Cain, Jaeyeon Kim, Nancy Paz, Ashish Kalra, David Jaffray, Jonathan Fitzgerald. Irinotecan sucrosofate liposome injection, MM-398, demonstrates superior activity and control of hypoxia as measured through longitudinal imaging using [18F]FAZA PET compared to free irinotecan in a colon adenocarcinoma xenograft model. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C293.

MM-398 (aka PEP02) is a stable, nanotherapeutic encapsulation of the pro-drug CPT-11 (irinotecan) that is currently in clinical development. In preclinical experiments, treatment with MM-398 resulted in significantly higher intratumoral concentrations of both irinotecan (142-fold) and its major metabolite, SN-38 (9-fold) and exhibited superior anti-tumor activity compared to free irinotecan in multiple tumor xenografts. Subsequently, multiple phase 1 and 2 studies have established a pharmacokinetic and safety profile that supports continued clinical development, including in pancreatic, gastric, colorectal and potentially other cancers. Because current evidence suggests that resistance to pancreatic cancer is driven largely by inadequate drug penetration into these often poorly vascularized, stromally dense and hypoxic tumors, we sought to better understand how this relatively large (100nm) liposomal nanotherapeutic could potentially result in increased efficacy in very advanced gemcitabine-resistant pancreatic cancer and other cancer types. We developed a mechanism-based PK model of MM-398 and free irinotecan designed to predict intratumor SN-38 levels. Sensitivity analysis revealed that for MM-398 local activation of irinotecan to SN-38 was a far more important parameter than systemic activation. Through cellular uptake studies, we demonstrated in vitro that MM-398 was preferentially internalized by phagocytic macrophage/monocyte cell lines and, to a far lesser extent, by tumor cell lines. Furthermore, tumor microdistribution studies by flow cytometry and IHC showed uptake of MM-398 liposomes in both tumor cells and tumor-associated macrophages with more liposomal material being present in the macrophages. This distribution also suggests that macrophages may contribute to the postulated rate limiting process of irinotecan activation. The sensitivity analysis also suggested that tumor permeability and vascularization are important determinants of tumor-associated SN-38 levels for both free irinotecan and MM-398. To determine the effect of MM-398 on these parameters we treated mice bearing HT29 (colorectal cancer) xenografts with a single dose of MM-398 and measured hypoxic markers (CAIX) and microvessel density (CD31) by IHC. Tumors treated with MM-398 showed a greater degree of CD31 staining and lower CAIX staining, indicating that MM-398 may be able to affect tumor characteristics that traditionally have contributed to therapy resistance and limited the delivery of cancer therapeutics and resistance. In summary, encapsulation of irinotecan alters rate-limiting processes that determine tumoral SN-38 levels. Delivery of MM-398 is believed to alter tumor microvessel density and decrease hypoxia. These intriguing mechanisms of action findings support the continued clinical development of MM-398 as a differentiated therapeutic for several cancer types. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C207.