Cyclophosphamide (CPA) and doxorubicin (DXR)-containing sterically stabilized liposomes (DXR-SL) have a proven clinical activity. We propose that a metronomic CPA dosing schedule enhances accumulation of DXR-SL in solid tumors, because it causes apoptosis in the endothelial cells of the growing tumor vasculature and thereby may increase the permeability of the tumor microvessels. To establish the validity of this hypothesis we investigated the therapeutic benefits of metronomic CPA dosing (p.o.) combined with DXR-SL (i.v.) in a Lewis lung carcinoma, subcutaneously growing in C57BL/6 mouse. The metronomic CPA dosing clearly promoted accumulation and subsequent deep diffusion of SL in the solid tumor as a result of rather a transient increase in the density of CD31+-microvessels, which shows high permeability to SL. It appears that the enhancing effect of metronomic CPA dosing is strongly dependent on the dose of CPA as well as on the time at which the treatment was initiated. Our study indicates that the use of metronomic chemotherapy combined with nanocarriers may be of significant clinical and practical importance in treating intractable solid tumors.

8553 Background: Aurora A kinase is a serine/threonine protein kinase that is essential for the successful transit of cells through mitosis. MLN8237 is a selective small molecule inhibitor of Aurora A kinase that has demonstrated anti-tumor activity in animal models of solid human tumors. In this study we explored the anti-tumor effect of MLN8237 in vivo in pre-clinical models of human Diffuse Large B-cell Lymphoma (DLBCL) both as a single agent and in combination with the anti-CD20 monoclonal antibody Rituximab. Methods: Three human DLBCL models were examined in SCID mice. In two of the models (Ly19 & WSU) the tumor cells expressed a constitutively active luciferase, enabling tumor burden analysis in either a subcutaneous or disseminated setting. The third model was a primary DLBCL recently obtained from a patient. Tumor bearing animals were treated for 21 days with MLN8237 (QD PO dosing at 3–20mg/kg), Rituximab (Q7D IV dosing at 10 mg/kg), or the two agents combined. Tumor burden was measured in the disseminated models as a function of luciferase-induced photon flux, and in the subcutaneous models using vernier calipers. Results: MLN8237 induced anti-tumor activity that was dose-dependent in all three models. In LY19 disseminated model, 3 mg/kg of MLN8237 combined with Rituximab induced synergistic anti-tumor activity (n=2); while 10 mg/kg MLN8237 (dosed QD) combined with Rituximab was additive. Importantly, combining MLN8237 with Rituximab led to complete cures in 100% of the animals. In the WSU model, combining MLN8237(3–10mg/kg) with Rituximab resulted in additive tumor growth inhibition. The mean survival endpoint was significantly longer (p=0.003 and <0.001 respectively) in the combination group when compared to the individual group. In the primary lymphoma model, MLN8237(10–20mg/kg) caused a significant anti-tumor effect during treatment period (TGI = 83–95%). Combining MLN8237 and Rituximab in primary model resulted in additive anti-tumor effect. Conclusions: MLN8237 combined with Rituximab was found to reduce tumor burden in an additive and/or synergistic mechanism in multiple DLBCL tumor models. MLN8237 is currently being tested as a single agent in a phase I clinical trail in patients with DLBCL. [Table: see text]

13059 Background: The mitotic kinase Aurora A is implicated in the development of multiple tumor types. MLN8054 is an oral, potent and selective small-molecule inhibitor of Aurora A with broad efficacy in preclinical models of cancer. Inhibition of Aurora A by MLN8054 induces accumulation of mitotic cells, followed by apoptosis. This study explores relationships between Aurora A inhibition, mitotic index, and tumor growth inhibition for xenograft models with different sensitivity to MLN8054. The marker response in mouse skin was also studied. Methods: Mice bearing subcutaneous xenografts were dosed orally qd or bid with MLN8054 for 21 days. Pharmacodynamic markers were studied after 1–2 doses. Formalin-fixed xenograft tissues were stained with the mitotic markers pHisH3 and MPM2, or with an antibody to the T288 autophosphorylation site on Aurora A. Tumor growth inhibition (TGI) was calculated using the formula 100 - [ΔT/ΔC * 100], where ΔT is the volume change for treated tumors, and ΔC is the volume change for control tumors. Results: HCT116 human colon xenografts were sensitive to MLN8054 on a qd or bid schedule (84% and 96% TGI respectively for 30mg/kg dose). The T288 autophosphorylation site was used to directly demonstrate inhibition of Aurora A, which resulted in dose-dependent duration of the elevation in mitotic index. Efficacy was similar for qd vs bid dosing of 30mg/kg MLN8054, and accordingly we found that a single dose was sufficient to elevate the mitotic index for about 20–24h in this model. SW480 human colon xenografts have MLN8054 sensitivity similar to that of HCT116, but more modest effects on mitotic index were observed. The mitotic index profile of SW480 is similar to that of MDA-MB-231 xenografts, the most insensitive model studied. Elevated mitotic index was also observed in mouse skin. Conclusions: We found that mitotic index measurements coupled with the T288 autophosphorylation site as a direct marker of Aurora A activity are useful for monitoring inhibition of Aurora A by MLN8054 in tumor and/or skin biopsies. In a sensitive model, greater duration of mitotic index elevation results in greater efficacy. Our continuing work aims to better understand the differences in marker and efficacy responses between xenograft lines, incorporating the pT288 antibody as a direct marker of Aurora A inhibition. [Table: see text]