In vitro and in vivo evaluation of tumor targeting styrene‐maleic acid copolymer‐pirarubicin micelles: Survival improvement and inhibition of liver metastases

Pirarubicin is a derivative of doxorubicin with improved intracellular uptake and reduced cardiotoxicity. We have prepared a micellar formulation of pirarubicin using styrene‐maleic acid copolymer (SMA) of mean molecular weight of 1.2 kDa, which exhibits a mean diameter of 248 nm in solution. Being a macromolecule, SMA‐pirarubicin micelles exhibit excellent tumor targeting capacity due to the enhanced permeability and retention (EPR) effect. Here we report the antitumor activity of SMA‐pirarubicin micelles on human colon and breast cancer cell lines in vitro , and a murine liver metastasis model in vivo . Metastatic tumor microvasculature, necrosis, apoptosis, proliferation, and survival were also investigated using immunohistochemistry for Ki‐67, active caspase‐3, and CD34, respectively. Drug cytotoxicity in vitro was assessed using MTT (3‐[4,5‐dimethyl‐2‐thiazolyl]‐2, 5‐diphenyl‐2H‐tetrazolium bromide) assay. In vivo , SMA‐pirarubicin was administered at 100, 150, or 200 mg/kg (pirarubicin equivalent). Tumor microvasculature was also assessed using scanning electron microscopy. Styrene‐maleic acid copolymer (SMA)‐pirarubicin micelles were toxic against human colorectal and breast cancer cells in vitro . IC 50 was at or below 1 μM, free pirarubicin equivalent. In vivo , SMA‐pirarubicin at 100 mg/kg reduced tumor volume by 80% and achieved a survival rate of 93% at 40 days after tumor inoculation. Styrene‐maleic acid copolymer (SMA)‐pirarubicin micelles demonstrated potent antitumor activity in this liver metastases model, contributing to prolonged survival. Histological examination of tumor nodules showed significant reduction and proliferation of tumor cells (>90%). The present results suggest that investigation of the effect of multiple dosing at later time points to further improve survival is warranted. ( Cancer Sci 2010)