Copoly(styrene-maleic acid)-pirarubicin micelles: high tumor-targeting efficiency with little toxicity

The copolymer of styrene-maleic acid (SMA) was used to construct micelles containing pirarubicin (4'-O-tetrahydropyranyladriamycin, or THP) as a new anticancer drug formulation. The procedure for the preparation of the micelles was simple, the component consisting of only SMA and pirarubicin in a noncovalent association, possibly by hydrophobic interaction between the styrene portion of SMA and pirarubicin chromophore. This method ensures more than 80% recovery of pirarubicin by weight, and 60% of drug loading (by weight) was achieved. The micelles obtained (SMA-THP) showed high solubility in water and a constant pirarubicin release rate of about 3-4%/day in vitro. SMA-THP micelles had an average molecular size of about 34 kDa according to gel chromatography; this size is a marked increase from the 627.6 Da of free THP, which suggests the formation of a micellar structure. When albumin was added, the molecular size of the micelles increased to about 94 kDa, which indicates binding to albumin, a unique characteristic of SMA. SMA-THP micelle preparation had a cytotoxic effect (93-101%) on MCF-7 breast cancer cells and SW480 human colon cancer cells in vitro that was comparable to that of free THP. An in vivo assay of SMA-THP at doses of 20 mg/kg in ddY mice bearing S-180 tumor revealed complete tumor eradication in 100% of tested animals. Mice survived for more than 1 year after treatment with micellar drug doses as high as 100 mg/kg pirarubicin equivalent. This marked antitumor activity can be attributed to the enhanced permeability and retention (EPR) effect of macromolecular drugs seen in solid tumors, which enables selective delivery of drugs to tumor and thus much fewer side effects. Complete blood counts, liver function test, and cardiac histology showed no sign of adverse effects for intravenous doses of the micellar preparation. These data thus suggest that intravenous administration of the SMA-THP micellar formulation can enhance the therapeutic effect of pirarubicin more than 50-fold.