ABA and BAB type triblock copolymers of PEG and PLA: a comparative study of drug release properties and "stealth" particle characteristics.

We synthesized two types of triblock copolymers containing PEG and PLA as controlled release carriers of hydrophobic drugs: these are the ABA type (PLA-PEG-PLA) and the BAB type (PEG-PLA-PEG). These polymers are amphiphilic and can form nanomicelles (40-200nm) in aqueous medium. On the surface of PLA-PEG-PLA (ABA) type nanomicelles, the PEG content was enhanced somewhat over the bulk amount; whereas in the PEG-PLA-PEG (BAB type), surface segregation was much higher. The copolymers tested can entrap 35% of paclitaxel by weight on the average. In general, the diffusion-controlled release of paclitaxel is slower for the BAB polymers; furthermore, the actual release rates are influenced by the PLLA lengths in the BAB copolymers. Surface PEG contents influence the "stealth" characteristics of the nanomicelles. Compared with PLA particles, all nanomicellar particles tested, of both BAB and ABA types, showed a four-fold reduction in monocyte cell uptake, with the BAB type copolymer exhibiting a lesser uptake.