What are determining factors for stable drug incorporation into polymeric micelle carriers? Consideration on physical and chemical characters of the micelle inner core.

Partially benzyl-esterified poly(ethylene glycol)-b-poly(aspartic acid) (PEG-P(Asp(Bzl))) having different hydrophobic inner-core structure were synthesized and analyzed. We obtained two types of the block copolymers for formation of polymeric micelle drug carriers; one had an amide-bond ratio of 1:3 (alpha/beta) in the poly(aspartic acid) residues through alkaline hydrolysis, and the other one had 100% of the alpha-amide through acid hydrolysis. Subsequently, we prepared partially benzyl-esterified block copolymers with an esterification degree of 40 to 100% in the aspartic acid residue. Regarding camptothecin (CPT) incorporation into polymeric micelles, we evaluated effects that block copolymers' inner hydrophobic block structures have on CPT behavior. Regarding CPT-incorporation stability, PEG-P(alpha,beta-Asp(Bzl) block copolymers with the alpha and beta-amides were found to exhibit higher CPT-incorporation stability. Using fluorescent probes, we evaluated the properties of inner-core blocks such as hydrophobicity and mobility/rigidity, and the findings implied that stable CPT incorporation could be obtained by an adequate balance between the micelle inner core's hydrophobicity and the micelle inner core's rigidity or between the micelle inner core's hydrophobicity and steric configuration of the hydrophobic block chain.