Purpose. To study the effects of hydrophobicity of the micelle-formingblock copolymeric drug conjugate, methotrexate (MTX) esters ofpoly-(ethylene oxide)-block-poly(2-hydroxyethyl-L-aspartamide) (MTXesters of PEO-b-PHEA), on the stability of micelles and on drug release.
Methods. MTX esters of PEO-b-PHEA with three levels of MTXconjugation were synthesized. Size distribution of the micelles wasmeasured by dynamic light scattering (DLS). The critical micelleconcentration (CMC) was determined by a light scattering study. Sizeexclusion high performance liquid chromatography (SEC-HPLC) wasused to study the equilibrium between unimers and micelles, and releaseof MTX at pH 7.4.
Results. MTX esters of PEO-b-PHEA with MTX substitution of 7.4%,22%, and 54% were prepared. The conjugates formed micelles basedon DLS. The stability of the micelles correlated with the level of MTXconjugation. The conjugate with 54% MTX had a lower CMC (0.019mg/mL) than the conjugates with 22% MTX (0.081 mg/mL) or 7.4%MTX (0.14 mg/mL). Micelle dissociation was significantly slower forthe conjugate with 54% MTX than that with 22% and 7.4% MTX.Slower release of MTX from the micelles was also observed for theconjugate with the higher MTX attachment.
Conclusions. MTX esters of PEO-b-PHEA can be structurallymodulated by varying the degree of MTX substitution, which in turn changesthe hydrophobicity of the conjugate, thereby modifying micelle stabilityand controlling drug release.