Structure-Dependent Incorporation of Terpenes into Amphiphilic Poly(2-oxazoline) Micelles

Poly(2‑oxazoline) (POx) amphiphilic block copolymers are highly effective carriers for poorly water‑soluble small molecules, yet structural features that promote drug incorporation remain unclear. Motivated by the unusually high loading of diterpene taxanes in POx micelles, we examined whether shared terpene motifs support solubilization with POx. Representative mono‑, di‑, and triterpenes,α‑pinene, carnosic acid (CA), carnosol (CARN), squalene, lupeol, betulin, and ursolic acid,were tested for micellization with a well‑defined POx triblock copolymer. Only the diterpenes formed small, uniform, and stable micelles. CA and CARN showed the best performance: CA produced reproducible 22–29 nm micelles at a 2/10 (w/w) CA/polymer feed, achieving ~17% loading capacity with quantitative efficiency. Higher feed ratios yielded large, polydisperse aggregates by DLS and TEM. Other terpenes displayed minimal or unstable incorporation. CA‑loaded micelles remained colloidally stable for 48 h at room temperature, though CA’s chemical stability varied by batch. Lyophilization improved CA stability, enabling 67–89% recovery after reconstitution. pH‑dependent studies showed increasing particle size with higher pH for CA/POx micelles, consistent with CA ionization and altered hydrogen bonding. Physiological NaCl suppressed this pH sensitivity for CA, indicating ionic‑strength‑mediated stabilization. Overall, the rigid diterpene scaffold and ortho‑hydroxyl groups appear key for POx incorporation, highlighting CA as a promising candidate for further formulation development.