Effect of Alkyl Length of Peptide–Polymer Amphiphile on Cargo Encapsulation Stability and Pharmacokinetics of 3‑Helix Micelles

3-Helix micelles have demonstrated excellent in vitro and in vivo stability. Previous studies showed that the unique design of the peptide–polymer conjugate based on protein tertiary structure as the headgroup is the main design factor to achieve high kinetic stability. In this contribution, using amphiphiles with different alkyl tails, namely, C16 and C18, we quantified the effect of alkyl length on the stability of 3-helix micelles to delineate the contribution of the micellar core and shell on the micelle stability. Both amphiphiles form well-defined micelles, <20 nm in size, and show good stability, which can be attributed to the headgroup design. C18-micelles exhibit slightly higher kinetic stability in the presence of serum proteins at 37 °C, where the rate constant of subunit exchange is 0.20 h^–1 for C18-micelles vs 0.22 h^–1 for C16-micelles. The diffusion constant for drug release from C18-micelles is approximately half of that for C16-micelles. The differences between the two micelles are significantly more pronounced in terms of in vivo stability and extent of tumor accumulation. C18-micelles exhibit significantly longer blood circulation time of 29.5 h, whereas C16-micelles have a circulation time of 16.1 h. The extent of tumor accumulation at 48 h after injection is ∼43% higher for C18-micelles. The present studies underscore the importance of core composition on the biological behavior of 3-helix micelles. The quantification of the effect of this key design parameter on the stability of 3-helix micelles provides important guidelines for carrier selection and use in complex environment.