Cothreading of Unmodified and Monoazidated β-Cyclodextrins in Polyrotaxanes for Orthogonal Modification of Cell-Penetrating Peptides via Click Chemistry

To establish a method for the orthogonal chemical functionalization of β-cyclodextrin (β-CD)-based polyrotaxane (PRX), unmodified and monoazidated β-CDs (N3-β-CDs) were utilized as cyclic host molecules for synthesizing PRXs. PRXs were successfully synthesized by using N3-β-CD, and the molar ratio of unmodified β-CD and N3-β-CD in the resulting PRXs could be regulated by the feed ratio of unmodified β-CD and N3-β-CD. It was revealed that all PRX molecules contained N3-β-CD, indicating that unmodified and monoazidated β-CDs were cothreaded onto a single polymer axis. Physicochemical characterization of β-CD/N3-β-CD cothreaded PRXs suggested that their crystalline structures were unaffected by the molar ratio of N3-β-CD, whereas their thermal stability was slightly improved with increasing molar ratio of N3-β-CD. To facilitate cellular internalization of PRXs, threaded N3-β-CD in PRXs was modified with cell-penetrating octaarginine (R8) peptides via a copper(I)-catalyzed azide–alkyne click reaction. The cellular internalization efficiency of the R8-modified PRXs increased with the number of R8 peptides modified on PRXs. However, PRXs modified with a large number of R8 peptides precipitated in serum solutions and exhibited high cytotoxicity. PRXs modified with the optimal number of R8 peptides did not exhibit precipitation and showed high cellular uptake compared to unmodified PRXs without cytotoxicity. Consequently, PRXs modified with the optimal number of R8 peptides showed superior cholesterol-reducing effects in fibroblasts derived from patients with Niemann–Pick type C disease. As demonstrated here, the number of modified functional molecules plays a crucial role in the treatment effects of PRXs, and β-CD/N3-β-CD cothreaded PRXs can offer potential for orthogonal postmodification. Thus, β-CD/N3-β-CD cothreaded PRXs could introduce benefits that enable the material applications of PRXs.