A pseudopolyrotaxane (PPRX) comprising 3-carboxy-5-nitrophenylboronic acid modified γ-cyclodextrin (NPBA-γ-CyD) and naphthalene modified polyethylene glycol (Naph-PEG) as a sugar-responsive supramolecular structure is prepared. The binding of sugar by the NPBA group induced disintegration of the Naph-PEG/NPBA-γ-CyD PPRX, allowing the components to be dissolved. The Naph-PEG/NPBA-γ-CyD PPRX exhibited better sensitivity compared to that of a PPRX based on 4-carboxyphenylboronic acid modified γ-cyclodextrin (PBA-γ-CyD). We have previously reported the unique structure of Naph-PEG/PBA-γ-CyD PPRX, which formed an inclusion complex with a single-stranded PEG chain being threaded through the γ-CyD rings, with the remaining internal space being occupied by the sugar-sensing PBA moiety from a neighboring ring, thus shielding it from sugar molecules and reducing the sugar sensitivity of the PPRX. In contrast, structural analyses in this study revealed that the sugar-sensing NPBA moiety in the Naph-PEG/NPBA-γ-CyD PPRX is not included in the neighboring NPBA-γ-CyD. This spatial arrangement and the high affinity of NPBA for sugar contributed to the improved sugar responsivity. The enhanced NPBA-γ-CyD was then applied to a PPRX containing Naph-PEG-appended insulin (Naph-PEG-Ins) that showed an improved response for glucose-induced insulin release.

We have designed a pseudopolyrotaxane (PPRX), known as a molecular necklace, consisting of phenylboronic acid-modified γ-cyclodextrin (PBA-γ-CyD) and naphthalene-modified polyethylene glycol (Naph-PEG) for developing sugar-responsive insulin delivery systems. Interestingly, structural analyses show that the Naph-PEG/PBA-γ-CyD PPRX obtained by our method was single stranded, whereas ordinary PPRXs using parent γ-CyD were double stranded. The Naph-PEG/PBA-γ-CyD PPRX was poorly water soluble at pH 7.4; however, sugar addition induced disintegration of the PPRX, and the components were dissolved, suggesting that the PBA moiety acts as a sugar sensor. We also have developed a PPRX consisting of Naph-PEG-appended insulin (Naph-PEG-Ins) and PBA-γ-CyD and have confirmed that the release rate of Naph-PEG-Ins was accelerated following sugar addition.