Drug Delivery System for the Anticancer Drug Paclitaxel Using Lipocalin-Type Prostaglandin D Synthase Conjugated to a Tumor-Targeting Peptide.

Many drugs currently under development tend to have poor water solubility and large molecular weights, necessitating the development of systems for solubilizing and delivering these drugs to lesion sites. In this study, we attempted to utilize lipocalin-type prostaglandin D synthase (L-PGDS), which has a typical β-barrel structure and secretory lipid transport activity, as a carrier for the anticancer drug paclitaxel (PTX), which has a molecular weight of 854 and extremely low water solubility. Docking simulations and tryptophan residue fluorescence quenching showed that PTX binds to the upper side of the barrel via mainly hydrophobic interactions. To efficiently deliver L-PGDS to cancer cells and promote its internalization, we conjugated the tumor-targeting peptide CRGDK, which binds to neuropilin-1 (NRP-1), to the C-terminus of L-PGDS (L-PGDS-CRGDK). This conjugation did not significantly affect function, resulting in dissociation constants of 10.9 ± 1.1 μM and 8.7 ± 1.5 μM for PTX binding to L-PGDS-CRGDK and L-PGDS, respectively. L-PGDS-CRGDK translocated into the cytoplasm of MDA-MB-231 human breast cancer cells, which express NRP-1, but not MDA-MB-468 cells, which do not express NRP-1. L-PGDS itself did not translocate into either cell type. In a mouse model implanted with MDA-MB-231 tumors, the commercially available PTX formulation, Taxol, showed a significant antitumor effect during administration, but this effect weakened after discontinuation, with tumor volumes approaching those of the control group 16 days after discontinuation. In contrast, PTX/L-PGDS and PTX/L-PGDS-CRGDK maintained their effects even after discontinuation, reducing tumor volumes to 66% and 52%, respectively, of those of the control group 16 days after discontinuation. While the antitumor effect of PTX/L-PGDS was comparable to that of Taxol during administration, PTX/L-PGDS-CRGDK exhibited superior tumor suppression compared with Taxol and PTX/L-PGDS. These results suggest that L-PGDS-CRGDK, which can bind relatively large molecules and specifically target tumors, is a promising drug delivery system carrier for the anticancer drug PTX.