Novel cathepsin B-sensitive paclitaxel conjugate: Higher water solubility, better efficacy and lower toxicity

In order to realize the targeted delivery of paclitaxel (PTX) to tumor through an environment-sensitive mechanism, increase its solubility in water and reformulate without toxic excipients, a novel PTX conjugate, PEG–VC–PABC–PTX was designed and synthesized in this study, using p-aminobenzylcarbonyl (PABC), a spacer, and valine–citrulline (VC), a substrate of cathepsin B (CB), to link polyethylene glycol (PEG) and PTX. Pegylated PTX (PEG-PTX) which was synthesized and Taxol formulation were prepared as controls. The conjugates were purified and characterized by melting points, 1H-NMR, ESI-MS or MALDI-TOF-MS. The two conjugates were similar in particle size, water solubility and their effects on MCF-7 cell line in vitro, and both of them induced no obvious toxicity in vivo. The release of PTX from PEG-PTX was faster due to its ester bond, while PEG–VC–PABC–PTX was proved to be CB-sensitive in terms of PTX release and its effect on cell cycle. Additionally, PEG–VC–PABC–PTX exhibited significant effects of antitumor, anti-angiogenesis and anti-proliferation in vivo, while the control conjugate was almost inefficacious at the same in vivo test. On the other hand, PTX conjugates demonstrated a thousand-time or more improvement in water solubility compared to PTX, suggesting a very easy way in the preparation and use of its injection. Without involvement of Cremophore EL and ethanol, the PTX conjugate will guarantee less adverse effects as frequently reported for Taxol formulation. Taxol formulation had a higher cytoxicity in vitro than PEG–VC–PABC–PTX likely because of toxic additives. Importantly, the CB-sensitive conjugate indicated a similar in vivo efficacy with the Taxol control, but much lower in vivo toxicity at the same doses evidenced by body weight, animal status, liver toxicity and blood count. Moreover, at the tolerant dose, this novel conjugate exhibited significantly better antitumor effect than that of Taxol formulation. In general, the PEG–VC–PABC–PTX conjugate designed in this study demonstrated significant advantages in terms of high water solubility, no toxic surfactant or organic solvent, tumor environment-sensitivity and high therapeutic index.

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