Development of a nattokinase–polysialic acid complex for advanced tumor treatment

Cancer-associated thrombus (CAT) impedes delivery of nanoparticles to tumor sites and also inhibits the ability of immune cells to detect and attack these tumors, particularly in advanced tumors with old thrombi. Nattokinase (NK) is an extract from a popular Japanese food, natto, which consists of boiled soybeans fermented with bacteria. Nattokinase exerts strong fibrinolytic and thrombolytic activities and can unblock blood vessels. To deliver NK to thrombus sites in tumors, we modified the surface of NK with polysialic acid (PSA), which formed complexes via electrostatic interactions, resulting in NK–PSA. Particle size and zeta potential of NK–PSA were evaluated, and differential scanning calorimetry, Fourier-transform infrared spectroscopy, and morphological analyses of NK–PSA were performed. To determine the efficacy of the NK–PSA complex on delivery of nanoparticulate drugs, sialic acid-modified doxorubicin liposomes (DOX-SAL) were used as a model drug. In vivo pharmacokinetic and tissue distribution analyses showed that the blood clearance rate of DOX-SAL was significantly enhanced by NK–PSA, and NK–PSA increased accumulation of 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindotricarbocyanine iodide (DiR) labeled SAL (DiR-SAL) in tumors. Analysis of anti-tumor efficacy showed that the combination of NK–PSA and DOX-SAL enhanced anti-tumor activity. These results suggested that NK–PSA combined with DOX-SAL may be an effective strategy to clear CAT and increase the ability of nanoparticles and immune cells to reach tumors.