Determinants for in vivo anti-tumor effects of PEG liposomal doxorubicin: importance of vascular permeability within tumors.

To elucidate the determinants of the in vivo anti-tumor efficacy of polyethylene glycol (PEG)-modified liposomal doxorubicin (DOX), we examined its anti-tumor effect against three different tumor cell lines (Lewis lung cancer (LLC), Colon-26 (C26) and B16BL6 melanoma (B16)) in vitro and in vivo. In vitro, LLC was the most sensitive tumor to DOX and liposomal DOX based on the MTT assay. However, the strongest in vivo anti-tumor effect was observed in the C26 tumor-bearing mice. The in vivo accumulation of radiolabelled PEG liposome in the C26 tumor after intravenous injection was significantly larger than in other tumors. The extent of vascularity assessed by immunohistochemical staining of CD31 was not directly related with the tumor accumulation of PEG liposome. On the other hand, Evans blue extravasation and secretion of VEGF in C26 tumors were higher than in LLC tumors, clearly demonstrating that the vasculature permeability was higher within C26 tumors. These results indicated that the vascular permeability within the tumor substantially affects the tumor accumulation of PEG liposome and may be one of the important determinants in the in vivo anti-tumor efficacy of PEG liposomal DOX.

Fetuin mediates hepatic uptake of negatively charged nanoparticles via scavenger receptor.

We tried to evaluate the possible involvement of fetuin in the scavenger receptors (SRs)-mediated hepatic uptake of polystyrene nanospheres with the size of 50 nm (NS-50), which has surface negative charge (zeta potential=-21.8+/-2.3 mV). The liver perfusion studies in rats revealed that the hepatic uptake of NS-50 pre-coated with fetuin (NS-50-fetuin) was significantly inhibited by poly inosinic acid (poly I), a typical inhibitor of SRs, whereas that of plain NS-50 or NS-50 pre-coated with BSA (NS-50-BSA) was not. The uptake of NS-50-fetuin by cultured Kupffer cells was also significantly inhibited by poly I, and anti-class A scavenger receptors (SR-A) antibody, suggesting that fetuin on NS-50 mediated the recognition and internalization of NS-50 by Kupffer cells and at least SR-A would be responsible for the uptake. Taken that Western blot analysis confirmed that fetuin certainly adsorbed on the surface of NS-50 after the incubation of NS-50 with serum, the results obtained in the present study indicate that fetuin would be one of the serum proteins that were substantially involved in the hepatic uptake of NS-50 via SRs.

Pre-coating with serum albumin reduces receptor-mediated hepatic disposition of polystyrene nanosphere: implications for rational design of nanoparticles.

We evaluated the in vivo disposition characteristics of polystyrene nanospheres (NS) with the particle size of 50 nm (NS-50) pre-coated with human serum albumin (HSA) after intravenous administration in rats. HSA-coated NS-50 showed much longer blood-circulating property and the hepatic uptake clearance for HSA-coated NS-50 was about 1/5 of that for NS-50. In parallel with the results obtained in the in vivo study, liver perfusion experiments also showed that the hepatic disposition of HSA-coated NS-50 was much less than that of NS-50 in the presence of serum in the perfusate. To unravel the mechanism behind the less affinity of HSA-coated NS-50 to the liver, serum proteins associated on the surface was quantitatively and qualitatively assessed. The results indicated that pre-coated HSA impaired subsequent association of serum proteins onto the surface, suggesting that the association of a given serum protein with opsonic activity might be suppressed by HSA pre-coating. From these findings, pre-coating of nanoparticles with serum albumin could be useful to prevent their rapid clearance by mononuclear phagocyte system in vivo.