In vivo fate of folate-targeted polyethylene-glycol liposomes in tumor-bearing mice.

PURPOSE: To compare the in vivo tissue distribution of folate-targeted liposomes (FTLs) injected i.v. in mice bearing folate receptor (FR)-overexpressing tumors (mouse M109 and human KB carcinomas, and mouse J6456 lymphoma) to that of nontargeted liposomes (NTLs) of similar composition. EXPERIMENTAL
DESIGN: A small fraction of a folate-polyethylene-glycol (PEG)-distearoyl-phosphatidylethanolamine conjugate was incorporated in FTLs. Both FTLs and NTLs were PEGylated with a PEG-distearoyl-phosphatidylethanolamine conjugate to prolong circulation time. Liposomes were labeled with [(3)H]cholesterol hexadecyl ether with or without doxorubicin loading. Liposome levels in plasma, tissues, or ascites were assessed by the number of [(3)H] counts. For doxorubicin-loaded formulations, we also determined the tissue doxorubicin levels by fluorimetry. To estimate the amount of liposomes directly associated with tumor cells in vivo, we determined the [(3)H]radiolabel counts in washed pellets of ascitic tumor cells using the ascitic J6456 lymphoma
RESULTS: FTLs retained the folate ligand in vivo, as demonstrated by their ability to bind ex vivo to FR-expressing cells after prolonged circulation and extravasation into malignant ascitic fluid. In comparison with NTLs, FTLs were cleared faster from circulation as a result of greater liver uptake. Despite the lower plasma levels, tumor levels of FTL-injected mice were not significantly different from those of NTL-injected mice. When NTLs and FTLs were loaded with doxorubicin, liver uptake decreased because of liver blockade, and uptake by spleen and tumor increased. When tumor-to-tissue liposome uptake ratios were analyzed, the targeting profile of FTLs was characterized by higher tumor:skin, and tumor:kidney ratios but lower tumor:liver ratio than NTLs. After a concomitant dose of free folic acid, FTLs (but not NTLs) plasma clearance and liver uptake were inhibited, indicating that accelerated clearance was mediated by the folate ligand. Surprisingly tumor uptake was not significantly affected by a codose of folic acid. In the J6456 ascitic tumor model, tumor cell-associated liposome levels were significantly greater for FTL-injected mice than for NTL-injected mice, despite slightly higher levels of the latter in whole ascites.
CONCLUSIONS: Whereas folate targeting does not enhance overall liposome deposition in tumors, the targeting profile of tumor versus other tissues is substantially different and intratumor liposome distribution in ascitic tumors is affected favorably with a selective shift toward liposome association with FR-expressing cells.