Tumor-imaging potential of liposomes loaded with In-111-NTA: biodistribution in mice

EMT6 tumors in BALB/c mice have been successfully imaged with small (less than 0.1 mu), unilamellar lipid vesicles (SUVs) loaded with In-111 nitrilotriacetic acid (In-111 NTA). Neutral SUVs prepared from distearoyl phosphatidylcholine (DSPC) and cholesterol (CH) (ratio 2:1) delivered sufficient radioactivity to allow tumor visualization 24 hr after i.v. injection; so did positively and negatively charged SUVs with the ratio 4:1:1 for DSPC:CH:X, were X was stearylamine or dicetyl phosphate. Other SUVs containing a 6-aminomannose or 6-aminomannitol derivative of cholesterol did not cause significant tumor accumulation of In-111 NTA, and tumor images were not readily discernible. The maximum tumor-associated radioactivity, 18.5% of injected dose per gram of tissue, was achieved with neutral SUVs. This level of tumor-associated In-111 was over 4 times that observed when unencapsulated In-111 NTA was injected. Neutral SUVs also gave the lowest specific activities in the liver and spleen (14.6% and 18.8% of dose respectively).