INTERNALIZATION OF INDIUM-LABELED LDL THROUGH A LIPID CHELATING ANCHOR IN HUMAN PANCREATIC-CANCER CELLS AS A POTENTIAL RADIOPHARMACEUTICAL FOR TUMOR-LOCALIZATION

Low-density lipoproteins (LDL) labeled with indium via a lipid-chelati ng agent, the bis(stearylamide) of diethylenetriaminepentaacetic acid (L), were evaluated as a potential radiopharmaceutical (In-111-L-LDL) for tumor localization by studying their internalization in human panc reatic cancer cells (Capan-1). Using Dil-LDL ioctadecyl-3,3,3',3'-tetr amethylindodicarbocyanine perchlorate-LDL), this cell line was shown t o bind human LDL with a high-affinity saturable component and a low-af finity non-saturable (40%) component, The single saturable high-affini ty binding site had a K-D of 27.5 +/- 2.1 mu g/ml and a maximal bindin g of 610 +/- 7.5 ng/ml protein. Electron-microscopic examination of th e In-L-LDL particles revealed the peripheral distribution of the elect ron-dense indium atoms at the outer surface of LDL. The modified LDL w ere then shown to be internalized by the cells. After conjugation of I n-L-LDL to colloidal gold to follow the different stages of internaliz ation, electron-microscopic examination showed that the In-L-LDL gold conjugates were stuck to the external sheet of the plasma apical and m icrovilli membrane, into earlier and later endosomes and into multives icular bodies, suggesting the penetration of the In-L-LDL particles in to lysosomal vacuoles. The observation of In-L-LDL-gold conjugates in deep-seated cytoplasm suggests that LDL could be employed as a drug-tr ansport vehicle for targeting cytotoxics or radionuclides close to the cell nucleus. (C) 1997 Wiley-Liss, Inc.