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-chelating agent, the bis(stearylamide) of diethylenetriaminepentaacetic acid (L), were evaluated as a potential radiopharmaceutical (111In-L-LDL) for tumor localization by studying their internalization in human pancreatic cancer cells (Capan-1). Using Dil-LDL (l,l′-dioctadecyl-3,3,3′,3′-tetramethylindodicarbocyanine perchlorate-LDL), this cell line was shown to bind human LDL with a high-affinity saturable component and a low-affinity non-saturable (40%) component. The single saturable high-affinity binding site had a KD of 27.5 ± 2.1 μg/ml and a maximal binding of 610 ± 7.5 ng/ml protein. Electron-microscopic examination of the In-L-LDL particles revealed the peripheral distribution of the electron-dense indium atoms at the outer surface of LDL. The modified LDL were then shown to be internalized by the cells. After conjugation of In-L-LDL to colloidal gold to follow the different stages of internalization, electron-microscopic examination showed that the In-L-LDL gold conjugates were stuck to the external sheet of the plasma apical and microvilli membrane, into earlier and later endosomes and into multi-vesicular bodies, suggesting the penetration of the In-L-LDL particles into lysosomal vacuoles. The observation of In-L-LDL-gold conjugates in deep-seated cytoplasm suggests that LDL could be employed as a drug-transport vehicle for targeting cytotoxics or radionuclides close to the cell nucleus. Int. J. Cancer, 70:315–322, 1997. © 1997 Wiley-Liss, Inc.