Indium-111 Labeling of Low Density Lipoproteins with the DTPA−Bis(stearylamide):  Evaluation as a Potential Radiopharmaceutical for Tumor Localization

Abstract

In order to use the LDL receptor pathway to target radionuclides to cancer sites for imaging and diagnostic purposes, a labeling procedure of LDL with 111In using the DTPA-bis(stearylamide) (L) has been developed. This bifunctional ligand is intended to be incorporated into the phospholipid monolayer of LDL and to specifically chelate the In3+ cation at the surface. The ligand was incorporated into LDL in buffered medium with a 65-80% yield. The L-LDL samples are stable over a 24 h period when examined by dialysis, allowing their storage before indium-111 radiolabeling. In vitro studies of In-L-LDL particles show that indium labeling is rapidly achieved (1 h). More than 85% of the indium atoms are bound to the chelating functions of the incorporated DTPA derivatives and less than 10% to the nonspecific complexation sites of LDL (e.g., protein residues). After incubation in human serum, the indium activity recovered in the LDL fraction of In-L-LDL samples (95%) is much higher than in In-LDL samples (35%), pointing out the strong stabilizing chelating effect of the ligand. Competitive binding studies show that In-L-LDL are recognized by LDL receptors of A549 cells like native LDL when the In-L/LDL ratio varies from 5 to 30. All these in vitro experiments demonstrate that the In-L-LDL conjugates possess properties suitable for further work with in vivo experiments.

Low density lipoprotein and liposome mediated uptake and cytotoxic effect of N4-octadecyl-1-β-D-arabinofuranosylcytosine in Daudi lymphoma cells

“…This is consistent with findings for other lipophilic compounds like indium complexed to DTPA-stearylamide, which did not hinder LDL-specific interactions with cells after incorporation into the lipophilic part of the LDL particles (Jasanada et al, 1996). Yen and co-workers (1995) reported that LDL receptors on Daudi cells were already up-regulated which was reflected by elevated mRNA for LDL receptors and insignificant increase of receptor density after incubation in lipoprotein-deficient medium.…”

Tissue biodistribution and blood clearance rates of intravenously administered carbon nanotube radiotracers

“…It is commercially available (Aldrich), highly water-soluble, and allows for a simple conjugation chemistry through linkage to the amino functions at the CNT surface. Also, DTPA rapidly cages indium with a highly thermodynamic equilibrium constant (44 Fmoc-Phe-OH (5 equiv) in DMF (6 ml) was added in the presence of Bop͞HOBt͞diisopropylethylamine (DIEA) (5͞5͞15 equiv). The coupling was repeated twice for 20 min.…”

“…Chelation of 111 In with DTPA and other chelating agents form very stable complexes that are commonly used for in vivo biodistribution studies of various therapeutic molecules and their delivery systems (45)(46)(47). The stability constant between DTPA and 111 In has been determined by others to be 1.5 ϫ 10 29 (48), and previous studies have shown that the [ 111 In]-DTPA ligand has a strong chelating effect on the radiometal in human serum (44). Moreover, we characterized the In-DTPA complex using nonradioactive indium by HPLC without any loss of indium, concluding that the chelate is stable under physiological conditions.…”

Internalization of indium-labeled LDL through a lipid chelating anchor in human pancreatic-cancer cells as a potential radiopharmaceutical for tumor localization

“…A lipophilic derivative of diethylenetriaminepentaacetic acid (DTPA), the N,N9-bis(stearylamide) of DTPA (L), was first incorporated into LDL giving L-LDL conjugates before adding 111-indium to obtain the final tracer 111 In-L-LDL. In vitro experiments demonstrated the stability of the lipophilic anchorage of the chelator, the stability of the indium-111 complexation in plasma, while binding studies showed recognition of the 111 In-L-LDL particles by LDL receptors of A549 cells (Jasanada et al, 1996). These preliminary studies demonstrated that In-L-LDL particles possess suitable properties for evaluation as a potential radiopharmaceutical for tumor localization.…”

“…For the radiometals detectable by gamma scintigraphy, the radioelement was affixed directly or indirectly to protein residues of Apoprotein B-100 (ApoB), the binding ligand to the LDL cell receptor (R-LDL) (Rosen et al, 1990;Virgolini et al, 1991;Ponty et al, 1993). In these procedures, the radiometal/LDL ratio was kept low to avoid disrupting the 3-dimensional structure of the recognition protein.To localize the radionuclide preferentially on the lipid layer and to minimize the structural modification of ApoB, we have developed a new labeling method using a lipid-chelating anchor (Jasanada et al, 1996). A lipophilic derivative of diethylenetriaminepentaacetic acid (DTPA), the N,N9-bis(stearylamide) of DTPA (L), was first incorporated into LDL giving L-LDL conjugates before adding 111-indium to obtain the final tracer 111 In-L-LDL.…”