Preliminary evaluation of 177Lu-labeled knottin peptides for integrin receptor-targeted radionuclide therapy
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Cystine knot peptides (knottins) 2.5D and 2.5F were recently engineered to bind integrin receptors with high affinity and specificity. These receptors are overexpressed on the surface of a variety of malignant human tumor cells and tumor neovasculature. In this study, 2.5D and 2.5F were labeled with a therapeutic radionuclide, 177Lu, and the resulting radiopeptides were then evaluated as potential radiotherapeutic agents in a murine model of human glioma xenografts.
Knottins 2.5D and 2.5F were synthesized using solid phase peptide synthesis, folded in vitro, and site-specifically coupled with 1,4,7,10-tetraazacyclododecane-N,N′,N′′,N′′′-tetraacetic acid (DOTA) at their N terminus for 177Lu radiolabeling. The stability of the radiopeptides 177Lu-DOTA-2.5D and 177Lu-DOTA-2.5F was tested in both phosphate-buffered saline (PBS) and mouse serum. Cell uptake assays of the radiolabeled peptides were performed in U87MG integrin-expressing human glioma cells. The biodistribution studies of both 177Lu-DOTA-2.5D and 177Lu-DOTA-2.5F were examined in U87MG tumor-bearing athymic nu/nu mice. Radiation absorbed doses for the major tissues of a human adult male were calculated based on the mouse biodistribution results.
DOTA-2.5D and DOTA-2.5F were labeled with 177Lu at over 55% efficiency. High radiochemical purity for both radiocomplexes (> 95%) could be achieved after high performance liquid chromatography (HPLC) purification. Both radiopeptides were stable in PBS and mouse serum. Compared to 177Lu-DOTA-2.5D (0.39 and 0.26 %ID/g at 2 and 24 h, respectively), 177Lu-DOTA-2.5F showed much higher tumor uptake (2.16 and 0.78 %ID/g at 2 and 24 h, respectively). It also displayed higher tumor to blood ratios than that of 177Lu-DOTA-2.5D (31.8 vs 18.7 at 24 h and 52.6 vs 20.6 at 72 h). Calculation of radiodosimetry for 177Lu-DOTA-2.5D and 177Lu-DOTA-2.5F suggested that tumor and kidney were tissues with the highest radiation absorbed doses. Moreover, 177Lu-DOTA-2.5F had a higher tumor to kidney radiation absorbed dose ratio than that of 177Lu-DOTA-2.5D.
Cystine knot peptides can be successfully radiolabeled with 177Lu for potential therapeutic applications. Knottin 2.5F labeled with 177Lu exhibits favorable distribution in murine U87MG xenograft model; thus, it is a promising agent for radionuclide therapy of integrin-positive tumors.
Keywords177Lu Cystine knot peptide Integrin Radionuclide therapy
This work was supported, in part, by the National Cancer Institute (NCI) 5R01 CA119053 (ZC), and the In Vivo Cellular Molecular Imaging Center (ICMIC) grant P50 CA114747 (SSG), the Edward Mallinckrodt Jr. Foundation (JRC), as well as a Ph.D. student fellowship from the China Scholarship Council (LJ).
Conflicts of interest
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