Preliminary evaluation of 177Lu labeled knottin peptides as novel integrin targeted agents in a human glioma xenografted mice model

Objectives Previously, engineered cystine-knot (knottin) peptides of 2.5D and 2.5F have been developed to possess high affinity and specificity to αvβ3 integrin receptors that over-expressed on the surface of many malignant human tumor cells and the tumor neovasculatures. In this study, 2.5D and 2.5F derivatives were labeled with a therapeutic radionuclide, 177Lu, and the resulting radiopeptides were evaluated whether they could be used as potential radiotherapeutic agents in a human glioma xenografted mice model.

Methods Two knottin peptides, 2.5D and 2.5F were synthesized and labeled with 177Lu after site-specific conjugation DOTA to the N-terminus of these peptides. The stability of the radiocomplexes was tested through mixing with PBS buffer or mouse serum. Cell uptake assays were performed in U87MG cell lines. The biodistribution studies of both 177Lu-DOTA-2.5D and 177Lu-DOTA-2.5F were examined in U87MG tumor-bearing nude mice.

Results Knottin peptides were labeled with 177Lu in good radiochemical yield (~63.85% and ~59.71%, respectively) and high purity (both, >90%). Both radiolabeled knottin peptides were stable in PBS buffer for 24h and mouse serum for 1h. Compared to 177Lu-DOTA-2.5D, 177Lu-DOTA-2.5F shows much better tumor uptakes (2.16 ± 0.29 and 0.78 ± 0.20 %ID/g at 2 h and 24 h, respectively). 177Lu-DOTA-2.5F also displays better tumor-to-blood ratio than that of 177Lu-DOTA-2.5D (31.8±12.4 vs. 18.7±6.8 at 24h, and 52.6±4.1 vs. 20.6±2.6 at 72h).

Conclusions Knottin peptide 2.5F labeled with 177Lu exhibits good tumor uptake and retention, and it is a promising integrin targeted radiotherapeutic agents for human glioblastoma