In vitro and in vivo evaluation of 177Lu- and 90Y-labeled E. coli heat-stable enterotoxin for specific targeting of uroguanylin receptors on human colon cancers

The human E. coli heat-stable enterotoxin (STh, amino acid sequence N1SSNYCCELCCNPACTGCY19) binds specifically to the guanylate cyclase C (GC-C) receptor, which is present in high density on the apical surface of normal intestinal epithelial cells as well as on the surface of human colon cancer cells. Analogs of STh are currently being used as vectors targeting human colon cancers. Previous studies in our laboratory have focused on development of 111Indium-labeled STh analogs for in vivo imaging applications. Here, we extend the scope of this work to include targeting of the therapeutic radionuclides 90Y and 177Lu. The peptide DOTA-F19-STh(1–19) was synthesized using conventional Fmoc-based solid-phase techniques and refolded in dilute aqueous solution. The peptide was purified by RP-HPLC and characterized by MALDI-TOF MS and in vitro receptor binding assay. The DOTA-conjugate was metallated with nonradioactive Lu(III)Cl3 and Y(III)Cl3, and IC50 values of 2.6±0.1 and 4.2±0.9 nM were determined for the Lu- and Y-labeled peptides, respectively. 177Lu(III)Cl3 and 90Y(III)Cl3 labeling yielded tracer preparations that were inseparable by C18 RP-HPLC, indicating that putative differences between Lu-, Y- and In coordination spheres are not observed in the context of labeled STh peptides. In vivo biodistribution studies of the 177Lu-labeled peptide in severe combined immunodeficient (SCID) mice bearing T-84 human cancer tumor xenografts showed rapid clearance from the bloodstream, with >90 %ID in the urine at 1 h pi. Localization of the tracer within tumor xenografts was 1.86±0.91 %ID/g at 1 h pi, a value higher than for all other tissues with the exception of kidney (2.74±0.24 %ID/g). At 24 h pi, >98 %ID was excreted into the urine, and 0.35±0.23 %ID/g remained in tumor, again higher than in all other tissues except kidney (0.91±0.46 %ID/g). Biodistribution results at 24 h pi for the 90Y-labeled peptide mirrored those for the 177Lu analog, in agreement with the identical behavior of the labeled analogs by C18 RP-HPLC. These results demonstrate the ability of 177Lu- and 90Y-labeled STh molecules to specifically target GC-C receptors expressed on T-84 human colon cancer cells.