Synthesis and in vitro evaluation of an In-111-labeled ST-peptide enterotoxin (ST) analogue for specific targeting of guanylin receptors on human colonic cancers

Background: Human colonic cancer cells are known to express guanylate cycla se C (GC-C) receptors for guanylin and uroguanylin. E.Coli ST is a peptide with high metabolic stability that specifically binds to GC-C receptors. An in vitro evaluation of a new synthetic indium-111 labeled ST conjugate for specific targeting of human colonic cancers that express GC-C receptors wa s performed. Materials and Methods: A DOTA conjugated ST analogue DOTA-NCS- 6-Ahx-Phe(19)-ST[1-19] (DOTA-NCS-ST) was synthesized and labeled with indiu m-111. The non-radioactive indium analogue (In-DOTA-NCS-ST) was also prepar ed in macroscopic quantities. In-111-DOTA-NCS-ST was produced as a single s pecies (> 80% RCP) and purified by HPLC. Human colon cancer CaCO-2 and T-84 cells were used to evaluate the in vitro IC50 values for GC-C receptor bin ding and determine the cell uptake and retention of radioactivity. Results: The DOTA-NCS-ST and In-DOTA-NCS-ST conjugates exhibit high in vitro bindin g affinity for GC-C receptors with IC50 values < 10 nM. The in vitro cell b inding studies with the In-111-DOTA-NCS-ST conjugate demonstrated that In-1 11-label ST internalizes in human colon cancer cells and exhibits long-term retention. Conclusion: The combination of radiolabeling efficacy and speci fic in vitro cell uptake and retention suggests that the DOTA-NCS-ST constr uct holds potential for the development of diagnostic or therapeutic radiop harmaceuticals labeled with trivalent radiometals for specific targeting of human colonic cancers.