Synthesis and in vitro evaluation of an 111in-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 cyclase 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 was 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 indium-111. 1. The non-radioactive indium analogue (In-DOTA-NCS-ST) was also prepared in macroscopic quantities. In-DOTA-NCS-ST was produced as a single species (>80% RCP) and purified by HPLC. Human colon cancer CaCO-2 and T-84 cells were used to evaluate the in vitro IC 50 values for GC-C receptor binding and determine the cell uptake and retention of radioactivity. Results: The DOTA-NCS-ST and In-DOTA-NCS-ST conjugates exhibit high in vitro binding affinity for GC-C receptors with IC 50 values <10 nM. The in vitro cell binding studies with the In-DOTA-NCS-ST conjugate demonstrated that 111 In-label ST internalizes in human colon cancer cells and exhibits long-term retention. Conclusion: The combination of radiolabeling efficacy and specific in vitro cell uptake and retention suggests that the DOTA-NCS-ST construct holds potential for the development of diagnostic or therapeutic radiopharmaceuticals labeled with trivalent radiometals for specific targeting of human colonic cancers.