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In Vivo Imaging of Human Colon Cancer Xenografts in Immunodeficient Mice Using a Guanylyl Cyclase C–Specific Ligand

¹ Research and Development Department, Targeted Diagnostics & Therapeutics, Inc., West Chester, Pennsylvania
² Research Department, Nycomed Amersham plc, Little Chalfont, Amersham, United Kingdom
³ Department of Synthetic Chemistry, Nycomed Amersham plc, Oslo, Norway
⁴ Biological Research, Nycomed Amersham Inc., Wayne, Pennsylvania
⁵ Biological Research, Nycomed Amersham plc, Durham, North Carolina
⁶ Departments of Medicine and Biochemistry and Molecular Pharmacology, Thomas Jefferson University, Philadelphia, Pennsylvania

Guanylyl cyclase C (GC-C) is a transmembrane receptor expressed by human intestinal cells and primary and metastatic colorectal adenocarcinomas but not by extraintestinal tissues or tumors. The Escherichia coli heat-stable enterotoxin analog, STa (5–18), is a 14–amino acid peptide that selectively binds to the extracellular domain of GC-C with subnanomolar affinity. This study examined the utility of a radiolabeled conjugate of STa (5–18) to selectively target and image extraintestinal human colon cancer xenografts in vivo in nude mice. Methods: The STa conjugate, ethoxyethyl-mercaptoacetamidoadipoylglycylglycine-STa (5–18) (NC100586), was synthesized and labeled with ^99mTc to produce ^99mTc-NC100586. This compound was intravenously administered to nude mice bearing human colon cancer xenografts, and specific targeting was evaluated by biodistribution and gamma camera imaging. Results: In CD-1 nude mice, biodistribution and scintigraphic imaging analyses showed selective uptake of ^99mTc-NC100586 into human colon cancer xenografts that express GC-C but not into normal tissues that do not express GC-C. Similarly, ^99mTc-NC100586 injected intravenously into CD-1 nude mice with human colon cancer hepatic metastases selectively accumulated in those metastases, and 5-mm foci of tumor cells were visualized after ex vivo imaging of excised livers. Accumulation of ^99mTc-NC100586 in human colon cancer xenografts reflected binding to GC-C because ^99mTc-NC100588, an inactive analog that does not bind to GC-C, did not selectively accumulate in cancer xenografts compared with normal tissues. Also, coadministration of excess unlabeled STa (5–18) prevented accumulation of ^99mTc-NC100586 in human colon cancer xenografts. Furthermore, ^99mTc-NC100586 did not selectively accumulate in Lewis lung tumor xenografts, which do not express GC-C. Conclusion: This study showed that intravenously administered STa (5–18) selectively recognizes and binds to GC-C expressed by human colon cancer cells in vivo. Also shown was the ability to exploit this selective interaction to target imaging agents to extraintestinal human colon tumors in nude mice. These results suggest the utility of STa and GC-C for the development of novel targeted imaging and therapeutic agents with high specificity for metastatic colorectal tumors in humans.

Key Words: guanylyl cyclase C • STa • colorectal cancer • enterotoxin

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