Comparative Evaluation of Three 64Cu-Labeled E. coli Heat-Stable Enterotoxin Analogues for PET Imaging of Colorectal Cancer

Analogues of the E. coli heat-stable enterotoxin (STh) are currently under study as both imaging and therapeutic agents for colorectal cancer. Studies have shown that the guanylate cyclase C (GC-C) receptor is commonly expressed in colorectal cancers. It has also been shown that STh peptides inhibit the growth of tumor cells expressing GC-C. The ability to determine GC-C status of tumor tissue using in vivo molecular imaging techniques would provide a useful tool for the optimization of GC-C-targeted therapeutics. In this work, we have compared receptor binding affinities, internalization/efflux rates, and in vivo biodistribution patterns of an STh analogue linked to N-terminal DOTA, TETA, and NOTA chelating moieties and radiolabeled with Cu-64. The peptide F19−STh(2−19) was N-terminally labeled with three different chelating groups via NHS ester activation and characterized by RP-HPLC, ESI-MS, and GC-C receptor binding assays. The purified conjugates were radiolabeled with Cu-64 and used for in vitro internalization/efflux, in vivo biodistribution, and in vivo PET imaging studies. In vivo experiments were carried out using SCID mice bearing T84 human colorectal cancer tumor xenografts. Incorporation of DOTA-, TETA-, and NOTA-chelators at the N-terminus of the peptide F19-STh(2−19) resulted in IC50s between 1.2 and 3.2 nM. In vivo, tumor localization was similar for all three compounds, with 1.2−1.3%ID/g at 1 h pi and 0.58−0.83%ID/g at 4 h pi. The principal difference between the three compounds related to uptake in nontarget tissues, principally kidney and liver. At 1 h pi, 64Cu-NOTA-F19-STh(2−19) demonstrated significantly (p < 0.05) lower uptake in liver than 64Cu-DOTA-F19-STh(2−19) (0.36 ± 0.13 vs 1.21 ± 0.65%ID/g) and significantly (p < 0.05) lower uptake in kidney than 64Cu-TETA-F19-STh(2−19) (3.67 ± 1.60 vs 11.36 ± 2.85%ID/g). Use of the NOTA chelator for coordination of Cu-64 in the context of E. coli heat-stable enterotoxin analogues results in higher tumor/nontarget tissue ratios at 1 h pi than either DOTA or TETA macrocycles. Heat-stable enterotoxin-based radiopharmaceuticals such as these provide a means of noninvasively determining GC-C receptor status in colorectal cancers by PET.