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 F(19)-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 F(19)-STh(2-19) resulted in IC(50)s 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, (64)Cu-NOTA-F(19)-STh(2-19) demonstrated significantly (p < 0.05) lower uptake in liver than (64)Cu-DOTA-F(19)-STh(2-19) (0.36 +/- 0.13 vs 1.21 +/- 0.65%ID/g) and significantly (p < 0.05) lower uptake in kidney than (64)Cu-TETA-F(19)-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.