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 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.
Liu
Dijie
D
Harry S. Truman Memorial Veterans Administration Hospital, Columbia, Missouri 65201, USA.
Overbey
Douglas
D
Watkinson
Lisa D
LD
Smith
Charles J
CJ
Daibes-Figueroa
Said
S
Hoffman
Timothy J
TJ
Forte
Leonard R
LR
Volkert
Wynn A
WA
Giblin
Michael F
MF
D000818:Animals; D001427:Bacterial Toxins / Q000737:chemistry*; D015179:Colorectal Neoplasms / Q000175:diagnosis*; D003301:Copper Radioisotopes / Q000737:chemistry*; D004768:Enterotoxins / Q000737:chemistry*; D029968:Escherichia coli Proteins; D005260:Female; D006801:Humans; D051379:Mice; D008813:Mice, Inbred ICR; D016513:Mice, SCID; D009368:Neoplasm Transplantation; D049268:Positron-Emission Tomography / Q000379:methods*; D013194:Staining and Labeling
D001427:Bacterial Toxins; D003301:Copper Radioisotopes; D004768:Enterotoxins; D029968:Escherichia coli Proteins; C034151:heat stable toxin (E coli)