title: Selective targeting of E. coli heat-stable enterotoxin analogs to human colon cancer cells.
abstract: BACKGROUND
  Radiolabeled analogs of the E. coli heat-stable enterotoxin (ST(h)) are currently under study as imaging and therapeutic agents for colorectal cancer. The aim of these studies is to compare in vitro and in vivo characteristics of two novel ST(h) analogs with appended DOTA chelating moieties.
  
  
  METHODS
  ST(h) analogs were synthesized with pendant N-terminal DOTA moieties and radiolabeled with indium-111. In vitro cell binding was studied using cultured T-84 human colorectal cancer cells, and in vivo biodistribution studies were carried out using T-84 human colorectal tumor xenografts in SCID mice.
  
  
  RESULTS
  Competitive radioligand binding assays employing T-84 human colon cancer cells demonstrated similar IC50 values for the F19-ST(h)(2-19) and F9-ST(h)(6-19) analogs. Addition of DOTA to the N-terminus of these peptides elicited distinctly different effects on binding affinities in vitro, effects that were largely unchanged by metallation with nonradioactive (nat)In. In vivo pharmacokinetic studies in SCID mice bearing T-84 human colon cancer-derived tumor xenografts demonstrated tumor uptake of 0.74 +/- 0.1% ID/g at 4 h post-injection (p.i.) for the 111In-DOTA-F19-ST(h)(2-19) analog, and significantly reduced tumor localization (0.27 + 0.08 % ID/g) for the 111In-DOTA-F9-ST(h)(6-19) analog.
  
  
  CONCLUSIONS
  These results demonstrate that placement of a DOTA moiety immediately adjacent to Cys 6 in ST(h) significantly inhibits receptor binding in vitro and in vivo, highlighting the need for intervening spacer residues between the pharmacophore and the DOTA chelating moiety in effective ST(h)-based radiopharmaceutical constructs.
authors.affiliation: Harry S. Truman Memorial Veterans Administration Hospital, 800 Hospital Dr., Columbia, MO 65201, USA. giblinm@health.missouri.edu
authors.forename: Michael F
authors.initials: MF
authors.lastname: Giblin
authors.forename: Gary L
authors.initials: GL
authors.lastname: Sieckman
authors.forename: Lisa D
authors.initials: LD
authors.lastname: Watkinson
authors.forename: Said
authors.initials: S
authors.lastname: Daibes-Figueroa
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Hoffman
authors.forename: Leonard R
authors.initials: LR
authors.lastname: Forte
authors.forename: Wynn A
authors.initials: WA
authors.lastname: Volkert
mesh_terms: D000818:Animals; D001427:Bacterial Toxins / Q000493:pharmacokinetics* / Q000627:therapeutic use; D001667:Binding, Competitive; D003110:Colonic Neoplasms / Q000000981:diagnostic imaging / Q000188:drug therapy* / Q000378:metabolism; D004768:Enterotoxins / Q000737:chemistry / Q000493:pharmacokinetics* / Q000627:therapeutic use; D029968:Escherichia coli Proteins / Q000493:pharmacokinetics* / Q000627:therapeutic use; D005260:Female; D006573:Heterocyclic Compounds, 1-Ring; D006358:Hot Temperature; D006801:Humans; D007205:Indium Radioisotopes / Q000493:pharmacokinetics / Q000627:therapeutic use; D008024:Ligands; D051379:Mice; D008813:Mice, Inbred ICR; D016513:Mice, SCID; D011485:Protein Binding; D011489:Protein Denaturation; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000737:chemistry / Q000493:pharmacokinetics / Q000627:therapeutic use; D014183:Transplantation, Heterologous; D014407:Tumor Cells, Cultured
chemical_list: D001427:Bacterial Toxins; D004768:Enterotoxins; D029968:Escherichia coli Proteins; D006573:Heterocyclic Compounds, 1-Ring; D007205:Indium Radioisotopes; D008024:Ligands; D019275:Radiopharmaceuticals; C071349:1,4,7,10-tetraazacyclododecane- 1,4,7,10-tetraacetic acid