title: Synthesis and radiopharmacological evaluation of a high-affinity and metabolically stabilized 18F-labeled bombesin analogue for molecular imaging of gastrin-releasing peptide receptor-expressing prostate cancer.
abstract: BACKGROUND
  Bombesin (BBN) and BBN analogues have attracted much attention as high-affinity ligands for selective targeting of the gastrin-releasing peptide (GRP) receptor. GRP receptors are overexpressed in a variety of human cancers including prostate cancer. Radiolabeled BBN derivatives are promising diagnostic probes for molecular imaging of GRP receptor-expressing prostate cancer. This study describes the synthesis and radiopharmacological evaluation of various metabolically stabilized fluorobenzoylated bombesin analogues (BBN-1, BBN-2, BBN-3).
  
  
  METHODS
  Three fluorobenzoylated BBN analogues containing an aminovaleric (BBN-1, BBN-2), or an aminooctanoic acid linker (BBN-3) were tested in a competitive binding assay against (125)I-[Tyr(4)]-BBN for their binding potency to the GRP receptor. Intracellular calcium release in human prostate cancer cells (PC3) was measured to determine agonistic or antagonistic profiles of fluorobenzoylated BBN derivatives. Bombesin derivative BBN-2 displayed the highest inhibitory potency toward GRP receptor (IC50 = 8.7 ± 2.2 nM) and was subsequently selected for radiolabeling with fluorine-18 ((18)F) through acylation with N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB). The radiopharmacological profile of (18)F-labeled bombesin [(18)F]BBN-2 was evaluated in PC3 tumor-bearing NMRI nude mice involving metabolic stability studies, biodistribution experiments and dynamic small-animal PET studies.
  
  
  RESULTS
  All fluorobenzoylated BBN derivatives displayed high inhibitory potency toward the GRP receptor (IC50=8.7-16.7 nM), and all compounds exhibited antagonistic profiles as determined in an intracellular calcium release assay. The (18)F-labeled BBN analogue [(18)F]BBN-2 was obtained in 30% decay-corrected radiochemical yield with high radiochemical purity >95% after semi-preparative HPLC purification. [(18)F]BBN-2 showed high metabolic stability in vivo with 65% of the radiolabeled peptide remaining intact after 60 min p.i. in mouse plasma. Biodistribution experiments and dynamic small-animal PET studies demonstrated high tumor uptake of [(18)F]BBN-2 in PC3 xenografts (2.75 ± 1.82 %ID/g after 5 min and 2.45 ± 1.25 %ID/g after 60 min p.i.). Specificity of radiotracer uptake in PC3 tumors was confirmed by blocking experiments.
  
  
  CONCLUSIONS
  The present study demonstrates that (18)F-labeled BBN analogue [(18)F]BBN-2 is a suitable PET radiotracer with favorable metabolic stability in vivo for molecular imaging of GRP receptor-positive prostate cancer.
authors: Department of Oncology, University of Alberta, Cross Cancer Institute, Edmonton, Canada T6G 1Z2.
authors: Susan
authors: S
authors: Richter
authors: Melinda
authors: M
authors: Wuest
authors: Stephanie S
authors: SS
authors: Krieger
authors: Buck E
authors: BE
authors: Rogers
authors: Matthias
authors: M
authors: Friebe
authors: Ralf
authors: R
authors: Bergmann
authors: Frank
authors: F
authors: Wuest
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001667:Binding, Competitive; D001839:Bombesin / Q000031:analogs & derivatives / Q000378:metabolism* / Q000493:pharmacokinetics; D002118:Calcium / Q000378:metabolism; D045744:Cell Line, Tumor; D002471:Cell Transformation, Neoplastic; D060326:Chemistry Techniques, Synthetic; D004355:Drug Stability; D005462:Fluorine Radioisotopes; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D042541:Intracellular Space / Q000378:metabolism; D008297:Male; D051379:Mice; D008819:Mice, Nude; D049268:Positron-Emission Tomography / Q000379:methods*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000378:metabolism / Q000473:pathology; D011854:Radiochemistry; D019275:Radiopharmaceuticals / Q000737:chemistry / Q000378:metabolism / Q000493:pharmacokinetics; D018004:Receptors, Bombesin / Q000378:metabolism*
chemical_list: D005462:Fluorine Radioisotopes; D019275:Radiopharmaceuticals; D018004:Receptors, Bombesin; D001839:Bombesin; D002118:Calcium
keywords: (18)F; Bombesin (BBN); Gastrin-releasing peptide (GRP) receptor; N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB); Positron emission tomography (PET); Prostate cancer