title: Approaches to improve metabolic stability of a statine-based GRP receptor antagonist.
abstract: The bombesin receptor family, in particular the gastrin-releasing peptide receptor (GRPr), is an attractive target in the field of nuclear oncology due to the high density of these receptors on the cell surface of several human tumors. The successful clinical implementation of64Cu-CB-TE2A-AR06,68Ga-RM2 and68Ga-NODAGA-MJ9, prompted us to continue the development of GRPr-antagonists. The aim of the present study was to assess if N-terminal modulations of the statine-based GRPr-antagonist influence the binding affinity, the pharmacokinetic performance and the in vivo metabolic stability.
  
  
  
  The GRPr-antagonist (D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2) was functionalized with the chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) via the spacer 4-amino-1-carboxymethyl-piperidine (Pip) and the amino acid N-Methyl-β-Ala, to obtain NMe-RM2 and labeled with68Ga and177Lu. The GRPr affinity of the corresponding metalloconjugates determined using [125I-Tyr4]-BN as radioligand. In vitro evaluation included internalization studies using PC3 cells. The68Ga-conjugate was evaluated in PC3 xenografts by biodistribution and PET studies, while investigations on the metabolic stability and plasma protein binding were performed.
  
  
  
  The half maximum inhibitory concentrations (IC50) of the metalloconjugates, using [125I-Tyr4]-BN, are in the low nanomolar range. PC3-cell culture binding studies of both metallated NMe-RM2 and RM2 show high GRPr-bound activity and low internalization. Metabolic studies showed that68Ga-NMe-RM2 and68Ga-RM2 are being cleaved in a similar fashion into three metabolites, with a good proportion of about 50% of the remaining blood activity at 15min post injection (p.i.) being represented by the intact radiotracer.68Ga-NMe-RM2 was shown to target specifically PC3 xenografts, with high and sustained tumor uptake of about 13% IA/g within a time frame of 3h. The PET images clearly visualized the tumor.
  
  
  
  The relatively high percentage of the remaining intact radiotracer in blood 15min post injection sufficiently enables in vivo targeting of GRPr positive tumors, finding which has been also shown in clinical trials.
authors.affiliation: Department of Nuclear Medicine, University Hospital Freiburg, Germany.
authors.forename: Ilinca
authors.initials: I
authors.lastname: Popp
authors.affiliation: Department of Nuclear Medicine, University Hospital Freiburg, Germany; German Cancer Consortium (DKTK), Heidelberg, Germany; German Cancer Research Center (DKFZ), Heidelberg, Germany.
authors.forename: Luigi
authors.initials: L
authors.lastname: Del Pozzo
authors.affiliation: Department of Pathology, University Hospital Bern, Bern, Switzerland.
authors.forename: Beatrice
authors.initials: B
authors.lastname: Waser
authors.affiliation: Department of Pathology, University Hospital Bern, Bern, Switzerland.
authors.forename: Jean Claude
authors.initials: JC
authors.lastname: Reubi
authors.affiliation: Department of Nuclear Medicine, University Hospital Freiburg, Germany; German Cancer Consortium (DKTK), Heidelberg, Germany; German Cancer Research Center (DKFZ), Heidelberg, Germany.
authors.forename: Philipp T
authors.initials: PT
authors.lastname: Meyer
authors.affiliation: Department of Nuclear Medicine, University Hospital Freiburg, Germany.
authors.forename: Helmut R
authors.initials: HR
authors.lastname: Maecke
authors.affiliation: Department of Nuclear Medicine, University Hospital Freiburg, Germany; German Cancer Consortium (DKTK), Heidelberg, Germany; German Cancer Research Center (DKFZ), Heidelberg, Germany,. Electronic address: eleni.gourni@medisin.uio.no.
authors.forename: Eleni
authors.initials: E
authors.lastname: Gourni
mesh_terms: D000595:Amino Acid Sequence; D000596:Amino Acids / Q000737:chemistry* / Q000378:metabolism* / Q000493:pharmacokinetics / Q000494:pharmacology; D000818:Animals; D045744:Cell Line, Tumor; D004355:Drug Stability; D005260:Female; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D051379:Mice; D049268:Positron-Emission Tomography; D021381:Protein Transport; D011854:Radiochemistry; D018004:Receptors, Bombesin / Q000037:antagonists & inhibitors*; D014018:Tissue Distribution
chemical_list: D000596:Amino Acids; D018004:Receptors, Bombesin; C013165:statine
keywords: (68)Ga; Gastrin-releasing peptide receptor antagonists; Metabolic stability; PET-imaging