Development of a potent DOTA-conjugated bombesin antagonist for targeting GRPr-positive tumours.
OBJECTIVE
Radiolabelled somatostatin-based antagonists show a higher uptake in tumour-bearing mouse models than agonists of similar or even distinctly higher receptor affinity. Very similar results were obtained with another family of G protein-coupled receptor ligands, the bombesin family. We describe a new conjugate, RM2, with the chelator DOTA coupled to D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH(2) via the cationic spacer 4-amino-1-carboxymethyl-piperidine for labelling with radiometals such as (111)In and (68)Ga.


METHODS
RM2 was synthesized on a solid support and evaluated in vitro in PC-3 cells. IC(50) and K(d) values were determined. The antagonist potency was evaluated by immunofluorescence-based internalization and Ca(2+) mobilization assays. Biodistribution studies were performed in PC-3 and LNCaP tumour-bearing mice with (111)In-RM2 and (68)Ga-RM2, respectively. PET/CT studies were performed on PC-3 and LNCaP tumour-bearing nude mice with (68)Ga-RM2.


RESULTS
RM2 and (111)In-RM2 are high-affinity and selective ligands for the GRP receptor (7.7 ± 3.3 nmol/l for RM2; 9.3 ± 3.3 nmol/l for (nat)In-RM2). The potent antagonistic properties were confirmed by an immunofluorescence-based internalization and Ca(2+) mobilization assays. (68)Ga- and (111)In-RM2 showed high and specific uptake in both the tumour and the pancreas. Uptake in the tumour remained high (15.2 ± 4.8%IA/g at 1 h; 11.7 ± 2.4%IA/g at 4 h), whereas a relatively fast washout from the pancreas and the other abdominal organs was observed. Uptake in the pancreas decreased rapidly from 22.6 ± 4.7%IA/g at 1 h to 1.5 ± 0.5%IA/g at 4 h.


CONCLUSIONS
RM2 was shown to be a potent GRPr antagonist. Pharmacokinetics and imaging studies indicate that (111)In-RM2 and (68)Ga-RM2 are ideal candidates for clinical SPECT and PET studies.
Mansi
Rosalba
R
Division of Radiological Chemistry, University Hospital Basel, Basel, Switzerland.
Wang
Xuejuan
X
Forrer
Flavio
F
Waser
Beatrice
B
Cescato
Renzo
R
Graham
Keith
K
Borkowski
Sandra
S
Reubi
Jean Claude
JC
Maecke
Helmut R
HR
D000818:Animals; D001839:Bombesin / Q000037:antagonists & inhibitors*; D045744:Cell Line, Tumor; D055808:Drug Discovery; D005260:Female; D057809:HEK293 Cells; D006573:Heterocyclic Compounds, 1-Ring / Q000737:chemistry*; D006801:Humans; D008297:Male; D051379:Mice; D009842:Oligopeptides / Q000737:chemistry / Q000378:metabolism* / Q000493:pharmacokinetics / Q000494:pharmacology*; D011471:Prostatic Neoplasms / Q000378:metabolism* / Q000473:pathology; D021381:Protein Transport; D018004:Receptors, Bombesin / Q000378:metabolism*; D023041:Xenograft Model Antitumor Assays
D006573:Heterocyclic Compounds, 1-Ring; D009842:Oligopeptides; D018004:Receptors, Bombesin; C071349:1,4,7,10-tetraazacyclododecane- 1,4,7,10-tetraacetic acid; D001839:Bombesin