Macrocyclic chelator-coupled gastrin-based radiopharmaceuticals for targeting of gastrin receptor-expressing tumours.
OBJECTIVE
Diethylenetriamine-pentaacetic acid (DTPA)-coupled minigastrins are unsuitable for therapeutic application with the available beta-emitting radiometals due to low complex stability. Low tumour-to-kidney ratio of the known radiopharmaceuticals is further limiting their potency. We used macrocyclic chelators for coupling to increase complex stability, modified the peptide sequence to enhance radiolytic stability and studied tumour-to-kidney ratio and metabolic stability using (111)In-labelled derivatives.


METHODS
Gastrin derivatives with decreasing numbers of glutamic acids were synthesised using (111)In as surrogate for therapeutic radiometals for in vitro and in vivo studies. Gastrin receptor affinities of the (nat)In-metallated compounds were determined by receptor autoradiography using (125)I-CCK as radioligand. Internalisation was evaluated in AR4-2J cells. Enzymatic stability was determined by incubating the (111)In-labelled peptides in human serum. Biodistribution was performed in AR4-2J-bearing Lewis rats.


RESULTS
IC(50) values of the (nat)In-metallated gastrin derivatives vary between 1.2 and 4.8 nmol/L for all methionine-containing derivatives. Replacement of methionine by norleucine, isoleucine, methionine-sulfoxide and methionine-sulfone resulted in significant decrease of receptor affinity (IC(50) between 9.9 and 1,195 nmol/L). All cholecystokinin receptor affinities were >100 nmol/L. All (111)In-labelled radiopeptides showed receptor-specific internalisation. Serum mean-life times varied between 2.0 and 72.6 h, positively correlating with the number of Glu residues. All (111)In-labelled macrocyclic chelator conjugates showed higher tumour-to-kidney ratios after 24 h (0.37-0.99) compared to (111)In-DTPA-minigastrin 0 (0.05). Tumour wash out between 4 and 24 h was low. Imaging studies confirmed receptor-specific blocking of the tumour uptake.


CONCLUSIONS
Reducing the number of glutamates increased tumour-to-kidney ratio but resulted in lower metabolic stability. The properties of the macrocyclic chelator-bearing derivatives make them potentially suitable for clinical purposes.
Division of Radiological Chemistry, University Hospital Basel, Petersgraben 4, 4031 Basel, Switzerland.
Stephan
S
Good
Martin A
MA
Walter
Beatrice
B
Waser
Xuejuan
X
Wang
Jan
J
Müller-Brand
Martin P
MP
Béhé
Jean-Claude
JC
Reubi
Helmut R
HR
Maecke
D000818:Animals; D045744:Cell Line, Tumor; D002614:Chelating Agents / Q000737:chemistry; D016503:Drug Delivery Systems / Q000379:methods*; D005755:Gastrins / Q000737:chemistry / Q000493:pharmacokinetics*; D047028:Macrocyclic Compounds / Q000737:chemistry / Q000493:pharmacokinetics; D008657:Metabolic Clearance Rate; D009928:Organ Specificity; D010190:Pancreatic Neoplasms / Q000000981:diagnostic imaging* / Q000378:metabolism*; D004369:Pentetic Acid / Q000737:chemistry* / Q000493:pharmacokinetics; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000138:chemical synthesis / Q000493:pharmacokinetics; D051381:Rats; D011917:Rats, Inbred Lew; D043887:Receptor, Cholecystokinin B / Q000378:metabolism*; D014018:Tissue Distribution
D002614:Chelating Agents; D005755:Gastrins; D047028:Macrocyclic Compounds; D019275:Radiopharmaceuticals; D043887:Receptor, Cholecystokinin B; C008530:minigastrin; D004369:Pentetic Acid