Chemical and biological characterization of new Re(CO)3/[99mTc](CO)3 bombesin analogues.
BACKGROUND
Bombesin, a neuropeptide with potential for breast and prostate tumor targeting, is rapidly metabolized in vivo, and as a result, uptake in tumor xenografts in mice is poor. An improvement can be expected from the introduction of nonnatural amino acids and spacers. Leu13 was replaced by cyclohexylalanine and Met14 by norleucine. Two spacers, -betaAla-betaAla- and 3,6-dioxa-8-aminooctanoic acid, were inserted between the receptor-binding amino acid sequence (7-14) of bombesin (BBS) and the retroN(alpha)-carboxymethyl histidine chelator used for labeling with the [99mTc](CO)3 core and the rhenium (Re) congener.


METHODS
The biological characterization of the new compounds was performed both in vitro on prostate carcinoma PC-3 cells (binding affinity, internalization/externalization) and in vivo (biodistribution in nude mice with tumor xenografts). The stability was also investigated in human plasma. The Re analogues were prepared for chemical characterization.


RESULTS
The nonnatural amino acids led to markedly slower degradation in human plasma and PC-3 cell cultures. The receptor affinity of the new technetium 99m ([99mTc])-labeled BBS analogues was similar to the unmodified compound with Kd<1 nM. Uptake in the pancreas and in PC-3 tumor xenografts in nude mice was blocked by unlabeled BBS. The best target-to-nontarget uptake ratio was clearly due to the presence of the more polar spacer, -betaAla-betaAla-.


CONCLUSIONS
The different spacers did not have a significant effect on stability or receptor affinity but had a clear influence on the uptake in healthy organs and tumors. Uptake in the kidneys was lower than in the liver, which is likely to be due to the lipophilicity of the compounds. A specific, high uptake was also observed in the gastrin-releasing peptide receptor-rich pancreas. Thus, with the introduction of spacers the in vivo properties of the compounds can be improved while leaving the affinity unaffected.
García Garayoa
Elisa
E
Paul Scherrer Institute, Centre for Radiopharmaceutical Science, CH-5232 Villigen PSI, Switzerland. elisa.garcia@psi.ch
Rüegg
Dominique
D
Bläuenstein
Peter
P
Zwimpfer
Martin
M
Khan
Irfan Ullah
IU
Maes
Veronique
V
Blanc
Alain
A
Beck-Sickinger
Annette G
AG
Tourwé
Dirk A
DA
Schubiger
P August
PA
D000230:Adenocarcinoma / Q000000981:diagnostic imaging* / Q000378:metabolism*; D000818:Animals; D001839:Bombesin / Q000493:pharmacokinetics*; D002254:Carbonates / Q000493:pharmacokinetics; D045744:Cell Line, Tumor; D004355:Drug Stability; D005260:Female; D006801:Humans; D008657:Metabolic Clearance Rate; D051379:Mice; D008819:Mice, Nude; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000138:chemical synthesis / Q000493:pharmacokinetics; D012211:Rhenium / Q000493:pharmacokinetics; D013667:Technetium / Q000493:pharmacokinetics*; D014018:Tissue Distribution
D002254:Carbonates; D019275:Radiopharmaceuticals; D012211:Rhenium; D013667:Technetium; D001839:Bombesin