Homodimeric forms of bombesin act as potent antagonists of bombesin on Swiss 3T3 cells

Bioconjugated chelates based on (methylpyridinyl)tacn: synthesis, 64Cu labeling and in vitro evaluation for prostate cancer targeting

Three new bifunctional copper chelators based on the 1,4,7-triazacyclononane (tacn) platform have been synthesized and conjugated to peptide. The first one is constituted of the tacn with two methylpyridinyl and one methylthiazolyl carboxylic acid pendant arms, while, in the second and third ones, the macrocycle is functionalized by three methylpyridinyl groups, with an additional hexynoic acid chain on a carbon of one or two pyridine rings. These three bifunctional chelators have been conjugated to the antagonist JMV594 peptide for targeting the gastrin releasing peptide receptor (GRP-r), which is overexpressed in prostate cancer. The resulting monomeric bioconjugates have shown their efficiency to be radiolabeled with β+ emitter 64Cu, and the hydrophilicity and PC-3 cell internalisation properties of these radiolabeled conjugates have been studied. PC-3 cell binding affinity of mono- and dimeric metal-free and natCu metallated conjugates have been evaluated by IC50 measurements. The results demonstrate the potential of these methylpyridinyl tacn derivatives for radiopharmaceutical applications.

Monomeric and Dimeric 68Ga-Labeled Bombesin Analogues for Positron Emission Tomography (PET) Imaging of Tumors Expressing Gastrin-Releasing Peptide Receptors (GRPrs)

The GRPr, highly expressed in prostate PCa and breast cancer BCa, is a promising target for the development of new PET radiotracers. The chelator HBED-CC ( N, N'-bis[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine- N, N'-diacetic acid) was coupled to the bombesin peptides: HBED-C-BN(2-14) 1, HBED-CC-PEG₂-[d-Tyr⁶,β-Ala¹¹,Thi¹³,Nle¹⁴]-BN(6-14) 2, HBED-CC-Y-[d-Phe⁶,Sta¹³,Leu¹⁴]-BN(6-14) (Y = 4-amino-1-carboxymethylpiperidine) 3, and HBED-CC-{PEG₂-Y-[d-Phe⁶,Sta¹³,Leu¹⁴]-BN(6-14)}₂ 4 (homodimer). Compounds 1-4 presented high binding affinities for GRPr (T47D, 0.56-3.51 nM; PC-3, 2.12-4.68 nM). In PC-3 and T47D cells, agonists [⁶⁸Ga]1 and [⁶⁸Ga]2 were mainly internalized while antagonists [⁶⁸Ga]3 and [⁶⁸Ga]4 were surface bound. Cell-related radioactivity reached a maximum after 45 min, while tracer levels followed GRPr expression (PC-3 > T47D > LNCaP > MDA-MB-231). [⁶⁸Ga]4 showed the highest cell-bound radioactivity (PC-3 and T47D). In vivo, tumor (PC-3) targeting for [⁶⁸Ga]3 and [⁶⁸Ga]4 increased over time, with dynamic μPET showing clearer tumors images at later time points. [⁶⁸Ga]3 and [⁶⁸Ga]4 can be considered suitable PET tracers for imaging PCa and BCa expressing GRPr.

Comparative study of 64Cu/NOTA-[D-Tyr6,βAla11,Thi13,Nle14]BBN(6-14) monomer and dimers for prostate cancer PET imaging

Background

Gastrin-releasing peptide receptors [GRPR] are highly over-expressed in multiple cancers and have been studied as a diagnostic target. Multimeric gastrin-releasing peptides are expected to have enhanced tumor uptake and affinity for GRPR. In this study, a ⁶⁴Cu-labeled 1,4,7-triazacyclononane-1,4,7-triacetic acid [NOTA]-monomer and two NOTA-dimers of [D-Tyr⁶,βAla¹¹, Thi¹³, Nle¹⁴]bombesin(6-14) ] [BBN(6-14)] were compared.

Methods

Monomeric and dimeric peptides were synthesized on solid phase support and radiolabeled with ⁶⁴Cu. NOTA-dimer 1 consists of asymmetrically linked BBN(6-14), while NOTA-dimer 2 has similar spacer between the two BBN(6-14) ligands and the chelator. In vitro GRPR-binding affinities were determined with competitive binding assays on PC3 human prostate cancer cells. In vivo stability and biodistribution of radiolabeled compounds were assessed in Balb/c mice. Cellular uptake and efflux were measured with radiolabeled NOTA-monomer and NOTA-dimer 2 on PC3 cells for up to 4 h. In vivo biodistribution kinetics were measured in PC3 tumor-bearing Balb/c nude mice by μ-positron emission tomography [μPET] imaging and confirmed by dissection and counting.

Results

NOTA-monomer, NOTA-dimers 1 and 2 were prepared with purity of 99%. The inhibition constants of the three BBN peptides were comparable and in the low nanomolar range. All ⁶⁴Cu-labeled peptides were stable up to 24 h in mouse plasma and 1 h in vivo. ⁶⁴Cu/NOTA-dimer 2 featuring a longer spacer between the two BBN(6-14) ligands is a more potent GRPR-targeting probe than ⁶⁴Cu/NOTA-dimer 1. PC3 tumor uptake profiles are slightly different for ⁶⁴Cu/NOTA-monomer and ⁶⁴Cu/NOTA-dimer 2; the monomeric BBN-peptide tracer exhibited higher tumor uptake during the first 0.5 h and a fast renal clearance resulting in higher tumor-to-muscle ratio when compared to ⁶⁴Cu/NOTA-dimer 2. The latter exhibited higher tumor-to-blood ratio and was retained longer at the tumor site when compared to ⁶⁴Cu/NOTA-monomer. Lower ratios of tumor-to-blood and tumor-to-muscle in blocking experiments showed GRPR-dependant tumor uptake for both tracers.

Conclusion

Both ⁶⁴Cu/NOTA-monomer and ⁶⁴Cu/NOTA-dimer 2 are suitable for detecting GRPR-positive prostate cancer in vivo by PET. Tumor retention was improved in vivo with ⁶⁴Cu/NOTA-dimer 2 by applying polyvalency effect and/or statistical rebinding.

Is there a role for agonist gastrin-releasing peptide receptor radioligands in tumour imaging?

Gastrin-releasing peptide (GRP) has been shown to be a tumour growth stimulating agent for a number of normal and human cancer cell lines. The tumour growth effect is a direct result of GRP binding to membrane G-protein coupled GRP receptors (GRP-R) on the cell surface. Available data on the role of GRP and GRP-R in human lung, prostate, breast, colorectal and gastric carcinoma are reviewed and it is suggested that radiolabelled agonists are preferable to antagonists for imaging and therapy as they appear to be internalised, yielding a higher target/background ratio. The use of rhenium or indium radiolabels for therapy may provide a new approach to GRP/bombesin expressing tumours.