DOTA Conjugate of Bisphosphonate and PSMA-Inhibitor: A Promising Combination for Therapy of Prostate Cancer Related Bone Metastases

Abstract

Prostate cancer (PCa) is one of the most common cancer types worldwide. 90% of men with late stage PCa will develop bone metastases. Since the expression level of PSMA (prostate-specific membrane antigen) in bone metastases can vary significantly, a compound is being searched for which accumulates in bone metastases independently of PSMA level. With DOTA-L-Lys(SA.Pam)-PSMA-617, we present a compound that, in addition to a PSMA inhibitor as a target vector, also contains a bisphosphonate that is established as a bone tracer and thus combines the advantages of PSMA targeting and bone targeting. This is a class of small molecules combining targeting of two different targets with the potential advantages for treatment of biologically heterogeneous bone metastasis from prostate cancer. The molecule can be labeled with lutetium-177 and used for the therapy of PCa-related bone metastases. DOTA-L-Lys(SA.Pam)-PSMA-617 was synthesized and radiolabelled in 1 M ammonium acetate buffer pH 5.5 at 95°C. Different amounts of precursor were evaluated. Complex stability was evaluated in three different media. LogD7.4 value was evaluated via the determination of the equilibrium distribution in a PBS/n-octanol mixture. A hydroxyapatite binding assay was used to evaluate the potential binding to bone metastases. In vitro affinity was determined and Ki value was evaluated. To evaluate the binding potential in mice, ex vivo biodistribution studies were carried out in LNCaP tumor-bearing Balb/c mice. [177Lu]Lu-labeling of DOTA-L-Lys(SA.Pam)-PSMA-617 showed quantitative RCY within 10 min and high complex stability over 14 days. The lipophilicity of the labeled compound was similar to the lipophilicity of the reference compound [177Lu]Lu-PSMA-617 and showed an excellent and selective HAP binding of 98.2 ± 0.11%. With a Ki of 42.3 ± 7.7 nM PSMA binding affinity is lower in comparison to [177Lu]Lu-PSMA-617. First ex vivo biodistribution studies with LNCaP tumor-bearing Balb/c mice showed a PSMA dependent tumor accumulation of 4.2 ± 0.7%ID/g and a femur accumulation of 3.4 ± 0.4%ID/g. [177Lu]Lu-DOTA-L-Lys(SA.Pam)-PSMA-617 is a promising compound for therapy of PCa related bone and tissue metastases. Accumulation on the bone metastases via two mechanisms also enables the treatment of bone metastases that show little or no PSMA expression.

Lu-177-Labeled Hetero-Bivalent Agents Targeting PSMA and Bone Metastases for Radionuclide Therapy

“…36 Additionally, Grus et al reported a PSMA derivative containing an additional bisphosphonate group, [ 177 Lu]Lu-DOTA-L-Lys(SA.Pam)-PSMA-617. 30 This ligand is based on PSMA-617 and pamidronate-squaric acid. It was successfully tested in mice as a hetero-bivalent therapy agent targeting both PSMA and bone metastasis for prostate cancer.…”

“…Especially, [ 68 Ga]Ga-P15-041 (Figure 2) showed high and selective accumulation in bone lesions, indicating that it is an excellent bone imaging agent for cancer patients. 25,26 To develop radionuclide therapeutic agents, Lu-177 bisphosphonates and several other related analogues, [ 177 Lu]Lu-EDTMP, 27 [ 177 Lu]Lu-ibandronate (IBA), 28 [ 177 Lu]Lu-DOTA-zoledronate, 29 [ 177 Lu]Lu-DOTA-L-Lys(SA.Pam)-PSMA-617, 30 metastasis have been introduced for clinical studies, suggesting that this is a viable treatment strategy. 31,32 Examples reported in the literature suggest that dual targeting could be useful to improve targeted radionuclide therapy.…”

“…It was successfully tested in mice as a hetero-bivalent therapy agent targeting both PSMA and bone metastasis for prostate cancer. 30 Recently, a new PSMA-targeting imaging agent, [ 68 Ga]Ga-P16-093, with a novel Glu-NH-CO-NH-Lys-Phe-O-carboxymethyl)-Tyr-HBED-CC group (Figure 1) was successfully evaluated in humans. 37,39,41,45,46 By modifying the chelating group for Lu(III) chelation while maintaining the same PSMA binding moiety and linker (X = Glu-NH-CO-NH-Lys-Phe-Ocarboxymethyl)-Tyr-) of this diagnostic imaging agent, [ 68 Ga]Ga-P16-093 was successfully transformed to the radionuclide therapy agents, [ 177 Lu]Lu-P17−087, [ 177 Lu]Lu-P17-088 44 (Approach #1) and [ 177 Lu]Lu-P19-065 43 (Approach #2) (see Figure 2).…”

“…Radioactive bisphosphonates, [ 68 Ga]­Ga-BPAMD, [ 68 Ga]­Ga-DOTA-(ZOL), and [ 68 Ga]­Ga-HBED-CC-BP ([ 68 Ga]­Ga-P15-041), have been reported as bone imaging agents. Especially, [ 68 Ga]­Ga-P15-041 (Figure ) showed high and selective accumulation in bone lesions, indicating that it is an excellent bone imaging agent for cancer patients. , To develop radionuclide therapeutic agents, Lu-177 bisphosphonates and several other related analogues, [ 177 Lu]­Lu-EDTMP, [ 177 Lu]­Lu-ibandronate (IBA), [ 177 Lu]­Lu-DOTA-zoledronate, [ 177 Lu]­Lu-DOTA-L-Lys­(SA.Pam)-PSMA-617, and related bisphosphonates targeting bone metastasis have been introduced for clinical studies, suggesting that this is a viable treatment strategy. , …”

“…For example, a hetero-bivalent agent targeting fibroblast activation protein alpha (FAPα) and PSMA, both abundantly expressed on or near prostate tumor cells, may enhance cancer detection and therapy. − Another paper investigated a hetero-bivalent agent that targeted both PSMA and the gastrin-releasing peptide receptor (GRPR) for the development of imaging agents by increasing its local concentration and its receptor binding . Additionally, Grus et al reported a PSMA derivative containing an additional bisphosphonate group, [ 177 Lu]­Lu-DOTA-L-Lys­(SA.Pam)-PSMA-617 . This ligand is based on PSMA-617 and pamidronate-squaric acid.…”