Macrocyclic chelator assembled RGD multimers for tumor targeting

DOTA-Based Plectin-1 Targeted Contrast Agent Enables Detection of Pancreatic Cancer in Human Tissue

Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive and lethal malignancy with extremely poor patient survival rates. A key reason for the poor prognosis is the lack of effective diagnostic tools to detect the disease at curable, premetastatic stages. Tumor surgical resection is PDAC's first-line treatment, however distinguishing between cancerous and healthy tissue with current imaging tools remains a challenge. In this work, we report a DOTA-based fluorescent probe targeting plectin-1 for imaging PDAC with high specificity. To enable heterogeneous functionalization of the DOTA-core with multiple targeting peptide units and the fluorophore, a novel, fully clickable synthetic route that proceeds in one pot was developed. Extensive validation of the probe set the stage for PDAC detection in mice and human tissue. Altogether, these findings may pave the way for improved clinical understanding and early detection of PDAC progression as well as more accurate resection criteria.

DOTAM-Based, Targeted, Activatable Fluorescent Probes for the Highly Sensitive and Selective Detection of Cancer Cells

The utilization of an activatable, substrate-based probe design in combination with a cellular targeting approach has been rarely explored for cancer imaging on a small-molecule basis, although such probes could benefit from advantages of both concepts. Cysteine proteases like cathepsin S are known to be involved in fundamental processes associated with tumor development and progression and thus are valuable cancer markers. We report the development of a combined dual functional DOTAM-based, RGD-targeted internally quenched fluorescent probe that is activated by cathepsin S. The probe exhibits excellent in vitro activation kinetics which can be fully translated to human cancer cell lines. We demonstrate that the targeted, activatable probe is superior to its nontargeted analog, exhibiting improved uptake into α_νβ₃-integrin expressing human sarcoma cells (HT1080) and significantly higher resultant fluorescence staining. However, profound activation was also found in cancer cells with a lower integrin expression level, whereas in healthy cells almost no probe activation could be observed, highlighting the high selectivity of our probe toward cancer cells. These auspicious results show the outstanding potential of the dual functionality concept combining a substrate-based probe design with a targeting approach, which could form the basis for highly sensitive and selective in vivo imaging probes.

Strained cyclooctyne as a molecular platform for construction of multimodal imaging probes

Small-molecule-based multimodal and multifunctional imaging probes play prominent roles in biomedical research and have high clinical translation ability. A novel multimodal imaging platform using base-catalyzed double addition of thiols to a strained internal alkyne such as bicyclo[6.1.0]nonyne has been established in this study, thus allowing highly selective assembly of various functional units in a protecting-group-free manner. Using this molecular platform, novel dual-modality (PET and NIRF) uPAR-targeted imaging probe: (64)Cu-CHS1 was prepared and evaluated in U87MG cells and tumor-bearing mice models. The excellent PET/NIRF imaging characteristics such as good tumor uptake (3.69%ID/g at 2 h post-injection), high tumor contrast, and specificity were achieved in the small-animal models. These attractive imaging properties make (64)Cu-CHS1 a promising probe for clinical use.

Chelators for copper radionuclides in positron emission tomography radiopharmaceuticals

The development of chelating agents for copper radionuclides in positron emission tomography radiopharmaceuticals has been a highly active and important area of study in recent years. The rapid evolution of chelators has resulted in highly specific copper chelators that can be readily conjugated to biomolecules and efficiently radiolabeled to form stable complexes in vivo. Chelators are not only designed for conjugation to monovalent biomolecules but also for incorporation into multivalent targeting ligands such as theranostic nanoparticles. These advancements have strengthened the role of copper radionuclides in the fields of nuclear medicine and molecular imaging. This review emphasizes developments of new copper chelators that have most greatly advanced the field of copper-based radiopharmaceuticals over the past 5 years.

RGD-based PET tracers for imaging receptor integrin αv β3 expression

Positron emission tomography (PET) imaging of receptor integrin αv β3 expression may play a key role in the early detection of cancer and cardiovascular diseases, monitoring disease progression, evaluating therapeutic response, and aiding anti-angiogenic drugs discovery and development. The last decade has seen the development of new PET tracers for in vivo imaging of integrin αv β3 expression along with advances in PET chemistry. In this review, we will focus on the radiochemistry development of PET tracers based on arginine-glycine-aspartic acid (RGD) peptide, present an overview of general strategies for preparing RGD-based PET tracers, and review the recent advances in preparations of (18) F-labeled, (64) Cu-labeled, and (68) Ga-labeled RGD tracers, RGD-based PET multivalent probes, and RGD-based PET multimodality probes for imaging receptor integrin αv β3 expression.

Cyclization of RGD peptide sequences via the macrocyclic chelator DOTA for integrin imaging

Two bicyclic compounds containing Arg-Gly-Asp (RGD) motifs (RGDf and RGD) were synthesized by cyclizing the peptide sequence across the macrocyclic ring of DOTA via two non-adjacent carboxylate pendent arms. The Lu(3+) or Cu(2+) complexes of these compounds, c(DOTA-RGDf) and c(DOTA-RGD), showed a metal dependent affinity towards integrin α(v)β(3)in vitro and the (177)Lu(3+) or (64)Cu(2+) labelled derivatives showed specific tumour uptake in MCF7 and U87MG tumour bearing mice.

The efficient synthesis and biological evaluation of novel bi-functionalized sarcophagine for (64)cu radiopharmaceuticals

Purpose We and others have reported that Sarcophagine-based bifunctional chelators could be effectively used in the syntheses of ⁶⁴Cu radiopharmaceuticals. The resulted ⁶⁴Cu-Sarcophagine complexes demonstrated great in vivo stability. The goal of this study was to further derivatize Sarcophagine cage with amino and maleimide functional groups for conjugation with bioligands.

Methods Starting from DiAmSar, three novel chelators (AnAnSar, BaMalSar, and Mal₂Sar) with two functional groups have been synthesized. Among those, BaMalSar and Mal₂Sar have been conjugated with cyclic peptide c(RGDyC) (denoted as RGD) and the resulted conjugates, BaMalSar-RGD and Mal₂Sar-RGD₂ have been labeled with ⁶⁴Cu. The tumor targeting efficacy of ⁶⁴Cu-labeled RGD peptides were evaluated in a subcutaneous U87MG glioblastoma xenograft model.

Results The conjugates, BaMalSar-RGD and Mal₂Sar-RGD₂ could be labeled with ⁶⁴CuCl₂ in 10 min with high purity (>98%) and high radiochemical yield (>90%). Both ⁶⁴Cu-BaMalSar-RGD and ⁶⁴Cu-Mal₂Sar-RGD₂ exhibited high tumor uptake and tumor-to-normal tissue ratios.

Conclusion Three novel chelators with two functional groups have been developed based on Sarcophagine cage. The platform developed in this study could have broad applications in the design and synthesis of ⁶⁴Cu-radiopharmaceuticals.

Multivalent bifunctional chelator scaffolds for gallium-68 based positron emission tomography imaging probe design: signal amplification via multivalency

The role of the multivalent effect has been well recognized in the design of molecular imaging probes toward the desired imaging signal amplification. Recently, we reported a bifunctional chelator (BFC) scaffold design, which provides a simple and versatile approach to impart multivalency to radiometal based nuclear imaging probes. In this work, we report a series of BFC scaffolds ((t)Bu(3)-1-COOH, (t)Bu(3)-2-(COOH)(2), and (t)Bu(3)-3-(COOH)(3)) constructed on the framework of 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) for (68)Ga-based PET probe design and signal amplification via the multivalent effect. For proof of principle, a known integrin α(v)β(3) specific ligand (c(RGDyK)) was used to build the corresponding NOTA conjugates (H(3)1, H(3)2, and H(3)3), which present 1-3 copies of c(RGDyK) peptide, respectively, in a systematic manner. Using the integrin α(v)β(3) binding affinities (IC(50) values), enhanced specific binding was observed for multivalent conjugates (H(3)2: 43.9 ± 16.1 nM; H(3)3: 14.7 ± 5.0 nM) as compared to their monovalent counterpart (H(3)1: 171 ± 60 nM) and the intact c(RGDyK) peptide (204 ± 76 nM). The obtained conjugates were efficiently labeled with (68)Ga(3+) within 30 min at room temperature in high radiochemical yields (>95%). The in vivo evaluation of the labeled conjugates, (68)Ga-1, (68)Ga-2, and (68)Ga-3, was performed using male severe combined immunodeficiency (SCID) mice bearing integrin α(v)β(3) positive PC-3 tumor xenografts (n = 3). All (68)Ga-labeled conjugates showed high in vivo stability with no detectable metabolites found by radio-HPLC within 2 h postinjection (p.i.). The PET signal amplification in PC-3 tumor by the multivalent effect was clearly displayed by the tumor uptake of the (68)Ga-labeled conjugates ((68)Ga-3: 2.55 ± 0.50%ID/g; (68)Ga-2: 1.90 ± 0.10%ID/g; (68)Ga-1: 1.66 ± 0.15%ID/g) at 2 h p.i. In summary, we have designed and synthesized a series of NOTA-based BFC scaffolds with signal amplification properties, which may find potential applications as diagnostic gallium radiopharmaceuticals.