Coordination-Mediated Synthesis of Purification-Free Bivalent 99mTc-Labeled Probes for in Vivo Imaging of Saturable System

Design of a New 99mTc-radiolabeled Cyclo-peptide as Promising Molecular Imaging Agent of CXCR4 Receptor: Molecular Docking, Synthesis, Radiolabeling, and Biological Evaluation

INTRODUCTION

C-X-C Chemokine receptor type 4 (CXCR4) is often overexpressed or overactivated in different types and stages of cancer disease. Therefore, it is considered a promising target for imaging and early detection of primary tumors and metastasis. In the present research, a new cyclo-peptide radiolabelled with 99mTc, 99mTc-Cyclo [D-Phe-D-Tyr-Lys (HYNIC)- D-Arg-2-Nal-Gly-Lys(iPr)], was designed based on the parental LY251029 peptide, as a potential in vivo imaging agent of CXCR4-expressing tumors.

METHODS

The radioligand was successfully prepared using the method of Fmoc solid-phase peptide synthesis and was evaluated in biological assessment. Molecular docking findings revealed high affinity (binding energy of -9.7 kcal/mol) and effective interaction of Cyclo [D-Phe- D-Tyr-Lys (HYNIC)-D-Arg-2-Nal-Gly-Lys(iPr)] in the binding pocket of CXCR4 receptor (PDB code: 3OE0) as well.

RESULT

The synthesized peptide and its purity were assessed by both reversed-phase high-performance liquid chromatography (RP-HPLC) and mass spectroscopy. High stability (95%, n = 3) in human serum and favorable affinity (Kd = 28.70 ± 13.56 nM and Bmax = 1.896 ± 0.123 fmol/mg protein) in the B16-F10 cell line resulted. Biodistribution evaluation findings and planar image interpretation of mice both showed high affinity and selectivity of the radiotracer to the CXCR4 receptors.

CONCLUSION

Therefore, the findings indicate this designed radioligand could be used as a potential SPECT imaging agent in highly proliferated CXCR4 receptor tumors.

Development of a hydroxamamide-based bifunctional chelating agent to prepare technetium-99m-labeled bivalent ligand probes

Hydroxamamide (Ham) is a thiol-free chelating agent that forms technetium-99m (^99mTc)-complexes with a metal-to-ligand ratio of 1:2 under moderate reaction conditions. Therefore, Ham-based chelating agents will produce ^99mTc-labeled compounds with a bivalent targeting scaffold. For their universal usage, we developed a novel Ham-based bifunctional chelating agent, “Ham-Mal”, with a maleimide group that can easily conjugate with a thiol group, for to preparing ^99mTc-labeled bivalent ligand probes. Ham-Mal was synthesized by a four-step reaction, and then reacted with cysteine or c(RGDfC) to produce Ham-Cys or Ham-RGD. These precursors were reacted with ^99mTcO₄^- for 10 min under room temperature to obtain ^99mTc-(Ham-Cys)₂ and ^99mTc -(Ham-RGD)₂. The cellular uptake level of ^99mTc-(Ham-RGD)₂ by U87MG (high Integrin ɑ_vβ₃ expression) cells was significantly higher than that by PC3 (low Integrin ɑ_vβ₃ expression) cells at 60 min after the incubation, and the uptake was significantly suppressed by pre-treatment for 15 min with excess c(RGDfK) peptide. In the in vivo study with U87MG/PC3 dual xenografted BALB/c-nu mice, the radioactivity of U87MG tumor tissue was significantly higher than that of PC3 tumor tissue at 360 min after the administration of ^99mTc-(Ham-RGD)₂. These results suggest Ham-Mal may have potential as a bifunctional chelating agent for ^99mTc-labeled bivalent ligand probes.

Advances in Development of Radiometal Labeled Amino Acid-Based Compounds for Cancer Imaging and Diagnostics

Radiolabeled biomolecules targeted at tumor-specific enzymes, receptors, and transporters in cancer cells represent an intensively investigated and promising class of molecular tools for the cancer diagnosis and therapy. High specificity of such biomolecules is a prerequisite for the treatment with a lower burden to normal cells and for the effective and targeted imaging and diagnosis. Undoubtedly, early detection is a key factor in efficient dealing with many severe tumor types. This review provides an overview and critical evaluation of novel approaches in the designing of target-specific probes labeled with metal radionuclides for the diagnosis of most common death-causing cancers, published mainly within the last three years. Advances are discussed such traditional peptide radiolabeling approaches, and click and nanoparticle chemistry. The progress of radiolabeled peptide based ligands as potential radiopharmaceuticals is illustrated via novel structure and application studies, showing how the molecular modifications reflect their binding selectivity to significant onco-receptors, toxicity, and, by that, practical utilization. The most impressive outputs in categories of newly developed structures, as well as imaging and diagnosis approaches, and the most intensively studied oncological diseases in this context, are emphasized in order to show future perspectives of radiometal labeled amino acid-based compounds in nuclear medicine.

Aryl isocyanide derivative for one-pot synthesis of purification-free 99mTc-labeled hexavalent targeting probe

INTRODUCTION

^99mTc-labeled hexavalent probes can be readily synthesized by the coordination of six equivalent isocyanide ligands towards Tc^I, and alkyl isocyanide ligands have been extensively used for preparing such probes. However, high ligand concentration (>1 mM) is generally required due to their insufficient coordination ability to Tc^I.

METHODS AND RESULTS

In this study, we revealed that aryl isocyanide ligands, which have greater π-accepting ability compared with alkyl ones, provided ^99mTc-labeled hexavalent probes in high radiochemical yields (>95%) even at low ligand concentration (50 μM). We applied this finding to the synthesis of a ^99mTc-labeled hexavalent RGD probe, targeting integrin α_vβ₃. This ^99mTc-labeled probe was prepared in a 5 min reaction at ligand concentration of 50 μM, and exhibited high tumor localization in vivo without post-labeling purification.

CONCLUSION

The present findings indicate that aryl isocyanide ligands would be a useful precursor to a variety of ^99mTc-labeled hexavalent targeting probes for molecular imaging of saturable systems.

ADVANCES IN KNOWLEDGE

Aryl isocyanide is a better precursor than alkyl isocyanide for preparing ^99mTc-labeled hexavalent targeting probe.

IMPLICATION FOR PATIENT CARE

This work provides a straightforward method to prepare molecular imaging agents of high target uptake, which would facilitate nuclear medicine imaging in clinical settings.

Manipulating Pharmacokinetics of Purification-Free 99mTc-Labeled Bivalent Probes for In Vivo Imaging of Saturable Targets

The accumulation of ^99mTc-labeled probes targeting saturable systems of the body is hindered by the presence of a large excess of unlabeled ligands needed to ensure high radiochemical yields in a short reaction time. To address the issue, we recently reported a novel concept of a metal-coordination-mediated synthesis of a bivalent ^99mTc-labeled probe from a monovalent ligand using d-penicillamine (Pen) as a chelating molecule and c(RGDfK) as a model targeting device. The Pen-conjugated c(RGDfK) via a hexanoate linkage (Pen-Hx-c(RGDfK)) provided a bivalent [^99mTc]Tc-[(Pen-Hx-c(RGDfK))₂ that possessed much higher integrin α_vβ₃ binding affinity than Pen-Hx-c(RGDfK) and visualized a murine tumor without purification. However, high radioactivity levels were observed in the abdominal regions, which necessitated improved pharmacokinetics of the probes for practical applications. In this study, a pharmacokinetic (PK) modifier was introduced to manipulate the pharmacokinetics of the ^99mTc-Pen₂-based bivalent probe. The Hx linkage in Pen-Hx-c(RGDfK) was replaced with acetyl-d-serine-d-serine-glycine (Ac-ssG) or hexanoyl-d-serine-d-serine-d-serine (Hx-sss) to prepare Pen-Ac-ssG-c(RGDfK) or Pen-Hx-sss-c(RGDfK). Pen-Ac-ssG-c(RGDfK) impaired the complexation ability of Pen-Hx-c(RGDfK), and a monovalent ^99mTc-labeled compound was generated at low ligand concentration. However, Pen-Hx-sss-c(RGDfK) provided the objective bivalent ^99mTc-labeled probe in high radiochemical yields at a concentration similar to that of Pen-Hx-c(RGDfK). [^99mTc]Tc-[Pen-Hx-sss-c(RGDfK)]₂ also possessed stability and integrin α_vβ₃ binding affinity similar to those of [^99mTc]Tc-[Pen-Hx-c(RGDfK)]₂. As a result, [^99mTc]Tc-[Pen-Hx-sss-c(RGDfK)]₂ exhibited tumor and abdominal radioactivity levels similar to and significantly lower than those of [^99mTc]Tc-[Pen-Hx-c(RGDfK)]₂. These findings indicate the incorporation of a tripeptide PK modifier to Pen-Hx-c(RGDfK) preserved the complexation ability and improved the pharmacokinetics of the resulting ^99mTc-labeled bivalent probe without impairing the targeting ability. Thus, the [Pen-Hx-(PK modifier)-(targeting device)] would constitute a basic formulation for preparing the ^99mTc-Pen₂-based bivalent probes for imaging saturable targets of the body.

Zn Complex of Diaminedithiol Tetradentate Ligand as a Stable Precursor for 99mTc-Labeled Compounds

The diaminedithiol (N₂S₂) tetradentate ligand constitutes a useful chelating molecule for preparing ^99mTc-labeled compounds of high in vivo stability in high radiochemical yields. However, since the thiol groups in the N₂S₂ ligand are easy to be oxidized to disulfide bonds, they need to be protected with an appropriate protecting group, which hinders the broad applications of the N₂S₂ ligand for radiopharmaceuticals. In this study, a Zn chelate of N₂S₂ was evaluated as a precursor for purification-free ^99mTc-labeled N₂S₂ under the mild and simple procedure. Zn-N₂S₂ was prepared by reacting Zn acetate with N₂S₂, and the Zn-N₂S₂ remained stable under aerobic conditions at room temperature. ^99mTc-N₂S₂ was obtained over 90% radiochemical yields at room temperature by a one-pot reaction, consisting of Zn-N₂S₂ (10⁻⁵ M), ^99mTcO₄⁻, ethylenediaminetetraacetic acid (EDTA), and a reducing agent (Sn²⁺) at pH = 5.5 to 7.5. ^99mTc-N₂S₂ was also obtained over 90% radiochemical yields when the reaction was conducted in the presence of an equimolar amount of IgG antibody. These findings indicate the Zn complex of N₂S₂ ligand constitutes a stable and useful precursor to prepare ^99mTc-labeled N₂S₂ compounds in high yields under the mild and simple procedure.

C-terminal-modified LY2510924: a versatile scaffold for targeting C-X-C chemokine receptor type 4

C-X-C chemokine receptor type 4 (CXCR4) constitutes a promising target for tumor diagnosis and therapy. Herein, we evaluate a new 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-conjugated CXCR4 antagonist derived from LY2510924, FRM001, and its metal complexes as CXCR4-targeting probes. FRM001 was synthesized by modifying the C-terminus of LY2510924 with maleimido-mono-amide-DOTA via a cysteine linker. FRM001 exhibited CXCR4-specific binding with an affinity similar to that of the parental LY2510924. The binding affinity of FRM001 remained unchanged after complexation with Ga, Lu, and Y. The internalization of ⁶⁷Ga-FRM001 into the cells was hardly observed. In mice biodistribution studies, ⁶⁷Ga-FRM001 exhibited high accumulation in the tumor and the liver with rapid elimination rates from the blood. The hepatic accumulation of ⁶⁷Ga-FRM001 was preferentially and significantly reduced by co-injecting a CXCR4 antagonist, AMD3100. The C-terminal-modified LY2510924 would constitute a versatile scaffold to develop CXCR4-targeting probes or therapeutics for tumor imaging or therapy.

The synthesis of a 99mTc-labeled tetravalent targeting probe upon isonitrile coordination to 99mTcI for enhanced target uptake in saturable systems

The presence of excess unlabeled ligands in the injectate hinders the target uptake of 99mTc-labeled targeting vectors. To address the issue, we previously developed a chemical design which provides a 99mTc-labeled trivalent RGD probe upon CN-βAla-Gly-Gly-c(RGDfK) (Lβ) coordination to [99mTc][Tc(CO)3]+ core at pH 6.0. In this study, we extended our coordination mediated synthesis of the trivalent RGD probe to that of a tetravalent one. Our initial attempts reacting Lβ with [99mTc][Tc(CO)3]+ core at pH 8.0 failed to provide [99mTc][Tc(CO)2(Lβ)4]+ due to the formation of multiple side products. A γ-aminobutylic acid (GABA) based isonitrile ligand CN-GABA-Gly-Gly-c(RGDfK) (LG), on the other hand, avoided the side reaction and selectively provided [99mTc][Tc(CO)2(LG)4]+ (99mTc-[LG]4) at pH 8.0. 99mTc-[LG]4 exhibited higher binding affinity to integrin αvβ3 than its unlabeled ligand, and visualized U87MG tumor without tedious post-labeling purification. These results indicate that the metal coordination-mediated syntheses of 99mTc-labeled multivalent probes have been successfully applied to a tetravalent one, which would allow a wider range of choices for designing novel 99mTc-labeled multivalent probes of high in vivo target uptake.

Radioactive Transition Metals for Imaging and Therapy

Nuclear medicine is composed of two complementary areas, imaging and therapy. Positron emission tomography (PET) and single-photon imaging, including single-photon emission computed tomography (SPECT), comprise the imaging component of nuclear medicine. These areas are distinct in that they exploit different nuclear decay processes and also different imaging technologies. In PET, images are created from the 511 keV photons produced when the positron emitted by a radionuclide encounters an electron and is annihilated. In contrast, in single-photon imaging, images are created from the γ rays (and occasionally X-rays) directly emitted by the nucleus. Therapeutic nuclear medicine uses particulate radiation such as Auger or conversion electrons or β^- or α particles. All three of these technologies are linked by the requirement that the radionuclide must be attached to a suitable vector that can deliver it to its target. It is imperative that the radionuclide remain attached to the vector before it is delivered to its target as well as after it reaches its target or else the resulting image (or therapeutic outcome) will not reflect the biological process of interest. Radiochemistry is at the core of this process, and radiometals offer radiopharmaceutical chemists a tremendous range of options with which to accomplish these goals. They also offer a wide range of options in terms of radionuclide half-lives and emission properties, providing the ability to carefully match the decay properties with the desired outcome. This Review provides an overview of some of the ways this can be accomplished as well as several historical examples of some of the limitations of earlier metalloradiopharmaceuticals and the ways that new technologies, primarily related to radionuclide production, have provided solutions to these problems.