Versatile Diphosphine Chelators for Radiolabeling Peptides with ^99mTc and ⁶⁴Cu

Area of Science:

• Radiochemistry and Nuclear Medicine
• Molecular Imaging
• Bioconjugation Chemistry

Background:

• Development of novel chelating agents is crucial for radiolabeling peptides for molecular imaging.
• Technetium-99m (^99mTc) and copper-64 (⁶⁴Cu) are valuable radionuclides for SPECT and PET imaging, respectively.
• Peptide-based radiotracers offer targeted delivery for specific disease biomarkers.

Purpose of the Study:

• To develop a versatile diphosphine (DP) platform for radiolabeling peptides with ^99mTc and ⁶⁴Cu.
• To synthesize and characterize DP-peptide bioconjugates for molecular imaging.
• To evaluate the radiolabeling efficiency, stability, and in vivo performance of the novel radiotracers.

Main Methods:

• Synthesis of two diphosphine derivatives: 2,3-bis(diphenylphosphino)maleic anhydride (DP^Ph) and 2,3-bis(di-p-tolylphosphino)maleic anhydride (DP^Tol).
• Bioconjugation of DP derivatives with a Prostate Specific Membrane Antigen-targeted dipeptide (PSMAt) and an integrin-targeted cyclic peptide (RGD).
• Radiolabeling of bioconjugates with ^99mTc and ⁶⁴Cu, followed by characterization of complex formation, radiochemical yield (RCY), and metabolic stability.
• In vivo SPECT imaging in mice to assess biodistribution and clearance pathways.

Main Results:

• DP-PSMAt conjugates formed stable cis/trans-[MO₂(DP^X-PSMAt)₂]⁺ complexes (M = ^99mTc, ^99gTc, ^natRe; X = Ph, Tol).
• Kit-formulations enabled rapid preparation of ^99mTc-labeled radiotracers with high RCYs (81% for DP^Ph-PSMAt, 88% for DP^Tol-PSMAt) within 5 minutes at 100 °C.
• ⁶⁴Cu-labeled complexes were formed rapidly with high RCYs (>95%) under mild conditions.
• ^99mTc-labeled radiotracers demonstrated high metabolic stability and rapid renal clearance in vivo.
• DP^Tol-PSMAt exhibited higher reactivity, leading to consistently higher RCYs compared to DP^Ph-PSMAt.

Conclusions:

• The developed diphosphine (DP) platform is versatile for straightforward functionalization of targeting peptides.
• The platform enables simple and efficient radiolabeling with both SPECT (^99mTc) and PET (⁶⁴Cu) radionuclides.
• The DP platform is amenable to derivatization for optimizing chelator reactivity and radiotracer hydrophilicity, offering potential for new receptor-targeted molecular imaging agents.