COMPARATIVE RADIOLABELING OF THE CYCLIC PEPTIDE CTHRSSVVC WITH [68GA]GALLIUM AND [18F]FLUORINE. A POTENTIAL PROBE FOR MOLECULAR IMAGING OF CD163⁺ MACROPHAGES

Introduction/Justification

CD163⁺ macrophages play a critical role in chronic inflammation, cancer, and hematologic disorders, making them a promising target for molecular imaging. These cells contribute to tumor immunosuppression, disease progression, and poor prognosis in solid and hematologic tumors. Recent studies indicate that CD163 is a relevant biomarker in Hodgkin's lymphoma, multiple myeloma, and leukemias, which are directly associated with tumor resistance and immune evasion. The cyclic peptide CTHRSSVVC has been identified as a CD163 ligand, showing high reactivity with inflammatory and atherosclerotic lesions, suggesting its potential for targeting CD163⁺ macrophages. In vitro assays demonstrated that [111In]In-DOTA-CTHRSSVVC binds to atherosclerotic plaques, further supporting its applicability in molecular imaging of inflammation and cancer. Cyclic peptides are widely used in radiotracer development due to their high specificity, enzymatic stability, and resistance to degradation. Radiolabeling of these peptides with PET radioisotopes such as 68Ga3+ and [¹⁸F]AlF2+ expands their potential applications in tracking inflammatory processes and hematologic malignancies.

Objectives

To evaluate the radiolabeling efficiency and chemical stability of the NOTA-CTHRSSVVC cyclic peptide with 68Ga3+ and [18F]AlF2+, aiming to develop a novel radiopharmaceutical for molecular imaging of CD163⁺ macrophages.

Materials and Methods

The NOTA-CTHRSSVVC conjugate was radiolabeled [68Ga]Ga(AcO)3 or [18F]AlF2+, which were prepared in 0.2 M sodium acetate buffer (pH 4.1); reactions carried out under different peptide amounts. When necessary, the final products were purified using solid phase columns. The radiochemical efficiency was assessed by HPLC coupled with a gamma radiation detector, while chemical stability was evaluated in the labeling solution for up to 4 hours. The partition coefficient (logP) was determined in n-octanol/water system, in triplicate.

Results

The NOTA-CTHRSSVVC peptide was successfully radiolabeled and purified with the [68Ga]Ga-NOTA-CTHRSSVVC exhibited a radiochemical purity of 97.8% (n = 3), while [18F]AlF-NOTA-CTHRSSVVC reached 95.5% (n = 3). Both radiolabeled peptides demonstrated high chemical stability, maintaining their integrity for up to 4 hours in physiological solution. The logP analysis indicated a hydrophilic profile with the value of -3.08 ± 0.16.

Conclusion

The radiolabeling of the NOTA-CTHRSSVVC peptide with 68Ga3+ and [18F]AlF2+ was efficient and stable, demonstrating chemical feasibility for the development of a novel radiopharmaceutical. Given the potential interaction of the peptide with CD163, future investigations may focus on assessing its biological affinity and molecular imaging applications for CD163⁺ macrophages in hematologic and inflammatory diseases.