High-Contrast CXCR4-Targeted ¹⁸F-PET Imaging Using a Potent and Selective Antagonist

Abstract

C-X-C chemokine receptor 4 (CXCR4) is highly expressed in cancers, contributing to proliferation, metastasis, and a poor prognosis. The noninvasive imaging of CXCR4 can enable the detection and characterization of aggressive cancers with poor outcomes. Currently, no 18F-labeled CXCR4 positron emission tomography (PET) radiotracer has demonstrated imaging contrast comparable to [68Ga]­Ga-Pentixafor, a CXCR4-targeting radioligand. We, therefore, aimed to develop a high-contrast CXCR4-targeting radiotracer by incorporating a hydrophilic linker and trifluoroborate radioprosthesis to LY2510924, a known CXCR4 antagonist. A carboxy-ammoniomethyl-trifluoroborate (PepBF3) moiety was conjugated to the LY2510924-derived peptide possessing a triglutamate linker via amide bond formation to obtain BL08, whereas an alkyne ammoniomethyl-trifluoroborate (AMBF3) moiety was conjugated using the copper-catalyzed [3+2] cycloaddition click reaction to obtain BL09. BL08 and BL09 were radiolabeled with [18F]­fluoride ion using 18F–19F isotope exchange. Pentixafor was radiolabeled with [68Ga]­GaCl3. Side-by-side PET imaging and biodistribution studies were performed on immunocompromised mice bearing Daudi Burkitt lymphoma xenografts. The biodistribution of [18F]­BL08 and [18F]­BL09 showed tumor uptake at 2 h postinjection (p.i.) (5.67 ± 1.25%ID/g and 5.83 ± 0.92%ID/g, respectively), which were concordant with the results of PET imaging. [18F]­BL08 had low background activity, providing tumor-to-blood, -muscle, and -liver ratios of 72 ± 20, 339 ± 81, and 14 ± 3 (2 h p.i.), respectively. [18F]­BL09 behaved similarly, with ratios of 64 ± 20, 239 ± 72, and 17 ± 3 (2 h p.i.), respectively. This resulted in high-contrast visualization of tumors on PET imaging for both radiotracers. [18F]­BL08 exhibited lower kidney uptake (2.2 ± 0.5%ID/g) compared to [18F]­BL09 (7.6 ± 1.0%ID/g) at 2 h p.i. [18F]­BL08 and [18F]­BL09 demonstrated higher tumor-to-blood, -muscle, and -liver ratios compared to [68Ga]­Ga-Pentixafor (18.9 ± 2.7, 95.4 ± 36.7, and 5.9 ± 0.7 at 2 h p.i., respectively). In conclusion, [18F]­BL08 and [18F]­BL09 enable high-contrast visualization of CXCR4 expression in Daudi xenografts. Based on high tumor-to-organ ratios, [18F]­BL08 may prove a valuable new tool for CXCR4-targeted PET imaging with potential for translation. The use of a PepBF3 moiety is a new approach for the orthogonal conjugation of organotrifluoroborates for 18F-labeling of peptides.

Selection for constrained peptides that bind to a single target protein

“…Indeed, while our original intent was to discover peptides that block Spike binding to ACE2, the peptide appears to bind in a region distinct from ACE2 and even outside of the regions bound to by therapeutic antibodies. Still, there are therapeutic modalities that do not require binding to a specific position, such as PROTACs (PROteolysis Targeting Chimeras) that make use of a weak (as high as 10 µM) peptide bindingmoiety to bring E3 ubiquitin ligase to the protein target [41][42][43][69][70][71] . The modified peptides could also be applicable to diagnostics, for example, when conjugated to gold nanoparticles they have been explored as diagnostics in point-of-care devices used to detect viral epitopes in HIV 72 .…”

“…For many applications, it is desirable to identify a peptide that binds to a single target. For example, this could be to detect a protein as a diagnostic 41,42 or to degrade a therapeutic target by directing it to cellular proteolytic machinery 43,44 . Here, we present a selection strategy to identify RiPPs that bind to a single target-of-interest, without specifying its binding position.…”

[99mTc]Tc-Galacto-RGD2 integrin αvβ3-targeted imaging as a surrogate for molecular phenotyping in lung cancer: real-world data

“…CXCR4 and integrin might synergistically promote lymphatic metastasis in lung cancer, and act as clinical predictors of lymph node metastasis in non-SCLC [ 29 , 30 ]. High expression levels of chemokines are related to a poor prognosis and a poor chemotherapy tolerance in cancer patients [ 31 – 34 ]. CXCR4 is a chemokine receptor that plays a critical role in the process of lymphocyte homing to lymphatic vessels and secondary lymphoid organs, including the lymph nodes [ 35 ].…”

99mTc-Galacto-RGD2 Integrin αvβ3 Targeted Imaging as a Surrogate for Molecular Phenotyping in Lung Cancer: Prospective Study in the Real World

“…CXCR4 and integrin might synergistically promote lymphatic metastasis in lung cancer, and might act as clinical predictors of lymph node metastasis in NSCLC [38][39][40]. High expression levels of chemokines are related to a poor prognosis and chemotherapy tolerance in cancer patients [41][42][43][44]. CXCR4 is a chemokine receptor that plays a critical role in the process of lymphocyte homing to lymphatic vessels and secondary lymphoid organs, including the lymph nodes [45].…”