ImmunoPET of CXCR7: a pilot imaging study in breast cancer

Objectives The CXCL12/CXCR4 pathway regulates cancer initiation, progression, metastasis, and response to therapy in more than 20 different cancers, identifying this chemokine signaling axis as a key imaging and therapeutic target. CXCR7 is an atypical chemokine receptor which does not activate G-proteins after binding with its cognate ligands. With a 10-fold higher affinity for CXCL12 than CXCR4, CXCR7 can both serve as a CXCL12 scavenger to regulate CXCR4 activity and enhance CXCL12 signaling (resulting in cell survival, adhesion, invasion, or angiogenesis) by itself or through interactions with CXCR4 and other receptors. These properties of CXCR7 motivate efforts to develop imaging methods to accurately identify localization of this receptor and inform mechanisms of action in vivo. Our goal here is to develop a positron emission tomography (PET) tracer for imaging of CXCR7 and use it to further elucidate CXCR7 functions in cancer.

Methods A monoclonal antibody for CXCR7 (named 11G8) was produced, conjugated to 2-S-(4-Isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (p-SCN-Bn-NOTA), and subsequently labeled with ⁶⁴Cu. Flow cytometry, fluorescence microscopic studies, and cell kinetic assessments were performed in breast cancer cell line MDA-MB-231 with or without stable transduction of CXCR7. PET imaging, organ distribution, and immunohistological assessment were performed in orthotopic MDA-MB-231 tumor-bearing NSG mice (with or without CXCR7) to evaluate the capacity and specificity of ⁶⁴Cu-NOTA-11G8 to target CXCR7 in vivo.

Results FACS analyses and fluorescence microscopy examination in CXCR7-expressing MDA-MB-231 cells revealed no significant loss of CXCR7 binding affinity/specificity for 11G8 after NOTA conjugation. ⁶⁴Cu-labeling was achieved with high yield and specific activity and ⁶⁴Cu-NOTA-11G8 maintained excellent stability in mouse serum. Cell kinetic studies showed that approximately 35% of 64Cu-NOTA-11G8 was internalized in CXCR7-transfected MDA-MB-231 cells after a 24-h incubation. PET imaging revealed that uptake of ⁶⁴Cu-NOTA-11G8 in CXCR7-transfected MDA-MB-231 tumors was significantly higher than that in MDA-MB-231 tumors without CXCR7 (10.1 ± 1.4 %ID/g for CXCR7 tumor and 4.8 ± 1.1 %ID/g for control tumor, n = 3 at 48 h p.i.). Blocking studies with excess unlabeled 11G8 decreased the uptake of ⁶⁴Cu-NOTA-11G8 in CXCR7-expressing MDA-MB-231 tumors (< 4.5%ID/g at every time point, n = 3) without significant impact of uptake in control tumors. Organ distribution via gamma counting corroborated PET data, and histology studies showed strong correlations between CXCR7 expression in tumor tissues and tumor uptake of ⁶⁴Cu-NOTA-11G8.

Conclusions Highly specific detection of the CXCR7 was achieved in vivo by the utilization of ⁶⁴Cu-NOTA-11G8. This novel PET imaging agent can provide useful information on CXCR7 in various biological process, including but not limited to cancer.

• Download figure
• Open in new tab
• Download powerpoint