[44Sc]Sc-CHX-A″-DTPA-RCCB6: A PET Tracer for Imaging CD70 Expression Across Latency Types of Burkitt Lymphoma

CD70 is a promising target for advancing the diagnosis and treatment of Burkitt lymphoma (BL). A ⁴⁴Sc-labeled single-domain antibody fragment tracer, [⁴⁴Sc]Sc-CHX-A″-DTPA-RCCB6, was developed and assessed for its potential in CD70-targeted immuno-PET imaging using BL models. Methods: RCCB6 single-domain antibody was conjugated with CHX-A″-DTPA and radiolabeled with ⁴⁴Sc. The final tracer, [⁴⁴Sc]Sc-CHX-A″-DTPA-RCCB6, was assessed for stability both in vitro and in vivo. Cellular uptake, binding, and internalization assays were conducted using type III and type I latency BL cell lines to confirm the tracer's specificity for CD70. Immuno-PET and biodistribution studies were performed in type III and type I latency BL models, whereas near-infrared fluorescence imaging was used to validate tumor accumulation. Finally, immunohistochemistry analysis was conducted on tumor tissues from both latency types to correlate between tracer accumulation and CD70 expression. Results: Radiolabeling of CHX-A″-DTPA-RCCB6 with ⁴⁴Sc achieved high radiochemical yield and specific activity. The tracer was highly stable both in vitro and in vivo. In vitro cellular uptake and internalization assays confirmed the specific binding of [⁴⁴Sc]Sc-CHX-A″-DTPA-RCCB6 to CD70 in type III latency BL cells. An inhibitory concentration of 50% of 16.45 ± 2.82 nM for RCCB6 and 38.74 ± 4.66 nM for CHX-A″-RCCB6 was determined from competition binding studies. Saturation binding studies determined the maximum number of binding sites, the association constant, and receptor density values for [⁴⁴Sc]Sc-CHX-A″-DTPA-RCCB6 in type III latency BL cells to be 4.83 ± 0.52 pM, 19.75 ± 5.97 nM, and (2.36 ± 0.26) × 10⁶ receptors per cell, respectively. Immuno-PET imaging and ex vivo biodistribution revealed high tracer accumulation in type III latency BL tumors, with sustained retention up to 6 h after injection (2.85 ± 0.84 %ID/g). Tracer uptake was minimal in both the blocked group and in type I latency BL tumors, with values of 0.35 ± 0.03 %ID/g and 0.58 ± 0.16 %ID/g, respectively. Near-infrared fluorescence imaging further confirmed tracer accumulation in type III latency BL tumors. Immunohistochemistry analysis supported these results, showing more intense CD70 staining in type III latency BL tumors compared with type I latency BL tumors. Conclusion: This work highlights the robust capability of [⁴⁴Sc]Sc-CHX-A″-DTPA-RCCB6 in delineating differential CD70 expression in BL models, demonstrating promising findings that underscore the necessity for clinical studies to validate its translational potential.