Novel Peptide-Based PET Probe for Non-invasive Imaging of C-X-C Chemokine Receptor Type 4 (CXCR4) in Tumors.

The recently reported CXCR4 antagonist 3 (Ac-Arg-Ala-[DCys-Arg-2Nal-His-Pen]-CO ₂ H) was investigated as a molecular scaffold for a CXCR4-targeted positron emission tomography (PET) tracer. Toward this end, 3 was functionalized with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and 1,4,7-triazacyclononanetriacetic acid (NOTA). On the basis of convincing affinity data, both tracers, [ ⁶⁸ Ga]NOTA analogue ([ ⁶⁸ Ga]- 5 ) and [ ⁶⁸ Ga]DOTA analogue ([ ⁶⁸ Ga]- 4 ), were evaluated for PET imaging in " in vivo " models of CHO-hCXCR4 and Daudi lymphoma cells. PET imaging and biodistribution studies revealed higher CXCR4-specific tumor uptake and high tumor/background ratios for the [ ⁶⁸ Ga]NOTA analogue ([ ⁶⁸ Ga]- 5 ) than for the [ ⁶⁸ Ga]DOTA analogue ([ ⁶⁸ Ga]- 4 ) in both in vivo models. Moreover, [ ⁶⁸ Ga]- 4 and [ ⁶⁸ Ga]- 5 displayed rapid clearance and very low levels of accumulation in all nontarget tissues but the kidney. Although the high tumor/background ratios observed in the mouse xenograft model could partially derive from the hCXCR4 selectivity of [ ⁶⁸ Ga]- 5 , our results encourage its translation into a clinical context as a novel peptide-based tracer for imaging of CXCR4-overexpressing tumors.