CXCR4 and neoplasm: By this measure, affinity for CXCR4 was in the order [68Ga]Pentixafor > [18F]RPS-534 > [18F]RPS-544 > [18F]RPS-547, and the correlation between PC3-CXCR4 tumor uptake and mass of AMD-3100 required to reduce uptake by 50% was linear (R2 > 0.95).