[¹⁸F]RPS-544: A PET tracer for imaging the chemokine receptor CXCR4

CXCR4 specific [¹⁸F]-labeled positron emission tomography (PET) imaging agents are needed which would enable general distribution of the radiotracer for clinical investigation. We sought to synthesize, radiolabel and evaluate [¹⁸F]RPS-544, a novel non-peptide CXCR4 antagonist as a CXCR4 specific probe. We compared [¹⁸F]RPS-544 with the previously published [¹⁸F]-3 ([¹⁸F]RPS-510 in this paper) in a bi-lateral tumor model of differential CXCR4 expression for its ability to selectively target CXCR4 expression.

Radiolabeling of [¹⁸F]RPS-544 and [¹⁸F]RPS-510 was performed by aromatic substitution on a 6-nitropyridyl group using no-carrier-added [¹⁸F]fluoride under basic conditions. ¹⁸F incorporation was determined by radioHPLC. Semi-preparative HPLC was used to purify the final product prior to reformulation. Imaging and biodistribution was performed in nude mice with bilateral PC3 (CXCR4+ and WT) xenograft tumors at 1, 2 and 4 h post injection.

RPS-544 bound CXCR4 with an IC50 of 4.9 ± 0.3 nM. [¹⁸F]RPS-544 showed preferential uptake in CXCR4+ tumors, with a CXCR4/WT ratio of 3.3 ± 1.3 at 1 h p.i. and 2.3 ± 0.5 at 2 h p.i. Maximum uptake in the CXCR4+ tumors was 3.4 ± 1.2%ID/g at 1 h p.i., significantly greater (p = 0.003) than the uptake in the WT tumor. Tumor/blood ratios were 2.5 ± 0.4 and 3.6 ± 0.3 at 1 and 2 h p.i. Tumor/muscle ratios were >4 at all time-points. Tumor/lung ratios were >2 at 1 h and 2 h p.i. Substantial uptake was observed in the liver (15-25%ID/g), kidneys (25-35%ID/g), the small intestine (1-7%ID/g) and the large intestine (1-12%ID/g). Blood concentrations varied over time (0.5-2%ID/g). All other organs showed uptake of <1%ID/g at all time points studied with clearance profiles similar to blood clearance.

Here we present, to the best of our knowledge, the first high affinity [18F]-labeled tracer, suitable for in vivo PET imaging of CXCR4. [18F]RPS-544 displayed high affinity for CXCR4 and good tumor uptake with a maximum uptake at 1 h p.i.. CXCR4 dependent uptake was demonstrated using bilateral tumors with differential CXCR4 expression as well as pharmacological blockade using the known CXCR4 antagonist, AMD-3100. Tissue contrast as judged by tumor to normal tissue ratios was positive in several key tissues. The structural and pharmacological similarities between [18F]RPS-544 and the approved drug AMD-3465, combined with the ease of synthesis and high molar activity (>185 GBq/μmol) achieved during radiosynthesis could lead to accelerated translation into the clinic.