Comparison of (18)F-labeled CXCR4 antagonist peptides for PET imaging of CXCR4 expression

CXCR4 is overexpressed on tumor cells from many types of human cancers. A high level of CXCR4 expression often correlates with poor prognosis, chemotherapy resistance, and metastasis. The development of CXCR4-specific radiotracers for positron emission tomography (PET) imaging will allow in vivo evaluation of receptor expression level for diagnosis or therapeutic evaluation.

Two new (18)F-labeled radiotracers based on an Ac-TC14012 peptide, [(18)F]FP-Ac-TC14012 and [(18)F]FB-Ac-TC14012, were synthesized and characterized. The affinities of the 2-fluoropropionate (FP)-conjugated or 4-fluorobenzoate (FB)-conjugated peptides to CXCR4-transfected Chinese hamster ovarian (CHO) cells were evaluated in a competitive binding assay with [(125)I]CXCL12 radioligand. The cell uptake and retention of [(18)F]FP-labeled and [(18)F]FB-labeled peptides were measured. The tumor targetability and pharmacokinetics of these two tracers were also evaluated by microPET imaging and biodistribution studies.

The labeled peptides retained high binding affinity to CXCR4 and showed much higher uptake in CXCR4-positive CHO cells than in CXCR4-negative cells in vitro. The smaller and more hydrophilic [(18)F]FP prosthetic group resulted in higher affinity and lower nonspecific cell uptake compared to the [(18)F]FB-labeled peptide. Both radiotracers showed much higher accumulation in CXCR4-positive than CXCR4-negative tumor xenografts in mice and allowed clear visualization of CXCR4 expression by PET. Among the two, [(18)F]FP-Ac-TC14012 showed higher tumor uptake and better tumor-to-background contrast. Unlike their N-terminal 4-F-benzoate analogs, these two tracers had minimal blood retention, likely due to reduced red blood cell binding. Metabolic organs, such as the liver and kidney, also showed high uptake. When blocked with low-dose cold peptide (10 microg), the tumor uptake was significantly increased, most likely due to the increased concentration in blood circulation, as evidenced by decreased liver uptake.

These results demonstrate that the [(18)F]FP-labeled Ac-TC14012 peptide with high tumor uptake, low nonspecific binding, and good tumor-to-background contrast promises [(18)F]FP-Ac-TC14012 as a PET tracer for in vivo PET imaging of CXCR4 expression.