Preliminary evaluation of 18 F-labeled LLP2A-trifluoroborate conjugates as VLA-4 (α 4 β 1 integrin) specific radiotracers for PET imaging of melanoma.

INTRODUCTION: The transmembrane α4 β1 integrin receptor, or very-late antigen 4 (VLA-4), is associated with tumor metastasis and angiogenesis, the development of chemotherapeutic drug resistance, and is overexpressed in multiple myelomas, osteosarcomas, lymphomas, leukemias, and melanomas. The peptidomimetic, LLP2A, is a high-affinity ligand with specificity for the extracellular portion of VLA-4 and several conjugates have been evaluated in vivo by NIR-fluorescence, 111 In-SPECT and 68 Ga- and 64 Cu-PET imaging, but to date, not with 18 F-PET.

METHODS: Using two highly stable organotrifluoroborate prosthetic groups: ammoniumdimethyl-trifluoroborate (AMBF3 ) and a new N-pyridinyl-para-trifluoroborate (N-Pyr-p-BF3 ), both capable of facile aqueous 18 F-labeling by isotope exchange (IEX), we present the first PET imaging evaluations of two [18 F]R-BF3 - -PEG2 -LLP2A tracers using VLA-4 overexpressing B16-F10 murine melanoma tumor mouse models.

RESULTS: Here, we demonstrate successful one-step 18 F-labeling of both conjugates with wet NCA [18 F]F- in radiochemical yields of up to 11.6% within 75 min at molar activities of 40-100 GBq/μmol. Average tumor uptake values based on ex vivo biodistribution values were 4.4%ID/g (11) and 2.8%ID/g (12) using 18 F-labeled LLP2A-conjugates with the two prosthetic groups: N-Pyr-p-BF3 (5) and alkyl-N,N-dimethylammonio-BF3 (AMBF3 ) (7), respectively, and was found to be target-specific as evidenced by in vivo blocking controls. Dynamic PET scanning and biodistribution studies revealed slow clearance of the [18 F]R-BF3 - -PEG2 -LLP2A tracers from the tumors, and also substantial uptake in the intestines, gall bladder, liver and bladder. Observed bone uptake was blockable, consistent with known VLA-4 expression in hematopoietic stem cells found in bone marrow.

CONCLUSIONS: These studies show that these [18 F]R-BF3 - -PEG2 -LLP2A conjugates (11 and 12) are promising VLA-4 targeting radiotracers, yet, further optimization will be required to reduce uptake in the gastro-intestinal tract.