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

Introduction: The transmembrane αβ 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, In-SPECT and Ga- and Cu-PET imaging, but to date, not with F-PET.

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

Results: Here, we demonstrate successful one-step F-labeling of both conjugates with wet NCA [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 F-labeled LLP2A-conjugates with the two prosthetic groups: N-Pyr-p-BF (5) and alkyl-N,N-dimethylammonio-BF (AMBF) (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 [F]R-BF-PEG-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 [F]R-BF-PEG-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.