Preliminary evaluation of 18F-labeled LLP2A-trifluoroborate conjugates as VLA-4 (α4β1 integrin) specific radiotracers for PET imaging of melanoma

Abstract

Introduction: The transmembrane α₄β₁ 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, ¹¹¹In-SPECT and ⁶⁸Ga- and ⁶⁴Cu-PET imaging, but to date, not with ¹⁸F-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 ¹⁸F-labeling by isotope exchange (IEX), we present the first PET imaging evaluations of two [¹⁸F]R-BF₃^--PEG₂-LLP2A tracers using VLA-4 overexpressing B16-F10 murine melanoma tumor mouse models.

Results: Here, we demonstrate successful one-step ¹⁸F-labeling of both conjugates with wet NCA [¹⁸F]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 ¹⁸F-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 [¹⁸F]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 [¹⁸F]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.

Keywords: (18)F-trifluoroborates; LLP2A peptidomimetic; Melanoma; Positron emission tomography; VLA-4; α(4)β(1) integrin.