The case for DOTA as a pharmacokinetic modulator for 18F-labeled peptides: DOTA-[18F]AMBF3-LLP2A for improved PET imaging of VLA-4 over-expression in murine melanoma

1008 Introduction: DOTA is a mainstay radioprosthetic group for use in radiometallation. However, in the absence of a radiometal, its role in favoring renal clearance represents an underutilized approach for 18F-labeled tracers. To show the benefits of using a DOTA moiety to favor renal clearance, we used the peptide LLP2A that recognizes the transmembrane protein very-late antigen 4 (VLA-4) that is overexpressed by many cancers. Previously, we showed that [18F]RBF3-PEG2-LLP2A derivatives gave low tumor uptake values and significant GI tract accumulation. Here, we designed a new RBF3-LLP2A bioconjugate with an appended DOTA moiety, which increased tumor uptake and reduced GI accumulation. Methods: A modified LLP2A-PEG2-NH2 conjugate equipped with an 18F-trifluoroborate radioprosthetic, AMBF3, and a DOTA moiety was synthesized. The DOTA-AMBF3-PEG2-LLP2A was radiolabeled by isotope exchange and was purified by semi-prep HPLC and C18 cartridge elution. Male C57BL/6J mice bearing B16-F10 melanoma tumors that overexpress the VLA-4 target were used to evaluate DOTA-[18F]AMBF3-PEG2-LLP2A using a combination of static and dynamic PET scans, biodistribution studies and blocking controls at 1h post injection (p.i.). Results: Precursor peptide was synthesized and 18F-labeled to provide formulations with mean (±SD) radiochemical purities of 95.9 ± 1.8 %, in radiochemical yields of 4.8 ± 2.9 % having molar activities of 131.7 ± 50.3 GBq/μmol. In vivo static PET images of [18F]DOTA-AMBF3-PEG2-LLP2A provided clear tumor visualization, and biodistribution studies showed that tumor uptake was 9.46 ± 2.19 percent injected dose per gram of tissue (%ID/g) with high tumor:muscle and tumor:blood contrast ratios of ~8 and ~10, respectively. Blocking confirmed the specificity of [18F]DOTA-AMBF3-PEG2-LLP2A to VLA-4 in the tumor and the bone marrow. Dynamic PET showed clearance of [18F]DOTA-AMBF3-PEG2-LLP2A mainly via the renal pathway, wherein accumulation in the intestines was reduced ~10-fold compared to our previously investigated LLP2A’s, while spleen uptake was at levels similar to previously reported LLP2A-chelator radiotracers. Conclusions: [18F]DOTA-AMBF3-PEG2-LLP2A represents a promising VLA-4 radiotracer and demonstrates how a DOTA appendage can favor urinary excretion.