Synthesis and preliminary preclinical evaluation of a F-18-fluorinated quaternary alpha-amino acid-based arginase inhibitor

Introduction: Arginase is an enzyme responsible for the hydrolytic cleavages of arginine to ornithine and urea, being vital for the excretion of metabolic waste from organisms. Recently, the recognition of the role of this enzyme has been extended with the finding that a series of pathogenic processes (from cardiovascular, immune-mediated or inflammatory conditions to tumor cell metabolism) are interrelated with its upregulation.(1) Since positron emission tomography (PET) has already shown high sensitivity and specificity in measuring the activity of certain enzymes, we postulate that this technique can also be valuable for the elucidation of some arginase-related pathophysiological conditions. With PET and a suitable radiolabeled arginase inhibitor, the regional distribution and the changes in expression of this enzyme may be assessed in living mammals. Objective: No positron-emitting arginase inhibitors have been synthesized to date. Herein, we report on a 18F-fluorinated quaternary α-amino acid-based molecule ([18F]ABF2), equivalent to a late-generation of arginase inhibitors recently described.(2) Furthermore, some preliminary in vitro, ex vivo and in vivo assessments are being performed on arginase overexpressing models to elucidate the pharmacokinetics and potential role of this radiotracer for arginase mapping. Methods: An arylboronic acid pinacol ester derived precursor for the 18F-fluorinated arginase inhibitor was synthesized and then radiolabeled via Cu mediated fluorodeboronation.(3) This strategy unlocks the access to 18F-fluorination of aromatic rings lacking electron withdrawing groups. Further deprotection of the amino acid moiety was achieved by hydrolysis and the final radiotracer was purified by solid phase extraction cartridges or semipreparative HPLC to obtain, after reformulation, a physiological and injectable solution. Preliminary in vitro competition assays using arginase-expressing human prostate carcinoma cell lines (PC3) were performed and supported additional in vivo studies with PC3 xenografted immune-deficient mice. Two potent arginase inhibitors, 2-amino-6-boronohexanoic acid (ABH) and 2-amino-6-borono-2-(l-(4-chlorobenzyl)piperidin-4-yl)hexanoic acid (ABCl) were used for in vivo competition tests. Results: In summary, 18F-fluorodeborination yield ranged from 30% to 70% when varying the amount of precursor from 4.5 µmol to 15 µmol. The final 18F-fluorinated compound was obtained as an injectable solution with radiochemical purity ≥95% in an overall yield up to 5% (d.c., non-automated synthesis, approx. 6 GBq/µmol). The radiotracer was stable for 4 h in a formulated solution, and when incubated in human serum. The experimental log D was found to be -0.67±0.05. The in vitro assay showed an inhibition of [18F]ABF2 uptake in arginase expressing cancer cells (PC3) when selective blocking competitors were present. A preliminary in vivo study with PC3-xenografted immune-deficient mice indicated a fast clearance of the tracer reaching its maximum at 0.25 min p.i. and decreasing exponentially thereafter in blood with a half-life of 14 min. Tracer accumulation in the PC3 tumors was clearly visualized, reaching a maximum at 8.5 min and decreasing thereafter with a half-life of 103 min. The in vivo uptake of [18F]ABF2 in PC3 xenografts was also found to be moderately suppressed Conclusions: ABF2 was efficiently 18F-fluorinated and isolated in suitable yields for pre-clinical assays. Preliminary in vitro and in vivo studies using PC3 cell lines, revealed a cellular and tumor uptake of the radiotracer that was blocked by other boronic acid-based reversible arginase inhibitors. Further studies are ongoing to evaluate off-target activity and isozyme specificity of [18F]ABF. References: (1) Caldwell RB et al. [2015], Trends Pharmacol Sci, 36(6):395; (2) Golebiowski A et al. [2013], Bioorg Med Chem Lett, 23:4837; (3) Tredwell M et al. [2014], Angew Chem Int Ed, 53(30):7751.