Towards the development of a PET radioligand for imaging gliomas bearing mIDH1

Introduction
Isocitrate dehydrogenase mutation 1 (mIDH1, IDH1R132H) is commonly reported in gliomas and is proposed as an interesting target for the diagnosis, prognosis and therapy for patients with gliomas. Some preclinical evaluations of 125I- or 18F-labeled mIDH1 ligands are reported for non-invasive imaging of mIDH1-bearing gliomas,1-3 however, to date, no radiotracers for PET imaging of mIDH1 are clinically available owing to hitherto low potency, selectivity and metabolic stability. Herein, the efforts to develop a novel mIDH1 selective PET radiotracer for imaging gliomas with mIDH1 are reported.
Methods
Compound GSK321 bearing the absolute configuration 3-R, 27-S (Fig.1) has been selected as the starting point to develop an 18F-labelled PET radioligand for imaging of mIDH1 due to the high potency (IC50 mIDH1 = 4.6 nM).4 A non-enantioselective synthesis of GSK321 was envisaged to get access to all stereoisomers for biological evaluation. The separation of the four stereoisomers of GSK321 was attempted via isocratic semi-preparative chiral HPLC using a CHIRALPAK IA column (10*250mm, 5µm) and an eluent mixture of ACN/H2O (1/1, v/v) at a flow rate of 3 mL/min. The IC50 values of separated stereoisomers of GSK321 were measured by enzymatic assays for wildtype IDH1 and IDH1R132H coupled to diaphorase. The introduction of the 18F-label into the most potent and selective stereoisomer of GSK321 will be performed via the copper-mediated radiofluorination of the corresponding boronic acid pinacol ester.
Results
The stereoisomeric mixture of GSK321 was synthesized from N-boc-protected piperidone with an overall yield of 22 % over seven steps. In a first attempt to separate the stereoisomers by semi-preparative chiral HPLC, only one enantiomeric pair [(3-R,27-S)- and (3-S,27-R)-GSK321)]was obtained in pure form which was further characterized by NMR spectroscopy. For the isolation of other enantiomeric pair [(3-R,27-S)- and (3-S,27-R)-GSK321], further chiral HPLC methods will be tested in the future. For (3-R,27-S)-GSK321, IC50 values of 6.5 and 477 nM were determined towards mIDH1 and wildtype IDH respectively. The opposite enantiomer, (3-S,27-R)-GSK321 inhibits mIDH1 with an IC50 of 242 nM.
Conclusions
Compound (3-R,27-S)-GSK321 proved as a suitable candidate for the development of an 18F-labeled radioligand for imaging of mIDH1 in glioma with PET. In the next steps, an enantioselective synthesis will be performed to give easy access to substantial amounts of (3-R,27-S)-GSK321 and the enantiomerically pure precursor for radiofluorination.