¹⁸F-Labeling and Radiopharmacological Evaluation of Novel Purinedione Multi-Eph Inhibitors

Objectives: The overexpression of various Eph receptors in tumors provokes the recent interest in highly affine inhibitors as attractive leads for the development of new targeted radioligands to image cancer [1]. Selective Ephtyrosine
kinase inhibitors based on the purinedione skeleton have been explored in the past as potential probes for imaging of EphB4 [2] and a SNEW peptide for EphB2 [3]. However up to now, there is still no optimal radiotracer
available. Herein, we report the synthesis, radiofluorination and biological evaluation of two novel purinedione derivatives as potential multi Eph inhibitor radioligands.
Methods: Based on known positions for affinity-related interactions of the lead structure with the receptor [4] two positions are favorable for the labeling with fluorine-18. Two precursors 1 and 3 as well as their reference compounds 2 and 4 were prepared. The radiolabeling was done in dry ACN at 100°C for 30 min. First cell association studies were performed using various Eph expressing melanoma cells (A375^wt/mock, A375^EphB4,
A375^EphB6, A375^EphB4) and Eph-negative controls (HL-60).
Results: After labeling, both tracers [¹⁸F]2 and [¹⁸F]4 were obtained in 10 – 15 % RCY (n.d.c.) after HPLC separation (RCP: > 95%). Cell experiments in vitro revealed a substantial cell association of both [¹⁸F]2 and [¹⁸F]4 ranging from 40 to 50 %ID/mg protein at 120 min in all cell lines used. The lack of any significant difference between wild type, recombinant and control cells is indicative for cell association of, as expected, low selectivity, but also of low specificity. The latter is consistent with the observation that preincubation with 100 μM of nonradioactive compound did not result in substantial inhibition of cell association.
Conclusions: [¹⁸F]2 and [¹⁸F]4 were synthesized successfully and first in vitro experiments were accomplished showing substantial cell association for both tracers in various melanoma cells. However, the cell experiments revealed data on specificity of purinedione derivatives that are contradictory to data from literature [4]. These observations will be elucidated in ongoing studies.
Acknowledgements
References
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[2] Mamat, C. et al. (2012), ChemMedChem, 7, 1991–2003;
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(2009) J. Med. Chem., 52, 6433–6446.