Introduction. Activin receptor-like kinase 2 (ALK2) is a pharmaceutical target of interest for the rare pediatric cancer diffuse intrinsic pontine glioma (DIPG), which currently does not have any effective chemotherapies. The previous lead radiotracer targeting ALK2, [¹¹C]M4K2127, demonstrated high brain uptake and moderate washout kinetics in rodents, but was subsequently deemed unsuitable due to the significant presence of radiometabolites in the brain. The goal of the present work is to develop novel radiolabeled 3,5-diphenylpyridines with improved metabolic stability compared to the first-generation of ALK2 radiotracers. Methods. M4K2009 and M4K2176 were synthesized as previously described. Radiolabeling was performed in a one-pot, two-step fashion using a [¹⁸F]fluoroethyl tosylate intermediate. Reactions were automated on a GE Tracerlab FXN synthesis module, with non-decay corrected radiochemical yields (RCY) calculated relative to dried [¹⁸F]fluoride. Radio-HPLC was used to confirm radiotracer identity, molar activities (A_m), and radiochemical purities (RCP). Digital autoradiography (ARG) was performed in fresh-frozen rodent tissue (10 μm thick) to assess total and non-specific binding in rodent tissue. Preclinical PET imaging was carried out in healthy rodents (i.e. Sprague Dawley rats, C57BL/6 mice) and radiometabolite analysis was conducted using column-switching HPLC. Results. [¹⁸F]M4K2176 was synthesized in average RCY of 1.7%, A_m of 284 GBq/μmol, and RCP of >95% (n = 4). [¹⁸F]M4K2176 displays good specific binding in rodent tissue (~70%, n = 3). Preliminary rodent imaging shows moderate maximum brain uptake (SUV 1.6-1.8) with good washout kinetics and minimal bone uptake, signifying minimal defluorination (n = 6). Results from ongoing radiometabolite and further imaging studies will be presented. Conclusion. [¹⁸F]M4K2176 is a promising second-generation radiotracer for neuroimaging ALK2 in DIPG.

<b>Figure 1. (A)</b> Radiosynthetic scheme for [<sup>18</sup>F]M4K2176. <b>(B)</b> Total and non-specific binding in rodent brain tissue (fresh-frozen, 10 um thick). Representative PET/CT summation images (0-120 min) in <b>(C)</b> Sprague-Dawley rat and <b>(D)</b> C57BL/6 mouse.

Figure 1. (A) Radiosynthetic scheme for [¹⁸F]M4K2176. (B) Total and non-specific binding in rodent brain tissue (fresh-frozen, 10 um thick). Representative PET/CT summation images (0-120 min) in (C) Sprague-Dawley rat and (D) C57BL/6 mouse.

Melissa Chasse

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