Preparation of Metal-Protein Aggregate Radioligand [¹¹C]HQ415 and Evaluation by Small Animal PET Imaging

Objectives: Metal ions are found in abnormally high concentrations in protein aggregates associated with different neurological diseases such as AD, DLB, and ALS.¹ Therefore, we believe it will be possible to identify and differentiate disease states using PET radiotracers based upon various metal chelating scaffolds. Our previous work has shown that metal-chelator [¹⁸F]FL2-b has good brain uptake and exhibits dose dependent specific binding in AD, ALS and DLB tissue.² Moving from the N,N-bidentate stilbene scaffold of L2-b, the potential of the hydroxyquinoline scaffold is also of interest. [¹⁸F]CABS-13 was synthesized but has poor brain uptake in nonhuman primates.³ However, another hydroxyquinoline lead compound, HQ415, has been identified in screening, and the objective of this study is to synthesize [11C]HQ415 and conduct preliminary evaluation of brain uptake and imaging properties in non-human primates. Methods: The compound HQ415 was discovered by high throughput screening and was shown to reduce toxicity of TDP-43 and alpha-synuclein aggregation in yeast models.⁴ Synthesis of a desmethyl precursor and reference standard for [¹¹C]HQ415 was accomplished by reacting 2-amine-4-methylpyridine with either 3-ethoxy-4-methoxybenzaldehyde (for the standard) or 3-ethoxy-4-hydroxybenzaldehyde (for the precursor), followed by addition of ethanolic 8-hyrdoxyquinoline at room temp. To produce [¹¹C]HQ415, precursor (1mg) and NaH (1mg) in 100µL of DMF was reacted with [¹¹C]CH₃I for 5 minutes at 80°C in an automated synthesis module then quenched with 1mL of HPLC buffer (40% EtOH, 10mM NaOAc, pH 4.5) and purified by semipreparative HPLC (3mL/min). The product peak (rt~4 min.) was collected and manually formulated with saline (10%ethanol). PET imaging with a mature rhesus macaque female was performed to evaluate distribution and brain uptake of [¹¹C]HQ415. Results: [¹¹C]HQ415 was isolated in 1.3% radiochemical yield (40.8mCi from 3Ci of [¹¹C]CO₂) in high specific activity (7,000 Ci/mmol). PET imaging in a non-human primate demonstrated moderate brain uptake and efflux for [¹¹C]HQ415 . The peak SUV was determined to be 1.7 in the cerebellum and thalamus at 375s post injection. Conclusions: The synthesis of [¹¹C]HQ415 is reported herein and its evaluation by PET imaging reveals that it is the first BBB permeable hydroxyquinoline with reasonable brain uptake and good brain distribution. Future work using radiolabeled hydroxyquinolines to evaluate the metal hypothesis of neurodegeneration is now feasible and this work is ongoing in our laboratory, including the potential of [¹¹C]HQ415 to image TDP-43 and alpha-synuclein. References:1. Roos, P.M., Vesterberg, O., Syversen, T. et al. Biol Trace Elem Res, 2013, 151, 159. 2. Cary, B. P.; Brooks, A. F. et. al. ACS Med. Chem. Lett., 2015, 6, 112. 3. a) Liang et al., ACS Med. Chem. Neurosci., 2015, 6, 535; b) Vasdev and Scott, Unpublished results. 4. David, D.J., Lena R., Bannister T. et al. Bio & Med Chem, 2013, 21, 6642.

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