TY - JOUR T1 - <strong>Development of <sup>18</sup>F-labeled gabapentin for imaging α2δ-1 receptors in neuropathic pain</strong> JF - Journal of Nuclear Medicine JO - J Nucl Med SP - 68 LP - 68 VL - 61 IS - supplement 1 AU - Yu-Peng Zhou AU - Pedro Brugarolas Y1 - 2020/05/01 UR - http://jnm.snmjournals.org/content/61/supplement_1/68.abstract N2 - 68Introduction: Previous studies indicate that the expression of α2σ-1 subunit of voltage-dependent calcium channels (VDCCs) increases 2-10 fold in the spinal cord and dorsal roots during neuropathic pain1. Gabapentin, which targets α2δ-1 protein, has been successfully used to treat neuropathic pain2. We hypothesize that radiolabeled gabapentin could be a potential radiotracer for imaging pain. Previously reported structure-activity relationship (SAR) studies on gabapentin have shown that 4-methyl and 4-ethyl derivatives of gabapentin are active whereas modification in the 2 position reduce the binding affinity of the molecules3. Based on these results, we propose to synthesize gabapentin derivative with 18F in the 4 position.Methods: The synthesis starts from 2-azaspiro[4.5]decane-3,8-dione (1). The amide is protected by tert-Butyloxycarbonyl (Boc) giving compound 2. The reduction of compound 2 with 2 equivalents of sodium borohydride (NaBH4) in methanol (MeOH) yields the hydroxyl compound 3. The precursor for 18F labeling, mesylate compound 4, is obtained by the reaction of compound 3 with methanesulfonyl chloride (MsCl) in dichloromethane (DCM) in the presence of triethylamine (NEt3) as base. The mesylate compound 4 is isolated after flash chromatography. [18F]KF is made by bombardment of a H218O with high energy protons in cyclotron. [18F]KF is loaded onto a QMA cartridge and then eluted with kryptofix-2.2.2.(K222)/K2CO3 water/acetonitrile solution (1:1). The mixture is dried under nitrogen flow and then ~0.5 mg of precursor 4 dissolved in 0.3 mL DMSO is added. The 18F labeled Boc protected gabapentin lactam (5) is obtained by the reaction [18F]KF and 4 in the presence of kryptofix-2.2.2.(K222)/K2CO3 at 110Co for 30 min. 18F-labeled gabapentin (7) is obtained by the hydrolysis and deprotection of compound 5 using NaOH and HCl, respectively. Reversed-phase HPLC is used for the purification of 18F-labeled gabapentin (7) using XBridge 5μm C18 semiprep-column (10 x 250 mm). The mobile phase is NaH2PO4 EtOH-H2O (10mM, v/v = 5:95) solution at flow rate of 4 mL/min. For the autoradiography experiment, mouse spinal cords sections are fixed in formalin for 10 min and washed with PBS. Sections are incubated with 10 μCi of 18F-labeled gabapentin (7) with pregabalin (0 - 5,000 nM) in PBS for 30 min and then washed with ice-cold PBS (3 x 0.5 min). After washing, the tissues are dried, exposed to phosphor screen and imaged 24 hours later. Results: The mesylate precursor 4 was synthesized through three steps reaction in ~40% combined yield and characterized by 1H and 13C NMR. 18F-labeled gabapentin derivative 7 was synthesized in ~20% radiochemical yield based on radio-HPLC. The identity and purity of 7 was confirmed by analytical HPLC. Autoradiography in mouse spinal cord sections shows binding of 7 in dorsal horn region consistent with the reported expression of α2δ-1 receptors4. Over 85% of the binding could be displaced by 5 μM of the selective α2δ-1 drug pregabalin demonstrating high specificity. Conclusion: A new radiotracer 7 for the α2δ-1 subunit of VDCCs was developed. The high specific binding of 18F-labeled gabapentin (7) to α2δ-1 subunit was confirmed by autoradiography and competition binding. Acknowledgements: This work is supported by Tosteson &amp; Fund for Medical Discovery (FMD) of MGH. We also want to thank the cyclotron team of GCMI for the [18F]KF production. References: [1] J. Neurosci. 21, 1868-1875 (2001); [2] Nat. Rev. Neurosci. 13, 542-555 (2012); [3] J. Med. Chem. 41, 1838-1845, (1998); [4] Neuroscience 155, 510-521 (2008) ER -