PT - JOURNAL ARTICLE AU - Neumann, Kiel AU - Thompson, Charles AU - Zinn, Kurt AU - Chao, Chih-kai AU - Blecha, Joseph AU - Huynh, Tony AU - Gerdes, John AU - VanBrocklin, Henry TI - Synthesis and Evaluation of 2-[<sup>18</sup>F]-fluororethyl-4-nitrophenyl-methylphosphonate: A tracer to assess therapeutic countermeasures to organophosphate exposure DP - 2017 May 01 TA - Journal of Nuclear Medicine PG - 869--869 VI - 58 IP - supplement 1 4099 - http://jnm.snmjournals.org/content/58/supplement_1/869.short 4100 - http://jnm.snmjournals.org/content/58/supplement_1/869.full SO - J Nucl Med2017 May 01; 58 AB - 869Objectives: Organophosphates (OP) are a class of compounds used as insecticides and chemical nerve agents that are known as potent cholinesterase inhibitors. As part of our ongoing program to investigate dynamic in vivo acetylcholinesterase (AChE) inhibitor profiles, we previously reported 2-[18F]fluoroethyl-4-nitrophenyl-methylphosphonate (OP4) (James, et al., ACS Chem Neurosci, 5:519-24, 2014) as a first-in-class OP PET tracer. These tracer studies when correlated to non-radioactive (cold) in vitro AChE evaluations (Chao, et al., ACS Chem Res Tox, 29:1810-17, 2016) reveal that OP4 forms a covalent AChE adduct, which does not undergo facile aqueous hydrolysis nor aging decomposition. Thus, the in vivo OP4-AChE adduct is suitable for assessing countermeasures of AChE-OP inhibition and related modes of action. Herein, we report an improved radiosynthetic preparation of OP4, dose formulation assessments, in addition to baseline tracer tissue biodistribution (BioD) and blood profiles in normal rats.Methods: OP4 was prepared by reacting [18F]fluoroethanol with bis-(4-nitrophenyl)-methylphosphonate at room temperature and was purified by semi-preparative reversed phase HPLC. Formulation vehicles were either 5% ethanol/saline/0.1% ascorbic acid or 10% acetonitrile (ACN)/PBS. Sprague Dawley (SD) male rats were used for microPET/CT imaging (Siemens Inveon) with time-dependent arterial blood sampling and BioD tissue profiling. Up to 1 mCi of OP4 was given i.v. with 90min dynamic imaging. Arterial blood samples were processed (centrifugation, blood protein precipitation) with serum fractions analyzed by reversed phase HPLC. BioD in SD rats was performed at 5, 30 and 90 min post-injection.Results: OP4 was prepared in 12 ± 4 % (n=12) yield in 65 min, chemical and radiochemical purities, &gt; 95%; and specific activity, 3,174 ± 345 Ci/mmol. OP4 dose stability (25 °C) was found to be pH sensitive, &gt; 4 hours in the ethanol/saline/ascorbate (pH &amp;#8804; 7.4) versus degradation in ACN/PBS (pH &amp;#8804; 7.4). With microPET imaging significant brain and spine penetration occurs 蠅 1 min post injection with little activity washout over 90 min for either formulation. Thoracic activity uptake in the heart and lungs was also found. OP4 in ACN/ PBS at pH 6.8 or 7.4 resulted in higher brain uptake vs. the ethanol/saline/ascorbate vehicle. Concomitant arterial blood sampling with microPET imaging confirmed high red blood cell and plasma protein activity 1 min post-injection. Serum metabolite analyses revealed &lt; 5% unbound OP4 in blood and the major (99%) free fraction metabolite was the corresponding hydrolyzed phosphonic acid. OP4 BioD profiles (ethanol/ saline/ascorbate vehicle) were found consistent with the microPET imaging activity values, including low liver uptake and clearance from the blood, heart and kidneys. Bone activity indicated no tracer defluorination.Conclusion: We have developed a more optimal higher-yielding and robust radiosynthesis of OP4 that routinely enables in vivo PET imaging and ex vivo studies. Significant and sustained brain uptake of OP4 was found with all dose formulations at pH &amp;#8804; 7.4, with higher brain activity for the ACN/PBS formulation. Activity distributions were found consistent between microPET imaging profiles vs. tissue and blood BioD over time. The established radiosynthetic approach is considered a viable platform that should enable preparations of other OP-labeled fluorine-18 and carbon-11 tracers. The in vivo and ex vivo methods established here are in use for evaluations of therapeutic countermeasures against OP tracer exposures. Research Support: This work was supported by U01NS092495-01.