Positron emission tomography studies of organophosphate chemical threats and oxime countermeasures

Abstract

There is a unique in vivo interplay involving the mechanism of inactivation of acetylcholinesterase (AChE) by toxic organophosphorus (OP) compounds and the restoration of AChE activity by oxime antidotes. OP compounds form covalent adducts to this critical enzyme target and oximes are introduced to directly displace the OP from AChE. For the most part, the in vivo inactivation of AChE leading to neurotoxicity and antidote-based therapeutic reversal of this mechanism are well understood, however, these molecular-level events have not been evaluated by dynamic imaging in living systems at millimeter resolution. A deeper understanding of these critically, time-dependent mechanisms is needed to develop new countermeasures. To address this void and to help accelerate the development of new countermeasures, positron-emission tomography (PET) has been investigated as a unique opportunity to create platform technologies to directly examine the interdependent toxicokinetic/pharmacokinetic and toxicodynamic/pharmacodynamic features of OPs and oximes in real time within live animals. This review will cover two first-in-class PET tracers representing an OP and an oxime antidote, including their preparation, requisite pharmacologic investigations, mechanistic interpretations, biodistribution and imaging.

Keywords: Acetylcholinesterase; Aging; Biodistribution; Carbon-11; Fluorine-18; Imaging; Inhibition; Organophosphate/organophosphorus; Oxime; Pharmacodynamics/toxicodynamics; Pharmacokinetics/toxicokinetics; Positron emission tomography (PET); Reactivation.

Fig. 1.

Examples of organophosphorus structures representing CWAs, insecticides, novichok, and chemical agent surrogates.

Fig. 2.

Structures of the fluoro surrogates and the expected OP-AChE adducts formed. PNP = p-nitrophenyl.

Fig. 3.

SUV (normalized) radioactivity vs. time curves found in rat brain after i.v. injection of [¹⁸F]VX-surrogate. Solid lines are tracer alone and broken lines are tracer and non-radioactive F-VX surrogate at 2.0 mg/kg (10 min prior). Legend: hind brain/brain stem (BS), caudate-putamen (CP), frontal cortex (FrCtx), mid-brain (MB), TH thalamus (TH), and cerebellum (CE). From James et al. ACS Chem Neurosci. 2014;5(7):519–24. Epub 2014/04/11. doi: https://doi.org/10.1021/cn500024c. PubMed PMID: 24716794; PMCID: 4102964. https://pubs.acs.org/doi/10.1021/cn500024c.

Fig. 4.

Rat sagittal brain and partial spine view with PET radioactivity averaged over all timeframes (0–120 min) post [¹⁸F]VX-surrogate i.v. injection. Image is displayed using NIH color table (0.0–4.0 SUV global thresholds); cerebral regions shown as ellipses and labeled per Fig. 3 definitions. From James et al. ACS Chem Neurosci. 2014;5(7):519–24. Epub 2014/04/11. doi: https://doi.org/10.1021/cn500024c. PubMed PMID: 24716794; PMCID: 4102964. https://pubs.acs.org/doi/10.1021/cn500024c.

Fig. 5.

Typical standard uptake value (SUV) vs. time curves for select peripheral rat tissues (heart, lung and liver; per legend) after administration of [¹⁸F]VX-surrogate. From James et al. ACS Chem Neurosci. 2014;5(7):519–24. Epub 2014/04/11. doi: https://doi.org/10.1021/cn500024c. PubMed PMID: 24716794; PMCID: 4102964. https://pubs.acs.org/doi/10.1021/cn500024c.

Fig. 6.

Tracer alone as a baseline biodistribution (2–60 min) of decay-corrected radioactivity after [¹¹C]2-PAM tracer i.v. injection in naïve rats. From: A First-In-Class Antidote Tracer for Organophosphate Intoxication. From: Neumann et al. ACS Chem Neurosci. 2018. Epub 2018/08/04. doi: https://doi.org/10.1021/acschemneuro.8b00212. PubMed PMID: 30071719. https://pubs.acs.org/doi/10.1021/acschemneuro.8b00212.

Fig. 7.

Structure and distribution of metabolites present in the plasma free-fraction at 5 min after tracer injection.

Fig. 8.

Biodistribution comparison for [¹¹C]2-PAM (baseline, aqua) and [¹¹C]2-PAM co-injected with 30 mg/kg 2-PAM (purple) at 5 min. From: Neumann et al. ACS Chem Neurosci. 2018. Epub 2018/08/04. doi: https://doi.org/10.1021/acschemneuro.8b00212. PubMed PMID: 30071719. https://pubs.acs.org/doi/10.1021/acschemneuro.8b00212 (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 9.

PET imaging detected radioactivity vs. time (min) in rats. Panel A: [¹¹C]2-PAM tracer alone (baseline). Panel B: tracer co-injected with 30 mg/kg 2-PAM. From: Neumann et al. ACS Chem Neurosci. 2018. Epub 2018/08/04. doi: https://doi.org/10.1021/acschemneuro.8b00212. PubMed PMID: 30071719. https://pubs.acs.org/doi/10.1021/acschemneuro.8b00212.

Fig. 10.

Expanded view PET imaging time-activity curves in brain for [¹¹C]2-PAM tracer alone (baseline; open circles and solid line) and tracer co-injected with 30 mg/kg 2-PAM (solid points and dashed line). From: Neumann et al. ACS Chem Neurosci. 2018. Epub 2018/08/04. doi: https://doi.org/10.1021/acschemneuro.8b00212. PubMed PMID: 30071719. https://pubs.acs.org/doi/10.1021/acschemneuro.8b00212.

Fig. 11.

PET-CT sagittal view of a 30 min summed radioactivity (SUV color bar) after injection of [¹¹C]2-PAM tracer alone (baseline). From: Neumann et al. ACS Chem Neurosci. 2018. Epub 2018/08/04. doi: https://doi.org/10.1021/acschemneuro.8b00212. PubMed PMID: 30071719. https://pubs.acs.org/doi/10.1021/acschemneuro.8b00212.

Scheme 1.

Inhibition of AChE by OPs and the possible post-inhibition processes: reactivation and aging.

Scheme 2.

Synthesis of fluorine-18 organophosphate surrogates by transesterification and alkylation.

Scheme 3.

Synthesis of carbon-11 organophosphate surrogates by reaction of ¹¹C-alkyl iodides with the cesium salt of O-p-nitrophenyl methylphosphonic acid.

Scheme 4.

Synthesis of ¹¹[C]-2-PAM.

Scheme 5.

Mechanism of AChE inhibition by the F-VX surrogate.

Scheme 6.

Inhibition, reactivation and non-aging determinations and processes for HuAChE and HuBChE inhibited by F-VX surrogate.