Inhibitor Report for: MFPhCh

Organophosphorus (OP) nerve agents were used for chemical warfare, assassination, and attempted murder of individuals. Therefore, forensic methods are required to identify known and unknown incorporated OP poisons. Serum is tested for the presence of covalent reaction products (adducts) of the toxicant with, e.g., butyrylcholinesterase (BChE) typically by targeted analysis, thus only detecting known OP adducts. We herein present a nontargeted two-step mass spectrometry (MS)-based workflow taking advantage of a high-resolution (HR) Orbitrap mass spectrometer and its option for in-source collision-induced dissociation (IS-CID) highly valuable for the detection of unknown agents. BChE adducts are extracted by immunomagnetic separation and proteolyzed with pepsin yielding a phosphylated nonapeptide (NP) biomarker NP(OP). In step 1, the sample is separated by micro liquid chromatography (microLC) detecting the NP(OP) by nontargeted HR MS followed by data-dependent tandem-MS (ddMS2). Extracted ion chromatograms of diagnostic product ions at m/z 778.33661, 673.29402, and 602.25690 reveal the accurate mass of the NP(OP) precursor ion as well as the elemental composition of the adducted phosphyl moiety. Considering this information, a second microLC run is performed (step 2) for nonselective IS-CID of NP(OP) yielding the cleaved charged phosphyl moiety. This fragment ion is immediately subjected to targeted CID in parallel reaction monitoring (PRM). The accurate mass of its product ions allows the determination of their elemental composition and thus supports its structural elucidation. The described workflow was exemplarily applied to NP(OP) of three Tamelin esters and VX providing highly appropriate abilities for the detection of adducts even of unknown OP poisons like Novichok agents.

A great number of structurally different organophosphorus compounds (OPs) was synthesized in the past decades to be used as pesticides or chemical warfare agents. Methyl-fluorophosphonylcholines were found to be highly toxic OPs and the acetylcholinesterase (AChE) reactivator pralidoxime was shown to be unable to reactivate inhibited AChE. In the course of the development of more effective AChE reactivators, we have determined the reactivation rate constants of various oximes with human AChE inhibited by methylfluorophosphonylcholine (MFPCh), methylfluoro-beta-phosphonylcholine (MFP beta Ch) and methylfluorophosphonylhomocholine (MFPhCh). In addition, we investigated the potential influence of aging phenomena on the oxime efficacy. Human AChE inhibited by MFPCh, MFP beta Ch or MFPhCh was extremely resistant towards reactivation by oximes. Nevertheless, the newer compounds, HLo 7 and HI 6, were substantially more potent reactivators than obidoxime and pralidoxime. The low oxime efficacy was not due to rapid aging since no decrease in reactivatability was found over 96 h at 37 degrees C. Within this period a substantial spontaneous reactivation was observed, with MFPCh >MFP beta Ch >MFPhCh, which did not follow pseudo-first-order kinetics. In conclusion, the unexpected results, i.e., high resistance of inhibited AChE towards oxime reactivation and aging, and much lower resistance towards spontaneous reactivation, calls for further experiments at a molecular level for a better understanding of the interactions among AChE, its inhibitors and reactivators.