Identifying safer anti-wear triaryl phosphate additives for jet engine lubricants

Abstract

Individuals aboard jet aircraft may be exposed to potentially toxic triaryl organophosphate anti-wear lubricant additives (TAPs) that are converted by cytochromes P450 into toxic metabolites. Consequences of exposure could be reduced by using less toxic TAPs. Our goal was to determine whether an in vitro assay for inhibition of butyrylcholinesterase (BChE) by bioactivated TAPs would be predictive of inhibition of serine active-site enzymes in vivo. The in vitro assay involved TAP bioactivation with liver microsomes and NADPH, followed by incubation with human BChE and measurement of BChE activity. Of 19 TAPs tested, tert-butylated isomers produced the least BChE inhibition. To determine the relevance of these results in vivo, mice were exposed to Durad 125 (D125; a commercial mixture of TAP esters) or to TAPs demonstrating low or no BChE inhibition when assayed in vitro. Inhibition of BChE by bioactivated TAPs in vitro correlated well with inhibition of other serine active-site enzymes in vivo, with the exception of brain acetylcholinesterase and neuropathy target esterase (NTE), which were not inhibited by any TAP tested following single exposures. A recombinant catalytic domain of NTE (rNEST) exhibited classical kinetic properties of NTE. The metabolite of tri-(o-cresyl) phosphate (ToCP), 2-(o-cresyl)-4H-1,3,2-benzodioxaphosphoran-2-one (CBDP), inhibited rNEST in vitro, but with an IC50 value almost 6-times higher than for inhibition of BChE. Physiologically-relevant concentrations of the flavonoid, naringenin, dramatically reduced D125 bioconversion in vitro. The in vitro assay should provide a valuable tool for prescreening candidate TAP anti-wear additives, identifying safer additives and reducing the number of animals required for in vivo toxicity testing.

Phosphate Esters, Thiophosphate Esters and Metal Thiophosphates as Lubricant Additives

“…A more recent concern has been the possibility of triaryl phosphate entering the cabin of jet turbine aircraft as a part of the bleed air [85]. This has accelerated the search for safer phosphate ester additives [86].…”

Contaminated aircraft cabin air

“…Preliminary studies have shown that dermal exposure by not only the TOCP isomer, but also the nonortho isomers (meta and para) caused sensorimotor deficits and neuropathological lesions in the brain [37]. Importantly, bioactivated Durad 125 (the TCP formulation used in gas turbine engine oils) and tri-para-cresyl phosphate (TPCP) have been found to be inhibitory to all enzymes tested [38]. TCP with less than 0.1% TOCP has been reported as a reproductive toxicant [39][40][41][42].…”

Exposure to triaryl phosphates: metabolism and biomarkers of exposure

“…Several of the potential triaryl phosphates did not generate esterase (BChE) inhibitor when bioactivated in the in vitro microsomal systems. Follow-on in vivo experiments with mice exposed (via gavage) to two of the candidate TAPs and the currently used commercial mix of TAP esters showed that one compound did not inhibit BChE in vivo , but modestly inhibited other esterases, while another compound that did not inhibit BChE either in vitro or in vivo was a potent inhibitor of carboxylesterase in vivo (Baker et al, 2011). We feel that expansion of this approach for characterizing lubricant TAP additives will accelerate the development of safer additives.…”

“…The resulting cyclic ester is known as [2-( o -cresyl)-4H-1,3,2-benzodioxaphosphoran-2-one] (CBDP) and is a very potent inhibitor of serine active site enzymes such as lipases, esterases and proteases. This highly toxic metabolite is also inactivated by an as yet unknown reaction(s) (broken arrow) (Aldridge, 1954; Baker et al, unpublished results). …”