Immuno-mass spectrometry assay of EPI-HNE4, a recombinant protein inhibitor of human elastase

In order to increase the sensitivity of liquid chromatography/mass spectrometry (LC/MS) assays of recombinant proteins for pharmacokinetics studies, we have developed an immuno-mass spectrometry assay for EPI-hNE4, a 6237 Da protein currently developed for respiratory distress syndromes. After immunocapture of the analyte in human plasma with magnetic beads coated with anti-EPI-hNE4 antibodies, the intact protein was eluted and separated in reversed-phase LC and then analysed by tandem mass spectrometry (MS/MS) in selected reaction monitoring (SRM) mode. The problem of analytical interference due to endogenous binding antibodies was addressed by successive steps of acidification and neutralisation before immunocapture. Furthermore, potential variations in the recovery of analyte during sample extraction were compensated for by addition of an internal standard recognised by the antibodies. The precision of the assay remained therefore below 15%. A significant increase in assay sensitivity was achieved since the extraction step allowed sample concentration and removal of matrix components interfering with the electrospray ionisation process. Using 0.4 mL of plasma, a limit of quantification at 0.5 ng/mL (80 pM) was reached, which represents a 10-fold improvement in sensitivity over our previous work using sample precipitation. This technique was able to monitor EPI-hNE4 kinetics in the plasma of human subjects for 36 h after an intravenous administration of 0.125 mg/kg.