Academic HTS: diverse portraits

Screening Dyrk1A inhibitors by MALDI-QqQ mass spectrometry: systematic comparison to established radiometric, luminescence, and LC-UV-MS assays

Enzyme-catalyzed reactions play key roles in disease pathology, thus making them relevant subjects of therapeutic inhibitor screening experiments. Matrix-assisted laser desorption/ionization (MALDI) assays have been demonstrated to be able to replace established screening approaches. They offer increased sample throughput, but care must be taken to avoid instrumental bias from differences in ionization efficiencies. We compared a MALDI-triple-quadrupole (QqQ) method for the Dyrk1A peptide substrate woodtide to LC-MS, liquid chromatography with ultraviolet detection (LC-UV), luminescence, and radiometric assays. MALDI measurements were performed on a MALDI-QqQ instrument in the multiple-reaction monitoring mode. Different MALDI conditions were investigated to address whether matrix type, sample support, and MRM- or SIM-based detection conditions can be used to accommodate the molar responses of substrate peptide and its phosphorylated form. UV detection served as a reference method. The impact of MALDI matrix on IC50 values was small, even considering that matrix preparations were used that are known to alleviate response differences. IC50 values determined by MALDI were ca. 2-fold lower than those determined by LC-UV. Although MALDI generated lower ion yields for the phosphorylated peptide than for the peptide substrate, we found that a correction of compound potencies was readily possible using correction factors based on unbiased LC-UV results. A thorough method development delivered a robust assay with excellent performance (Z' > 0.91) that was close to that seen for LC-UV.

Extending matrix-assisted laser desorption/ionization triple quadrupole mass spectrometry enzyme screening assays to targets with small molecule substrates

Mass spectrometry (MS)-based high-throughput screening (HTS) has tremendous potential as an alternative to current screening methods due to its speed, sensitivity, reproducibility and label-free readout. We recently reported that a new generation matrix-assisted laser desorption/ionization triple quadrupole (MALDI-QqQ) mass spectrometer is ideally suited for a variety of enzyme assays and screening protocols. However, all the targets measured to date had peptide substrates that were easily monitored by selected ion monitoring (SIM) without interference from the MALDI matrix. To further extend the application to enzymes with small molecule, non-peptide substrates, we evaluated this method for measuring enzyme activity and inhibition of acetylcholinesterase (AChE). Due to the potential of MALDI matrix interference, multiple reaction monitoring (MRM) was investigated for selective MS/MS transitions and to accurately measure the conversion of acetylcholine into choline. Importantly, ionization, detection and MRM transition efficiency differences between the substrate and product can be overcome by pre-balancing the MRM transitions during method development, thus allowing for a direct readout of the enzyme activity using the ratio of the substrate and product signals. Further validation of the assay showed accurate concentration-dependent inhibition measurements of AChE with several known inhibitors. Finally, a small library of 1008 drug-like compounds was screened at a single dose (10 microM) and the top 10 inhibitors from this primary screen were validated in a secondary screen to determine the rank order of inhibitory potency for each compound. Collectively, these data demonstrate that a MALDI-QqQMS-based readout platform is amenable to measuring small molecule substrates and products and offers significant advantages over current HTS methods in terms of speed, sensitivity, reproducibility and reagent costs.