The 40 selected compounds from the single concentration screen were followed-up in concentration-response mode (100-1.56 µM) with analysis by RapidFire TM MS. The crosslinking yield was found to decrease with increasing concentration of elaborated compounds, consistent with competitive displacement (Figure ). The data showed good reproducibility across three replicates and the pIC50 values generated were measured across a range of 4-6 (Figure ). Values were corrected using the Cheng-Prusoff equation to obtain pKi values, based on the pKD value of 2 measured previously (pKD=3.7, Figure ). To validate the resulting binding affinities, the 40 compounds were also screened in the TR-FRET assay. The assays showed good correlation (R 2 =0.72) confirming that the PhABit-based displacement screen was a reliable screening method (Figure ).