A New Platform for Label-Free, Proximal Cellular Pharmacodynamic Assays: Identification of Glutaminase Inhibitors Using Infrared Matrix-Assisted Laser Desorption Electrospray Ionization Mass Spectrometry

Cellular pharmacodynamic assays are crucial aspects of lead optimization programs in drug discovery. These assays are sometimes difficult to develop, oftentimes distal from the target and frequently low throughput which necessitates their incorporation in the drug discovery funnel later than desired. The earlier direct pharmacodynamic modulation of a target can be established, the less resources are wasted on compounds that are acting via an off-target mechanism. Mass spectrometry is a versatile tool that is often used for direct, proximal cellular pharmacodynamic assay analysis but liquid chromatography-mass spectrometry methods are low throughput and unable to fully support structure-activity relationships efforts in early medicinal chemistry programs. Infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) is an ambient ionization method amenable to high throughput cellular assays, capable of diverse analyte detection, ambient and rapid laser sampling process, and low cross contamination. Here we demonstrate the capability of IR-MALDESI for detection of diverse analytes directly from cells and report the development of a high throughput label free, proximal cellular pharmacodynamic assay using IR-MALDESI for discovery of glutaminase inhibitors and a biochemical assay for hit confirmation. We demonstrate the throughput with a [~]100,000 compound cellular screen. Hits from the screening were confirmed by retesting in dose-response with mass spectrometry-based cellular and biochemical assays. A similar workflow can be applied to other targets with minimal modifications, which will speed up discovery of cell active lead series and minimize wasted chemistry resources on off-target mechanisms.