Label-free whole-cell assays: expanding the scope of GPCR screening

A new class of instruments offers an unprecedented combination of label-free detection with exquisite sensitivity to live-cell responses. These instruments can quantify G-protein-coupled receptor (GPCR) signaling through G(s), G(i) and G(q) pathways and in some cases distinguish G-protein coupling, with sensitivity high enough to detect endogenous receptors. Here, we review emerging data evaluating impedance- and optical-based label-free instruments for GPCR drug discovery. In comparison with traditional GPCR assays, we highlight strengths, weaknesses and future opportunities for label-free biosensors. The ability to qualitatively distinguish G-protein coupling has groundbreaking potential for assessing functional selectivity, a concept that is changing the way GPCR pharmacology is defined and screening strategies are designed.