An important field of study in pharmacology comprises the investigation of drug-target interaction kinetics. Thus, assessing both the lifetime of a drug on its receptor (i.e., drug-target residence time; Copeland, 2016) and the magnitude of drug-mediated receptor activation (i.e., drug efficacy) across the time are critical to understand in vivo pharmacological activity of small-molecule drugs. Of note, while classical in vitro methods view drug-receptor interaction in terms of equilibrium affinity, the residence time model considers the dynamics of receptor conformational rearrangements, which affect drug association and dissociation. Although, classical binding experiments can also address kinetics questions, they are tedious and very time consuming. Accordingly, monitoring drug-receptor interaction dynamics by means of receptor biosensors has become fundamental for understanding how drugs trigger receptor activity over the time. Precisely, in the last years, a number of Fluorescence Resonance Energy Transfer (FRET)-based assays have been developed to accurately display drug-receptor interaction in real time (Lohse et al., 2012). Indeed, one of the most outstanding methods consists of assessing intramolecular conformational rearrangements upon receptor challenge by monitoring intramolecular FRET changes (Vilardaga et al., 2009). Thus, a FRET-based receptor biosensor is built by fusing both donor and acceptor fluorophores to the receptor sequence (Vilardaga et al., 2009). Importantly, a general consensus has prompted to basically attach these molecules (i.e., cyan and yellow fluorescent proteins, CFP and YFP, respectively) intracellularly, this is, in the cytosolic side of the receptor's structure (Figure 1A). Accordingly, when the receptor is activated and a conformational rearrangement occurs the distance and/or orientation of the fluorophores within the receptor biosensor changes and it is possible to monitor FRET changes in real time, thus permitting to finely characterize receptor's activation. Needless to say, although precision is higher than that obtained in classical binding assays, the present biosensors cannot discern between receptors expressed at the cell surface or intracellularly, thus much effort is needed in order to exactly elucidate ligand-receptor kinetics constants.