Abstract
The forward (k(on)) and reverse (k(off)) rate constants of drug-target interactions have important implications for therapeutic efficacy. Hence, time-resolved assays capable of measuring these binding rate constants may be informative to drug discovery efforts. Here, we used an ion channel activation assay to estimate the k(on)s and k(off)s of four dopamine D(2) receptor (D(2)R) agonists; dopamine (DA), p-tyramine, (R)- and (S)-5-OH-dipropylaminotetralin (DPAT). We further probed the role of the conserved serine S193(5.42) by mutagenesis, taking advantage of the preferential interaction of (S)-, but not (R)-5-OH-DPAT with this residue. Results suggested similar k(off)s for the two 5-OH-DPAT enantiomers at wild-type (WT) D(2)R, both being slower than the k(off)s of DA and p-tyramine. Conversely, the k(on) of (S)-5-OH-DPAT was estimated to be higher than that of (R)-5-OH-DPAT, in agreement with the higher potency of the (S)-enantiomer. Furthermore, S193(5.42)A mutation lowered the k(on) of (S)-5-OH-DPAT and reduced the potency difference between the two 5-OH-DPAT enantiomers. Kinetic K(d)s derived from the k(off) and k(on) estimates correlated well with EC(50) values for all four compounds across four orders of magnitude, strengthening the notion that our assay captured meaningful information about binding kinetics. The approach presented here may thus prove valuable for characterizing D(2)R agonist candidate drugs.