A new series of indole-based antagonists of the PGD2 receptor subtype 1 (DP1 receptor) was identified and the progress of the structure–activity relationship study to the identification of potent and selective antagonists is presented. Selective DP1 antagonists with high potency and selectivity were prepared. Of particular interest is the DP1 antagonist 26 with a Ki value of 1nM for the DP1 receptor and an IC50 value of 4.6 nM in a DP1 ...