A, Growing the aryl-alkyne-aryl spacer: template compound 1 (green), aryl-alkyne and aryl FRASEs (magenta), and TAM/FLT3 profiles of inhibitors (IC50 values given in nM) with a partial (2) and full (3) spacers. B, Filling the back-pocket by aromatic groups: template compound (green), source FRASEs (magenta) and sample inhibitors 4–6 not showing sufficient potency and selectivity. C, Filling the back-pocket with smaller aliphatic groups: template compound (green), source FRASEs (magenta) and sample inhibitors 7, 8 showing high potency and promising selectivity. D, Co-crystal structure of 7 in complex with MERTK and its major interactions with the adenine pocket (Pro672, Phe673 and Glu595) and back-pocket (Lys619).