Compound 2a, without any substituents on the phenyl ring, is slightly less active (IC 50 =14.5 µM, Tab. 2), indicating that the tested halogen substituents tend to increase inhibitory activity against TbPTR1. Notably, despite tiny activity differences, the activities of the compounds halogenated at the meta position are more favorable for each of the three tested halogens (Cl, Br, CF 3 ) than their para equivalents, which is consistent with the QM predictions (Fig. ).