Taken together, most of the new pteridine derivatives display 1-2-fold greater inhibition of TbPTR1 than LmPTR1 and are more or equally active against PTR1 than the reference compound 1b. The nanomolar to subnanomolar PTR1 inhibitors show improved selectivity for PTR1 over the off-target hDHFR by up to about 3 orders of magnitude (2c, 4e: TbPTR1 IC50 <0.1 nM; SI >400000) (Figure ). The IC50 against hDHFR is typically greater than 100 μM, whereas inhibitory activities against TbDHFR and LmDHFR are higher. For parasite DHFR, the compounds with the best inhibitory activities have similar IC50 values to 1b (e.g. LmDHFR IC50 4a: 0.13 μM, 4b: 0.15 μM and 1b: 0.26 μM). Thus, the newly designed compounds show improved target inhibitory profiles, particularly for the PTR1 targets, and overall good selectivity for the parasitic proteins.