Glutaminase-IN-1 (CPD20), a CB839 derivative, is an allosteric inhibitor of 1,3,4-selenadiazole-containing kidney-type glutaminase (KGA), with an IC50 of 1 nM. Glutaminase-IN-1 shows improved cellular uptake and antitumor activity. The IC50 values of Glutaminase-IN-1 are 17 nM, 6.78 μM, 19 nM and 9 nM in A549, H2, Caki-1 and HCT116 cell lines, respectively. Glutaminase-IN-1 has better KGA inhibitory activity than the corresponding BPTES and CB839.