There is no change in [Cl]i in HEK293-cl cells when incubated with CLP257, indicating inactivity on NKCC1, KCC1, KCC3 or KCC4. Oocyte pre-incubation with CLP257 (200 nM) increases KCC2 transport activity by 61%, but causes no change in other CCCs. Functional, dose-dependent antagonism is also observed between CLP257 and the recently characterized KCC2 antagonist VU024055119. CLP257 (50 μM) provokes < 0.2% of the effect of 5 μM muscimol in CHO cells transduced with recombinant α1β2γ2 GABAA receptors, indicating negligible agonist activity of CLP257 on GABAA receptors.