NSC23925 is a specific Pgp1 inhibitor. It reverses MDR1 (Pgp1) but does not inhibit MRP or BCRP-mediated MDR. NSC23925 itself is not a substrate of Pgp1 but stimulates ATPase activity of Pgp, directly inhibiting the function of Pgp1 in a dose-dependent manner without altering the total expression level of Pgp1.