ZK168281 is a 25-carboxylic ester 1α,25(OH) 2 D 3 analog and a pure VDR antagonist with a K d value of 0.1 nM. ZK168281 is an effective inhibitor of the coactivator (CoA) interaction of its receptor In Vitro ZK168281 resembles more the mouse constitutive androstane receptor (CAR) inverse agonist Androstanol in its ability to recruit corepressor (CoR) proteins. A salt bridge between the CoR and a conserved lysine in helix 4 of the nuclear receptor (NR) is central to this interaction, but also helix 12 was stabilized by direct contacts with residues of the CoR. Fixation of helix 12 in the antagonistic/inverse agonistic conformation prevents an energetically unfavorable free floatation of the C terminus. MCE has not independently confirmed the accuracy of these methods. They are for reference only. Form:Solid IC50& Target:Kd: 0.1 nM (VDR)