Ezatiostat HCl (TLK199). (1) Ezatiostat enters the cell and is hydrolyzed to its corresponding diacid. (2) The diacid form of ezatiostat binds to GST P1-1 and disrupts the interaction of GST P1-1 with JNK. (3) JNK is activated by phosphorylation and in turn activates c-Jun. (4) c-Jun and other regulators promote transcription of genes that lead to proliferation and differentiation of normal hematopoietic cells or apoptosis of malignant cells. (5) Signaling pathways are activated to promote growth and differentiation of hematologic progenitors that form neutrophils, erythrocytes, and platelets.