Ezatiostat causes dissociation of the enzyme from the jun-N-terminal kinase/c-Jun (JNK/JUN) complex, leading to JNK activation by phosphorylation. The therapeutic action of ezatiostat appears to include both proliferation of normal myeloid progenitors as well as apoptosis of the malignant clone.
Selection of a resistant clone of an HL60 tumor cell line through chronic exposure to Ezatiostat (TLK199) results in cells with elevated activities of c-Jun NH2 terminal kinase (JNK1) and ERK1/ERK2, and allowes the cells to proliferate under stress conditions that induced high levels of apoptosis in the wild type cells.

Administration of Ezatiostat (TLK199), stimulates both lymphocyte production and bone marrow progenitor (colony-forming unit-granulocyte macrophage) proliferation, but only in glutathione S-transferase P1-1 (GSTP1 ^+/+ ) and not in GSTP1 ^-/- animals.