R-348

R438 is a selective Janus tyrosine kinase 3 (JAK3) inhibitor prodrug which is converted to its active metabolite R333. JAKs are cytoplasmic tyrosine kinases that are involved in immune system activation and cytokine signaling. They achieve their effects when coupled with signal tranducers and activators of transcription (STATs). JAK3 is critical to immune system activation, and high levels of JAK3 are expressed in cells and thyocytes, and are inducible in T and B cells. JAK3 is also found in nonhematopoietic cells, but its function in these cells is currently undetermined. JAK3 inhibitors impede the development of CD8 memory cells by attenuating the interleukin 7 (IL7) and interleukin 15 (IL-15) pathways, making it a promising drug for treatment of autoimmune disorders.