SUMMARY: MyeloFibrosis (MF) is a MyeloProliferative Neoplasm (MPN) characterized by a ineffective hematopoiesis, progressive fibrosis of the bone marrow and potential for leukemic transformation. This stem cell disorder is Philadelphia Chromosome negative and manifestations include anemia, splenomegaly and related symptoms such as abdominal distension and discomfort with early satiety. Cytokine driven debilitating symptoms such as fatigue, fever, night sweats, weight loss, pruritus and bone or muscle pain can further impact an individual’s quality of life. Myelofibrosis can be primary (PMF) or secondary to Polycythemia Vera (PV) or Essential Thrombocythemia (ET). The JAK-STAT signaling pathway has been implicated in the pathogenesis of Myelofibrosis. 
This pathway normally is responsible for passing information from outside the cell through the cell membrane to the DNA in the nucleus for gene transcription. Janus Kinase (JAK) family of tyrosine kinases are cytoplasmic proteins and include JAK1, JAK2, JAK3 and TYK2. JAK1 helps propagate the signaling of inflammatory cytokines whereas JAK2 is essential for growth and differentiation of hematopoietic stem cells. These tyrosine kinases mediate cell signaling by recruiting STAT’s (Signal Transducer and Activator of Transcription), with resulting modulation of gene expression. In patients with MPN, the aberrant myeloproliferation is the result of dysregulated JAK2-STAT signaling as well as excess production of inflammatory cytokines associated with this abnormal signaling. These cytokines contribute to the symptoms often reported by patients with MF. JAK2 mutations such as JAK2 V617F are seen in approximately 60% of the patients with PMF and ET and 95% of patients with PV. Unlike CML where the BCR-ABL fusion gene triggers the disease, JAK2 mutations are not initiators of the disease and are not specific for MPN. Further, several other genetic events may contribute to the abnormal JAK2-STAT signaling. JAKAFI® is a potent JAK1 and JAK2 inhibitor and exerts its mechanism of action by targeting and inhibiting the dysregulated JAK2-STAT signaling pathway. The FDA approval of JAKAFI® for the treatment of Intermediate and high risk Myelofibrosis was based on 2 phase III trials – COMFORT (Controlled Myelofibrosis Study with Oral JAK1/JAK2 Inhibitor Treatment) – I and COMFORT-II studies. In COMFORT-I study, 309 intermediate or high risk patients were randomized to receive either JAKAFI® (N=155) or Placebo (N=154). The primary end point of a 35% or more reduction in spleen size at 24 weeks was noted in 42% of those who received JAKAFI® vs 0.7% in the placebo group (P<0.0001). Most patients in the JAKAFI® group had some reduction in the spleen volume whereas majority of those in the placebo arm had increase in splenomegaly. There was a 46% reduction in the TSS (Total Symptom Score) at week 24 in the JAKAFI® group compared to 5% in the placebo group and majority of patients in the later group had worsening of symptoms (P<0.0001). When JAKAFI® was compared to Best Available Therapy (BAT) in the COMFORT-II study, 28% of the patients in the JAKAFI® group met the primary endpoint of a 35% or more reduction in the spleen volume at 48 weeks compared to none in the BAT group (P<0.0001). Over 55% had a mean decrease in spleen size in the JAKAFI® compared to a 4% mean increase in the BAT group. The 2 year follow up analyses from both these trials showed improved overall survival and a reduction in the risk of death for patients randomized to JAKAFI®, compared to those in the control groups. There was weight gain with alleviation of cachexia and improvements in splenomegaly and symptoms were durable. This benefit was seen in patients regardless of JAK mutations. It remains to be seen if JAKAFI® will benefit patients with Polycythemia Vera and Essential Thrombocythemia. Kantarjian HM, Silver RT, Komrokji RS, et al. Clinical Lymphoma Myeloma and Leukemia 2013; 13:638-645
