Myelofibrosis Drug Development through the Decade: Novel JAK Inhibitors
Authors: Khan Ali Younas , Tariq Muhammad Junaid , Usman Muhammad , Ijaz Awais , Rafae Abdul , Malik Mustafa Nadeem , Fazeel Hafiz Muhammad , Nunes Cavalcante Parr Nadia Carenina , Fraz Muhammad Asad , Khalil Muhammad Jahanzeb , Jose Jemin Aby , Durer Seren , Durer Ceren , Anwer Faiz 
Published: October 2019

Abstract
Introduction
Myelofibrosis (MF), a BCR-ABL negative myeloproliferative neoplasm (MPN), has an annual incidence of 1 in 100000 for the primary MF and 0.3-0.7 in 100000 for secondary MF in the USA. MF patients have a median survival of 6.5 years. The primary mutation, JAK2V617F, occurs in 40-60% of MF cases. Ruxolotinib, a JAK inhibitor, has been the mainstay in treating high risk, debilitating MF but largely clinical needs are unmet.
Methods
A comprehensive literature research was performed using PubMed, Cochrane, EMBASE, Web of Science and Clinicaltrials.gov. We included all trials that were under development in phase I/II/III trials. Our search identified 1642 full-length manuscripts or abstracts with published results in the last decade ( Jan 2007 till Dec 2017) were screened for relevant studies. After screening by 2 independent reviewers, 212 articles were finalized for our final analyses. We have reviewed the mechanism of action, safety and efficacy of 2nd generation JAK inhibitors in this review.
Results
JAK1 inhibitor: Itacitinib reduced total symptom score (TSS) ≥ 50% in 15/42 (36%) patients. Mild gastrointestinal (GI) disturbances and some grade 3-4 myelosuppression (anemia: 33%, thrombocytopenia: 29%) were reported.
JAK2 inhibitors: In PERSIST-1, pacritinib when compared to best available therapy (BAT) showed SVR ≥ 35% in 19.1% vs. 4.7% patients, with lower rates of myelosuppression (thrombocytopenia: 17%, anemia: 11%). In PERSIST-2, a phase III trial of pacritinb vs. BAT in patients with baseline cytopenias, similar efficacy was demonstrated (SVR ≥ 35%: 18% vs. 3%). Increasing rates of heart failure and intracranial hemorrhages led to a temporary hold which was lifted in August 2017. Lestaurtinib showed CI in 7 (44%) patients in a phase I trial (n=16) and 6 (27%) patients in a phase II trial (n=22). Most notable toxicities were G 1/2 GI disturbances, anemia occurred in 14% and thrombocytopenia in 23% of patients. In a phase III trial (n=193), fedratinib showed a SVR ≥ 35% and a TSS ≥ 50% in 40% and 36% patients, respectively. However, incidence of significant neurotoxicity and Wernicke's encephalopathy led to its suspension. Similarly, a trial of XL019 was terminated due to emergence of central and peripheral neurotoxicity. In a phase I trial (n=48), NS-018 exhibited a spleen length reduction (SLR) ≥ 50% in 20 (56%) patients along with prompt improvement in bone marrow fibrosis (37%). Anemia and thrombocytopenia were reported in 15% and 27% of patients, respectively. Dizziness (23%) and nausea (19%) were also reported. Gandotinib demonstrated SLR ≥50% in 62% patients, in a phase I trial (n=38). G1 diarrhea (55.3%) and nausea (42.1%) were the most common toxicities.
JAK 1/2 inhibitors: SIMPLIFY-1 (S1), a phase III clinical trial (n=432) of momelotinib vs. ruxolotinib in JAK inhibitor-naïve patients, demonstrated non-inferiority for momelotinib, in spleen volume reduction (SVR) ≥ 35% (26.5% vs. 29%; p=0.01). However, SIMPLIFY-2 trial (S2), that compared these two drugs in JAK inhibitor exposed patients did not achieve similar responses with momelotinib (6.7% vs. 5.8%; p=0.90). Interestingly, momelotinib excelled at achieving transfusion independency in both trials (S1: 66.5% vs. 49.3%; p=0.001, S2: 43.3% vs 21.2%; p=0.001). Grade ≥ 3 infections and peripheral neuropathy were the major toxicities noted. These trials were suspended after 89% of patients failed to achieve the primary endpoint of SVR. AZD1480 demonstrated clinical improvement (CI) in four (11%) patients in a phase I trial (n=35). Most common adverse events included grade (G) 1-2 dizziness and anemia.
Conclusion
Novel JAK pathway inhibitors have shown promising efficacy in MF but safety concerns regarding the hematological (cytopenias) and non-hematological adverse effects needs to be addressed until their use in clinical practice is established. Momelotinib success in achieving anemia related endpoints is note-worthy and should be further explored in this regard. A phase II study [NCT03165734] evaluating pacritinib monotherapy as a second line treatment in patients with baseline thrombocytopenia is ongoing.

Disclosures
No relevant conflicts of interest to declare.