RUXOLITINIB: THE JACK FOR JAK INHIBITION

Ruxolitinib: Inhibition of the Janus Kinases

The janus kinases are a sub family of the protein kinases and are refer to as non receptor tyrosine kinases. Signals from the Janus kinases regulate several different types of proteins such as the cytokine receptor family (interferon), the GM-CSF family and the GP130 receptor family.[1] The JAK kinases occur in four isoforms, with two matching phosphorylation domains, one for activity one for regulation. JAK2 in been shown to be mutated in several conditions including thrombocthemia and myeloprliferation disorders [2]. In relation to haematological maliganacies the JAK2 mutation have been shown to essential for tumor growth and proliferation [3-5]. Inhibiting the JAK2 kinase offers a potential mechanism for chemotherapeutic action. Ruxolitinib is a small, molecule inhibitor that has been established to inhibit the Janus kinases; early clinic work established that Ruxolitinib has sufficient anti-tumor activity to warrant further investigation. [5]

Ruxolitinib: Properties and Availability

Ruxolitinib JAK inhibitor was originally researched under the codes INCB018424 or INC 424 and is being developed by Novartis (US name Incyte). Ruxolitinib structure clearly indicates that it is a mono substituted purine with activity for 2 of the 3 known ligands for Janus kinases. Ruxolitinib is also available as the phosphate salt and some clinical formulations use this rather than the free base. The Ruxolitinib IC50 for its targets are JAK1 (3.3 nM) and JAK2 (2.8 nM), but Ruxolitinib has also been shown to be active against Tyk2 (19 nM) and JAK 3 (323 nM) [6]. Ruxolitinib solubility in water and ethanol is extremely poor for the free base but Ruxolitinib free base is soluble in DMSO, however no details are given. Ruxolitinib phosphate salt is soluble in DMSO and water with concentrations achievable of 200 and 8 mg/ml respectively. Ruxolitinib stability is listed for its powdered free base and phosphate salt form and this can be stored for upwards of 2 years if kept at -20^oC. Researchers can buy Ruxolitinib free base or phosphate salt from a variety of Ruxolitinib suppliers although Ruxolitinib price is dependent on the supplier. The price of a 10 mg vial can range from $109 up to $300, researchers are advised to shop very carefully for this product.

RUXOLITINIB: Preclinical testing

Ruxolitinib has been tested in mouse models of myeloproliferative neoplasm which demonstrate significant improvement in spleen volume and reduction of neoplastic cells / inflammatory cytokines. It also has been demonstrated to inhibit cytokine induced JAK signalling pathways in lymphocytes.[7] Interestingly a topical application of Ruxolitinib resulted in significant changes in cellular proliferation and edema.[7;8] In arthritis rat models Ruxolitinib efficacy was improved over standard treatments based on partial or intermittent treatment and no toxicity linked to humoral immunity suppression or adverse hematological effects. However, most emphasisis has been placed on research in to myeloproliferative neoplasm since this tumor type expresses the JAK2V617F mutation.[8] Ruxolitinib is particularly effective against this mutation hinting at a possible role in treating this disease.[9] Based on these results phase 1 and 2 clinical trials were initiated.

 

CLINICAL ACCOMPLISHMENTS OF RUXOLITINIB:

Ruxolitinib has been initially reported for activity in myeloproliferative neoplasms where modest reductions in the size of the spleen and other criteria were observed [10]. However, in a communication to the editor of the Journal the trial was published indications were given that in reality of the 51 patient’s enrolled long term treatment did not show any significant difference to standard therapy but did alleviate constitutional symptoms. In addition reductions in spleen proved to be temporary and the drug demonstrated significant withdraw toxicity [10-12].  However, phase ½ clincal trials have continued most notable with COMFORT1 and COMFORT2 trials demonstrating the 42% Ruxolitinib treated patients had >35% reduction in spleen volume with clear indications of improved symptoms. In direct comparison to the best available therapy Ruxolitinib was highly significantly better. The results of these two trials have enbled Ruxolitinib to achieve FDA approval for intermediate or high risk myleofibrosis. Ruxolitinib is currently undergoing clinical trials at phase 3 investigating the Efficacy and Safety in Polycythemia vera subjects who are resistant to or intolerant of hydroxyurea.

References

 

   1.   Bole-Feysot C, Goffin V et al. Prolactin (PRL) and its receptor: actions, signal transduction pathways and phenotypes observed in PRL receptor knockout mice. Endocr Rev 1998; 19(3):225-268.

   2.   Percy MJ, McMullin MF. The V617F JAK2 mutation and the myeloproliferative disorders. Hematol Oncol 2005; 23(3-4):91-93.

   3.   Borie DC, Si MS et al. JAK3 inhibition as a new concept for immune suppression. Curr Opin Investig Drugs 2003; 4(11):1297-1303.

   4.   Atallah E, Verstovsek S. Prospect of JAK2 inhibitor therapy in myeloproliferative neoplasms. Expert Rev Anticancer Ther 2009; 9(5):663-670.

   5.   Ferrajoli A, Faderl S et al. The JAK-STAT pathway: a therapeutic target in hematological malignancies. Curr Cancer Drug Targets 2006; 6(8):671-679.

   6.   Shilling AD, Nedza FM et al. Metabolism, excretion, and pharmacokinetics of [14C]INCB018424, a selective Janus tyrosine kinase 1/2 inhibitor, in humans. Drug Metab Dispos 2010; 38(11):2023-2031.

   7.   Fridman JS, Scherle PA et al. Preclinical evaluation of local JAK1 and JAK2 inhibition in cutaneous inflammation. J Invest Dermatol 2011; 131(9):1838-1844.

   8.   Fridman JS, Scherle PA et al. Selective inhibition of JAK1 and JAK2 is efficacious in rodent models of arthritis: preclinical characterization of INCB028050. J Immunol 2010; 184(9):5298-5307.

   9.   Verstovsek S. Therapeutic potential of JAK2 inhibitors. Hematology Am Soc Hematol Educ Program 2009;636-642.

10.   Barosi G, Rosti V. Novel strategies for patients with chronic myeloproliferative disorders. Curr Opin Hematol 2009; 16(2):129-134.

11.   Tefferi A, Litzow MR et al. Long-term outcome of treatment with ruxolitinib in myelofibrosis. N Engl J Med 2011; 365(15):1455-1457.

12.   Tefferi A, Pardanani A. Serious adverse events during ruxolitinib treatment discontinuation in patients with myelofibrosis. Mayo Clin Proc 2011; 86(12):1188-1191.