Ruxolitinib is a kinase inhibitor that is selective for the Janus Associated Kinases (JAK) 1 and 2. These kinases are responsible for the mediation of cytokine and growth factor signalling which in turn effect immune function and hematopoiesis. The signalling process involves signal transducers and transcription activators (STAT) which modulate gene expression. Patients with myelofibrosis have abnormal JAK1 and JAK2 activity thus ruxolitinib works to regulate this.
The mean half-maximal inhibitory concentration (IC50) for JAK 1 and JAK 2 are 2.8 nmol/L and 3.3 nmol/L respectively. After administration of ruxolitinib, a decrease in levels of phosphorylated STAT (marker for JAK activity) in a dose-dependent manner can be observed. Pharmacodynamic resistance has not been observed.
Ruxolitinib is metabolized by CYP3A4. Less potent active metabolites forms as a result.
Thrombocytopenia was the dose-limiting toxicity.