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Activating point mutations in the JAK2 kinase were identified in BCR-ABL-negative myeloproliferative neoplasms, including polycythemia vera, essential thrombocythemia and primary myelofibrosis (MF). This encouraged the development of several small-molecule JAK2 tyrosine kinase inhibitors, of which ruxolitinib (formerly known as INCB018424) was approved by the US Food and Drug Administration for the treatment of patients with intermediate or high-risk MF, including primary MF, postpolycythemia vera MF and post-essential thrombocythemia MF. Another JAK2 inhibitor, SAR302503 (formerly known as TG101348), is in advanced clinical trials. Both drugs inhibit JAK2 kinase activity in vitro and JAK2-dependent proliferation of cell lines with IC50 values in the low nanomolar concentration range. 8,9 Among the four kinases of the JAK family (JAK1, JAK2, JAK3 and TYK2), SAR302503 also inhibits JAK1, Tyk2 and JAK3, albeit with B30-, B100and B300-fold weaker efficiency than JAK2, respectively. Ruxolitinib inhibits JAK1 and JAK2 equally well, and targets TYK2 410-fold and JAK3 B100-fold weaker. As both drugs were only tested for inhibition of a few dozen unrelated kinases, accounting for only a small portion of the 518 human kinases, comprehensive data on their specificity are missing. In addition, no structural data of ruxolitinib or SAR302503 bound to the JAK2 kinase domain that would reveal their binding modes and molecular mechanism-of-action are available. Of note, ruxolitinib is the only FDA-approved kinase inhibitor for which no co-crystal structure with its target kinase has been published. Here, we present a near-kinome-wide survey of the specificity of ruxolitinib and SAR302503 and determine their binding modes to the JAK2 kinase domain by extensive sampling using molecular dynamics (MD) simulations. For specificity testing, we used a panel consisting of 368 recombinant human kinases (including 70 kinase mutants relevant to human disease), thereby covering B60% of the human kinome. Inhibition of the kinase activity in vitro was assayed for both drugs in parallel at a concentration of 1.0 mM. Ruxolitinib inhibited the activity of 33 kinases (including 11 kinase mutants) by X50%, whereas 54 kinases (including 14 kinase mutants) were inhibited by SAR302503 (Table 1a and Supplementary Data). Eleven and 14 kinases (including 2 and 4 kinase mutants, respectively) were inhibited by ruxolitinib and SAR302503, respectively, by X80% (Table 1a). We subsequently determined the IC50 values for kinases that showed profound inhibition in the tested panel. We concentrated on known oncogenes and/or validated drug targets in cancer and other diseases. These included the receptor tyrosine kinases ALK, RET, TRK-B, the cytoplasmic tyrosine kinases ACK1, FAK, LCK and the serine/threonine kinase JNK1. Ruxolitinib strongly inhibited TRK-B (IC501⁄4 11 nM), as well as ACK1, ALK and RET with IC50 values below 300 nM. SAR302503 inhibited LCK and RET with IC50 values B500 nM, and ACK1, FAK and JNK1 with IC50 values B200 nM (Table 1b) in addition to the previously described inhibition of FLT3 and BCR-ABL. We were intrigued when we found LRRK2 and several of its pathogenic mutants, which are common causes of familial Parkinson’s disease, to be profoundly inhibited by both JAK2 inhibitors (Table 1a). We then used in vitro kinase inhibition assays for LRRK2 and monitored the cellular phosphorylation of LRRK2 at Ser-910/935 as a pharmacodynamic marker of LRRK2 kinase activity. LRRK2 kinase activity was inhibited in vitro with IC50 values 820 nM and 1.8mM for ruxolitinib and SAR302503, respectively (Table 1b), but both drugs were not able to strongly inhibit the LRRK2 phosphorylation at Ser-910/935 in cells (data not shown). SAR302503 inhibits a much larger number of off-target tyrosine kinases than ruxolitinib (31 vs 15, excluding the JAK kinases), whereas the number of off-target serine-/threonine kinases is similar for both drugs. The tyrosine kinases that are targeted by SAR302503, but not by ruxolitinib, include the SRC family kinases LCK and FGR, the T-cell kinase ITK, as well as the KIT and FLT3 receptor kinases, all of which are critical for hematopoietic cell signaling. In addition, SAR302503 targets kinases that are predominantly expressed in non-hematopoietic cells, such as PDGFR members and DDR2. Those are thought to contribute to the side-effect profile of BCR-ABL tyrosine kinase inhibitors. Based on these observations, one may speculate on a higher incidence of adverse events in patients treated with SAR302503, as compared with ruxolitinib. The detailed knowledge of the binding mode of a kinase inhibitor at the atomic level is essential to understand its mechanism-of-action, interpret its specificity, predict and rationalize its resistance mechanisms, and suggest points of chemical derivatization for improved potency and specificity. To shed light on the binding mode of ruxolitinib and SAR302503 to JAK2, we carried out multiple runs of MD simulations (simulation protocols and analyses of MD trajectories are in the Supplementary Information). MD is a computational method to assess the structure and flexibility of proteins and their interactions with ligands. Notably, we performed simulations with explicit solvent and full flexibility of both JAK2 and inhibitor, that is, taking into account not only enthalpic but also entropic contributions of drug binding. It is important to note that MD simulations are significantly more accurate (albeit computationally more expensive) than the commonly used docking with rigid protein targets. Following a similar MD-based simulation protocol, we previously predicted the binding mode of a potent ATP-competitive inhibitor of the EphB4 tyrosine kinase, which is essentially identical to the subsequently determined crystal structure. The MD simulations (cumulative sampling of 1.5 and 9.6ms for ruxolitinib and SAR302503, respectively) suggest that both drugs inhibit JAK2 by a so-called type I binding, in which the inhibitor targets the ATPbinding site of the kinase in its active conformation and the DFGmotif at the base of the activation loop is in its inward-facing conformation. Importantly, the analysis of the free-energy surface (Supplementary Figures S2 and S3) and displacement from the starting poses (Supplementary Figure S8) indicate that there are multiple poses, that is, multiple positions, orientations and/or conformations, of each of the two drugs in the ATP-binding site (Figure 1 and Supplementary Figures S4 and S7). The double-ring system (7H-pyrrolo[2,3-d]pyrimidin) of ruxolitinib is involved in two persistent hydrogen bonds with the so-called hinge region,

Erratum: Specificity and mechanism-of-action of the JAK2 tyrosine kinase inhibitors ruxolitinib and SAR302503 (TG101348)