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The fine equilibrium of bone homeostasis is maintained by bone-forming osteoblasts and bone-resorbing osteoclasts. Here, we show that TAM receptors MERTK and TYRO3 exert reciprocal effects in osteoblast biology: Osteoblast-targeted deletion of MERTK promotes increased bone mass in healthy mice and mice with cancer-induced bone loss, whereas knockout of TYRO3 in osteoblasts shows the opposite phenotype. Functionally, the interaction of MERTK with its ligand PROS1 negatively regulates osteoblast differentiation via inducing the VAV2-RHOA-ROCK axis leading to increased cell contractility and motility while TYRO3 antagonizes this effect. Consequently, pharmacologic MERTK blockade by the small molecule inhibitor R992 increases osteoblast numbers and bone formation in mice. Furthermore, R992 counteracts cancer-induced bone loss, reduces bone metastasis and prolongs survival in preclinical models of multiple myeloma, breast- and lung cancer. In summary, MERTK and TYRO3 represent potent regulators of bone homeostasis with cell-type specific functions and MERTK blockade represents an osteoanabolic therapy with implications in cancer and beyond.

BCR-ABL negative myeloproliferative neoplasms (MPNs) consist of essential thrombocythemia, polycythemia vera, and myelofibrosis. The majority of patients harbor the JAK2-activating mutation V617F. JAK2 inhibitors were shown to reduce symptom burden and splenomegaly in MPN patients. However, treatment options are limited after failure of JAK2 inhibitors. AXL, a member of the TAM family of receptor tyrosine kinases, mediates survival and therapy resistance of different myeloid cancers including acute myeloid leukemia and chronic myeloid leukemia. We studied the relevance of AXL as a target in MPN using primary patient cells and preclinical disease models. We found that AXL is abundantly activated in MPN cells and that its ligand growth arrest-specific gene 6 is upregulated in MPN patients. Pharmacologic and genetic blockade of AXL impaired viability, decreased proliferation and increased apoptosis of MPN cells. Interestingly, ruxolitinib treatment induced increased phosphorylation of AXL indicating that activation of AXL might mediate resistance to ruxolitinib. Consistently, the AXL inhibitor bemcentinib exerted additive effects with ruxolitinib via impaired STAT3, STAT5, and AKT signaling. Both agents had activity when employed alone and exerted an additive effect on survival and splenomegaly in vivo. Moreover, bemcentinib treatment normalized red blood cell count and hemoglobin levels in vivo. Thus, our data indicate that AXL inhibition represents a novel treatment option in MPN warranting clinical investigation.

BCR-ABL negative myeloproliferative neoplasms (MPNs) consist of essential thrombocythemia, polycythemia vera, and myelofibrosis. The majority of patients harbor the JAK2-activating mutation V617F. JAK2 inhibitors were shown to reduce symptom burden and splenomegaly in MPN patients. However, treatment options are limited after failure of JAK2 inhibitors. AXL, a member of the TAM family of receptor tyrosine kinases, mediates survival and therapy resistance of different myeloid cancers including acute myeloid leukemia and chronic myeloid leukemia. We studied the relevance of AXL as a target in MPN using primary patient cells and preclinical disease models. We found that AXL is abundantly activated in MPN cells and that its ligand growth arrest-specific gene 6 is upregulated in MPN patients. Pharmacologic and genetic blockade of AXL impaired viability, decreased proliferation and increased apoptosis of MPN cells. Interestingly, ruxolitinib treatment induced increased phosphorylation of AXL indicating that activation of AXL might mediate resistance to ruxolitinib. Consistently, the AXL inhibitor bemcentinib exerted additive effects with ruxolitinib via impaired STAT3, STAT5, and AKT signaling. Both agents had activity when employed alone and exerted an additive effect on survival and splenomegaly in vivo. Moreover, bemcentinib treatment normalized red blood cell count and hemoglobin levels in vivo. Thus, our data indicate that AXL inhibition represents a novel treatment option in MPN warranting clinical investigation.

Despite many therapeutic advances in recent years Multiple Myeloma (MM) still remains incurable in the majority of the patients. In addition, MM patients suffer significantly from co-morbidities including bone pain and renal insufficiency. Therefore, the development of novel treatments is warranted. The TAMR family consists of Tyro3, Axl and Mer which represent evolving targets in cancer. We demonstrated that the role of TAMR is non-redundant in hematologic malignancies, with Axl exerting an important function in AML, but not in MM, where Mer represents a novel target. Therefore, we tested the therapeutic potential of the Mer-inhibitor R992, which has an 8-fold selectivity over Tyro3 and a 13-fold selectivity over Axl in preclinical MM models (Rigel, San Francisco, USA). R992 exerted a dose-dependent growth inhibition of U266, JJN3 and RPMI8226 cells in vitro (n=3, *p Oral administration of 60mg/kg R992 BID to mice significantly reduced tumor burden in the U266 systemic myeloma mouse model. The λ light chain concentration and the CD138+ MM cell load was reduced 2-fold in R992 treated mice compared to placebo-treated mice 8 weeks after injection (n=5/5, *p For further phenotyping of the effects of R992 we performed microcomputed tomography (µCT) and histological analysis of the tibias in the U266 model. µCT analysis of proximal tibia metaphyses revealed, that bone volume and bone mineral density (BMD) were significantly increased by R992 (n=7/7, *p To directly assess the effect of R992 on osteoclasts, we treated osteoclast cultures with R992 and observed an inhibition of osteoclast differentiation by R992 alone and in co-culture with myeloma cells. Western blot analysis confirmed, that Mer phosphorylation was reduced by R992, whereas the phosphorylation of Tyro3 was not altered. Concomitantly, phosphorylation of p38 and activation of non-canonical NFκB pathway showed a dose dependent reduction after Mer blockade. Interestingly, R992 led also to increased osteoblast differentiation and could restore myeloma mediated osteoblast inhibition in co-cultures of MM cells and osteoblasts. In summary, our data suggest that Mer blockade leads to inhibition of MM and its associated bone disease. Furthermore, the function of Mer in bone homeostasis promoting osteoclast and inhibiting osteoblast activity leads to the potential application of Mer inhibitors also in osteolytic bone metastases or osteoporosis. Disclosures Darwish: Rigel Pharmaceuticals: Employment. Bhamidipati:Rigel Pharmaceuticals: Employment. Masuda:Rigel Pharmaceuticals: Employment, Equity Ownership. Loges:BerGenBio: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding.