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.

The TAM (TYRO3, MERTK, and AXL) family of receptor tyrosine kinases are pleiotropic regulators of adult tissue homeostasis maintaining organ integrity and self-renewal. Disruption of their homeostatic balance fosters pathological conditions like autoinflammatory or degenerative diseases including rheumatoid arthritis, lupus erythematodes, or liver fibrosis. Moreover, TAM receptors exhibit prominent cell-transforming properties, promoting tumor progression, metastasis, and therapy resistance in various cancer entities. Emerging evidence shows that TAM receptors are involved in bone homeostasis by regulating osteoblastic bone formation and osteoclastic bone resorption. Therefore, TAM receptors emerge as new key players of the regulatory cytokine network of osteoblasts and osteoclasts and represent accessible targets for pharmacologic therapy for a broad set of different bone diseases, including primary and metastatic bone tumors, rheumatoid arthritis, or osteoporosis.

7059 Background: The interplay with bone marrow stroma plays an important role in the pathobiology of MDS. Gas6 is secreted by mesenchymal bone marrow stroma cells and promotes survival and therapy resistance of AML cells expressing the Axl receptor. We hypothesized that inhibiting Axl by the small molecule inhibitor BGB324 might hold therapeutic potential in MDS. Methods: We investigated the inhibitory effect of BGB324 on primary bone marrow mononucleated cells (BMMNC) and mesenchymal stroma cells (MSC) from MDS patients in comparison to healthy donors. In the ongoing first-in-patient Phase 1a/b trial BGBC003 A standard 3 + 3 dose escalation study was performed to identify the maximum tolerated dose of BGB324 in patients with previously treated high risk MDS or AML. BGB324 was administered as an oral loading dose on days one and two followed by a reduced daily maintenance. Three dose levels were explored 400/100mg, 600/200mg and 900/300mg. Results: We found that BGB324 inhibited BMMNC from low- and high-risk MDS patients with an IC50 of 2.1 µM and 3.8 µM, respectively (n = 5). In comparison, BMNNC from healthy donors were resistant to BGB324 (IC50 9.4 µM, p < 0.05, n = 10). Axl expression was present in MSC isolated from the BM of MDS patients and BGB324 inhibited the proliferation of MSC from low- and high-risk MDS patients (IC50 2.5 µM and 2.7 µM, respectively; n = 7/5).To date, 3 patients with MDS were treated with 400 mg loading dose and 100 mg maintenance dose of BGB324. Therapy has been well-tolerated and the MTD has not yet been reached. The majority of adverse events reported have been Grade 1 and 2. The most common related adverse events are diarrhea and fatigue. One patient with MDS was treated for 80 weeks and experienced a PR. Evidence of target inhibition was demonstrated by almost complete inhibition of Axl phosphorylation accompanied by reduction in phosphoErk and phosphoAkt signalling at day 21 of treatment. Conclusions: BGB324 is well-tolerated and might represent a promising novel treatment approach in MDS. Safety and efficacy of BGB324 will be explored further in clinical trials. Clinical trial information: NCT02488408.

Accumulating evidence suggests that the immune system shows subtle but relevant differences between men and women. These differences may have an impact on cancer development and TME composition as well as responses to and adverse events elicited by immunotherapies. Several, albeit not all, clinical trials indicate a greater benefit from mono‐immunotherapies over chemotherapies for male patients than for female patients, especially in non‐small cell lung cancer and melanoma. Vice versa, female patients might benefit more from chemo‐immunotherapies. In human as well as animal models, sex differences in cancer microenvironment composition were described, with partially divergent results. Sex‐specific factors such as the levels of hormones, in particular testosterone and estrogen, or X‐ or Y‐chromosome associated genes are likely to drive the observed differences, but are often confounded by external influences such as smoking behavior, diet, or UV exposure. Therefore, large clinical and mechanistic knowledge gaps remain regarding the influence of sex on cancer immunotherapies and strategies to optimize response in either sex. More clinical as well as experimental research in this field is required to close these knowledge gaps, and clinical trials should include large enough groups of male and/or female patients to allow robust sex‐specific analyses.

Low‐dose antiangiogenic therapies have demonstrated the ability to enhance normalization of tumor vessels, consequently improving hypoxia levels, drug delivery, and promoting anticancer immune responses. Mast cells have been identified as contributors to resistance against antiangiogenic therapy and facilitators of abnormal neoangiogenesis. In this study, we demonstrate that by simultaneously targeting intratumoral mast cells with Imatinib and administering low‐dose anti‐VEGFR2 therapy, antitumor efficacy can be enhanced in preclinical models. Thus, combinatory treatment overcomes therapy resistance, while concurrently promoting tumor vessel normalization. Notably, histomorphometric analysis of tumor sections revealed that vessel perfusion could be improved through mast cell inhibition and, despite a significantly reduced microvessel density, the combination treatment did not result in elevated tumor hypoxia levels compared to anti‐VEGFR2 therapy alone. Short‐term Imatinib application effectively increased antitumor efficacy, and by prolonging the application of Imatinib tumor vessel normalization was additionally improved. The combination of mast cell depletion and antiangiogenic treatments has not been investigated in detail and promises to help overcoming therapy resistance. Further studies will be required to explore their impact on other treatment approaches, and subsequently to validate these findings in a clinical setting.

The TAM family of receptor tyrosine kinases were recently identified to regulate bone homeostasis by controlling osteoblasts and bone formation. Despite extensive knowledge of TAM receptor function in the mononuclear phagocyte system, the role of TAM receptors in osteoclasts remains largely unknown. Here, we identify a physiological regulatory system including MERTK and TYRO3 in osteoclasts controlling RHOA-ROCK-COFILIN/Myosin II signaling, thereby antagonistically regulating osteoclast-mediated bone remodeling to maintain bone homeostasis. Myeloid-specific lysozyme M -mediated deletion of Mertk led to increased bone mass in mice. In early stages of RANKL-induced osteoclast differentiation MERTK promotes amoeboid migration mode in osteoclast precursor cells by inducing RHOA-COFILIN-MLC2 pathway leading to increased osteoclast formation. In contrast, TYRO3 inhibits RHOA-ROCK signaling in osteoclast precursor cells thereby inhibiting these processes. Furthermore, we unraveled an inhibitory role of MERTK as well as TYRO3 in osteoclast differentiation and function. In line with this, mice with cathepsin K -mediated deletion of Mertk and Tyro3 exhibited an osteoporotic bone phenotype. We found that osteoclast precursor cell morphology dictates its fusion capacity and identified MERTK as a negative regulator of osteoclast fusion. In multinucleated cells, deletion of Mertk inhibits actin ring formation by mediating central actomyosin contraction and inactivation of COFILIN. In contrast, spatially well-ordered RHOA activation at adhesion structures, induced by loss of Tyro3, improves osteoclast biomechanotransduction to ameliorate podosome belt formation and enhance osteoclast function. By using a syngeneic breast cancer bone metastasis osteolysis models we identified TYRO3 as a bone protective receptor for osteolytic bone diseases, whereas MERTK represents a pharmacologic accessible target to inhibit osteoclast formation because its stimulatory effects of osteoclast precursors prevail the inhibitory effects on mature osteoclasts. Next to the recently uncovered role of MERTK as a target for osteoanabolic therapy MERTK may represent a one-drug two-target treatment strategy to increase osteoblast function and reduce osteoclast formation for treatments of bone diseases.