Blockade of Mer By the Small Molecule Inhibitor R992 Inhibits Multiple Myeloma and Its Associated Bone Disease By Restoring the Perturbed Bone Homeostasis

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<0.001). Mechanistically, Mer blockade inhibited proliferation in 5-bromo-2′-deoxyuridine assays and induced apoptosis as shown by increased numbers of Annexin V⁺ cells (n=3,*p<0.05 and *p<0.001, respectively). To delineate signaling pathways mediating the biological effects of Mer blockade in MM cells we investigated key mediators of MM cell proliferation and survival. Here, we found reduced phosphorylation of Akt upon Mer inhibition with R992. Furthermore, R992 inhibited mitogen-activated protein kinase (MAPK) pathways Erk and p38. Subsequently, we investigated whether inhibition of Mer signaling increases chemosensitivity of MM cells. Combination treatment of R992 with bortezomib and cyclophosphamide demonstrated that Mer inhibition significantly increased sensitivity of MM cells to these established MM therapies.

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<0.05 and n=4/5, *p<0.05, respectively). Importantly, treatment with R992 resulted in a significant prolongation of overall survival by 15 days in the U266 model (median OS 73 vs. 88 days (n=13/12, *p<0.05). In addition, treatment with R992 prolonged survival in the more aggressive JJN3 model (median OS 24 vs. 27 days, n=9/7, *p<0.005).

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<0.05). Moreover, analysis of the metaphyseal spongiosa showed that R992 could retard myeloma-mediated destruction of trabecular bone area measured by increased trabecular number and increased trabecular thickness (n=7/7, *p<0.05). Interestingly, R992 could also enlarge the metaphyseal diameter due to thickened cortical bone. Vice versa, overexpression of the Mer ligands Gas6 and Pros1 in U266 and JJN3 cells led to increased osteoclast and decreased osteoblast differentiation in vitro and more rapid and destructive myeloma bone disease in vivo. These data suggest that the expression of Mer ligands represent thus far unrecognized mediators of MM-induced perturbed bone homeostasis.

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.