The anti-mitotic agents PTC-028 and PTC596 display potent activity in pre-clinical models of multiple myeloma but challenge the role of BMI-1 as an essential tumour gene.

Future progress in the treatment of multiple myeloma (MM) requires both the characterisation of key drivers of the disease and novel, innovative approaches to tackle these vulnerabilities. The present study focussed on the pre-clinical evaluation of a novel drug class, BMI-1 modulators, in MM. We demonstrate potent activity of PTC-028 and PTC596 in a comprehensive set of in vitro and in vivo models, including models of drug resistance and stromal support. Treatment of MM cells with PTC-028 and PTC596 downregulated BMI-1 protein levels, which was found to correlate with drug activity. Surprisingly, BMI-1 was dispensable for the activity of BMI-1 modulators and MM cell growth. Our data rather point to mitotic arrest accompanied by myeloid cell leukaemia-1 (MCL-1) loss as key anti-MM mechanisms and reveal impaired MYC and AKT signalling activity due to BMI-1 modulator treatment. Moreover, we observed a complete eradication of MM after PTC596 treatment in the 5TGM.1 in vivo model and define epigenetic compounds and B cell leukaemia/lymphoma 2 homology domain 3 (BH3) mimetics as promising combination partners. These results bring into question the postulated role of BMI-1 as an essential MM gene and confirm BMI-1 modulators as potent anti-mitotic agents with encouraging pre-clinical activity that supports their rapid translation into clinical trials.

Maternal embryonic leucine zipper kinase is a novel target for proliferation-associated high-risk myeloma.

Treatment of high-risk patients is a major challenge in multiple myeloma. This is especially true for patients assigned to the gene expression profiling-defined proliferation subgroup. Although recent efforts have identified some key players of proliferative myeloma, genetic interactions and players that can be targeted with clinically effective drugs have to be identified in order to overcome the poor prognosis of these patients. We therefore examined maternal embryonic leucine zipper kinase (MELK) for its implications in hyper-proliferative myeloma and analyzed the activity of the MELK inhibitor OTSSP167 both in vitro and in vivoMELK was found to be significantly overexpressed in the proliferative subgroup of myeloma. This finding translated into poor overall survival in patients with high vs low MELK expression. Enrichment analysis of upregulated genes in myeloma cells of MELKhigh patients confirmed the strong implications in myeloma cell proliferation. Targeting MELK with OTSSP167 impaired the growth and survival of myeloma cells, thereby affecting central survival factors such as MCL-1 and IRF4 This activity was also observed in the 5TGM.1 murine model of myeloma. OTSSP167 reduced bone marrow infiltration and serum paraprotein levels in a dose-dependent manner. In addition, we revealed a strong link between MELK and other proliferation-associated high-risk genes (PLK-1, EZH2, FOXM1, DEPDC1) and MELK inhibition also impaired the expression of those genes. We therefore conclude that MELK is an essential component of a proliferative gene signature and that pharmacological inhibition of MELK represents an attractive novel approach to overcome the poor prognosis of high-risk patients with a proliferative expression pattern.

Maternal embryonic leucine zipper kinase inhibitor OTSSP167 has preclinical activity in multiple myeloma bone disease.

Multiple myeloma bone disease is characterized by an uncoupling of bone remodeling in the multiple myeloma microenvironment, resulting in the development of lytic bone lesions. Most myeloma patients suffer from these bone lesions, which not only cause morbidity but also negatively impact survival. The development of novel therapies, ideally with a combined anti-resorptive and bone-anabolic effect, is of great interest because lesions persist with the current standard of care, even in patients in complete remission. We have previously shown that MELK plays a central role in proliferation-associated high-risk multiple myeloma and its inhibition with OTSSP167 resulted in decreased tumor load. MELK inhibition in bone cells has not yet been explored, although some reports suggest that factors downstream of MELK stimulate osteoclast activity and inhibit osteoblast activity, which makes MELK inhibition a promising therapeutic approach. Therefore, we assessed the effect of OTSSP167 on bone cell activity and the development of myeloma-induced bone disease. OTSSP167 inhibited osteoclast activity in vitro by decreasing progenitor viability as well as via a direct anti-resorptive effect on mature osteoclasts. In addition, OTSSP167 stimulated matrix deposition and mineralization by osteoblasts in vitro This combined anti-resorptive and osteoblast-stimulating effect of OTSSP167 resulted in the complete prevention of lytic lesions and bone loss in myeloma-bearing mice. Immunohistomorphometric analyses corroborated our in vitro findings. In conclusion, we show that OTSSP167 has a direct effect on myeloma-induced bone disease in addition to its anti-multiple myeloma effect, which warrants further clinical development of MELK inhibition in multiple myeloma.