Combinational Therapy of Lenalidomide with Activin A Neutralizing Antibody; Preclinical Rationale for a Novel Anti-Myeloma Strategy
Authors: Scullen Tyler , Vallet Sonia , Santo Loredana , Fulciniti Mariateresa , Eda Homare , Cirstea Diana D. , Patel Kishan , Yee Andrew , Mahindra Anuj , Raje Noopur 
Published: October 2019

Abstract
Abstract 1871
Multiple myeloma (MM) is characterized by the vast majority of patients developing osteolytic bone lesions consequent to biochemical imbalances in bone resorption and formation. As such, compound combinations that succeed in selectively inhibiting processes relevant to both osteolysis as well as MM cell viability would be of great therapeutic potential to the MM patient.
Here we investigated the benefits of combining lenalidomide with an Activin A neutralizing antibody. Lenalidomide is an immunomodulatory drug which displays both direct anti tumor activity as well as indirect inhibition of osteoclastogenesis but lacks an anabolic bone effect. Conversely, Activin A, secreted predominantly by bone marrow stromal cells (BMSCs), is an extracellular protein complex with apparent paracrine functions critical in MM tumor-induced osteolysis whose sequestration has been recently demonstrated by us to restore bone architecture and reduce tumor burden in vivo. Given these properties, we hypothesized that an alleviation of the abnormal processes associated with the MM bone microenvironment via Activin A neutralizing antibody, combined with a selective targeting of MM cell growth and viability with lenalidomide could yield a positive outcome for MM patients with bone disease.
Interestingly, our studies revealed that lenalidomide stimulated Activin A secretion in BMSCs, leading us to search for potential underlying mechanisms. Due to previous implications in MM induced Activin A secretion, as well as the presence of a highly conserved c-Jun-binding sequence in the promoter of the constituent subunit of the peptide, INHβA, the JNK-dependent pathway was of particular interest. Accordingly, BMSCs treated with lenalidomide displayed an early activation of JNK signaling evidenced by increases in both JNK phosphorylation and c-Jun mRNA expression, an effect which was found to be AKT dependent. We next investigated and accordingly observed that lenalidomide had a deleterious effect on osteoblastogenesis as evidenced via significant decreases in alkaline phosphatase activity associated with a downregulation of DLX5 mRNA expression and an increase in SMAD2 signaling. Expectantly, further studies revealed that osteoblasts differentiated both alone as well as in co-culture with MM cells exhibited variable levels of extracellular Activin A concentrations which were inversely correlated with ALP activity when treated with lenalidomide. Furthermore, the addition of lenalidomide was in fact noted to augment existing Activin A secretion as well as the resultant ALP dysfunction. This could be overcome by the addition of Activin A neutralizing antibody, which efficiently sequestered Activin A without compromising the cytotoxic MM activity of lenalidomide.
Collectively, our results illustrate an Activin A mediated inhibition of osteoblastogenesis by lenalidomide, in which lenalidomide stimulates a JNK dependent secretion of Activin A by BMSCs. This however can be reversed via the addition of Activin A neutralizing antibody.
Our work therefore indicates the validity of combining Activin A neutralizing antibody with lenalidomide, and provides the basis for a currently accruing Phase I clinical trial in MM (NCT01562405).

Disclosures:
Raje: Celgene: Consultancy; Onyx: Consultancy; Millennium: Consultancy; Acetylon: Research Funding; Amgen: Research Funding; Eli-Lilly: Research Funding.