Novel Alkylating Agent, Ureidomustine Exhibit Pre-Clinical Efficacy in B-Cell Lymphoma with Minimal Dose-Limiting Myelotoxicity

Many patients with B-cell lymphomas, including mantle cell lymphoma (MCL) and diffuse large B-cell lymphoma (DLBCL), are not cured by conventional chemo-immunotherapy. One reason for this is because these drugs, while effective, are limited by their narrow therapeutic window and significant toxicities. B-cell lymphomas are highly dependent on DNA damage checkpoints, and hence are biologically responsive to drugs that trigger these checkpoints. Hence, In order to identify superior DNA damaging anti-lymphoma drugs we evaluated a series of novel, third generation, DNA-directed alkylating agents that have DNA specific binding domains chemically linked via urea, carbamate or hydrazinecarboxamide to N-mustard pharmacophores. The chemically favorable water-soluble ureidomustine (BO-1055) was evaluated for activity against 23 human lymphoma cell lines including MCL, DLBCL (GCB and ABC subtype) and a spontaneous murine B-cell lymphoma. Fifty % of these MCL and DLBCL cell lines exhibited BO-1055 IC50 values in the nanomolar range including even markedly chemo-resistant cell lines as OCI-Ly10 (IC50 0.117μM ±0.21). In marked contrast, against normal human tissues (lung fibroblast IMR90, kidney fibroblast CV1, mesenchymal stromal, bronchial epithelium, myofibroblast, bone marrow-derived endothelium) and hematopoietic cells (cord blood CD34+ cells, colony-forming assay for hematopoietic progenitors) and in cobblestone area-forming assay for hematopoietic stem cells, BO-1055 IC50s were > 10μM. Hence BO-1055 has a significant therapeutic window (50-100-fold) between its toxicity against B-cell lymphomas compared to normal human cells. We evaluated BO-1055 cardiotoxicity in the HL-1 cardiomyocyte line and observed a 227-fold less cytotoxicity compared to Doxorubicin. Drug combination studies with BO-1055 and an Hsp90 inhibitor (PU-H71), an Hsp70 inhibitor (TT46), doxorubicin and bortezomib (Velcade) demonstrated synergistic effects based on Compusyn analysis with very low combination indices in 50% of lymphoma cell lines. The synergistic effect was not observed in normal cells. Notably, BO-1055 caused downregulation of the critical lymphoma oncoproteins MYC and BCL6, but not Bcl2. BCL6 normally suppresses the ATR-driven S-phase checkpoint. Accordingly treatment with BO-1055 resulted in accumulation of cells in S-phase and up-regulation of proteins involved in DNA repair and intra-S-phase checkpoints [MRE11, p-P95/NBS1 (ser343), RAD50, p-ATR (ser428)]. Finally, xenograft experiments in NSG mice bearing MCL JEKO1 GFP/luciferase+ tumors treated with BO-1055 (30mg/kg) 3x/week showed complete tumor remission after 2 weeks of treatment as monitored by luminescent imaging. In summary, BO-1055 is emerging as a potent therapeutic agent for B-cell lymphomas, with little toxicity against normal tissues and hence potentially wider therapeutic window than current lymphoma drugs.