Synergistic action of kinase and BET bromodomain inhibitors in cell killing has been reported for a variety of cancers. Using the chemical scaffold of the JAK2 inhibitor TG101348 we developed and characterized single agents which potently and simultaneously inhibit BRD4 and a specific set of oncogenic tyrosine kinases including JAK2, FLT3, RET, and ROS1. Lead compounds showed on-target inhibition in several blood cancer cell lines and were highly efficacious at inhibiting the growth of hematopoietic progenitor cells from myeloproliferative neoplasm (MPN) patients. Screening across 931 cancer cell lines revealed differential growth inhibitory potential with highest activity against bone and blood cancers, and greatly enhanced activity over the single BET inhibitor JQ1. Gene-drug sensitivity analyses and drug combination studies indicate synergism of BRD4 and kinase inhibition as a plausible reason for the superior potency in cell killing. Combined, our findings indicate promising potential of these agents as novel chemical probes and cancer therapeutics.

Heightened matrix metalloproteinase (MMP) activity has been noted in the context of the tumor microenvironment for many years, and causal roles for MMPs have been defined across the spectrum of cancer progression. This is primarily due to the ability of the MMPs to process extracellular matrix (ECM) components and to regulate the bioavailability/activity of a large repertoire of cytokines and growth factors. These characteristics made MMPs an attractive target for therapeutic intervention but notably clinical trials performed in the 1990s did not fulfill the promise of preclinical studies. The reason for the failure of early MMP inhibitor (MMPI) clinical trials that are multifold but arguably principal among them was the inability of early MMP-based inhibitors to selectively target individual MMPs and to distinguish between MMPs and other members of the metzincin family. In the decades that have followed the MMP inhibitor trials, innovations in chemical design, antibody-based strategies, and nanotechnologies have greatly enhanced our ability to specifically target and measure the activity of MMPs. These advances provide us with the opportunity to generate new lines of highly selective MMPIs that will not only extend the overall survival of cancer patients, but will also afford us the ability to utilize heightened MMP activity in the tumor microenvironment as a means by which to deliver MMPIs or MMP activatable prodrugs.

Bone metastasis is common during breast cancer progression. Matrix metalloproteinase-2 (MMP-2) is significantly associated with aggressive breast cancer and poorer overall survival. In bone, tumor or host derived MMP-2 contributes to breast cancer growth and does so by processing substrates including type I collagen and transforming growth factorβ (TGFβ) latency proteins. These data provide strong rationale for the application of MMP-2 inhibitors to treat the disease. However, in vivo, MMP-2 is systemically expressed. Therefore, to overcome potential toxicities noted with previous broad-spectrum MMP inhibitors (MMPIs), we used highly selective bisphosphonic based MMP-2 inhibitors (BMMPIs) that allowed for specific bone targeting. In vitro, BMMPIs impacted the viability of breast cancer cell lines and osteoclast precursors but not osteoblasts. In vivo, we demonstrated using two bone metastatic models (PyMT-R221A and 4T1) that BMMPI treatment significantly reduced tumor growth and tumor associated bone destruction. Additionally, BMMPIs are superior in promoting tumor apoptosis compared to the standard of care bisphosphonate, zoledronate. We demonstrated MMP-2 selective inhibition in the bone microenvironment using specific and broad spectrum MMP probes. Further, compared to zoledronate, BMMPI treated mice had significantly lower levels of TGFβ signaling and MMP generated type I collagen carboxy-terminal (ICTP) fragments. Taken together, our data show the feasibility of selective inhibition of MMPs in the bone metastatic breast cancer microenvironment. We posit that BMMPIs could be easily translated to the clinical setting for the treatment of bone metastases given the well-tolerated nature of bisphosphonates.

Bone metastatic breast cancer is currently incurable and will be evident in more than 70% of patients that succumb to the disease. Understanding the factors that contribute to the progression and metastasis of breast cancer can reveal therapeutic opportunities. Matrix metalloproteinases (MMPs) are a family of proteolytic enzymes whose role in cancer has been widely documented. They are capable of contributing to every step of the metastatic cascade, but enthusiasm for the use of MMP inhibition as a therapeutic approach has been dampened by the disappointing results of clinical trials conducted more than 20 years ago. Since the trials, our knowledge of MMP biology has expanded greatly. Combined with advances in the selective targeting of individual MMPs and the specific delivery of therapeutics to the tumor microenvironment, we may be on the verge of finally realizing the promise of MMP inhibition as a treatment strategy. Here, as a case in point, we focus specifically on MMP-2 as an example to show how it can contribute to each stage of breast-cancer-to-bone metastasis and also discuss novel approaches for the selective targeting of MMP-2 in the setting of the bone-cancer microenvironment.