rrx-001 and Hypoxia

The endogenous mediator of vasodilation, nitric oxide (NO), has been shown to be a potent radiosensitizer. However, the underlying mode of action for its role as a radiosensitizer - while not entirely understood - is believed to arise from increased tumor blood flow, effects on cellular respiration, on cell signaling, and on the production of reactive oxygen and nitrogen species (RONS), that can act as radiosensitizers in their own right. NO activity is surprisingly long-lived and more potent in comparison to oxygen. Reports of the effects of NO with radiation have often been contradictory leading to confusion about the true radiosensitizing nature of NO. Whether increasing or decreasing tumor blood flow, acting as radiosensitizer or radioprotector, the effects of NO have been controversial. Key to understanding the role of NO as a radiosensitizer is to recognize the importance of biological context. With a very short half-life and potent activity, the local effects of NO need to be carefully considered and understood when using NO as a radiosensitizer. The systemic effects of NO donors can cause extensive side effects, and also affect the local tumor microenvironment, both directly and indirectly. To minimize systemic effects and maximize effects on tumors, agents that deliver NO on demand selectively to tumors using hypoxia as a trigger may be of greater interest as radiosensitizers. Herein we discuss the multiple effects of NO and focus on the clinical molecule RRx-001, a hypoxia-activated NO donor currently being investigated as a radiosensitizer in the clinic.

Topics: Animals; Antineoplastic Agents; Azetidines; Epigenesis, Genetic; Gamma Rays; Hemoglobins; Humans; Hypoxia; Mice; Models, Molecular; Neoplasms; Nitric Oxide; Nitric Oxide Donors; Nitric Oxide Synthase Type II; Nitro Compounds; Oxidation-Reduction; Oxygen; Protein Binding; Radiation-Sensitizing Agents

The hypoxic bone marrow (BM) microenvironment confers growth/survival and drug resistance in multiple myeloma (MM) cells. Novel therapies targeting the MM cell in its hypoxic BM milieu may overcome drug resistance. Recent studies led to the development of a novel molecule RRx-001 with hypoxia-selective epigenetic and nitric oxide-donating properties. Here, we demonstrate that RRx-001 decreases the viability of MM cell lines and primary patient cells, as well as overcomes drug resistance. RRx-001 inhibits MM cell growth in the presence of BM stromal cells. RRx-001-induced apoptosis is associated with: (i) activation of caspases; (ii) release of ROS and nitrogen species; (iii) induction of DNA damage via ATM/γ-H2AX; and (iv) decrease in DNA methyltransferase (DNMT) and global methylation. RNA interference study shows a predominant role of DNMT1 in MM cell survival versus DNMT3a or DNMT3b. The deubiquitylating enzyme USP7 stimulates DNMT1 activity, and conversely, USP7-siRNA reduced DNMT1 activity and decreased MM cell viability. RRx-001 plus USP7 inhibitor P5091 triggered synergistic anti-MM activity. MM xenograft studies show that RRx-001 is well tolerated, inhibits tumor growth and enhances survival. Combining RRx-001 with pomalidomide, bortezomib or SAHA induces synergistic anti-MM activity. Our results provide the rationale for translation of RRx-001, either alone or in combination, to clinical evaluation in MM.

Topics: Animals; Antineoplastic Agents; Apoptosis; Azetidines; Bortezomib; DNA (Cytosine-5-)-Methyltransferase 1; DNA (Cytosine-5-)-Methyltransferases; Drug Resistance, Neoplasm; Drug Synergism; Epigenomics; Heterografts; Humans; Hypoxia; Mice; Multiple Myeloma; Nitro Compounds; Thalidomide; Ubiquitin Thiolesterase; Ubiquitin-Specific Peptidase 7