Cancer-induced bone pain: Mechanisms and models

Abstract

Cancerous cells can originate in a number of different tissues such as prostate, breast and lung, but often go undetected and are non-painful. Many types of cancers have a propensity to metastasize to the bone microenvironment first. Tumor burden within the bone causes excruciating breakthrough pain with properties of ongoing pain that is inadequately managed with current analgesics. Part of this failure is due to the poor understanding of the etiology of cancer pain. Animal models of cancer-induced bone pain (CIBP) have revealed that the neurochemistry of cancer has features distinctive from other chronic pain states. For example, preclinical models of metastatic cancer often result in the positive modulation of neurotrophins, such as NGF and BDNF, that can lead to nociceptive sensitization. Preclinical cancer models also demonstrate nociceptive neuronal expression of acid-sensing receptors, such as ASIC1 and TRPV1, which respond to cancer-induced acidity within the bone. CIBP is correlated with a significant increase in pro-inflammatory mediators acting peripherally and centrally, contributing to neuronal hypersensitive states. Finally, cancer cells generate high levels of oxidative molecules that are thought to increase extracellular glutamate concentrations, thus activating primary afferent neurons. Knowledge of the unique neuro-molecular profile of cancer pain will ultimately lead to the development of novel and superior therapeutics for CIBP.

Keywords: ASIC; Cytokines; Glutamate; NGF; Oxidative Stress; Syngenic tumor model.

Figure 1

Diagram of a nociceptive fiber innervating the bone microenvironment demonstrating a number of nociceptive stimuli that act via their receptors that may cause bone cancer pain. Preclinical models have demonstrated that the nociceptive stimuli are released from either the tumor cells themselves or from a number of different tumor-associated immune cells within the bone microenvironment. An addition source of protons and proteases includes the osteoclasts. Preclinical models predict that such nociceptive stimuli may act directly to cause pain via ion channels on the nociceptive fibers or may act via second messengers resulting in the sensitization and increased excitability of the nociceptive fiber within and around the bone. (H+ = proton; NO = nitric oxide; O2⁻ = superoxide; ONOO = peroxynitrate, LPA = lysophosphatidic acid; Glu = glutamate; MCP-1 = monocyte chemottractant protein-1; MIP-1a = macrophage inflammatory protein-1α; IL-6 = interleukin-6; TNF α = tumor necrosis/factor- α; IL-1β = interleukin-1β; TGF-β = transforming growth factor-β; NGF = nerve growth factor; BDNF = brain-derived neurotrophic growth factor; NT-3 = neurotrophin-3; ASIC1 – acid sensing ion channel-1; ASIC3 = acid sensing ion channel-3; TRPV1 = transient receptor potential cation channel-subfamilyV1; NMDAR = N-methyl-D-aspartate receptor; AMPAR = α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor; TrkA, B, & C = tyrosine kinase receptors A, B & C)(figure made by AM Symons-Liguori).