Mechanisms Underlying Cancer-Induced Bone Pain

Abstract

Pain from bone metastases is multifaceted with clinical descriptors including ongoing pain, hypersensitivity to external stimuli and intermittent episodes of breakthrough pain characterized as a sudden and abrupt onset of severe pain on a background of well-controlled pain. Moreover, cancer-induced bone pain remains inadequately managed due to a myriad of side effects associated with the current pain relieving regimens, which primarily rely on administration of opiates. Despite advances made in cancer therapeutics, these patients experience an inferior quality of life with incapacitating pain with limited daily activities. Development of long-term novel, non-opiate mechanism-based therapeutics with limited side effects is considered beneficial in elevating the patients' quality of life. First part of this dissertation encompasses the role of p38 MAPK in a mouse model of cancer-induced bone pain in which breast cancer cells were injected and sealed into the femur. Our data demonstrated that both acute and prolonged inhibition of p38 MAPK blocked cancer-induced spontaneous pain but had no effect on the evoked pain indicating important differences in mechanisms mediating ongoing pain as opposed to evoked pain. Undermanaged control of breakthrough pain is attributed to poor understanding of underlying mechanisms and how they may differ from ongoing pain due, in part, to lack of a pre-clinical model in which these mechanisms can be studied. We have established a rat model of cancer-induced bone pain to examine ongoing pain and pain relief using conditioned place preference paradigm as well as breakthrough pain using palpation-induced conditioned place aversion. We have shown that while peripheral afferent input from the tumor-bearing tibia mediates cancer-induced ongoing pain and initiation of breakthrough pain, it does not contribute to the maintenance of breakthrough pain. These data suggest that molecular targets mediating these two mechanisms may be different. This hypothesis was confirmed by our findings in this model that acute blockade of interleukin-6 blocked movement-evoked breakthrough pain in tumor-bearing rats, but failed to block tumor-induced ongoing pain. Hence, we provide a platform to manipulate treatments that can be given alone or in combination with opiates in such a way that patients receive adequate control of breakthrough pain.