Modulation of Rat Cancer-Induced Bone Pain is Independent of Spinal Microglia Activity.

The dissemination of cancer to bone can cause significant cancer-induced bone pain (CIBP), severely impairing the patient's quality of life. Several rodent models have been developed to explore the nociceptive mechanisms of CIBP, including intratibial inoculation of breast carcinoma cells in syngeneic Sprague Dawley rats. Using this model, we investigated whether resident spinal microglial cells are involved in the transmission and modulation of CIBP, a long-debated disease feature. Immunohistochemical staining of ionizing calcium-binding adaptor molecule 1 (Iba-1) and phosphorylated p38-mitogen-activated protein kinase (P-p38 MAPK) showed no spinal microglial reaction in cancer-bearing rats, independently of disease stage, sex, or carcinoma cell line. As a positive control, significant upregulation of both Iba-1 and P-p38 was observed in a rat model of neuropathic pain. Additionally, intrathecal administration of the microglial inhibitor minocycline did not ameliorate pain-like behaviors in cancer-bearing rats, in contrast to spinal morphine administration. Our results indicate that microglial reaction is not a main player in CIBP, adding to the debate that even within the same models of CIBP, significant variations are seen in disease features considered potential drug targets. We suggest that this heterogeneity may reflect the clinical landscape, underscoring the need for understanding the translational value of CIBP models.

Cancer-induced Bone Pain Impairs Burrowing Behaviour in Mouse and Rat.

BACKGROUND: Cancer-induced bone pain remains a serious public health concern, with a need for translational behavioural tests in order to assess nociception in preclinical models of this condition. Burrowing is an innate, ethologically relevant rodent behaviour that has been proven sensitive to chronic pain conditions. Herein, we studied for the first time whether burrowing performance is altered in preclinical models of cancer-induced bone pain. MATERIALS AND METHODS: Mice and rats were inoculated with syngeneic breast cancer cells. Bone degradation was radiographically evaluated and nociception was assessed in limb-use and burrowing tests. RESULTS: Cancer-bearing rodents showed reduced relative bone density and limb-use scores, confirming disease development. Burrowing performance decreased over time in both rodent models. CONCLUSION: Burrowing performance was reduced in both rodent models, indicating that the burrowing test is a relevant and reproducible behavioural test for assessing disease development in both mouse and rat models of cancer-induced bone pain.

Author Correction: The Src family kinase inhibitor dasatinib delays pain-related behaviour and conserves bone in a rat model of cancer-induced bone pain.

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Decitabine attenuates nociceptive behavior in a murine model of bone cancer pain.

Bone cancer metastasis is extremely painful and decreases the quality of life of the affected patients. Available pharmacological treatments are not able to sufficiently ameliorate the pain, and as patients with cancer are living longer, new treatments for pain management are needed. Decitabine (5-aza-2'-deoxycytidine), a DNA methyltransferases inhibitor, has analgesic properties in preclinical models of postsurgical and soft-tissue oral cancer pain by inducing an upregulation of endogenous opioids. In this study, we report that daily treatment with decitabine (2 µg/g, intraperitoneally) attenuated nociceptive behavior in the 4T1-luc2 mouse model of bone cancer pain. We hypothesized that the analgesic mechanism of decitabine involved activation of the endogenous opioid system through demethylation and reexpression of the transcriptionally silenced endothelin B receptor gene, Ednrb. Indeed, Ednrb was hypermethylated and transcriptionally silenced in the mouse model of bone cancer pain. We demonstrated that expression of Ednrb in the cancer cells lead to release of ß-endorphin in the cell supernatant, which reduced the number of responsive dorsal root ganglia neurons in an opioid-dependent manner. Our study supports a role of demethylating drugs, such as decitabine, as unique pharmacological agents targeting the pain in the cancer microenvironment.

Chronic high dose P2X7 receptor inhibition exacerbates cancer-induced bone pain.

The functional role of P2X7 receptor (P2X7R) inhibition in cancer-induced bone pain has been highly contradictory. Whereas knockout studies have suggested pro-nociceptive effects, pharmacological studies suggest anti-nociceptive or no effect. The discrepancy is likely linked to the highly polymorphic nature of the P2X7R and the related functional differences in different tissue and conditions. In this study we tested the analgesic potential of AFC5261, a selective P2X7R antagonist, in a rat model of cancer-induced bone pain to evaluate if the opposing pro- and anti-nociceptive effects could be a consequence of long vs. short term inhibition of the P2X7R. Following intratibial inoculation of MRMT-1 carcinoma cells, movement-evoked and background pain was assessed with the limb use and weight-bearing test, and the effect of acute and chronic AFC5261-treatement evaluated. Bone degradation and tumor progression was in addition evaluated with x-ray densitometry and bioluminescence, respectively. In an acute treatment regime, a single administration of 300 mg/kg AFC5261 had no effect on either weight-bearing or limb use deficits. In contrast, morphine significantly increased both the limb use and weight-bearing ratio. In a chronic treatment study, BID administration of 300 mg/kg AFC5261 exacerbated the pain-related behavior, demonstrated by an earlier onset of both limb use and weight-bearing deficits without affecting the overall bone degradation or tumor progression. In contrast, 50 mg/kg and 100 mg/kg AFC5261 had no effect on the pain-related behavior. Overall, the data suggest that whereas acute P2X7R inhibition has no effect on the pain-related behavior, chronic inhibition exacerbate the cancer-induced bone pain.

The Src family kinase inhibitor dasatinib delays pain-related behaviour and conserves bone in a rat model of cancer-induced bone pain.

Pain is a severe and debilitating complication of metastatic bone cancer. Current analgesics do not provide sufficient pain relief for all patients, creating a great need for new treatment options. The Src kinase, a non-receptor protein tyrosine kinase, is implicated in processes involved in cancer-induced bone pain, including cancer growth, osteoclastic bone degradation and nociceptive signalling. Here we investigate the role of dasatinib, an oral Src kinase family and Bcr-Abl tyrosine kinase inhibitor, in an animal model of cancer-induced bone pain. Daily administration of dasatinib (15 mg/kg, p.o.) from day 7 after inoculation of MRMT-1 mammary carcinoma cells significantly attenuated movement-evoked and non-evoked pain behaviour in cancer-bearing rats. Radiographic - and microcomputed tomographic analyses showed significantly higher relative bone density and considerably preserved bone micro-architecture in the dasatinib treated groups, suggesting a bone-preserving effect. This was supported by a significant reduction of serum TRACP 5b levels in cancer-bearing rats treated with 15 mg/kg dasatinib. Furthermore, immunoblotting of lumbar spinal segments showed an increased activation of Src but not the NMDA receptor subunit 2B. These findings support a role of dasatinib as a disease modifying drug in pain pathologies characterized by increased osteoclast activity, such as bone metastases.