A rat model of bone cancer pain

This study describes the first known model of bone cancer pain in the rat. Sprague-Dawley rats receiving intra-tibial injections of syngeneic MRMT-1 rat mammary gland carcinoma cells developed behavioural signs indicative of pain, including: mechanical allodynia, difference of weight bearing between hind paws and mechanical hyperalgesia. The development of the bone tumour and structural damage to the bone was monitored by radiological analysis, quantitative measurement of mineral content and histology. Intra-tibial injections of 3 x 10(3) or 3 x 10(4) syngeneic MRMT-1 cells produced a rapidly expanding tumour within the boundaries of the tibia, causing severe remodelling of the bone. Radiographs showed extensive damage to the cortical bone and the trabeculae by day 10-14 after inoculation of 3 x 10(3) MRMT-1 cells, and by day 20, the damage was threatening the integrity of the tibial bone. While both mineral content and mineral density decreased significantly in the cancerous bone, osteoclast numbers in the peritumoural compact bone remained unchanged. However, tartarate-resistant acid phosphatase staining revealed a large number of polykariotic cells, resembling those of osteoclasts within the tumour. No tumour growth was observed after the injection of heat-killed MRMT-1 cells. Intra-tibial injections of 3 x 10(3) or 3 x 10(4) MRMT-1 cells, heat-killed cells or vehicle did not show changes in body weight and core temperature over 19-20 days. The general activity of animals after injection with live or heat-killed MRMT-1 cells was higher than that of the control group, however, the activity of the MRMT-1 treated group declined during the progress of the disease. Rats receiving intra-tibial injections of MRMT-1 cells displayed the gradual development of mechanical allodynia and mechanical hyperalgesia/reduced weight bearing on the affected limb, beginning on day 12-14 or 10-12 following injection of 3 x 10(3) or 3 x 10(4) cells, respectively. These symptoms were not observed in rats receiving heat-killed cells or vehicle. Behavioural data suggest a reasonable time window for evaluation of anti-nociceptive agents between day 14 and 20 after cancer cell inoculation in this model. Acute treatment with morphine (1-3mg/kg, subcutanously (s.c.)) produced a dose-dependent reduction in the response frequency of hind paw withdrawal to von Frey filament stimulation 17 or 19 days following intra-tibial injections of 3 x 10(3) MRMT-1 cells. A significant reduction in the difference in hind limb weight bearing was also observed. Acute treatment with celebrex (10-30 mg/kg, s.c.) did not affect mechanical allodynia or difference in weight bearing in rats 20 days following treatment with 3 x 10(3) MRMT-1 cells. Although the pathophysiology of cancer pain is largely unknown, significant enhancement of glial fibrillary acidic protein (GFAP) staining in the corresponding segments of the ipsilateral spinal cord highlights the possible involvement of astrocytes. In summary, the induction of bone cancer in the rat by the syngeneic MRMT-1 mammary tumour cell line provides a valid pre-clinical model for pain associated with bone metastases. Significant mechanical hyperalgesia and allodynia develops in association with the progression of the tumour in the bone marrow cavity, while the general condition of the animal remains satisfactory. While acute treatment with morphine has some analgesic effect on hind limb sparing the selective COX-2 inhibitor, celebrex, has no influence on the pain-related behavioural changes in this model.