The ingrowth of the pain-mediating nerve surrounding the lumbar vertebral body end plate or annulus fibrosus of the intervertebral disc into the vertebral body end plate and intervertebral disc (nerve ingrowth) is one of the causes of human intervertebral disc back pain [1]. Several cytokines including the neurotrophic factor (nerve growth factor and tumor necrosis factor [TNF-α]) influence the nerve ingrowth. Above all, TNF-α is one of the factors affecting human intervertebral disc back pain, because TNF-α induces inflammation in the degenerative intervertebral disc and promotes the ingrowth of the pain sensory nerve into the intervertebral disc [2]. Multiple subcutaneous injections of etanercept, a TNF-α inhibitor, are shown to be efficacious for intervertebral back pain, as reported in Europe and the United States. However, the efficacy of etanercept is currently controversial [34]. It is thought that the subcutaneous injection of etanercept is inefficacious because its local drug concentration in the intervertebral disc is low [5].

Considering this, we examined the efficacy of a single dose of TNF-α inhibitor (100 µg of etanercept) that was directly injected into the rat degenerative intervertebral disc. We previously observed a significant increase in the pain-related peptide level in the nerves of rat degenerative intervertebral discs compared with normal discs. In addition, etanercept administration could suppress the increase in the peptide level. We thus reported that a single dose of TNF-α inhibitor with direct injection into the rat degenerative intervertebral disc was efficacious [6]. In our facility, based on the results from these experiments, the TNF-α inhibitor was directly administered as a single dose to patients with intervertebral disc back pain after approval from our clinical study department. We reported that the injection of 10 mg of etanercept into the intervertebral disc was efficacious for human intervertebral disc back pain [7]. However, the appropriate dose of etanercept has not been determined in animal or clinical studies. In the present study, we aimed to determine the optimal dose of a single and direct injection of the TNF-α inhibitor etanercept by using the rat model of degenerative intervertebral disc from injury.

All protocols for the animal procedures were approved by the Ethics Committees of our institutions following the National Institutes of Health Guidelines for the Care and Use of Laboratory Animals (1996 revision).

The proportion of FG-labeled CGRP-immunoreactive neurons per total FG-labeled neurons from the L1 to L6 level was 43.1%±6.9% in the nonpunctured sham control group, 57.1%±9.1% in the puncture+saline group, 45.3%±9.7% in the puncture+10-µg etanercept group, 38.3%±4.1% in the puncture+100-µg etanercept group, and 26.6%±3.2% in the puncture+1,000-µg etanercept group (Figs. 1, 2). The proportion of CGRP-immunoreactive neurons was significantly higher in the puncture+saline group compared with the nonpuncture group (p<0.05). There were no significant differences between the puncture+saline group and the puncture+10-µg etanercept group (p>0.05). However, a significant decrease in the percentage of FG and CGRP double-positive cells in FG-positive cells was observed in the etanercept (100 µg and 1,000 µg)-administered groups in a dose-dependent manner (p<0.05).

In the present study, we observed an increase in the pain-related peptide level in the rat disc puncture model. In addition, when 10 µg of etanercept was directly administered as a single dose to the rat intervertebral disc, no suppressive effect on the pain-related peptide expression was observed in the administered group, indicating that a low dose of etanercept was inefficacious. This is consistent with the results previously reported by Cohen et al. [5]. They reported that a single dose of etanercept (0.1–1.5 mg) with direct injection into the intervertebral disc of patients with chronic intervertebral disc pain was inefficacious [5]. On the other hand, the pain-related peptide expression was suppressed in the etanercept (100 µg and 1,000 µg)-administered groups in a dose-dependent manner. It was reported that the subcutaneous injection of etanercept reduced the disease activity of patients with active rheumatoid arthritis (RA) in a dose-dependent manner [10]. However, the subcutaneous injection of 50 mg of etanercept to RA patients results in a significantly higher risk for adverse events compared to the 25 mg injection. For instance, upper respiratory infection particularly develops 4 times more often for etanercept treated RA patients. Therefore, we think that such a simple subcutaneous dose escalation is unfavorable from a viewpoint considering the possibility of complications [11]. Based on these results, a direct and single administration of a high dose of etanercept at 25 mg and 2.5 times as high as the previous clinical trial study into the intervertebral disc of patients with chronic intervertebral disc pain is currently being tested. The results so far suggest a higher efficacy of etanercept. In addition, we will carefully observe the adverse events such as infections that are associated with the increase in the dose of etanercept.

When a low dose of the TNF-α inhibitor (10 µg of etanercept) was directly administered to the rat intervertebral disc in the rat model of degenerative intervertebral disc from injury, no suppressive effect on the pain-related peptide expression was observed. However, when a higher dose of etanercept (100 µg and 1,000 µg) was administered, the pain-related peptide expression was suppressed in a dose-dependent manner. Based on these results, in our facility, a direct and single administration of a high dose of etanercept (25 mg, 2.5 times as high as the previous clinical trial study) is currently being tested after approval from our clinical study department.