Evaluating the effects of recombinant human bone morphogenetic protein-2 on pain-associated behaviors in a rat model following implantation near the sciatic nerve.

OBJECTIVE It has been hypothesized that the recombinant human bone morphogenetic protein-2 (rhBMP-2) amplification of the host inflammatory response interacts with nerves in the spine and contributes to the occurrence of new, postoperative complaints of radiculitis. This in vivo rat study was conducted to assess the capacity for rhBMP-2/ACS (rhBMP-2 applied to absorbable collagen sponge [ACS]) to stimulate pain-associated behaviors in the rat chronic constriction injury (CCI) model. METHODS Rats were randomly assigned to one of 14 treatment groups. Half of the animals underwent a sham procedure in which the left sciatic nerve was exposed and manipulated but no ligature was placed (Sham cohort), while the remaining animals had chromic gut sutures tied around the sciatic nerve to induce CCI (CCI cohort). The following test articles were applied to the sciatic nerve in each cohort: saline alone, saline applied to ACS, 0.1 mg/ml rhBMP-2 applied to ACS, or 1.0 mg/ml rhBMP-2 applied to ACS. The ACS was either wrapped around the sciatic nerve or implanted adjacent to the nerve. Thermal withdrawal latency was assessed on Days 7, 14, 21, and 28 postoperatively. Isolated nerves from selected rats in each group were examined and assessed for histopathological changes on Days 3, 7, 14, and 28. RESULTS CCI produced a significant pain behavioral response for all treatment groups at all time points. In the Sham cohort, 0.1 mg/ml rhBMP-2/ACS wrapped around the nerve (WRP) decreased thermal withdrawal on Day 28, and 1.0 mg/ml rhBMP-2/ACS placed adjacent to the nerve (ADJ) decreased thermal withdrawal on Days 21 and 28. Conversely, in the CCI cohort, 0.1 mg/ml rhBMP-2/ACS ADJ increased thermal withdrawal latencies on Day 7; 1.0 mg/ml rhBMP-2/ACS ADJ increased thermal withdrawal latencies on Day 7; and 1.0 mg/ml rhBMP-2/ACS WRP increased thermal withdrawal on Days 7 and 14. Histologically, the effect of rhBMP-2 on nerve inflammation was unclear, as inflammatory cell infiltration was similar in the rhBMP-2/ACS and saline/ACS groups. rhBMP-2 was variably associated with bone formation within the epineurium at 14 days, and more prevalently at 28 days, with no clear relationship between dose or ACS positioning. CONCLUSIONS In this study, rhBMP-2/ACS did not appear to induce pain independent of grossly visible ectopic bone formation. At the earliest time points, rhBMP-2 appeared to have a neuroprotective effect as evidenced by decreased pain exhibited by the rhBMP-2-treated animals in the CCI cohort, but this effect diminished over time, and by Day 28, the pain behavioral responses in the rhBMP-2-treated group were comparable to those in the group in which saline was applied to the nerve. In the Sham cohort, there was a dose-independent induction of pain at later time points, presumably due to new bone formation mechanically irritating the nerve. Histological examination revealed nerve lesions that appeared to be caused by mechanical trauma associated with surgical manipulation of the nerve during placement of the ACS and/or CCI sutures.

Tissue distribution of clonidine following intraforaminal implantation of biodegradable pellets: potential alternative to epidural steroid for radiculopathy.

BACKGROUND: Epidural steroid injections have shown efficacy in short-term pain relief, but often require repeated injections in order to provide continued pain relief. It has been suggested that a continuous, locally administered dose of an anti-inflammatory compound may provide sustained pain relief at doses lower than those needed with injections. OBJECTIVE: To evaluate the distribution of clonidine after transforaminal placement of a biodegradable drug delivery depot system. STUDY DESIGN: A preclinical animal study. METHODS: A biodegradable polymer drug depot designed to provide sustained delivery of clonidine was placed in or near a single lumbar neural foramen in 12 farm pigs. Clonidine tissue concentrations were measured at the implant site and at incremental distances from the implant over a time period of 12 weeks. Plasma clonidine levels were measured at 4 hours postimplantation on days 1, 2, 3, 5, and 7, and then weekly until the termination of the study. RESULTS: Clonidine was detectable up to 6 cm away from the drug depot. The highest concentrations of clonidine were present within the targeted spinal nerve; the concentration decreased with increasing distance from the depot. Clonidine was undetectable in plasma from all animals at all time points. LIMITATIONS: While clonidine was detected up to 6 cm from the drug depot, it is unknown if the drug concentration has clinical relevance. CONCLUSIONS: The results indicate that a biodegradable depot designed to be placed in a specific location to provide local sustained release of an anti-inflammatory and analgesic drug may be a feasible new approach to treat radicular pain associated with intervertebral disc pathology and other spinal conditions.

Effect of etanercept, a tumor necrosis factor-alpha inhibitor, on neuropathic pain in the rat chronic constriction injury model.

STUDY DESIGN: The effects of a low, local dose of a tumor necrosis factor-alpha (TNF-alpha) inhibitor on neuropathic pain behaviors in a rat chronic constriction injury model were evaluated. OBJECTIVE: To investigate whether a peripherally implanted polymer drug depot can deliver a dose of etanercept sufficient to reduce thermal hyperalgesia and mechanical allodynia in a rat model of neuropathic pain. SUMMARY OF BACKGROUND DATA: TNF-alpha inhibitors reduce pain-associated behavior in experimental models of neuropathic pain. Moreover, systemic injections of TNF-alpha inhibitors have suggested some efficacy in treating sciatic pain in limited, off-label clinical studies. Improvements in these results may be obtained by optimal dosing via targeted, sustained delivery at the site of disc-induced inflammation. METHODS: Unilateral chronic constriction injury was applied to the sciatic nerve of 56 male, Wistar rats. Four groups of animals (n = 7) received 0.5 mL phosphate-buffered saline every 3 days, 0.3 or 3 mg/kg etanercept every 3 days, or gabapentin (60 mg/kg) 1 hour before each behavioral test all via subcutaneous injection. Two groups of animals received 1.5 or 3.0 microg/h etanercept delivered by poly(lactic-co-glycolic acid) (PLGA) millicylinders (1 mm diameter x 10 mm long) implanted near the injured sciatic nerve. One group received a PLGA millicylinder implanted near the injured sciatic nerve. The final group received 3.0 microg/h etanercept via PLGA millicylinder implanted next to the uninjured, contralateral sciatic nerve. RESULTS: A low, local dose of etanercept (approximately 3 microg/h) delivered by a polymer depot significantly reduced (P < 0.05) thermal hyperalgesia for 57 days as compared to polymer depot without drug or an etanercept-loaded depot implanted near the contralateral sciatic nerve, and equivalent to a 10-fold higher dose delivered by repeat subcutaneous injection. CONCLUSION: This preclinical study indicates that delivering TNF-alpha inhibitors by means of a locally administered polymeric formulation provides long-lasting analgesia in an inflammatory neuropathic pain model.

Effect on bone induction of using contrast media to reconstitute recombinant human bone morphogenetic protein-2 in an ectopic model in rats.

OBJECT: In this study the authors tested the osteoinductive potential of recombinant human bone morphogenetic protein-2 (rhBMP-2) when combined with each of three commercially available contrast media (Conray, Omniscan, and Optiray). METHODS: Initial in vitro and cadaver tests verified the feasibility of using contrast media to visualize absorbable collagen sponge implants containing rhBMP-2 on fluoroscopic radiographic images. For the feasibility studies, lyophilized rhBMP-2 was prepared for injection by reconstitution with contrast media instead of sterile water. For the in vivo study, samples of an rhBMP-2 stock solution were diluted to 0.1 mg/ml by using three contrast media. In each sample, the final solution consisted of 97% contrast medium by volume. Recombinant human bone morphogenetic protein-2 diluted with sterile water for injection was used as a positive control. The rhBMP-2 solutions were applied to 0.5-cm3 collagen sponges and implanted subcutaneously on the thoracic cavity of athymic rats. At 4 weeks, the rats were killed, and the implants were removed. The explants were graded for degree of bone formation by using manual palpation and radiographic and histological assessments. CONCLUSIONS: By all methods of evaluation used, rhBMP-2 diluted with Omniscan was equivalent to rhBMP-2 diluted with sterile water in inducing bone formation. Both Conray and Optiray were shown to inhibit the osteoinductive potential of rhBMP-2.