Experimental studies of potential analgesics for the treatment of chemotherapy-evoked painful peripheral neuropathies

OBJECTIVE

We investigated potential analgesics for chemotherapy-evoked neuropathic pain using rats treated with paclitaxel.

DESIGN

Drugs were tested in a repeated dosing paradigm (four daily injections). Topiramate was tested with a long-term treatment paradigm (12 days). A literature search was performed to summarize prior data.

MEASURES

Mechanical stimulation of the hind paw was used to assay antiallodynic and antihyperalgesic effects acutely and 24 hours after injection.

RESULTS

Amitriptyline produced significant analgesia, but this was not apparent until after the second injection. Baclofen produced significant effects, but the response varied erratically. Mexiletine and NMED-126 (a mixed N- and T-type calcium channel blocker) produced consistent, significant analgesia when tested acutely, but the pain relief did not persist at 24 hours postinjection. Oxcarbazepine had no effect at any time. Tramadol produced consistent, near-complete analgesia when tested acutely, but the analgesia did not persist to 24 hours postinjection. Topiramate produced significant effects that were first evident after 6-8 days of dosing.

CONCLUSIONS

The present data and data from the literature review suggest that there are several potential treatments for chemotherapy-evoked neuropathic pain. Nonsteroidal anti-inflammatory drugs have little or no efficacy. Opioids have an effect, but probably only with high doses. At least some antidepressants are analgesic in these conditions. Some, but clearly not all, anticonvulsants and sodium channel blockers have efficacy. Tramadol is a particularly promising candidate. Topiramate, acetyl-L-carnitine, carbamazepine, and venlafaxine may have protective or restorative effects. Clinical trials of these candidates are needed to advance the treatment of chemotherapy-evoked pain.

Chemotherapy-induced neuropathy

Peripheral neuropathy is a common dose-limiting toxicity of chemotherapy. Chemotherapy-induced peripheral neuropathy (CIPN) causes numerous debilitating symptoms, impairs functional capacity, and results in dose reductions or possible cessation of chemotherapy. Analgesic or neurotropic agents are only modestly effective in treating neuropathic symptoms. Animal and human studies into the pathogenesis of CIPN have demonstrated heterogeneity in the mechanism(s) of nerve injury caused by individual agents, which may partly explain the wide variation in the resultant symptoms. Development of optimal therapeutic measures to treat CIPN requires continued research into the pathophysiologic basis of nerve injury, identification of risk factors for individual patients, development of reproducible and easy-to-use measurement scales, and finally, the performance of appropriately designed clinical trials to evaluate potentially promising agents.