Motor Evoked Potentials as a Biomarker in Alemtuzumab Treated Multiple Sclerosis Patients

Alemtuzumab pivotal studies have shown robust effect on relapse rate and MRI parametric. The effect on disease progression did not however reach statistical significance in CARE MS I Clinical trials, especially in the progressive forms of Multiple Sclerosis (MS) have been greatly hampered by the insensitivity of the Expanded Disability Status Scale (EDSS) especially at either ends of the scale. Other more recently introduced clinical scales such the timed 25 foot walk test (T25FWT), six minute walk test (6MWT) or the 9-hole peg test (9HPT) also lack in sensitivity and reliability. A validated, sensitive and reliable biomarker for disability progression or regression would greatly help clinical research in MS. Such a biomarker could also help in evaluating if and when patients would require retreatments with Alemtuzumab.

Motor evoked potentials (MEP's) measures the integrity and function of corticospinal tracts. The procedure uses an electromagnet to induce directly or via interneurons a small depolarizing current in the axon hillock of large pyramidal neurons in the motor cortex. A subsequent motor response can then be measured in the targeted limb muscle. The amplitude and latency of such response can be measured giving an indication of the conduction integrity of the corticospinal tracts (CST) from the cortex to the limb. MEP's are able to detect perhaps even before clinical measures, evidence of CST involvement and deterioration through increased central motor conduction time and/or reduced ratio of centrally elicited MEP amplitude to peripherally elicited compound motor amplitude potential (CMAP).

MEP's have been studied in a variety of pathologies involving CST's. MEP's have in patients with cervical spondylosis as well as with patients with amyotrophic lateral sclerosis, shown to being more sensitive than clinical examination in detecting CST involvement. MEP's have been shown in MS to correlate with EDSS scores. In studies looking at spinal cord injury patients MEP's were able to demonstrate an improvement in central motor conduction time (CMCT) induced by extended release fampridine.

The main critique of MEP's has been as in all electrophysiological tests one of high intra-patient variability which can be compounded by non-standardized techniques between centers. Newer MEP techniques using neuro-navigation, standardized facilitation techniques and choosing the best of three responses can significantly reduce the inherent variability of obtained values. We believe that with such techniques MEP's can become a useful surrogate biomarker in clinical trials of MS. The latter however needs to be validated in a prospective manner.