As the field of MS care turns its sights on addressing progressive disease, the need for more biomarkers of disease activity and therapeutic target engagement is perhaps greater than ever.
IN THE FIELD OF multiple sclerosis (MS) there has been an ongoing search for ideal biomarkers to aid in the diagnosis of MS, prognostication, evaluation for ongoing subclinical disease activity, assessment for evidence of progressive disease, determination of adequate treatment response, and guiding the choice of safe disease-modifying therapies (DMTs). A biomarker should, therefore, ideally correlate with disease activity, progression, and treatment effects. The utility of a biomarker is also determined by its cost effectiveness, safety, practicality, and availability in routine clinical practice. The value of a biomarker is its ability to predict, correlate, and serve as a surrogate for a clinical state or outcome. The relevance of reliable biomarkers has strong clinical implications and affects the timeliness of diagnosis; the selection of safe, personalized, and effective DMTs; and the characterization of the phenotypic profile of a patient with MS.
In this outline, we present a selection of different established and novel biomarkers that are expected to have increasing clinical applications in coming years.
The detection of oligoclonal immunoglobulin bands in the cerebrospinal fluid (CSF) by isoelectric focusing on the absence of corresponding bands in the serum indicates an intrathecal production of antibodies. As 2 or more isolated oligoclonal bands (OCBs) are present in more than 95% of persons with MS (PwMS),1 such presence of isolated OCB has been included in the 2017 modified McDonald diagnostic criteria as a substitute for the criterion of dissemination in time. Further, the absence of isolated CSF OCB has a high negative predictive value for MS.2,3 However, the presence of OCB is not specific to MS4 and can be found in a variety of inflammatory (autoimmune and paraneoplastic encephalitides, neurosarcoidosis, systemic lupus erythematosus, neurologic Behcet disease, neurologic Sjögren syndrome) and infectious (herpes encephalitis, neuroborreliosis, neurotuberculosis) central nervous system (CNS) diseases.3 High-positive results (≥10 isolated OCB in the CSF) also have a prognostic value as they have been shown to correlate with higher annualized clinical and radiographic relapse rates.5 The clear downside of OCB as a biomarker is that for clinicians to obtain CSF, patients need to undergo an invasive procedure.
Neurofilament light chains (NfLs) are neuronal cytoskeletal components involved in axonal transport and can be detected in the setting of axonal or neuronal damage. Although they are nonspecifically elevated in response to neuroaxonal damage, they are considered a promising surrogate marker for clinical and radiographic disease activity in MS. Even though their concentration is significantly lower in serum compared with CSF (approximately 1:200), NfLs can now be detected and quantified in the serum with the help of ultrasensitive single molecule arrays. Given a good correlation between serum and CSF levels, NfLs have significant potential as a serologic biomarker.6-8 High and increasing serum NfL levels have been shown to correspond to clinical (Expanded Disability Status Scale [EDSS]) and radiographic evidence of disease activity (number of gadolinium-enhancing lesions, new or enlarging T2/ fluid-attenuated inversion recovery [FLAIR] lesions, increased brain and spinal cord atrophy rates).9,10 They can serve as a marker of treatment response with a demonstrated decrease of NfL levels in response to treatment with a variety of DMTs and after autologous hematopoietic stem cell transplantation.7,11 Baseline NfL levels are also potential biomarkers of prognostication as PwMS with lower NfL baseline levels are less likely to develop disability (EDSS, ≥ 4) and transition to secondary progressive MS.12
Data from phase 3 trials in progressive disease show that high baseline NfL concentrations are associated with higher atrophy rates and disability worsening. Compared with placebo, a significant decrease in NfL has been shown with different DMTs in their respective phase 3 trials for primary progressive (ocrelizumab, fingolimod) and secondary progressive (natalizumab, siponimod) MS. This contrasts with the primary clinical end point, which was not reached with 2 of the trialed medications (natalizumab and fingolimod). This could put into question the clinical value of NfL levels at least as a tool of measurement of treatment response in progressive disease.13-17 Nonetheless, the utility of serum NfL as a biomarker appears high and with expected availability in different commercial laboratories, their use in clinical practice will likely become more frequent.
Another promising serological biomarker candidate is the astroglial cytoskeletal glial fibrillary acidic protein (GFAP) as a surrogate marker of reactive astrogliosis with good correlations of GFAP CSF and serum concentrations. Higher GFAP serum levels have been shown to be associated with disease severity, duration, and progressive disease course. In particular, GFAP may have potential as a disease severity marker in progressive disease.18-20
Ocular coherence tomography (OCT) is the most established and promising ocular biomarker in MS. Its clear advantage is its relative cost efficiency and availability, which makes it easy to obtain baseline and repeat assessments in the clinic setting. Retinal nerve fiber layer (RNFL) thinning has been repeatedly shown to be more severe in progressive compared with relapsing-remitting MS (RRMS),21-25 and in advanced stages of the disease.22 Furthermore, RNFL thinning correlates with clinical scores (EDSS, MS functional composite) as well as atrophy in magnetic resonance imaging (MRI),26,27 and is associated with an increased risk of disability worsening.28
Novel eye-tracking devices also could become a future biomarker as they can detect subclinical efferent dysfunction in both saccades and smooth pursuit that correlates to clinical scores (EDSS, timed 25-ft walk).29-31
T2/FLAIR hyperintense lesions and gadolinium-enhanced T1 lesions have been long recognized as an important biomarker for the diagnosis of MS and subsequent monitoring for ongoing disease activity. In the setting of a single clinical attack consistent of a typical MS syndrome, a diagnosis of clinically definite MS can be achieved by radiographic determination of dissemination in space (≥ 2 of the following specific areas of the CNS: periventricular, juxtacortical/cortical, infratentorial, and spinal cord) and time (coexistence of enhancing and nonenhancing lesions or interval development of a new lesion in MRI).2
The vein located in the center of MS lesions can frequently be determined by MRI, especially in periventricular lesions. Evidence is growing that this “central vein sign” has the potential to be a useful biomarker to distinguish MS lesions from other white matter lesions.32
Paramagnetic rim lesions are another finding that can help differentiate MS lesions from white matter lesions of a different etiology.33 Furthermore, they are a potential biomarker of chronic inflammation with decreased lesion volume shrinkage and increased interval development of T1 hypointensity. Histopathologically, they are the correlate of iron-laden inflammatory myeloid cells located at the rim of chronic demyelinated lesions.34
Spinal cord MRI can be an important prognostic tool as the presence of spinal cord lesions is associated with a higher MS risk in clinically isolated syndrome and a higher risk for development of MS-related disability.35,36 Furthermore, the presence of baseline spinal cord lesions is associated with later development of secondary progressive MS.37
MRI is a sensitive monitoring tool to evaluate for subclinical disease activity and adequate response to DMT. The goal of “no evidence of disease activity” is determined by the absence of 3 measures (NEDA-3): clinical relapses, clinical disability progression, and the radiographic absence of new T2 and/or gadolinium-enhanced T1 lesions. However, the absence of accelerated brain volume loss has been proposed as an important fourth criterion (NEDA-4).38 The determination of brain and spinal cord (mean upper cervical cord area) atrophy by segmentation tools has indeed become a common radiographic research metric for progressive MS trials. However, even though volumetric assessments are now commercially available, their utility in clinical practice is still severely limited due to significant noise and a high degree of variability across scanners and segmentation tools.
More than 20 DMTs are approved for use in different MS phenotypes, and these drugs come with a wide degree of variability in their mechanism of action. When considering DMT options, factors such as efficacy, tolerability, safety, and long-term outcomes are typically assessed. Biomarkers are often employed to determine the safety, biologic effects, and efficacy of DMTs prior to initiation and during therapy.
Natalizumab, a humanized antibody to α4 integrin, prevents the entry of lymphocytes into the CNS and was approved for the treatment of RRMS in 2004.39 Not long after its approval, an association with progressive multifocal leukoencephalopathy (PML), an opportunistic infection caused by the John Cunningham virus (JCV), emerged and led to the voluntary withdrawal of the drug from the market in 2005.40 After careful investigation of PML risk factors, the medication was reintroduced into the market with very clear guidelines to regulate its use. The presence of anti-JCV antibodies, treatment duration longer than 24 months, and prior exposure to immunosuppressive therapy have been identified as significant risk factors for the development of PML with the use of natalizumab and are used for risk stratification.40-42 The anti-JCV antibody index has emerged as a biomarker that reliably predicts the risk of natalizumab-associated PML. Hence it is used in clinical practice to guide treatment selection and monitored periodically during natalizumab therapy to mitigate the risk of PML. However, its utility in assessing PML risk with other DMTs such as fumarates, sphingosine-1 phosphate receptor modulators, and anti- CD20 agents is limited, and routine monitoring with these DMTs is overall not recommended.41,43
Certain DMTs have been associated with lymphopenia and the absolute lymphocyte count (ALC) serves as a safety biomarker during the use of these treatments. For instance, approximately 17% of patients treated with dimethyl fumarate (DMF) develop grade 2-3 lymphopenia.44 In PwMS treated with DMF, persistent lymphopenia has been found as a potential risk factor for PML.41,45,46 Therefore, ALC monitoring is necessary in patients receiving DMF to identify those at risk for complications. Other DMTs associated with lymphopenia include fingolimod and other sphingosine-1 phosphate receptor modulators, alemtuzumab, cladribine, and teriflunomide.47,48 With some DMTs, lymphopenia may be associated with complications, therefore the ALC is a reliable biomarker in risk stratification before and during therapy.
The CD19 lymphocyte count is a biomarker for treatment response in patients treated with B-lymphocyte depleting drugs such as ocrelizumab, rituximab, and ofatumumab, and this test can serve as a biomarker for individualized dosing strategies.49 Baseline CD19 count has been shown to predict B-lymphocyte repopulation in patients treated with ocrelizumab.50 Monitoring of the CD19 lymphocytes in these patients can aid in a more individualized dosing approach through determination of individual B-lymphocyte repopulation rates and detection of early repopulation.
Natalizumab, interferon β, and rituximab therapies have been associated with neutralizing antibodies (Nabs) in a subset of patients treated with these medications. The incidence of persistent antinatalizumab antibody is reported to be about 6% and associated with suboptimal clinical response and persistent infusion reactions.51 Neutralizing antibodies against interferon β have also been recognized as a factor that leads to poor efficacy.52 Evaluating for Nabs in patients treated with these DMTs may be relevant, as this test serves as a biomarker for efficacy and safety.
Biomarkers have strong clinical significance and are relevant in the diagnosis, prognostication, treatment selection and response, and safety monitoring in MS management. It is critically important for clinicians to understand how to utilize the currently available biomarkers to enhance clinical care and optimize the long-term outcomes of PwMS. Further research is ongoing to discover and develop novel biomarkers in multiple sclerosis.