Abstract
Neurofilaments are the main components of the axonal cytoskeleton, and these proteins are exclusive products of neuronal cells. Hence, their release into the extracellular space, and eventually into cerebrospinal fluid (CSF) and blood, is a direct measure of neuronal injury. 1
A number of studies have shown that neurofilament light chain (NfL) is increased in CSF in almost all neurological diseases that involve processes leading to neuronal degeneration, including multiple sclerosis (MS), and that levels correlate with the speed of this process. This means that NfL is not specific for the aetiology of disease, but sensitive and specific for its common outcome, neuronal damage.
In MS, CSF NfL was associated with a triad of key clinical features: (a) acute disease activity, (b) therapy response and (c) prediction for the course of disability progression. 1 For the first time, there was a perspective to measure disease activity in MS and to monitor therapy response in real-time fashion. The enthusiasm created led to the prediction of ‘resuscitation of the spinal tap’, 2 and a controversy was held in this journal in 2015 titled ‘The only certain measure of the effectiveness of multiple sclerosis drugs is cerebrospinal fluid level’ [of NfL]. 3 The that time opponents to this statement rightfully raised the need of two or more lumbar punctures as a major hurdle for a translation into standard clinical practice, as ‘monitoring’ inevitably implies repeat measurements, for example, before and after an intervention to quantitate its effect, here efficacy of a therapy on MS disease activity. Given the reluctance to accept CSF analysis as a diagnostic tool for MS in certain geographies, the perspective of even two lumbar punctures was prohibitive for a routine use of NfL in clinical practice.
The breakthrough came based on two crucial development steps. The advent of a next generation sandwich immunoassay platform, single molecule array (SIMOA) (https://www.quanterix.com), lowered the detection threshold by a factor of 125 and 25 vis-à-vis enzyme-linked immunosorbent assay (ELISA) or electrochemiluminescence, respectively. 4 The consequence was that NfL concentrations that are typically 50–100 times lower in serum and plasma compared to CSF could be reliably quantified down to physiological levels in blood-derived samples. Second, the establishment of a close relationship of CSF and serum/plasma levels allowed to use serum NfL (sNfL) levels as a proxy of processes within the central nervous system (CNS). 5
Meanwhile, based on the SIMOA technology, the triad of diagnostic signals NfL reflects has been confirmed in a series of retrospective analysis of phase 3 studies (e.g. FREEDOMS, TRANSFORMS, ASCEND, CARE-MS 1, EXPAND, INFORMS) and patient registries in plasma or serum. 6 Serum or plasma NfL has now become a standard for ‘measuring MS’ on the group level. Several additional aspects of monitoring have only recently emerged:
sNfL appears to be a sensitive indicator of suboptimal drug response 7 in cohorts. As a direct consequence, the discriminative capacity of sNfL for the degree of drug response could be used as an endpoint in phase 2 trials for specific features of ‘progression’, that is, on an aspect of MS where currently no trial paradigm is established. 8
Monitoring sNfL levels in individual patients was demonstrated recently to correlate with symptoms of activation of disease where routine clinical and magnetic resonance imaging (MRI) assessment delivered, in hindsight, false negative results. 9 This may have immediate consequences for our approach to initiate or escalate therapy with disease-modifying therapies (DMTs).
While until recently monitoring of sNfL was seen only for the short-term aspect of disease activity and drug response, a further indication for monitoring sNfL is to support the diagnosis of progressive multifocal leukoencephalopathy (PML). This severe side effect of DMTs for MS leads to strong and rapid increase of sNfL prior to onset of clinical symptoms, beyond levels seen in acute MS exacerbation; the increase seems to be highly correlated with the PML lesion volume. 10 Larger cohorts may be needed to further substantiate the case for sNfL as a safety biomarker for early identification of PML.
Our YES for the use of sNfL to monitor MS evolution for acute and long-term (disability) course and drug response comes with several caveats:
sNfL shows strong age-dependency with increase in adulthood, while the physiological levels in childhood, and specifically early childhood, are still not well defined.
Comorbidities of internal medicine (e.g. atrial fibrillation) or neurological (e.g. Alzheimer’s disease) nature correlate with an increase of sNfL. On the contrary, while there is clear distinction of sNfL levels between MS and controls, and between different stages of MS on the group level, the range of levels among individual patients shows relevant overlap. 5 Hence, the interpretation of individual sNfL values to monitor MS in a routine diagnostic setting can only be valid when these confounding factors are well controlled, and levels are interpreted in relation to age-matched physiological values.
An effort to establish a large normative database of age-related reference values, and to quantify the impact of comorbidities, specifically also in the typical age-range of progressive MS, is prerequisite to establish sNfL as a standard biomarker of MS in clinical practice. Furthermore, commutability of values across the different assay protocols and platforms needs to be established in order to be on common grounds with what is a ‘normal’ versus ‘pathologic’ sNfL value.
The excitement around the current successes to establish sNfL as a biomarker for MS and other neurological diseases has led to the expectation by some observers that sNfL may replace MRI as standard biomarker in MS. MRI is in fact an expensive biomarker and, compared to sNfL, is retrospective for structural damage that had already occurred, and is in clinical routine restricted to brain pathology. Instead, sNfL measures neuronal damage real-time and comprehensively, that is, includes spinal cord pathology. However, we believe that when sNfL will be established as a validated test for individual monitoring in a few years time, it will not be able to replace MRI for its unique diagnostic profile provided for MS patients. We, therefore, foresee a complementary use of imaging (MRI) and biofluid (sNfL, and potentially others) markers in view of translation from precision medicine to personalised medicine in MS.
In conclusion, there is a convincing conceptual basis for the use of sNfL in MS as the first blood-based monitoring biomarker to quantify the current state of disease, its long-term outcome, treatment efficacy, and lately for treatment-specific safety. The establishment of normal reference values and a broader technical-analytical validation is work in progress and a prerequisite for routine use in individual patients in the coming years.
Footnotes
Declaration of Conflicting Interests
The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: D.L. has been the Therapeutic Area Head at Novartis, Neuroscience Development Unit, until January 2019. He has received personal compensation for consulting and speaking, and travel reimbursement from Quanterix, Orion and Sanofi.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.