Abstract
Multiple sclerosis is the major demyelinating autoimmune disease of the central nervous system. Relapsing MS can be treated by a number of approved monoclonal antibodies that currently target: CD20, CD25 (withdrawn), CD49d and CD52. These all target potentially pathogenic memory B cell subsets and perhaps functionally inhibit pathogenic T cell function. These consist of chimeric, humanized and fully human antibodies. However, despite humanization it is evident that all of these monoclonal antibodies can induce binding and neutralizing antibodies ranging from
Introduction to multiple sclerosis
Multiple sclerosis (MS) is a major disease of the central nervous system that affects about 3 million people worldwide, including over 1 million within the United States and 130,000 people within the United Kingdom [1]. Disease is associated with blood-brain barrier dysfunction, mononuclear cell infiltration of the central nervous system (CNS) and the development of demyelination [2]. Disease is often associated with recurrent neurological attacks and the development of neuronal damage leading to progressive worsening of disability and significant loss of quality of life [2]. Although, there has been much focus on targeting T cells to control disease, all effective treatments seem to target an immune subset within the memory B cell pool [2, 3, 4]. Indeed, there are an increasing number of treatments for disease, many of them high-cost biologics [5], which can be effective at inhibiting relapsing attacks, particularly if initiated early following diagnosis [2, 6].
Therapeutic antibodies used in multiple sclerosis. Relapsing disease in MS occurs following entry of mononuclear cells into the central nervous system. This causes demyelination and nerve loss, leading to aberrant neurotransmission and clinical signs. This seems to relate of memory B cells accumulation into the brain/spinal cord, where they may activate damaging T cells and glia. These may also form ectopic B cell follicles that can produce intrathecal, oligoclonal antibodies, which can trigger progressive glial cell-induced neurodegeneration within the damaged nerve environment. These are targeted by fully human immunoglobulin or humanized or chimeric antibodies that are generated from murine (rat/mouse) clones. These IgG1 depleting antibodies target either CD20 or CD52 and the IgG4 blocking antibody targets CD49d to inhibit mononuclear cell migration in to the inflamed brain. Created with 
However, it was soon recognized that anti-drug antibodies (ADA) could inhibit treatment efficacy of biologic therapies [7, 8]. Mononuclear cell depleting and non-depleting monoclonal antibodies (mAb) are amongst the most potent treatments to inhibit relapsing and active progressive MS (Fig. 1) [2]. However, these have the potential to produce ADA that can impede function [9, 10]. This was clearly seen with original rodent antibodies and prompted the development of a variety of chimeric antibodies that consisted of rodent variable immunoglobulin heavy and light chain for antigen/target binding and the constant regions of human immunoglobulins [11]. This was developed further such that humanised antibodies only contained the rodent complementarity determining regions (CDRs) and human framework elements of the immunoglobulin (Fig. 1). This and the development of fully human antibodies were designed to reduce immunogenicity further (Fig. 1) [11, 12]. However, despite this, humanised and even human immunoglobulin-based therapeutics are to some extent immunogenic [12, 13, 14, 15]. As such antibodies used in the treatment of MS have some level of immunogenicity (Table 1). This not only depends on the degree of humanization, but many other factors including dose, route and target [12, 16].
ADA induced by antibody treatments of MS
ADA induced by antibody treatments of MS
Antibodies used in multiple sclerosis, their targets and range of anti-drug antibody frequency reported in clinical studies.
Natalizumab
Natalizumab was the first approved therapeutic monoclonal antibody for MS [2, 32]. This humanised IgG4 antibody (AN100226), was derived from a mouse mAb (AN100 226m) and blocks the binding of alpha 4 integrin (CD49d) to vascular cell adhesion molecule (CD105) on inflamed blood vessels within the CNS, during multiple sclerosis (Fig. 1) [39]. This prevents the entry of mononuclear cells into the CNS and prevents relapsing MS [32, 39]. This was originally administered as an intravenous 300 mg infusion monthly and is highly effective at disease control [32]. However, this is sometimes associated with the development of progressive multifocal leucoencephalopathy due to immunosuppression, which is associated with infection and destruction of oligodendrocytes by the John Cunningham (JC) virus [2, 40]. This prompted a risk mitigation scheme based on JC infection, titre and past history of DMT use leading to switching often within 24 months in JC-infected individuals [2, 41]. Natalizumab induces ADA that have been associated with anaphylactoid, infection reactions and treatment failure [32, 42, 43] (Table 1). The occurrence of natalizumab ADA varies between studies reported ranging from 4–14% [32, 33, 34]. but even up to 58% within 6 months of treatment onset [35]. The majority of natalizumab ADA bind to the CDR within the paratopes and have neutralizing potential, particularly those with slow dissociation rates [42, 44, 45]. Whilst these appear to decrease with time, but some people develop persistent ADA [34, 46, 47]. These can be associated with re-expression of functional surface CD49d and disease activity [42, 48, 49]. The presence of ADA was inversely correlated with serum natalizumab concentration and high antibody titres were associated with very low or undetectable serum natalizumab concentration and gadolinium-enhancing lesions and relapses [34, 35, 42]. Natalizumab has a half-life of about 16 days [41, 50] and following cessation of natalizumab therapy and loss of blockade of cell-surface alpha4, beta one integrin and cell trafficking, rebound disease (exaggerated relapse) can develop within about 12 weeks [51]. This can be prevented by switching to CD20 and CD52-depleting antibodies [52], indicating that these both target the central drivers of MS pathology. These agents both induce long-term depletion of memory B cells [3, 26] consistent with other high-efficacy treatments [37, 53].
Alemtuzumab
Alemtuzumab was the first humanised antibody and the CDRs from the rat IgG2a CAMPATH-1 clone were grafted onto human IgG1 [54]. This was developed to reduce immunogenicity associated with the rat IgG2b CAMPATH-1G [54]. Although initially developed for cancer therapy, where ADA occurred in few people [15], a different lower dosing schedule was developed and approved for the treatment of MS [55, 56]. This is a CD52, T and B, lymphocyte-depleting agent administered as 60 mg mAb within five 12 mg daily treatments and a year later and in subsequent years if required 36 mg over three treatments that provides long-term therapy [56]. Alemtuzumab ADA have been down played and implied to be of low frequency (0.5% above a threshold) [36], and of minor significance on lymphocyte populations or efficacy [36, 55, 57]. However, despite depleting both CD4, CD8 and CD19 B cells by over 80–90% [37], the vast majority of people generate binding ADA within 1 month of infusion and they occur in over 85% of people within 3 months of treatment (Table 1) [37]. This probably relates to the low-dose, short half-life and range of tissue expression of CD52, including expression by antigen presenting macrophages and dendritic cells [9, 57]. It was evident that marked neutralization occurred, which was functionally significant and accumulated with time [10, 37, 38]. This could lead to treatment failure [10, 38].
CD20-depleting antibodies
The other major target for therapeutic mAb in MS is CD20 [2, 58]. This is expressed within the B cell lineage with the exception of early stem cells and plasmablasts or long-lived plasma cells [3]. Thus depleting antibodies can prevent formation of novel B cell responses, but does not remove established antibody responses [25, 59].
Rituximab
Rituximab is a chimeric CD20-depleting mAb derived from the variable region of mouse clone 2B8 and the constant human IgG1/
Ocrelizumab
Ocrelizumab is a humanised CD20-depleting antibody (Fig. 1) that was developed and licenced for relapsing and active progressive MS [24, 58] possibly in the anticipation of the patent expiration of rituximab. This is administered as a 600 mg six-monthly infusion [24]. The mouse IgG2b/
Ublituximab
Ublituximab is chimeric human IgG1 glycoengineered mAb with enhanced ADCC killing [58, 73]. This is highly-active in MS and is currently undergoing regulatory approval [74]. The level of ADA induction has yet to be reported.
Ofatumumab
Ofatumumab is a fully human CD20-depleting antibody (Fig. 1), which has been licenced for active-relapsing MS [28, 58]. Ofatumumab was generated by immunising human Ig transgenic mice to generate clone 2F2 [75]. This mAb induces binding ADA in less than 0.2–3% of individuals and essentially no neutralizing antibodies (Table 1) [27, 28]. In contrast to the other CD20 agents, dosing is low (20 mg), frequent (monthly) and subcutaneous injection, to allow for home administration and more rapid repopulation of B cells following cessation [27, 28], which may offer many advantages in the era of COVID-19.
Treatment of MS in the COVID-19 era
Subcutaneous agents to avoid hospital visits
With the development of the COVID-19 pandemic following SARS-CoV-2 coronavirus infection, there was initial concern about the use of high-efficacy immunosuppression [26, 76]. This led to changes in clinical behaviours and the avoidance of hospital-treatment visits such as through telemedicine [77, 78] treatment-delays and extended-interval dosing of treatments [76, 79, 80], more-rapid infusions [81] and focus on the value of subcutaneous administration [28]. Alemtuzumab has been used subcutaneously for many years in solid organ transplantation and has been used off-label in MS [82, 83, 84]. Importantly, subcutaneous ofatumumab, a human CD20 specific IgG1 antibody, injection was recently approved for home use in active MS [28, 58]. Likewise, studies with subcutaneous ocrelizumab (NCT0397230) are ongoing and monthly 300 mg subcutaneous natalizumab has also been recently approved within Europe [85]. These are reported to induce no or a low frequency of ADA [58, 85]. It will be important to determine if this is maintained in real world use.
Daclizumab
Daclizumab was a CD25 IgG1 specific humanised antibody that was licenced for relapsing MS and was used as a monthly (Q4W) 150 mg subcutaneous injection [30]. However, it was withdrawn by the manufacturers due to the occurrence of adverse effects, notably serious inflammatory brain disorders [86]. Daclizumab induced ADA in about 4–19% of individuals and about 3–8% of people developed NAbs and these were relatively transient [15, 29, 30, 31] (Table 1). The frequency of this was higher than intravenous formulations and doses used in transplantation [15]. Subcutaneous alemtuzumab was again slightly more immunogenic than the intravenous delivery in cancer indications [87]. This is perhaps not surprising as the subcutaneous route is a well-known sensitizing route harbouring and exposes the agent to antigen presenting, dendritic cells that prime immune responses [88, 89, 90]. As such in the small number of studies where subcutaneous and intravenous routes are directly compared, the subcutaneous route often, but not always induced more ADA than following intravenous delivery [15]. It remains to be seen if subcutaneous administration of antibodies induces more antibodies, these have yet to be found following subcutaneous administration of natalizumab [85]. and ofatumumab [28]. This may change as these agents are used in real-world settings.
Future prospects
With the advent of remote, home delivery of therapeutic antibodies it will be important to develop methods to remotely monitor ADA. Thankfully, this technology has been developed and validated using finger-prick monitoring of capillary blood to monitor SARS-CoV-2 antibodies following infection/vaccination and demonstrate a strong correlation with analysis of serum [91, 92, 93]. Antibodies are stable for months and can be remotely sampled anywhere there is a postal service. Although at the population level ADA are likely to be of limited significance, it is clear at the individual level that ADA are part of treatment failure [10]. If levels that predict future treatment failure can be found it will serve in decisions to continue or switch treatment and aid patient management.
Footnotes
Acknowledgments
ANA received a Peter Enzor/Sir John Zochonis intercalated bursary given by Worshipful Company of Tallow Chandlers and a Rod Flowers Vacation Scholarship, for which she is extremely grateful. AE and ASK are recipients of an award from the National Institute of Health Research, “Rapid, Sensitive Detection of Resistance to Therapeutic Antibodies” Product Development NIHR201645.
Conflict of interest
DB has nothing relevant to declare, but has received consultancy from InMune Bio, Lundbeck, Merck, Novartis, Roche and Teva within the past 3 years. OQ and ANA have nothing to declare. AE is CEO of Camstech Ltd an early stage company developing novel biochemical sensing technologies. ASK has filed patents related to technology used for ADA assays.