Disease-Modifying Therapy

1. The Modern DMT Landscape

MS now has more than twenty licensed DMTs, more than any other neurological disease. They are conventionally grouped by route, mechanism, and approximate efficacy:

All licensed DMTs primarily target relapsing inflammation; only ocrelizumab is licensed for PPMS. Effects on brain atrophy and PIRA progression are smaller and require longer trials to detect.

2. Treatment Strategy — Escalation vs Early High-Efficacy

Two competing philosophies have organised MS prescribing for two decades:

• Escalation — start with platform DMT; switch to high-efficacy if breakthrough activity. Lower acute risk; patients accumulate damage during ineffective platform period.
• Early high-efficacy / induction — start with high-efficacy DMT (anti-CD20, natalizumab) at diagnosis. Higher initial risk profile; better long-term disability outcomes in observational cohorts.

Two large randomised trials are settling the question: TREAT-MS and DELIVER-MS, both reporting from 2024–2026. Observational evidence (Brown et al., JAMA 2019; Spelman et al., Neurology 2021) consistently favours early high-efficacy.

3. Injectables — Interferon-β and Glatiramer Acetate

Interferon-β-1a/1b were the first licensed DMTs in MS (IFNB MSSG, Neurology 1993; Jacobs et al., Ann Neurol 1996). They are pleiotropic immunomodulators:

• Shift T-cell cytokine balance from Th1/Th17 toward Th2; up-regulate IL-10.
• Reduce MMP-9-mediated BBB breakdown; reduce trafficking of activated lymphocytes.
• Avonex (IFN-β-1a IM weekly), Rebif (IFN-β-1a SC tiw), Betaseron / Extavia (IFN-β-1b SC qod), Plegridy (peg-IFN-β-1a SC q2wk).
• ARR reduction ~30%; effect on disability progression modest.
• Adverse effects: flu-like reactions, injection-site reactions, transaminitis, depression, leukopenia, thyroid dysfunction; neutralising antibodies in ~5–30%.

Glatiramer acetate (Copaxone) is a synthetic random copolymer of glutamic acid, lysine, alanine, tyrosine designed in 1967 by Sela and Arnon at the Weizmann Institute as a negative control for MBP-induced EAE — and unexpectedly suppressed the disease. Mechanism: induces Th2-biased glatiramer-specific T cells that cross the BBB and exert bystander suppression in the CNS. ARR reduction ~30%, comparable to IFN-β; benign safety profile (post-injection reaction the main issue). Daily 20 mg or 3×/week 40 mg SC.

4. Teriflunomide and the Fumarates

Teriflunomide (Aubagio) is the active metabolite of leflunomide; it inhibits dihydroorotate dehydrogenase (DHODH), blocking de novo pyrimidine synthesis in proliferating lymphocytes. Quiescent cells are unaffected (they use the salvage pathway). TEMSO and TOWER trials (O’Connoret al., NEJM 2011; Confavreux et al., Lancet Neurol 2014) showed ~31% ARR reduction. Long half-life (~2 weeks) requires cholestyramine washout for pregnancy.

Dimethyl fumarate (Tecfidera) activates the Nrf2 antioxidant pathway and has anti-inflammatory effects; the active metabolite is monomethyl fumarate. DEFINE and CONFIRM (Goldet al., NEJM 2012; Fox et al., NEJM 2012) showed ~50% ARR reduction. Adverse effects: flushing, GI symptoms, lymphopenia (~5% develop persistent CD4<500/µL); rare PML in patients with prolonged severe lymphopenia. Diroximel fumarate is a prodrug of monomethyl fumarate with reduced GI side effects.

5. S1P-Receptor Modulators — Fingolimod, Siponimod, Ozanimod, Ponesimod

Sphingosine-1-phosphate (S1P) gradients between blood (high) and lymph node (low) drive lymphocyte egress from lymphoid tissue via the S1P₁ receptor. S1P modulators bind S1P₁, induce its internalisation, and trap autoreactive lymphocytes in lymph nodes — a functional sequestration rather than depletion.

• Fingolimod (Gilenya) — first oral DMT (FREEDOMS, Kappos et al., NEJM 2010); ARR reduction ~54%. Non-selective (S1P_1,3,4,5); first-dose bradycardia mandates 6-h cardiac monitoring; macular oedema, hypertension, lymphopenia, herpes-virus reactivation.
• Siponimod (Mayzent) — selective S1P_1,5; first DMT licensed for active SPMS (EXPAND, Kappos et al., Lancet 2018; 21% reduction in 3-mo CDP). Requires CYP2C9 genotyping.
• Ozanimod (Zeposi) — selective S1P_1,5 (RADIANCE, Comi et al., Lancet Neurol 2019).
• Ponesimod (Ponvory) — selective S1P₁; OPTIMUM (Kappos et al., JAMA Neurol 2021) head-to-head vs teriflunomide showed superiority.
• Class effects: dose-titration, lymphopenia, transaminitis, hypertension, macular oedema, varicella reactivation; rare progressive multifocal leukoencephalopathy reported. Pregnancy contraindication and washout.

6. Cladribine — Pulse Immune Reconstitution

Cladribine (2-chlorodeoxyadenosine) is a purine analogue selectively cytotoxic to lymphocytes (high deoxycytidine kinase / low 5′-nucleotidase ratio). The MS regimen (Mavenclad) is two short oral courses 12 months apart, producing prolonged lymphocyte depletion followed by partial reconstitution — an «immune reset» strategy.

• CLARITY (Giovannoni et al., NEJM 2010) — ARR reduction ~58% over 96 weeks vs placebo.
• No ongoing dosing after year 2 in many patients; sustained efficacy 3–4 years post-treatment in subgroup analyses.
• Adverse effects: lymphopenia (predictable, dose-dependent), herpes-zoster reactivation, modest cancer signal historically (later attenuated in pooled safety analyses).

7. Natalizumab — the α4-Integrin Story and PML

Natalizumab (Tysabri) is a humanised IgG4 monoclonal against α4-integrin (VLA-4), preventing leukocyte transmigration across the BBB by blocking binding to VCAM-1. It was validated in EAE by Yednock et al. (Nature 1992) and licensed in 2004 on the basis of the AFFIRM trial (Polman et al., NEJM 2006): ~68% reduction in ARR, the largest single-trial effect ever reported.

• Standard regimen: 300 mg IV every 4 weeks; extended-interval dosing (every 6 weeks) reduces PML risk without loss of efficacy (NOVA trial, Foley et al., Lancet Neurol 2022).
• Subcutaneous formulation (Tyruko / Tysabri SC) approved in 2021.
• PML risk — progressive multifocal leukoencephalopathy due to JC virus reactivation in oligodendrocytes; the central safety concern.

8. Anti-CD20 — Rituximab, Ocrelizumab, Ofatumumab, Ublituximab

CD20 is a B-cell lineage marker expressed from pre-B through memory B cells (absent from plasma cells and stem cells). Anti-CD20 monoclonals deplete B cells via antibody-dependent cellular cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC) and direct apoptosis. The MS-licensed agents:

• ARR reductions ~46–50% vs IFN-β-1a or teriflunomide; superior MRI activity suppression.
• Ocrelizumab is the only DMT licensed for PPMS (ORATORIO).
• Adverse effects: infusion reactions (premedication), upper-respiratory infections, urinary tract infection, hypogammaglobulinaemia (cumulative, requires monitoring of IgG / IgM), opportunistic infections in long-term users.
• Hepatitis B reactivation possible; screen pre-treatment.
• Vaccine response is impaired post-CD20; complete vaccinations (including COVID-19, HPV) before initiation when possible.

9. Alemtuzumab — Anti-CD52 Immune Reconstitution

Alemtuzumab (Lemtrada) is a humanised anti-CD52 monoclonal that depletes T and B lymphocytes (and to lesser extent monocytes) for months, with skewed reconstitution (B cells repopulate first, regulatory T cells expand). Two short courses 12 months apart can produce sustained, sometimes years-long, disease suppression.

• CARE-MS I and CARE-MS II (Cohen et al., Lancet 2012; Coles et al., Lancet 2012) — superior to high-dose IFN-β-1a; ARR reduction ~50%.
• ~50% NEDA-3 at 5 years; durable MRI suppression.
• Secondary autoimmunity (the central safety concern): ~30% develop Graves disease, ~1–3% immune thrombocytopenia, ~0.3% anti-GBM disease — mediated by aberrant B-cell reconstitution. Five years of monthly thyroid/CBC/creatinine monitoring is mandated.
• Rare cardiovascular events (stroke, dissection) prompted EMA/FDA restriction in 2019 to relapsing MS with high activity despite a full and adequate course of at least one other DMT.

10. BTK Inhibitors — the New Frontier

Bruton tyrosine kinase (BTK) is a cytoplasmic Tec-family kinase essential for B-cell receptor signalling and also expressed in microglia. CNS-penetrant BTK inhibitors offer a unique combination: B-cell modulation (without depletion) plus direct microglial modulation within the CNS — biologically targeted at the compartmentalised inflammation of progressive MS.

• Evobrutinib — covalent BTK-i; phase II positive for ARR but phase III EVOLUTION-I/II vs teriflunomide failed primary endpoint (2023).
• Tolebrutinib — covalent, brain-penetrant; phase III HERCULES trial in non-relapsing SPMS reported (2024) a positive effect on disability progression — the first MS DMT to slow progression in non-active SPMS. Hepatotoxicity signals require monitoring.
• Fenebrutinib — reversible non-covalent BTK-i; phase III FENhance and FENtrepid ongoing.
• Remibrutinib — reversible covalent BTK-i; phase III REMODEL ongoing.

The class is positioned to address PIRA and SPMS — the unmet need for which the existing DMT armamentarium is insufficient.

11. Acute Relapse Management

For a confirmed relapse with significant functional impact:

• High-dose corticosteroids — methylprednisolone 1 g IV (or oral equivalent, the OMS trial showed bioequivalence) daily for 3–5 days; tapering oral course optional.
• Plasma exchange — for severe steroid-refractory relapses (Weinshenker et al., Ann Neurol 1999); 5–7 sessions over 10–14 days; particularly effective in NMOSD.
• IVIG — second-line; weak evidence in MS; main use is in MOGAD/NMOSD attacks or post-partum.
• Rule out infection, fever (pseudo-relapse) before treating; treatment shortens recovery but does not change ultimate outcome.

12. Choosing & Switching DMTs

Initial choice depends on disease activity, prognostic factors, comorbidity, fertility plans and patient preference. Switching is driven by:

• Suboptimal response — defined by ≥ 2 relapses, ≥ 3 new T2 lesions, ≥ 1 gadolinium-enhancing lesion, or confirmed disability progression in 12 months. Modified Rio score is one common framework.
• Adverse effects — severe lymphopenia, JCV seroconversion, hepatotoxicity, intolerable side effects.
• Pregnancy planning — teriflunomide (cholestyramine washout), fingolimod, S1P modulators, and natalizumab require specific peripregnancy planning. Glatiramer acetate and IFN-β are most favourable; ocrelizumab + planning windows is increasingly used.
• Switching from natalizumab — rebound disease activity occurs in ~20% if drug stopped without bridge; common bridges are ocrelizumab or fingolimod started during washout.
• De-escalation — in older patients (> 55) with stable disease, DISCOMS (Corboy et al., Lancet Neurol 2023) showed non-inferiority of DMT discontinuation in selected patients.

Vaccination, vitamin D supplementation, cardiovascular risk management and smoking cessation are now standard non-DMT components of MS care.