1. Introduction
MS is the most common autoimmune disease of the CNS, affecting around 2.8 million people worldwide. The first clinical symptoms typically appear between the ages of 20 and 40 and affect twice as many women as men [ 1 ]. Most patients experience an acute onset of the disease followed by a period of remission [ 2 ]. After relapses, the disease progresses to a chronic course, while a small percentage of patients develop this progressive phase from the outset. MS relapses are characterized by the activation and migration of peripheral immune cells into the CNS, leading to perivascular inflammatory lesions [ 3 ]. In contrast, the progression of MS without relapses is mainly driven by compartmentalized inflammation, in which T cells, B cells and myeloid cells are trapped in the brain and spinal cord behind a closed blood-brain barrier (BBB), and by degenerative disease mechanisms [ 4 ]. Several lines of evidence suggest that insidious progression occurs in the earliest phases of the disease [ 5 , 6 , 7 ] and becomes the main clinical feature of MS in later stages [ 4 ].
Over the past few decades, tremendous progress has been made in the development of disease-modifying therapies (DMTs) for the treatment of relapsing-remitting MS (RRMS). Although therapeutic options in patients with PMS are limited, recent research has led to the validation of new promising treatment strategies in clinical trials with PMS patients.
In this manuscript, we first describe the pathological mechanisms involved in disease progression. Furthermore, we summarize the current knowledge about siponimod and ocrelizumab, which are approved drugs for the treatment of secondary progressive MS (SPMS), while ocrelizumab is currently the only drug approved for the treatment of primary progressive MS (PPMS). Moreover, we provide an overview of the most promising therapeutics currently under investigation for the treatment of PMS. Finally, we discuss potential targets for novel treatment strategies in PMS.
2. MS Pathology and Pathogenesis–PMS
The pathological hallmark of RRMS is the presence of focal inflammatory lesions in the white matter, while the pathologic process in PMS is assumed to be more complex, consisting of a variety of different mechanisms and pathways. Besides neurodegeneration, this also includes inflammation, which takes place behind an intact BBB. This so-called compartmentalized inflammation consists of B cell, T cell and myeloid cell aggregates in the meninges and perivascular cuffs [ 8 , 9 , 10 ]. These meningeal cell aggregates secrete cytokines, free radicals and proteases [ 11 , 12 ] and are assumed to have important roles in cortical gray matter damage as they are associated with elevated subpial demyelination, a higher level of cortical lesion formation and increased neuronal degeneration [ 9 , 13 ]. Microglia and macrophages have a central role in the pathomechanisms of chronic active lesions, which are the most common in PMS patients and are also referred to as smoldering lesions [ 14 , 15 ]. Numerous activated, iron-positive microglia and macrophages can be found at the edges of slowly expanding lesions (SELs), a subgroup of chronic white matter lesions, which are suggested to promote lesion expansion [ 16 ]. Moreover, a correlation between activated microglia/macrophages and the extent of paranodal disruption, ongoing demyelination and neuroaxonal injury has been made, indicating a crucial role in neurodegeneration [ 17 , 18 ]. This process of neurodegeneration can be directly induced by microglia and macrophages upon the release of neurotoxic factors [ 11 , 19 , 20 ]. Activated microglia and macrophages can also release reactive oxygen species (ROS) and reactive nitrogen species (RNS), which are known to induce mitochondrial dysfunction and injury to cells, leading to the degeneration of oligodendrocytes and neurons [ 21 ]. Iron is known to be an essential cofactor in myelin formation by oligodendrocytes [ 22 , 23 ]. CNS inflammation can promote extracellular iron accumulation [ 24 ], resulting in cell injury through protein misfolding, glutamate excitotoxicity, oxidative stress and ferroptosis [ 25 ]. Remyelination is the natural repair mechanism of demyelination, showing its effectiveness in the early stages of disease, while it declines over time [ 26 ]. Moreover, it has been shown that the proportion of remyelinated lesions is lower in PMS than in RRMS [ 10 ]. This ineffective remyelination in PMS is attributed to persistent neuroinflammation, the loss of oligodendrocyte lineage cells and the pathogenic microenvironment [ 27 , 28 , 29 ].
3. Approved Therapeutics for MS Progression
Since the release of IFN β-1b in 1993, as the first approved disease-modifying treatment (DMT) for MS, many other drugs have been successfully developed for the treatment of RRMS. However, the use of these approved DMTs in progressive MS has shown largely disappointing results. The first DMT approved in Western Europe and North America for the use of SPMS was mitoxantrone in 2000 based on the MIMS trial [ 30 ]. Mitoxantrone showed positive effects on the relapse rate and the progression of disability, but it is currently no longer used in many countries due to its severe side effects [ 31 ]. Cladribine, a DNA synthesis inhibitor, which selectively diminishes lymphocyte numbers in the periphery by the induction of apoptosis, was also approved for the treatment of active SPMS by the FDA in 2019 based on the CLARITY clinical trial ( NCT00213135 ). A post hoc analysis showed that patients with relapsing MS had a lower risk of progression to SPMS within two years of treatment [ 32 ]. Although cladribine is able to cross the BBB [ 33 ] and in vitro data have shown microglia inhibition upon treatment in culture [ 34 ], it is questionable whether the brief drug administration regimen is able to cause long-lasting therapeutic effects on the CNS. Siponimod and ocrelizumab are two other therapeutic agents that are available for the treatment of PMS and whose properties we describe in more detail below.
3.1. Siponimod
Siponimod is a selective sphingosine-1-phosphate (S1P)-1 and S1P-5 modulator that is not only able to reduce the number of circulating lymphocytes by preventing their egress from the lymph nodes but can also pass through the BBB due to its small size and lipophilic nature [ 35 ]. Both target receptors can be found on CNS-resident cells, enabling siponimod to exert central effects in CNS autoimmune diseases [ 36 ]. In various animal models of MS, siponimod decreased meningeal inflammation and underlying cortical demyelination [ 37 ], attenuated microgliosis and astrogliosis [ 38 , 39 ], reduced oxidative stress and mitochondrial injury [ 37 ] and induced remyelination [ 40 ]. The phase III EXPAND trial investigated the efficacy of siponimod in 1651 patients. Siponimod reduced the risk of 3-month confirmed disability progression by 21% compared with placebo [ 41 ]. Moreover, the increase in T2 lesion volume from baseline was lower, and the brain volume was preserved to a higher degree in siponimod-treated patients, suggesting neuroprotective effects. Based on these findings, the FDA approved the use of siponimod for the treatment of SPMS in patients with active disease in 2019.
3.2. Ocrelizumab
B cells play an important pathogenic role in MS, and a substantial reduction in new lesion formation as well as the annual relapse rate could be shown in randomized clinical trials in RRMS using the monoclonal anti-CD20 antibodies rituximab and ocrelizumab [ 42 , 43 ]. With the OLYMPUS trial, rituximab was the first B cell-depleting therapy tested for its efficacy in PMS [ 44 ]. Although the OLYMPUS trial reported negative results, a subgroup analysis revealed reduced disease progression in patients younger than 52 years of age with active inflammatory lesions. The findings from the OLYMPUS trial had a significant impact on the design of the ORATORIO trial, the second study comparing ocrelizumab versus placebo in 732 PMS patients [ 45 ]. The inclusion criteria for this study were patients younger than 55 years of age, an Expanded Disability Status Scale (EDSS) score of 3.0 to 6.5 and a positive IgG index or evidence of cerebrospinal fluid (CSF)-specific oligoclonal bands. The primary endpoint of this study was met, showing lower rates of 12-week confirmed disability progression in ocrelizumab-treated patients as compared to placebo, with a relative risk reduction of 24%. Based on the positive results of the ORATORIO study, ocrelizumab was approved by the FDA and remains the only DMT approved for PPMS. To better understand the mechanism of action of ocrelizumab and the biology of B cells in RMS or PPMS, an open-label, multicenter biomarker study was conducted ( NCT02688985 ). In this study, the prognosis of nonrelapsing progressive disease was associated with neurofilament heavy-chain and activated glial markers [ 46 ]. In addition, a correlation was found between the long-term progression of multiple sclerosis and increases in glial fibrillary acidic protein in the CSF, suggesting that the biology of nonrelapsing progressive MS is related to insidious damage from neurodegeneration and/or the smoldering compartmentalized inflammation of the CNS [ 15 ]. In contrast to its beneficial use in young and active PMS patients, anti-CD20 treatment is therefore thought to be less effective in preventing clinical progression independent of relapse activity (PIRA), a process that could be linked to both smoldering inflammation and neurodegeneration [ 7 ]. An ongoing multicenter, randomized, double-blind, controlled phase III study is currently evaluating the efficacy, safety and pharmacokinetics of double or triple doses of ocrelizumab in adults with PPMS compared to the approved 600 mg dose ( NCT04548999 ).
4. Therapeutics under Investigation for MS Progression
Several therapeutics for the treatment of PMS are currently being evaluated in advanced clinical trials ( Table 1 ). Besides the modulation of CNS-compartmentalized inflammation, many of these therapeutics target alternative pathophysiological pathways to achieve neuroprotection and/or remyelination. In this section, we present some of the most promising candidates.
4.1. Bruton’s Tyrosine Kinase Inhibitors
Bruton’s tyrosine kinase (BTK) is a non-receptor, cytoplasmic protein tyrosine kinase expressed on most hematopoietic cell populations including B cells and myeloid cells [ 47 ]. In B cells, BTK is an essential component of multiple signaling pathways downstream of the B cell receptor controlling B cell survival, activation, maturation, cytokine production and the antigen-dependent stimulation of T lymphocytes [ 48 ]. In myeloid cells such as macrophages and microglia, BTK is involved in Fc receptor signaling regulating activation, cytokine secretion, ROS production and inflammasome activation [ 49 ]. Since BTK inhibition is an effective therapeutic strategy for the treatment of many B cell lymphomas, interest has grown in testing its efficacy in the treatment of MS, in which B cells play an important pathogenic role [ 48 ]. In the experimental autoimmune encephalomyelitis (EAE) model of MS, the BTK inhibitor evobrutinib diminished the antigen-dependent activation and maturation of B cells and decreased the release of pro-inflammatory cytokines [ 50 ]. In addition, evobrutinib reduced the ability of B cells to act as antigen-presenting cells in the development of encephalitogenic T cells, resulting in decreased CNS infiltration and clinical severity. Due to the molecular size and pharmacological properties, it is assumed that BTK inhibitors are able to cross the BBB [ 51 ] but with varying degrees of effectiveness [ 52 ]. Accordingly, BTK inhibitors provide an opportunity to modulate not only the peripheral immune system but through the BBB penetrance also CNS-compartmentalized B cell and myeloid cell inflammation, a hallmark of PMS. In a recent study of our group, the BTK inhibitor evobrutinib reduced the pro-inflammatory activity of microglia in the EAE model of MS [ 53 ]. Moreover, in a model of toxic demyelination induced by cuprizone, evobrutinib promoted the clearance of myelin debris by microglia, resulting in an accelerated remyelination. These findings indicate that BTK inhibition might be a promising therapeutic approach in targeting CNS-compartmentalized inflammation, leading to a decreased disease progression. The two BTK inhibitors tolebrutinib and fenebrutinib are currently being analyzed for their potential effects on PMS, which we discuss in more detail below.
4.2. Tolebrutinib
The covalent and irreversible BTK inhibitor tolebrutinib is being evaluated in two phase III trials, HERCULES in SPMS ( NCT04411641 ) and PERSEUS in PPMS ( NCT04458051 ). Both studies are double-blind, randomized, placebo-controlled trials and aim to investigate the effects of treatment on delaying the progression of disability. In the HERCULES trial, the efficacy and safety of tolebrutinib were investigated in 1131 patients with nonrelapsing SPMS, defined as no clinical relapses for the past 24 months, documented evidence of disability accumulation in the past 12 months and an EDSS score between 3.0 and 6.5. The study was recently completed and met its primary endpoint by demonstrating a 31% risk reduction in time to confirmed disability progression at 6 months in MS patients treated with tolebrutinib compared to placebo [ 54 ]. In addition, key secondary endpoints were met, including an increase in the probability of achieving confirmed improvement in disability at 6 months and a reduction in the annualized rate of new/enlarging T2 lesions. Of note, similar to evobrutinib in two phase III trials in RRMS ( NCT04338022 and NCT04338061 ), tolebrutinib failed to meet the primary endpoint of reducing the annualized adjudicated relapse rate in two recent phase III trials ( NCT04410978 and NCT04410991 ) in RRMS compared to teriflunomide [ 55 ]. Although the analysis of secondary endpoints revealed a 29% risk reduction in 6-month CDP, the number of new Gd-enhancing T1 lesions was higher in one treatment group receiving tolebrutinib than the one receiving teriflunomide. These results suggest that acute focal inflammation and smoldering neuroinflammation are two distinct biological processes and that BTKi would be much more suitable for the treatment of PMS than relapsing prophylaxis. Thus, tolebrutinib represents a breakthrough and may be the first therapy of its kind to offer clinically meaningful improvement in disability progression. It remains to be seen whether tolebrutinib can show equally positive results in the second trial, which is recruiting PPMS patients and is scheduled to end in July 2025.
4.3. Fenebrutinib
The non-covalent and reversible BTK inhibitor fenebrutinib is being evaluated for safety and efficacy in disease progression in patients with PPMS in the FENtrepid trial ( NCT04544449 ). A total of 985 participants were enrolled in this double-blind, randomized study, and the treatment effects of fenebrutinib are being compared with those of ocrelizumab, the only approved drug for the treatment of PPMS to date [ 56 ]. The primary outcome is the time to onset of composite 12-week confirmed disability progression, and the study is expected to be completed by the end of 2026.
4.4. Metformin
Metformin is a synthetic guanidine derivative that has a blood glucose-lowering effect [ 57 ] and is a first-line drug in the treatment of type 2 diabetes mellitus, especially in patients with obesity [ 58 ]. In addition to its blood glucose-lowering effect, metformin also has anti-inflammatory and antioxidant properties, which are attributed to the activation of adenosine monophosphate-activated protein kinase (AMPK) [ 59 , 60 ]. In EAE, metformin decreased immune cell infiltration and reduced the expression of pro-inflammatory cytokines, cell adhesion molecules, matrix metalloproteinase 9 and the chemokine RANTES in the CNS of EAE animals, resulting in the slowing of disease progression [ 61 ]. In the cuprizone model, metformin treatment reduced cuprizone-induced NF-κB activation by upregulating AMPK activity, downregulated the expression of pro-inflammatory microglia-associated genes, decreased inducible NO synthase (iNOS) protein levels, and increased the anti-inflammatory protein Trem2 in the corpus callosum [ 62 ]. Moreover, further studies have shown that metformin reduces endogenous ROS levels and associated DNA damage [ 63 ] and improves the remyelination capacity of aged human OPCs by restoring their responsiveness to pro-differentiation signals [ 64 ]. Based on its ability to cross the BBB [ 65 ] and exert antioxidant, neuroprotective and regulatory effects on neuroinflammation and microglia activity, metformin may be a suitable candidate for the treatment of PMS. Little is known so far about metformin treatment’s effects on MS patients. In a prospective cohort study including a total of 50 obese RRMS patients with metabolic syndrome, treatment with metformin led to a reduction in the number of new or enlarging T2 brain lesions on MRI in 20 patients [ 66 ]. In addition, the study demonstrated an increased level of AMPK expression as well as an elevated proportion of Treg cells and a decreased production of IFN-γ and IL-17 in MS patients treated with metformin. Further studies of metformin in the treatment of PMS are currently being prepared. In the USA, for example, the recruitment of patients for a randomized, placebo-controlled phase I clinical trial in PMS patients has recently begun ( NCT05349474 ). The safety and efficacy of the metformin add-on treatment will be investigated in an estimated 44 PMS patients for 12 months until the end of the study in mid-2026. A second clinical trial is currently recruiting 120 patients for a multicenter, randomized, placebo-controlled phase II clinical trial in PMS patients ( NCT05893225 ). The aim of this clinical trial is to demonstrate the neuroprotective and remyelinating effects of metformin in MS patients. The primary endpoint is the change in walking speed as measured by the 25-foot walk test (T25FW) between baseline and 96 weeks of treatment. The study is scheduled to be finished by the end of 2026. Another clinical trial is a large multicenter, multi-arm, multi-stage, randomized, placebo-controlled phase II/III clinical trial (OCTOPUS) currently recruiting 1200 patients (ISRCTN14048364). The aim of this clinical trial is to evaluate the safety and efficacy of several drugs, including metformin, in both SPMS and PPMS patients. The primary endpoint is the whole brain atrophy rate as measured by MRI and disability progression as measured by a multicomponent measure of disability progression that includes the EDSS, the T25FW and the 9-hole peg test (9HPT). The study is scheduled to be completed by the end of 2028.
4.5. Ibudilast
Ibudilast is an inhibitor for several cyclic nucleotide phosphodiesterases, toll-like receptor 4 and macrophage migration inhibitory factor [ 67 ]. In vitro, ibudilast decreased pro-inflammatory cytokine production such as that of tumor necrosis factor α (TNF-α), interleukin-1b (IL-1b) and IL-6; increased the secretion of the anti-inflammatory cytokine IL-10 as well as several neurotrophic factors by activated microglia [ 68 , 69 ] and inhibited the microglia-mediated destruction of neurons [ 70 ]. In addition, ibrutinib reduced spinal cord inflammation and improved clinical severity in EAE [ 71 ]. Due to its small size, ibrutinib is known to be able to cross the BBB [ 72 ], suggesting a positive effect on compartmentalized inflammation and neuroprotective properties. Ibudilast was tested in the double-blind, randomized and placebo-controlled phase II SPRINT-MS trial ( NCT01982942 ), which included a total of 255 patients with a similar number of SPMS and PPMS participants [ 73 ]. The primary endpoint was met, which was defined as a reduction in MRI-derived brain atrophy over 96 weeks. Post hoc analyses showed that the treatment effect was largely limited to people with PPMS and not SPMS, which may be related to the faster progression of brain atrophy in the PPMS placebo group than that in the SPMS placebo group [ 74 ]. Although ibudilast showed no effects on new or enlarging T1 and T2 lesions [ 75 ] and did not reduce neurofilament levels in serum or CSF [ 76 ], ibudilast was significantly associated with smaller SEL volumes, which were reduced by 23% [ 77 ]. In addition, ibudilast had a significant effect on reducing MRI changes in SLEs over time, suggesting a reduction in chronic active lesions and a treatment effect on compartmentalized inflammation. The effect of ibudilast on SELs in combination with the observed effect on brain atrophy make this therapeutic agent attractive for a phase III trial in PMS.
4.6. Vidofludimus Calcium (IMU-838)
Vidofludimus calcium is a small molecule that is currently being developed as an oral treatment option for MS or other chronic inflammatory and autoimmune diseases [ 78 ]. Similar to teriflunomide, which is already approved for MS, vidofludimus calcium inhibits the enzyme dihydroorotate dehydrogenase (DHODH) in the mitochondria and thus prevents the de novo synthesis of pyrimidine in activated lymphocytes, leading to altered cell proliferation and apoptosis [ 79 ]. In addition, vidofludimus calcium also activates the transcription factor Nuclear Receptor-Related 1 (Nurr1), which is mainly expressed in neurons, astrocytes and microglia and is associated with direct neuroprotective properties [ 80 , 81 ], suggesting it as a suitable candidate for the treatment of PMS. In vitro, vidofludimus calcium reduced the neurotoxic environment in human microglia as well as primary monocytes and protected neurons under pro-apoptotic conditions [ 82 ]. In EAE, vidofludimus calcium attenuated the severity of the disease and increased the expression of Nurr1 target genes in the CNS [ 82 ]. Vidofludimus calcium is currently being evaluated for efficacy, safety and tolerability in a multicenter, randomized, double-blind, placebo-controlled phase II CALLIPER trial in 467 patients with PPMS and SPMS who will be followed for up to 120 weeks ( NCT05054140 ). The primary endpoint is the annualized rate of percent change in brain volume during the main treatment period, and the study is expected to be completed by early 2025. A pre-planned interim analysis to assess serum neurofilament light chain (NfL) was performed after half of the study participants had completed the 24-week study treatment [ 83 ]. In 203 patients, composed of 61% nonactive SPMS, 10% active SPMS and 29% PPMS patients, the average serum Nfl in the vidofludimus calcium group decreased by 22.4% from baseline to week 24 compared to placebo. These results support the potential efficacy of vidofludimus calcium in slowing disease progression in PMS.
4.7. Hydroxychloroquine
Hydroxychloroquine (HCQ) is approved for the treatment or prophylaxis of malaria and for the treatment of autoimmune diseases such as chronic discoid lupus erythematosus, systemic lupus erythematosus in adults and rheumatoid arthritis [ 84 ]. HCQ is a small molecule and has been shown to cross the BBB [ 85 ]. In vitro studies have shown that HCQ inhibits the activity of human microglia and B cells and improves the iron-induced destruction of neurons [ 86 , 87 ]. In EAE, pretreatment with HCQ delayed the onset of the disease, reduced the number of IBA-1-positive microglia/macrophages and decreased white matter demyelination [ 86 ]. The combined use of HCQ with the anti-hypertensive drug indapamide, which scavenges oxygen radicals and protects neurons from iron-induced death, resulted in less oxidative and neuroaxonal damage in the lysolecithin model of spinal cord demyelination [ 88 ]. In a recently completed open-label, single-center, single-arm futility phase II trial, HCQ was analyzed to reduce the progression of disability in 35 patients with PPMS ( NCT02913157 ). The primary endpoint was the change in T25FW performance between 6 and 18 months of follow-up. Disease worsening occurred in 8 of the 35 participants, and based on the null hypothesis, the researchers concluded that treatment with HCQ results in less disability worsening in PPMS and should be further investigated in randomized controlled clinical trials [ 89 ].
5. Target Identification for Novel Treatment Strategies in PMS
Although drug development for PMS has increased significantly in recent years, there is still a need to find new therapeutic targets for the disease progression of MS. Direct therapeutic neuroprotection and/or remyelination remains an important research focus, even though several recent studies have shown negative results (lipoic acid, NCT03161028 [ 90 ]; simvastatin, NCT03387670 [ 91 ]).
The BBB-penetrating small molecule Sephin-1 , a selective inhibitor of holophosphatase [ 92 , 93 ], has been shown to prolong eIF2a phosphorylation in stressed primary oligodendrocyte cultures, leading to an increased synthesis of protective proteins and higher oligodendrocyte survival [ 94 ]. In addition, Sephin1 delayed the onset of clinical symptoms in EAE, which was correlated with a prolonged integrated stress response, reduced oligodendrocyte and axon loss and diminished T cell numbers in the CNS. Thus, neuroprotective treatment based on improving the integrated stress response could have positive effects on PMS.
Another strategy to promote neuroprotection and remyelination in PMS is the use of endogenous metabolites. Taurine is an ubiquitous, non-proteinaceous amino acid present in the brain in millimolar concentrations [ 95 ] and is known as an osmoregulator and neuromodulator [ 95 , 96 ]. Taurine is involved in the modulation of neuronal excitability, antioxidation [ 97 ] and, as recently shown, in the differentiation of oligodendrocytes [ 98 ]. As a drug, taurine is very well tolerated and is actively transported through the BBB [ 95 ]. Therefore, taurine supplementation could be a promising treatment strategy to improve neuroprotection and remyelination.
Another approach to promote the protection of neurons and oligodendrocytes in PMS could be to target novel cell death signaling pathways such as those of necroptosis and ferroptosis . During necroptosis, inflammatory stimuli such as activation of Toll-like receptor (TLR) 4 or TNF receptor 1 signaling activate receptor-interacting protein kinase (RIPK) 1, leading to necrosis by plasma membrane disruption and cell lysis [ 99 ].Ferroptosis is driven by iron-dependent phospholipid peroxidation and regulated by several cellular metabolic pathways, including redox homeostasis, iron handling, mitochondrial activity and the metabolism of amino acids, lipids and sugars, as well as various signaling pathways relevant to disease [ 100 ]. Necroptosis and ferroptosis have been linked to neurodegeneration in PMS [ 101 , 102 ] as well as other neurological diseases such as Alzheimer’s and Parkinson’s [ 99 , 103 ]. In addition, necroptosis and ferroptosis have been shown to contribute to the formation of neurotoxic microglia and astrocytes [ 101 , 104 ] and reduce the differentiation of OPCs and the ability to myelinate [ 101 , 105 ]. Attempts to inhibit necroptosis and ferroptosis with RIPK inhibitors or ferrostatin-1 analogs reduced neuronal loss and ameliorated disease progression in EAE [ 101 , 102 ], also suggesting positive effects on PMS.
Fibrinogen is a pleiotropic protein, which has essential roles in coagulation, inflammation and tissue repair [ 106 ]. If the BBB is disrupted, fibrinogen can enter the CNS and be converted into fibrin, which leads to the activation of immune responses [ 107 ]. Fibrin can be detected in the cortex of active but also chronic lesions of progressive MS, and fibrin deposition has been shown to promote neuroinflammation as well as glial scar formation through direct effects on microglia, astrocytes and neurons [ 108 , 109 , 110 ], resulting in decreased neuronal density [ 111 ]. Recently, a monoclonal antibody (5B8) was developed that specifically targets the cryptic fibrin epitope γ377–395 and selectively inhibits fibrin-induced inflammation and oxidative stress without interfering with clotting [ 112 ]. In several MS models, 5B8 entered the CNS and bound to parenchymal fibrin, resulting in reduced innate immune response, oxidative stress, demyelination, axonal damage and clinical severity. In PMS, however, the BBB is largely intact, which prevents antibodies from entering the CNS. This could be remedied by the use of shuttles that smuggle the drug into the brain or the fundamental development of new types of drugs with suitable chemical properties that effectively overcome the BBB.
Targeting the complement pathway is another treatment approach that may be useful in PMS. The complement system plays a crucial role in innate immunity and consists of a network of circulating and membrane-expressed proteins that recognize non-self-components [ 113 , 114 ]. After activation via the classical pathway, triggered by antigen-antibody complexes or alternative or lectin pathways, the complement system is critical for the clearance of pathogens as well as apoptotic and dead cells [ 115 ]. Several studies have shown that most complement components can also be produced in the CNS [ 116 ] and are upregulated in the more severely diseased MS brain [ 117 , 118 , 119 , 120 ], suggesting a crucial pathophysiological role in PMS. In EAE, the targeted inhibition of complement activation with complement receptor 2-conjugated inhibitors improved both the acute and chronic stages of disease [ 121 ]. Furthermore, the inhibition of the alternative complement pathway with a monoclonal antibody specific for complement factor B attenuated the chronic phase of disease, resulting in less inflammation and demyelination [ 122 ]. In addition, treatment with a monoclonal antibody that inhibits complement component (C)1q decreased the concentration of “inflamed” microglia in the hippocampi of mice during chronic EAE, suggesting that complements may play an important role in promoting an inflammatory environment [ 120 ]. However, complement components may also have neuroprotective and/or anti-inflammatory properties, as C1q increased the phagocytosis of apoptotic neurons by microglia, resulting in a lower release level of toxic intracellular contents and excitotoxic damage to neighboring neurons [ 123 ]. A recent study in PPMS patients found an association between an increased serum C3a/C3 ratio and a higher risk of disability progression, while increased CSF C1q levels tended to be associated with a lower risk of disability progression, suggesting that complement cascades may play a dual role in MS progression [ 124 ]. Due to the small number of patients studied, further research is needed to clarify a possible relationship between the complement system and disability progression and to determine whether targeting complement cascades could be a promising treatment strategy for PMS.
6. Conclusions
Several therapies effectively target the peripheral immune mechanisms involved in MS relapse biology. Nevertheless, there is a large unmet need in MS for therapies that halt the chronic progression of neurological disability. There is a lot of evidence to suggest that relapsing activity and disease progression are two distinct pathological mechanisms that also require specific, targeted treatment strategies. This could be achieved by new treatment targets that promote neuroprotection and/or remyelination, inhibit cell death signaling pathways and induce the inactivation of fibrinogen and the complement system. In addition, sequential therapy, in which individual drugs are used at certain stages of the disease, or combination therapy, in which therapeutics are used in parallel to treat relapses and disease progression simultaneously, could be a promising approach. The use of a combination therapy could also have positive additive or synergistic effects on disease severity. Recently, the BTK inhibitor tolebrutinib has shown promising results in a phase III trial in PMS and may be the first therapy of its kind to offer a clinically meaningful reduction in disability progression. The identification of further effective therapies for PMS could be supported by novel high-throughput in vitro assays and animal models that better reflect the key clinical features of PMS. In addition, there is an urgent need for reliable biomarkers to more accurately assess treatment response in clinical trials of progressive MS.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.
Author Contributions
Conceptualization, D.H. and M.S.W.; writing—original draft preparation, D.H. and M.S.W.; writing—review and editing, D.H. and M.S.W. All authors have read and agreed to the published version of the manuscript.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
No new data were created or analyzed in this study. Data sharing is not applicable to this article.
Conflicts of Interest
D.H. is supported by an intramural grant (Startförderung) of the Universitätsmedizin Göttingen. M.S.W. received research support from the National Multiple Sclerosis Society (NMSS; PP 1660), Novartis, TEVA, Biogen-Idec, Roche, Merck, the ProFutura Program of the Universitätsmedizin Göttingen and the Deutsche Forschungsgemeinschaft (WE 3547/4-1; WE 3547/5-1; WE3547/7-1), in association with SFB TRR 274.
References
1. Walton C. King R. Rechtman L. Kaye W. Leray E. Marrie R.A. Robertson N. La Rocca N. Uitdehaag B. van der Mei I. Rising prevalence of multiple sclerosis worldwide: Insights from the Atlas of MS, third edition Mult. Scler. J. 2020 26 1816 1821 10.1177/1352458520970841 33174475 PMC7720355
2. Dendrou C.A. Fugger L. Friese M.A. Immunopathology of multiple sclerosis Nat. Rev. Immunol. 2015 15 545 558 10.1038/nri3871 26250739
3. Bar-Or A. Li R. Cellular immunology of relapsing multiple sclerosis: Interactions, checks, and balances Lancet Neurol. 2021 20 470 483 10.1016/S1474-4422(21)00063-6 33930317
4. Lassmann H. Multiple Sclerosis Pathology Cold Spring Harb. Perspect. Med. 2018 8 a028936 10.1101/cshperspect.a028936 29358320 PMC5830904
5. Portaccio E. Bellinvia A. Fonderico M. Pasto L. Razzolini L. Totaro R. Spitaleri D. Lugaresi A. Cocco E. Onofrj M. Progression is independent of relapse activity in early multiple sclerosis: A real-life cohort study Brain 2022 145 2796 2805 10.1093/brain/awac111 35325059
6. University of California, San Francisco MS-EPIC Team Cree B.A. Hollenbach J.A. Bove R. Kirkish G. Sacco S. Caverzasi E. Bischof A. Gundel T. Zhu A.H. Silent progression in disease activity-free relapsing multiple sclerosis Ann. Neurol. 2019 85 653 666 10.1002/ana.25463 30851128 PMC6518998
7. Kappos L. Wolinsky J.S. Giovannoni G. Arnold D.L. Wang Q. Bernasconi C. Model F. Koendgen H. Manfrini M. Belachew S. Contribution of Relapse-Independent Progression vs Relapse-Associated Worsening to Overall Confirmed Disability Accumulation in Typical Relapsing Multiple Sclerosis in a Pooled Analysis of 2 Randomized Clinical Trials JAMA Neurol. 2020 77 1132 1140 10.1001/jamaneurol.2020.1568 32511687 PMC7281382
8. Kutzelnigg A. Lucchinetti C.F. Stadelmann C. Bruck W. Rauschka H. Bergmann M. Schmidbauer M. Parisi J.E. Lassmann H. Cortical demyelination and diffuse white matter injury in multiple sclerosis Brain 2005 128 Pt 11 2705 2712 10.1093/brain/awh641 16230320
9. Choi S.R. Howell O.W. Carassiti D. Magliozzi R. Gveric D. Muraro P.A. Nicholas R. Roncaroli F. Reynolds R. Meningeal inflammation plays a role in the pathology of primary progressive multiple sclerosis Brain 2012 135 Pt 10 2925 2937 10.1093/brain/aws189 22907116
10. Luchetti S. Fransen N.L. van Eden C.G. Ramaglia V. Mason M. Huitinga I. Progressive multiple sclerosis patients show substantial lesion activity that correlates with clinical disease severity and sex: A retrospective autopsy cohort analysis Acta Neuropathol. 2018 135 511 528 10.1007/s00401-018-1818-y 29441412 PMC5978927
11. Calabrese M. Magliozzi R. Ciccarelli O. Geurts J.J. Reynolds R. Martin R. Exploring the origins of grey matter damage in multiple sclerosis Nat. Rev. Neurosci. 2015 16 147 158 10.1038/nrn3900 25697158
12. Mendiola A.S. Ryu J.K. Bardehle S. Meyer-Franke A. Ang K.K. Wilson C. Baeten K.M. Hanspers K. Merlini M. Thomas S. Transcriptional profiling and therapeutic targeting of oxidative stress in neuroinflammation Nat. Immunol. 2020 21 513 524 10.1038/s41590-020-0654-0 32284594 PMC7523413
13. Magliozzi R. Howell O. Vora A. Serafini B. Nicholas R. Puopolo M. Reynolds R. Aloisi F. Meningeal B-cell follicles in secondary progressive multiple sclerosis associate with early onset of disease and severe cortical pathology Brain 2007 130 Pt 4 1089 1104 10.1093/brain/awm038 17438020
14. Frischer J.M. Weigand S.D. Guo Y. Kale N. Parisi J.E. Pirko I. Mandrekar J. Bramow S. Metz I. Bruck W. Clinical and pathological insights into the dynamic nature of the white matter multiple sclerosis plaque Ann. Neurol. 2015 78 710 721 10.1002/ana.24497 26239536 PMC4623970
15. Absinta M. Sati P. Masuzzo F. Nair G. Sethi V. Kolb H. Ohayon J. Wu T. Cortese I.C.M. Reich D.S. Association of Chronic Active Multiple Sclerosis Lesions with Disability In Vivo JAMA Neurol. 2019 76 1474 1483 10.1001/jamaneurol.2019.2399 31403674 PMC6692692
16. Lassmann H. Pathogenic Mechanisms Associated with Different Clinical Courses of Multiple Sclerosis Front. Immunol. 2018 9 3116 10.3389/fimmu.2018.03116 30687321 PMC6335289
17. Prineas J.W. Kwon E.E. Cho E.S. Sharer L.R. Barnett M.H. Oleszak E.L. Hoffman B. Morgan B.P. Immunopathology of secondary-progressive multiple sclerosis Ann. Neurol. 2001 50 646 657 10.1002/ana.1255 11706971
18. Nikic I. Merkler D. Sorbara C. Brinkoetter M. Kreutzfeldt M. Bareyre F.M. Bruck W. Bishop D. Misgeld T. Kerschensteiner M. A reversible form of axon damage in experimental autoimmune encephalomyelitis and multiple sclerosis Nat. Med. 2011 17 495 499 10.1038/nm.2324 21441916
19. Jackle K. Zeis T. Schaeren-Wiemers N. Junker A. van der Meer F. Kramann N. Stadelmann C. Bruck W. Molecular signature of slowly expanding lesions in progressive multiple sclerosis Brain 2020 143 2073 2088 10.1093/brain/awaa158 32577755
20. Dong Y. Yong V.W. When encephalitogenic T cells collaborate with microglia in multiple sclerosis Nat. Rev. Neurol. 2019 15 704 717 10.1038/s41582-019-0253-6 31527807
21. Faissner S. Plemel J.R. Gold R. Yong V.W. Progressive multiple sclerosis: From pathophysiology to therapeutic strategies Nat. Rev. Drug Discov. 2019 18 905 922 10.1038/s41573-019-0035-2 31399729
22. Stephenson E. Nathoo N. Mahjoub Y. Dunn J.F. Yong V.W. Iron in multiple sclerosis: Roles in neurodegeneration and repair Nat. Rev. Neurol. 2014 10 459 468 10.1038/nrneurol.2014.118 25002107
23. Lee N.J. Ha S.K. Sati P. Absinta M. Nair G. Luciano N.J. Leibovitch E.C. Yen C.C. Rouault T.A. Silva A.C. Potential role of iron in repair of inflammatory demyelinating lesions J. Clin. Investig. 2019 129 4365 4376 10.1172/JCI126809 31498148 PMC6763243
24. Urrutia P. Aguirre P. Esparza A. Tapia V. Mena N.P. Arredondo M. Gonzalez-Billault C. Nunez M.T. Inflammation alters the expression of DMT1, FPN1 and hepcidin, and it causes iron accumulation in central nervous system cells J. Neurochem. 2013 126 541 549 10.1111/jnc.12244 23506423
25. Zierfuss B. Wang Z. Jackson A.N. Moezzi D. Yong V.W. Iron in multiple sclerosis—Neuropathology, immunology, and real-world considerations Mult. Scler. Relat. Disord. 2023 78 104934 10.1016/j.msard.2023.104934 37579645
26. Goldschmidt T. Antel J. Konig F.B. Bruck W. Kuhlmann T. Remyelination capacity of the MS brain decreases with disease chronicity Neurology 2009 72 1914 1921 10.1212/WNL.0b013e3181a8260a 19487649
27. Stangel M. Kuhlmann T. Matthews P.M. Kilpatrick T.J. Achievements and obstacles of remyelinating therapies in multiple sclerosis Nat. Rev. Neurol. 2017 13 742 754 10.1038/nrneurol.2017.139 29146953
28. Plemel J.R. Liu W.Q. Yong V.W. Remyelination therapies: A new direction and challenge in multiple sclerosis Nat. Rev. Drug Discov. 2017 16 617 634 10.1038/nrd.2017.115 28685761
29. Lubetzki C. Zalc B. Williams A. Stadelmann C. Stankoff B. Remyelination in multiple sclerosis: From basic science to clinical translation Lancet Neurol. 2020 19 678 688 10.1016/S1474-4422(20)30140-X 32702337
30. Hartung H.P. Gonsette R. Konig N. Kwiecinski H. Guseo A. Morrissey S.P. Krapf H. Zwingers T. Mitoxantrone in progressive multiple sclerosis: A placebo-controlled, double-blind, randomised, multicentre trial Lancet 2002 360 2018 2025 10.1016/S0140-6736(02)12023-X 12504397
31. Cross A.H. Naismith R.T. Established and novel disease-modifying treatments in multiple sclerosis J. Intern. Med. 2014 275 350 363 10.1111/joim.12203 24444048
32. Vermersch P. Giovannoni G. Soelberg-Sorensen P. Rammohan K. Cook S. Keller B. Roy S. Reduction of risk of secondary progressive multiple sclerosis within 2 years of treatment with cladribine tablets: An analysis of the CLARITY study Jouenal Neurollogical Sci. 2019 405 328 10.1016/j.jns.2019.10.1444
33. Hermann R. Karlsson M.O. Novakovic A.M. Terranova N. Fluck M. Munafo A. The Clinical Pharmacology of Cladribine Tablets for the Treatment of Relapsing Multiple Sclerosis Clin. Pharmacokinet. 2019 58 283 297 10.1007/s40262-018-0695-9 29987837 PMC6373393
34. Singh V. Voss E.V. Benardais K. Stangel M. Effects of 2-chlorodeoxyadenosine (Cladribine) on primary rat microglia J. Neuroimmune Pharmacol. 2012 7 939 950 10.1007/s11481-012-9387-7 22821377
35. Bigaud M. Rudolph B. Briard E. Beerli C. Hofmann A. Hermes E. Muellershausen F. Schubart A. Gardin A. Siponimod (BAF312) penetrates, distributes, and acts in the central nervous system: Preclinical insights Mult. Scler. J. Exp. Transl. Clin. 2021 7 20552173211049168 10.1177/20552173211049168 34777855 PMC8573504
36. O’Sullivan S. Dev K.K. Sphingosine-1-phosphate receptor therapies: Advances in clinical trials for CNS-related diseases Neuropharmacology 2017 113 Pt B 597 607 10.1016/j.neuropharm.2016.11.006 27825807
37. Ward L.A. Lee D.S. Sharma A. Wang A. Naouar I. Ma X.I. Pikor N. Nuesslein-Hildesheim B. Ramaglia V. Gommerman J.L. Siponimod therapy implicates Th17 cells in a preclinical model of subpial cortical injury JCI Insight 2020 5 e132522 10.1172/jci.insight.132522 31821174 PMC7030818
38. Gentile A. Musella A. Bullitta S. Fresegna D. De Vito F. Fantozzi R. Piras E. Gargano F. Borsellino G. Battistini L. Siponimod (BAF312) prevents synaptic neurodegeneration in experimental multiple sclerosis J. Neuroinflamm. 2016 13 207 10.1186/s12974-016-0686-4 PMC5002118 27566665
39. Husseini L. Geladaris G. Steinleitner M. Grondey K. Koch J. Häusler D. Weber M.S. Siponimod inhibits IFN-gamma-induced microglial activation through a dual central and peripheral mechanism of action Mult. Scler. J. 2022 28 348 349
40. Mannioui A. Vauzanges Q. Fini J.B. Henriet E. Sekizar S. Azoyan L. Thomas J.L. Pasquier D.D. Giovannangeli C. Demeneix B. The Xenopus tadpole: An in vivo model to screen drugs favoring remyelination Mult. Scler. J. 2018 24 1421 1432 10.1177/1352458517721355 28752787
41. Kappos L. Bar-Or A. Cree B.A.C. Fox R.J. Giovannoni G. Gold R. Vermersch P. Arnold D.L. Arnould S. Scherz T. Siponimod versus placebo in secondary progressive multiple sclerosis (EXPAND): A double-blind, randomised, phase 3 study Lancet 2018 391 1263 1273 10.1016/S0140-6736(18)30475-6 29576505
42. Hauser S.L. Waubant E. Arnold D.L. Vollmer T. Antel J. Fox R.J. Bar-Or A. Panzara M. Sarkar N. Agarwal S. B-cell depletion with rituximab in relapsing-remitting multiple sclerosis N. Engl. J. Med. 2008 358 676 688 10.1056/NEJMoa0706383 18272891
43. Hauser S.L. Bar-Or A. Comi G. Giovannoni G. Hartung H.P. Hemmer B. Lublin F. Montalban X. Rammohan K.W. Selmaj K. Ocrelizumab versus Interferon Beta-1a in Relapsing Multiple Sclerosis N. Engl. J. Med. 2016 376 221 234 10.1056/NEJMoa1601277 28002679
44. Hawker K. O’Connor P. Freedman M.S. Calabresi P.A. Antel J. Simon J. Hauser S. Waubant E. Vollmer T. Panitch H. Rituximab in patients with primary progressive multiple sclerosis: Results of a randomized double-blind placebo-controlled multicenter trial Ann. Neurol. 2009 66 460 471 10.1002/ana.21867 19847908
45. Montalban X. Hauser S.L. Kappos L. Arnold D.L. Bar-Or A. Comi G. de Seze J. Giovannoni G. Hartung H.P. Hemmer B. Ocrelizumab versus Placebo in Primary Progressive Multiple Sclerosis N. Engl. J. Med. 2016 376 209 220 10.1056/NEJMoa1606468 28002688
46. Cross A.H. Gelfand J.M. Thebault S. Bennett J.L. von Budingen H.C. Cameron B. Carruthers R. Edwards K. Fallis R. Gerstein R. Emerging Cerebrospinal Fluid Biomarkers of Disease Activity and Progression in Multiple Sclerosis JAMA Neurol. 2024 81 373 383 10.1001/jamaneurol.2024.0017 38466277 PMC10928543
47. Geladaris A. Torke S. Weber M.S. Bruton’s Tyrosine Kinase Inhibitors in Multiple Sclerosis: Pioneering the Path Towards Treatment of Progression? CNS Drugs 2022 36 1019 1030 10.1007/s40263-022-00951-z 36178589 PMC9550714
48. Torke S. Weber M.S. Inhibition of Bruton s tyrosine kinase as a novel therapeutic approach in multiple sclerosis Expert. Opin. Investig. Drugs 2020 29 1143 1150 10.1080/13543784.2020.1807934 32772592
49. Rip J. Van Der Ploeg E.K. Hendriks R.W. Corneth O.B.J. The Role of Bruton’s Tyrosine Kinase in Immune Cell Signaling and Systemic Autoimmunity Crit. Rev. Immunol. 2018 38 17 62 10.1615/CritRevImmunol.2018025184 29717662
50. Torke S. Pretzsch R. Hausler D. Haselmayer P. Grenningloh R. Boschert U. Bruck W. Weber M.S. Inhibition of Bruton’s tyrosine kinase interferes with pathogenic B-cell development in inflammatory CNS demyelinating disease Acta Neuropathol. 2020 140 535 548 10.1007/s00401-020-02204-z 32761407 PMC7498502
51. Owens T.D. Smith P.F. Redfern A. Xing Y. Shu J. Karr D.E. Hartmann S. Francesco M.R. Langrish C.L. Nunn P.A. Phase 1 clinical trial evaluating safety, exposure and pharmacodynamics of BTK inhibitor tolebrutinib (PRN2246, SAR442168) Clin. Transl. Sci. 2022 15 442 450 10.1111/cts.13162 34724345 PMC8841436
52. Turner T.J. Brun P. Gruber R.C. Ofengeim D. Comparative CNS Pharmacology of the Bruton’s Tyrosine Kinase (BTK) Inhibitor Tolebrutinib Versus Other BTK Inhibitor Candidates for Treating Multiple Sclerosis Drugs R&D 2024 24 263 274 10.1007/s40268-024-00468-4 PMC11315827 38965189
53. Geladaris A. Torke S. Saberi D. Alankus Y.B. Streit F. Zechel S. Stadelmann-Nessler C. Fischer A. Boschert U. Hausler D. BTK inhibition limits microglia-perpetuated CNS inflammation and promotes myelin repair Acta Neuropathol. 2024 147 75 10.1007/s00401-024-02730-0 38656399 PMC11043151
54. Fox R.J. Bar-Or A. Traboulsee A. Oreja-Guevara C. Giovannoni G. Vermersch P. Syed S. Li Y. Vargas W.S. Turner T.J. Efficacy and Safety of Tolebrutinib Versus Placebo in Non-Relapsing Secondary Progressive Multiple Sclerosis: Results from the Phase 3 HERCULES Trial Proceedings of the 40th Congress of the European Committee for Treatment and Research in Multiple Sclerosis Copenhagen, Denmark 18–20 September 2024 Abstract 4027
55. Oh J. Arnold D.L. Cree A.C. Ionete C. Kim H.J. Sormani M.P. Syed S. Chen Y. Maxwell C.R. Benoit P. Efficacy and Safety of Tolebrutinib Versus Teriflunomide in Relapsing Multiple Sclerosis: Results from the Phase 3 GEMINI 1 and 2 Trials Proceedings of the 40th Congress of the European Committee for Treatment and Research in Multiple Sclerosis Copenhagen, Denmark 18–20 September 2024 Abstract 4026
56. Montalban X. Belachew S. Wolinsky J.S. Ocrelizumab in Primary Progressive and Relapsing Multiple Sclerosis N. Engl. J. Med. 2017 376 1694 10.1056/NEJMoa1606468 28445661
57. Nasri H. Rafieian-Kopaei M. Metformin: Current knowledge J. Res. Med. Sci. 2014 19 658 664 25364368 PMC4214027
58. UK Prospective Diabetes Study (UKPDS) Group Effect of intensive blood-glucose control with metformin on complications in overweight patients with type 2 diabetes (UKPDS 34) Lancet 1998 352 854 865 10.1016/S0140-6736(98)07037-8 9742977
59. Buczynska A. Sidorkiewicz I. Kretowski A.J. Adamska A. Examining the clinical relevance of metformin as an antioxidant intervention Front. Pharmacol. 2024 15 1330797 10.3389/fphar.2024.1330797 38362157 PMC10867198
60. Hawley S.A. Gadalla A.E. Olsen G.S. Hardie D.G. The antidiabetic drug metformin activates the AMP-activated protein kinase cascade via an adenine nucleotide-independent mechanism Diabetes 2002 51 2420 2425 10.2337/diabetes.51.8.2420 12145153
61. Nath N. Khan M. Paintlia M.K. Singh I. Hoda M.N. Giri S. Metformin attenuated the autoimmune disease of the central nervous system in animal models of multiple sclerosis J. Immunol. 2009 182 8005 8014 10.4049/jimmunol.0803563 19494326 PMC2965405
62. Abdi M. Pasbakhsh P. Shabani M. Nekoonam S. Sadeghi A. Fathi F. Abouzaripour M. Mohamed W. Zibara K. Kashani I.R. Metformin Therapy Attenuates Pro-inflammatory Microglia by Inhibiting NF-kappaB in Cuprizone Demyelinating Mouse Model of Multiple Sclerosis Neurotox. Res. 2021 39 1732 1746 10.1007/s12640-021-00417-y 34570348
63. Algire C. Moiseeva O. Deschenes-Simard X. Amrein L. Petruccelli L. Birman E. Viollet B. Ferbeyre G. Pollak M.N. Metformin reduces endogenous reactive oxygen species and associated DNA damage Cancer Prev. Res. 2012 5 536 543 10.1158/1940-6207.CAPR-11-0536 22262811
64. Neumann B. Baror R. Zhao C. Segel M. Dietmann S. Rawji K.S. Foerster S. McClain C.R. Chalut K. van Wijngaarden P. Metformin Restores CNS Remyelination Capacity by Rejuvenating Aged Stem Cells Cell Stem Cell 2019 25 473 485.e478 10.1016/j.stem.2019.08.015 31585093 PMC6863391
65. Labuzek K. Suchy D. Gabryel B. Bielecka A. Liber S. Okopien B. Quantification of metformin by the HPLC method in brain regions, cerebrospinal fluid and plasma of rats treated with lipopolysaccharide Pharmacol. Rep. 2010 62 956 965 10.1016/S1734-1140(10)70357-1 21098880
66. Negrotto L. Farez M.F. Correale J. Immunologic Effects of Metformin and Pioglitazone Treatment on Metabolic Syndrome and Multiple Sclerosis JAMA Neurol. 2016 73 520 528 10.1001/jamaneurol.2015.4807 26953870
67. Cho Y. Crichlow G.V. Vermeire J.J. Leng L. Du X. Hodsdon M.E. Bucala R. Cappello M. Gross M. Gaeta F. Allosteric inhibition of macrophage migration inhibitory factor revealed by ibudilast Proc. Natl. Acad. Sci. USA 2010 107 11313 11318 10.1073/pnas.1002716107 20534506 PMC2895110
68. Suzumura A. Ito A. Yoshikawa M. Sawada M. Ibudilast suppresses TNFα production by glial cells functioning mainly as type III phosphodiesterase inhibitor in the CNS Brain Res. 1999 837 203 212 10.1016/S0006-8993(99)01666-2 10434004
69. Kawanokuchi J. Mizuno T. Kato H. Mitsuma N. Suzumura A. Effects of interferon-β on microglial functions as inflammatory and antigen presenting cells in the central nervous system Neuropharmacology 2004 46 734 742 10.1016/j.neuropharm.2003.11.007 14996551
70. Mizuno T. Kurotani T. Komatsu Y. Kawanokuchi J. Kato H. Mitsuma N. Suzumura A. Neuroprotective role of phosphodiesterase inhibitor ibudilast on neuronal cell death induced by activated microglia Neuropharmacology 2004 46 404 411 10.1016/j.neuropharm.2003.09.009 14975696
71. Fujimoto T. Sakoda S. Fujimura H. Yanagihara T. Ibudilast, a phosphodiesterase inhibitor, ameliorates experimental autoimmune encephalomyelitis in Dark August rats J. Neuroimmunol. 1999 95 35 42 10.1016/S0165-5728(98)00251-3 10229113
72. Ledeboer A. Liu T. Shumilla J.A. Mahoney J.H. Vijay S. Gross M.I. Vargas J.A. Sultzbaugh L. Claypool M.D. Sanftner L.M. The glial modulatory drug AV411 attenuates mechanical allodynia in rat models of neuropathic pain Neuron Glia Biol. 2006 2 279 291 10.1017/S1740925X0700035X 18176632 PMC2174834
73. Fox R.J. Coffey C.S. Conwit R. Cudkowicz M.E. Gleason T. Goodman A. Klawiter E.C. Matsuda K. McGovern M. Naismith R.T. Phase 2 Trial of Ibudilast in Progressive Multiple Sclerosis N. Engl. J. Med. 2018 379 846 855 10.1056/NEJMoa1803583 30157388 PMC6172944
74. Goodman A.D. Fedler J.K. Yankey J. Klingner E.A. Ecklund D.J. Goebel C.V. Bermel R.A. Chase M. Coffey C.S. Klawiter E.C. Response to ibudilast treatment according to progressive multiple sclerosis disease phenotype Ann. Clin. Transl. Neurol. 2021 8 111 118 10.1002/acn3.51251 33460301 PMC7818089
75. Naismith R.T. Bermel R.A. Coffey C.S. Goodman A.D. Fedler J. Kearney M. Klawiter E.C. Nakamura K. Narayanan S. Goebel C. Effects of Ibudilast on MRI Measures in the Phase 2 SPRINT-MS Study Neurology 2021 96 e491 e500 10.1212/WNL.0000000000011314 33268562 PMC7905793
76. Fox R.J. Raska P. Barro C. Karafa M. Konig V. Bermel R.A. Chase M. Coffey C.S. Goodman A.D. Klawiter E.C. Neurofilament light chain in a phase 2 clinical trial of ibudilast in progressive multiple sclerosis Mult. Scler. J. 2021 27 2014 2022 10.1177/1352458520986956 33635141 PMC8387506
77. Nakamura K. Thoomukuntla B. Bena J. Cohen J.A. Fox R.J. Ontaneda D. Ibudilast reduces slowly enlarging lesions in progressive multiple sclerosis Mult. Scler. J. 2024 30 369 380 10.1177/13524585231224702 PMC11190892 38286755
78. Muehler A. Peelen E. Kohlhof H. Groppel M. Vitt D. Vidofludimus calcium, a next generation DHODH inhibitor for the Treatment of relapsing-remitting multiple sclerosis Mult. Scler. Relat. Disord. 2020 43 102129 10.1016/j.msard.2020.102129 32428844
79. Fitzpatrick L.R. Deml L. Hofmann C. Small J.S. Groeppel M. Hamm S. Lemstra S. Leban J. Ammendola A. 4SC-101, a novel immunosuppressive drug, inhibits IL-17 and attenuates colitis in two murine models of inflammatory bowel disease Inflamm. Bowel Dis. 2010 16 1763 1777 10.1002/ibd.21264 20310011
80. Saijo K. Winner B. Carson C.T. Collier J.G. Boyer L. Rosenfeld M.G. Gage F.H. Glass C.K. A Nurr1/CoREST pathway in microglia and astrocytes protects dopaminergic neurons from inflammation-induced death Cell 2009 137 47 59 10.1016/j.cell.2009.01.038 19345186 PMC2754279
81. Vietor J. Gege C. Stiller T. Busch R. Schallmayer E. Kohlhof H. Hofner G. Pabel J. Marschner J.A. Merk D. Development of a Potent Nurr1 Agonist Tool for In Vivo Applications J. Med. Chem. 2023 66 6391 6402 10.1021/acs.jmedchem.3c00415 37127285 PMC10184128
82. Peelen E. Wu H. Marschner J. Busch R. Jafari M. Herrmann A. Wulff T. Gege C. Muehler A. Vitt D. Vidofludimus Calcium Activity on Nurr1 in Preclinical Models: A Potential Neuroprotective Function in Multiple Sclerosis Proceedings of the 40th Congress of the European Committee for Treatment and Research in Multiple Sclerosis Copenhagen, Denmark 18–20 September 2024 Abstract 1165
83. Fox R.J. Wolf C. Ondruš M. Sciacca V. Kohlhof H. Vitt D. Muehler A. Serum Neurofilament Changes in Progressive MS: Exploring the Impact of Vidofludimus Calcium by Age and Disability in the CALLIPER Study Interim Analysis Proceedings of the 40th Congress of the European Committee for Treatment and Research in Multiple Sclerosis Copenhagen, Denmark 18–20 September 2024 Abstract 686
84. Ben-Zvi I. Kivity S. Langevitz P. Shoenfeld Y. Hydroxychloroquine: From malaria to autoimmunity Clin. Rev. Allergy Immunol. 2012 42 145 153 10.1007/s12016-010-8243-x 21221847 PMC7091063
85. Wei Y. Nygard G.A. Ellertson S.L. Khalil S.K. Stereoselective disposition of hydroxychloroquine and its metabolite in rats Chirality 1995 7 598 604 10.1002/chir.530070807 8593253
86. Koch M.W. Zabad R. Giuliani F. Hader W. Jr. Lewkonia R. Metz L. Wee Yong V. Hydroxychloroquine reduces microglial activity and attenuates experimental autoimmune encephalomyelitis J. Neurol. Sci. 2015 358 131 137 10.1016/j.jns.2015.08.1525 26344560
87. Faissner S. Mahjoub Y. Mishra M. Haupeltshofer S. Hahn J.N. Gold R. Koch M. Metz L.M. Ben-Hur T. Yong V.W. Unexpected additive effects of minocycline and hydroxychloroquine in models of multiple sclerosis: Prospective combination treatment for progressive disease? Mult. Scler. J. 2018 24 1543 1556 10.1177/1352458517728811 28857721
88. Brown D. Moezzi D. Dong Y. Koch M. Yong V.W. Combination of Hydroxychloroquine and Indapamide Attenuates Neurodegeneration in Models Relevant to Multiple Sclerosis Neurotherapeutics 2021 18 387 400 10.1007/s13311-020-01002-5 33410109 PMC8116375
89. Koch M.W. Kaur S. Sage K. Kim J. Levesque-Roy M. Cerchiaro G. Yong V.W. Cutter G.R. Metz L.M. Hydroxychloroquine for Primary Progressive Multiple Sclerosis Ann. Neurol. 2021 90 940 948 10.1002/ana.26239 34590328
90. Waslo C. Hildebrand A. Rooney W. Morris C. Mitchell J. Metz J. Soldan M.M.P. Freedman M.S. Repovic P. Solomon A. Lipoic Acid for Treatment of Progressive Multiple Sclerosis; Results of a Phase 2 Randomized Placebo-.Controlled Trial Proceedings of the 40th Congress of the European Committee for Treatment and Research in Multiple Sclerosis Copenhagen, Denmark 18–20 September 2024 Abstract 204
91. Chataway J. Williams T. Blackstone J. De Angelis F. Bianchi A. Calvi A. Doshi A. John N.A. Mangion S.A. Wade C. Evaluating the effectiveness of simvastatin in slowing the progression of disability in secondary progressive multiple sclerosis (MS-STAT2 trial): A multicentre, randomised, placebo-controlled, double-blind phase 3 clinical trial BMJ Open 2024 14 e086414 10.1136/bmjopen-2024-086414 PMC11409264 39284697
92. Das I. Krzyzosiak A. Schneider K. Wrabetz L. D’Antonio M. Barry N. Sigurdardottir A. Bertolotti A. Preventing proteostasis diseases by selective inhibition of a phosphatase regulatory subunit Science 2015 348 239 242 10.1126/science.aaa4484 25859045 PMC4490275
93. Pellegrini L. Bonfio C. Chadwick J. Begum F. Skehel M. Lancaster M.A. Human CNS barrier-forming organoids with cerebrospinal fluid production Science 2020 369 eaaz5626 10.1126/science.aaz5626 32527923 PMC7116154
94. Chen Y. Podojil J.R. Kunjamma R.B. Jones J. Weiner M. Lin W. Miller S.D. Popko B. Sephin1, which prolongs the integrated stress response, is a promising therapeutic for multiple sclerosis Brain 2019 142 344 361 10.1093/brain/awy322 30657878 PMC6351782
95. Albrecht J. Schousboe A. Taurine interaction with neurotransmitter receptors in the CNS: An update Neurochem. Res. 2005 30 1615 1621 10.1007/s11064-005-8986-6 16362781
96. Oja S.S. Saransaari P. Significance of Taurine in the Brain Adv. Exp. Med. Biol. 2017 975 Pt 1 89 94 10.1007/978-94-024-1079-2_8 28849446
97. Huxtable R.J. Physiological actions of taurine Physiol. Rev. 1992 72 101 163 10.1152/physrev.1992.72.1.101 1731369
98. Beyer B.A. Fang M. Sadrian B. Montenegro-Burke J.R. Plaisted W.C. Kok B.P.C. Saez E. Kondo T. Siuzdak G. Lairson L.L. Metabolomics-based discovery of a metabolite that enhances oligodendrocyte maturation Nat. Chem. Biol. 2018 14 22 28 10.1038/nchembio.2517 29131145 PMC5928791
99. Yu Z. Jiang N. Su W. Zhuo Y. Necroptosis: A Novel Pathway in Neuroinflammation Front. Pharmacol. 2021 12 701564 10.3389/fphar.2021.701564 34322024 PMC8311004
100. Jiang X. Stockwell B.R. Conrad M. Ferroptosis: Mechanisms, biology and role in disease Nat. Rev. Mol. Cell Biol. 2021 22 266 282 10.1038/s41580-020-00324-8 33495651 PMC8142022
101. Zelic M. Pontarelli F. Woodworth L. Zhu C. Mahan A. Ren Y. LaMorte M. Gruber R. Keane A. Loring P. RIPK1 activation mediates neuroinflammation and disease progression in multiple sclerosis Cell Rep. 2021 35 109112 10.1016/j.celrep.2021.109112 33979622 PMC8917516
102. Van San E. Debruyne A.C. Veeckmans G. Tyurina Y.Y. Tyurin V.A. Zheng H. Choi S.M. Augustyns K. van Loo G. Michalke B. Ferroptosis contributes to multiple sclerosis and its pharmacological targeting suppresses experimental disease progression Cell Death Differ. 2023 30 2092 2103 10.1038/s41418-023-01195-0 37542104 PMC10482919
103. Mitroshina E.V. Saviuk M. Vedunova M.V. Necroptosis in CNS diseases: Focus on astrocytes Front. Aging Neurosci. 2022 14 1016053 10.3389/fnagi.2022.1016053 36778591 PMC9911465
104. Ryan S.K. Zelic M. Han Y. Teeple E. Chen L. Sadeghi M. Shankara S. Guo L. Li C. Pontarelli F. Microglia ferroptosis is regulated by SEC24B and contributes to neurodegeneration Nat. Neurosci. 2023 26 12 26 10.1038/s41593-022-01221-3 36536241 PMC9829540
105. Li Y. Xiao D. Wang X. The emerging roles of ferroptosis in cells of the central nervous system Front. Neurosci. 2022 16 1032140 10.3389/fnins.2022.1032140 36590286 PMC9797129
106. Davalos D. Akassoglou K. Fibrinogen as a key regulator of inflammation in disease Semin. Immunopathol. 2012 34 43 62 10.1007/s00281-011-0290-8 22037947
107. Akassoglou K. Strickland S. Nervous system pathology: The fibrin perspective Biol. Chem. 2002 383 37 45 10.1515/BC.2002.004 11928820
108. Adams R.A. Bauer J. Flick M.J. Sikorski S.L. Nuriel T. Lassmann H. Degen J.L. Akassoglou K. The fibrin-derived γ377-395 peptide inhibits microglia activation and suppresses relapsing paralysis in central nervous system autoimmune disease J. Exp. Med. 2007 204 571 582 10.1084/jem.20061931 17339406 PMC2137908
109. Davalos D. Ryu J.K. Merlini M. Baeten K.M. Le Moan N. Petersen M.A. Deerinck T.J. Smirnoff D.S. Bedard C. Hakozaki H. Fibrinogen-induced perivascular microglial clustering is required for the development of axonal damage in neuroinflammation Nat. Commun. 2012 3 1227 10.1038/ncomms2230 23187627 PMC3514498
110. Schachtrup C. Ryu J.K. Helmrick M.J. Vagena E. Galanakis D.K. Degen J.L. Margolis R.U. Akassoglou K. Fibrinogen triggers astrocyte scar formation by promoting the availability of active TGF-β after vascular damage J. Neurosci. 2010 30 5843 5854 10.1523/JNEUROSCI.0137-10.2010 20427645 PMC2871011
111. Yates R.L. Esiri M.M. Palace J. Jacobs B. Perera R. DeLuca G.C. Fibrin(ogen) and neurodegeneration in the progressive multiple sclerosis cortex Ann. Neurol. 2017 82 259 270 10.1002/ana.24997 28719020
112. Ryu J.K. Rafalski V.A. Meyer-Franke A. Adams R.A. Poda S.B. Rios Coronado P.E. Pedersen L.O. Menon V. Baeten K.M. Sikorski S.L. Fibrin-targeting immunotherapy protects against neuroinflammation and neurodegeneration Nat. Immunol. 2018 19 1212 1223 10.1038/s41590-018-0232-x 30323343 PMC6317891
113. Walport M.J. Complement. First of two parts N. Engl. J. Med. 2001 344 1058 1066 10.1056/NEJM200104053441406 11287977
114. Walport M.J. Complement. Second of two parts N. Engl. J. Med. 2001 344 1140 1144 10.1056/NEJM200104123441506 11297706
115. Sarma J.V. Ward P.A. The complement system Cell Tissue Res. 2011 343 227 235 10.1007/s00441-010-1034-0 20838815 PMC3097465
116. Veerhuis R. Nielsen H.M. Tenner A.J. Complement in the brain Mol. Immunol. 2011 48 1592 1603 10.1016/j.molimm.2011.04.003 21546088 PMC3142281
117. Michailidou I. Willems J.G. Kooi E.J. van Eden C. Gold S.M. Geurts J.J. Baas F. Huitinga I. Ramaglia V. Complement C1q-C3-associated synaptic changes in multiple sclerosis hippocampus Ann. Neurol. 2015 77 1007 1026 10.1002/ana.24398 25727254
118. Watkins L.M. Neal J.W. Loveless S. Michailidou I. Ramaglia V. Rees M.I. Reynolds R. Robertson N.P. Morgan B.P. Howell O.W. Complement is activated in progressive multiple sclerosis cortical grey matter lesions J. Neuroinflamm. 2016 13 161 10.1186/s12974-016-0611-x 27333900 PMC4918026
119. Michailidou I. Naessens D.M. Hametner S. Guldenaar W. Kooi E.J. Geurts J.J. Baas F. Lassmann H. Ramaglia V. Complement C3 on microglial clusters in multiple sclerosis occur in chronic but not acute disease: Implication for disease pathogenesis Glia 2017 65 264 277 10.1002/glia.23090 27778395 PMC5215693
120. Absinta M. Maric D. Gharagozloo M. Garton T. Smith M.D. Jin J. Fitzgerald K.C. Song A. Liu P. Lin J.P. A lymphocyte-microglia-astrocyte axis in chronic active multiple sclerosis Nature 2021 597 709 714 10.1038/s41586-021-03892-7 34497421 PMC8719282
121. Hu X. Tomlinson S. Barnum S.R. Targeted inhibition of complement using complement receptor 2-conjugated inhibitors attenuates EAE Neurosci. Lett. 2012 531 35 39 10.1016/j.neulet.2012.10.012 23079547 PMC3498843
122. Hu X. Holers V.M. Thurman J.M. Schoeb T.R. Ramos T.N. Barnum S.R. Therapeutic inhibition of the alternative complement pathway attenuates chronic EAE Mol. Immunol. 2013 54 302 308 10.1016/j.molimm.2012.12.018 23337717 PMC3602149
123. Fraser D.A. Pisalyaput K. Tenner A.J. C1q enhances microglial clearance of apoptotic neurons and neuronal blebs, and modulates subsequent inflammatory cytokine production J. Neurochem. 2010 112 733 743 10.1111/j.1471-4159.2009.06494.x 19919576 PMC2809134
124. Lunemann J.D. Hegen H. Villar L.M. Rejdak K. Sao-Aviles A. Carbonell-Mirabent P. Sastre-Garriga J. Mongay-Ochoa N. Berek K. Martinez-Yelamos S. Association of Complement Factors With Disability Progression in Primary Progressive Multiple Sclerosis Neurol. Neuroimmunol. Neuroinflamm. 2024 11 e200270 10.1212/NXI.0000000000200270 38912898 PMC11226316