Part 1:A Trial Of Neuroprotective And Neuroregenerative Therapeutic Strategies in Multiple Sclerosis
Mar 22, 2022
Contact: joanna.jia@wecistanche.com / WhatsApp: 008618081934791
Niklas Huntemann1,2 · Leoni Rolfes1 · Marc Pawlitzki2 · Tobias Ruck1 · Stefen Pfeufer2 · Heinz Wiendl2 · Sven G. Meuth1
Accepted: 15 April 2021 / Published online: 4 June 2021
© The Author(s) 2021
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Abstract
In the recent past, a plethora of drugs have been approved for the treatment of multiple sclerosis (MS). These therapeutics are mainly confined to immunomodulatory or immunosuppressive strategies but do not sufficiently address remyelination and neuroprotection. However, several neurodegenerative agents have shown potential in pre-clinical research and entered Phase I to III clinical trials. Although none of these compounds have yet proceeded to approval, understanding the causes of failure can broaden our knowledge about neuroprotection and neuroregeneration in MS. Moreover, most of the investigated approaches are characterized by consistent mechanisms of action and proved convincing efficacy in animal studies. There- fore, learning from their failure will help us to enforce the translation of findings acquired in pre-clinical studies into clinical application. Here, we summarise trials on MS treatment published since 2015 that have either failed or were interrupted due to a lack of efficacy, adverse events, or for other reasons. We further outline the rationale underlying these drugs and analyze the background of failure to gather new insights into MS pathophysiology and optimize future study designs. For conciseness, this review focuses on agents promoting remyelination and medications with primarily neuroprotective properties or unconventional approaches. Failed clinical trials that pursue immunomodulation are presented in a separate article.
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Key Points
Innovative study designs with integrated clinical scores and longer observation periods are needed to depict neuroregenerative and neuroprotective effects.
In particular, add-on trials combining promising neuro-generative compounds with highly effective monoclonal antibody therapies could be favorable approaches.
Age-dependent effects, as well as the influence of dis-ease duration, need to be taken more into account in the design of studies evaluating neuroprotective and neuron-generative agents.
1 Introduction
The therapy of multiple sclerosis (MS) has experienced breakthroughs during the past 25 years, with many new agents being approved and multiplying therapeutic options for patient care [1]. The vast majority of these drugs were designed to modulate immune cell functions, predominantly benefiting patients with relapsing-remitting MS (RRMS). However, about 10–15% of patients show a primary chronic-progressive disease course (PPMS), and also in RRMS, a transition into secondary progressive MS (SPMS) frequently occurs [2]. The conversion to progressive MS is accompanied by a shift from neuroinflammation-driven disease pathogenesis towards neurodegeneration [3]. Consequently, the efficacy of immunomodulatory treatment is attenuated in patients with progressive disease courses [4, 5]. Thus, therapeutics supporting neuroregeneration are urgently required. These approaches likely include strategies that promote remyelination through inducing proliferation and different- initiation of oligodendrocyte precursor cells (OPCs) as well as the survival of mature oligodendrocytes [6]. Beyond remyelination, neuroregeneration can also be achieved by agents providing neuroprotection. These agents target a broad spectrum of detrimental processes such as excitotoxicity, mitochondrial dysfunction, or oxidative stress [7]. Unfortunately, at present none of these neurodegenerative compounds has made their way into clinical use[8].
Nonetheless, drugs that are tested in clinical trials are generally based on a conclusive rationale and were excessively evaluated in animal experiments. The reasons under- lying their failure are diverse.
In some studies, insights gained from animal models were just not translatable to clinical trials, even though they did not lack a thoughtful pathophysiological rationale. In other cases, unexpected adverse events (AEs) occurred or methodological insufficiencies obscured the potential beneficial effects of the investigated compounds. These limitations in trial design involve inappropriate study endpoints, insufficient follow-up durations, and samples biases. Therefore, analysis of these studies is crucial for advances in both pre-clinical and clinical research.
This review assesses neuroregenerative and alternative treatment strategies in MS since 2015 that entered clinical Phase I–III trials and either failed or were interrupted. First, we describe the mechanisms of action based on (pre-) clinical data underlying each approach. After delineating the respective trial, we identify key reasons for failure and outline consequences for future research on autoimmune neuroinflammation and MS.
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2 Methods
To identify relevant trials, we conducted a MEDLINE search for eligible articles as previously described [9]. We included articles that were published between 01 January 2015 and 31 December 2020. The terms used in Medical Subject Headings were ‘multiple sclerosis’, ‘trial’, and ‘therapy’ or ‘treatment’. Since trials with negative outcomes are merely published as abstracts or not at all, we additionally searched for relevant studies sourced from international conferences (Annual Meeting of the American Academy of Neurology [AAN], European/Americas Committee for Treatment and Research in Multiple Sclerosis [ECTRIMS/ACTRIMS]), consultation of national and international registries for clinical trials (United States National Library of Medicine; clinicaltrials.gov; European Union Drug Regulating Authorities Clinical Trials Database [EudraCT]), and personal communications with the authors. After removal of duplicates, we performed abstract or full-text screening to further exclude trials on compounds that either:
i. reached the primary study endpoint (PSE)
ii. were intended to treat secondary complications of MS (e.g., spasticity, fatigue)
iii. that were investigated in combination with other drugs if the other drug was not approved by the European Medicines Agency (EMA)/ U.S. Food and Drug Administration (FDA) for the treatment of MS.
If studies were conducted in cohorts with both relapsing MS or acute optic neuritis (AON) and progressive MS patients, the compound was assigned to whichever group predominated in number.
The outlined search returned 6656 records, 21 of which, with a total of 15 distinct agents, met our inclusion criteria (for details on the search strategy see Fig. 1; for details on individual compounds see Table 1).
3 Failed Clinical Trials in Acute Optic Neuritis and Relapsing Multiple Sclerosis
3.1 Neuroprotective Approaches
To some degree, MS can be understood as an interplay of neuroinflammation and neurodegeneration [10]. Thus, beyond immunomodulatory treatments, there is an urgent need for neuroprotective therapeutics to ameliorate neurodegenerative processes and to delay disability progression in MS. Neuroprotection involves a plethora of mechanisms ranging from the support of trophic factors and protection against excitotoxicity as well as oxidative stress to the restoration of mitochondrial function [7, 8]. Due to this variety of targets, a wide spectrum of neuroprotective agents has been tested in clinical trials.
3.1.1 Atorvastatin
3.1.1.1 Background Hydroxymethylglutaryl-CoA reductase inhibitors (here referred to as statins) are widely used
agents targeting dyslipidemia. Besides inhibition of the mevalonate pathway, statins were shown to prevent excitotoxicity [11], induce the secretion of neurotrophic factors [12], and reduce the release of free radicals due to suppression of inducible nitric oxide synthase [13]. Beyond neuroprotection, statins exhibit immunomodulatory properties. Among these are the restriction of antigen presentation [14], inhibition of leukocyte migration across the blood-brain barrier (BBB) [15], and induction of a shift of the inflammatory response towards T helper 2 (TH2) cells [16]. Considering these neuroprotective and immunomodulatory effects, statin treatment unsurprisingly proved convincing efficacy in experimental autoimmune encephalomyelitis (EAE) [15, 16]. Moreover, several small studies indicated a benefit of statins in RRMS, either alone or in combination with interferon-β [17–21].
3.1.1.2 Studies In 2016, the results of the ARIANNA study were published [22].
Enrolling 154 RRMS patients, atorvastatin was evaluated at a dose of 40 mg/day as an add-on therapy to interferon-β1b over 24 months in a double-blind, randomized, placebo-controlled manner. Atorvastatin failed to improve the pSE, i.e., the change in fractional brain volume. Also, secondary endpoints, including clinical (e.g., Expanded Disability Status Scale [EDSS] [23], annualized relapse rate [ARR]), cognitive (Rao battery test [24]), and magnetic resonance imaging (MRI) criteria (e.g., gadolinium-enhanced lesions [GELs]) were not met.
3.1.1.3 Comment The disappointing outcome of the ARI- ANNA study is in line with the failure of various other trials investigating the role of statins in RRMS [25–28, 33].
Foremost, the results of the largest trial in this context, the SIMCOMBIN study, are remarkable [29]. In this Phase IV trial, treatment with simvastatin as an add-on therapy to interferon-β1a failed to show any benefit. In addition, two meta-analyses confirmed that statin treatment does not improve clinical or MRI parameters in RRMS [30, 31]. Some trials even indicated an increased rate of T2 lesions or enhanced clinical disease activity in patients receiving statins [18, 28–30, 32, 33]. These observations might depend on statin-induced detrimental processes. These processes might potentially even aggravate neuronal- motion and degeneration. Underlying mechanisms involve enhanced secretion of pro-inflammatory cytokines [34] as well as depletion of OPCs and oligodendrocytes, thereby impairing remyelination [35].
Moreover, statins also mediate effects hampering their combination with interferon-β. Among these is the inhibition of interferon-induced phosphorylation of signal transducer and activator of transcription (STAT) 1 [36] and the counteraction of interferon-mediated suppression of matrix metalloproteinases (MMPs) [37]. Given these antagonistic actions, statins might better be combined with other disease-modifying therapies (DMTs) to create synergistic effects. Since the mechanism of action of interferon-β treatment is multifactorial and still not completely understood
[38], the investigation of promising compounds as add-on medications to interferon-β needs to be questioned in general. Instead of interferon-β, the combination with a highly effective immunomodulatory agent characterized by a clear mechanism of action seems to be more suitable to assess the effects of neuroprotection and neuroregeneration.
Notably, simvastatin was shown to reduce brain atrophy by 43% in a double-blind, placebo-controlled trial, including 140 patients with SPMS (MS-STAT) [39]. This promising outcome initiated an ongoing Phase III trial [40]. The convincing results of simvastatin in SPMS raise the question of whether the neuroprotective properties of statins might outweigh their anti-inflammatory capabilities. Noteworthy is a recent study that included patients from the ARIANNA trial and another study that assessed the impact of atorvastatin on RRMS as an add-on medication to interferon-β1a [41]. Forty-two patients who were allocated to placebo and 27 participants who received atorvastatin in the initial studies were followed up for about 8 years. Remarkably, the atorvastatin group displayed a reduced risk of EDSS progression; however, without significant amelioration of the relapse rate.
In conclusion, statins failed to provide benefits to RRMS patients in the short term. Nevertheless, long-term effects and the use in SPMS need to be further evaluated to confirm the initial positive results.
3.1.2 Lipoic Acid
3.1.2.1 Background
Lipoic acid (LA) is an endogenously produced antioxidant, that is used, for instance, for the treatment of diabetic peripheral neuropathy [42]. Anti-oxidative functions depend on a variety of processes. With its reduced form, dihydrolipoic acid, LA forms a potent redox couple [43]. Together, they regenerate other anti-oxidative agents [44], serve as chelators of metal ions [45], and improve mitochondrial activity [43]. Through the scavenging of free radicals [44], LA prevents reactive oxygen species-mediated dysfunction of the BBB and transendothelial monocytic migration [46]. Additionally, LA reduces cytokine secretion, proliferation, and central nervous system (CNS) in the traction of T cells [47, 48]. The impact on T cells seems to be mediated by an intracellular increase of cyclic adenosine monophosphate (cAMP) [49]. Several of the mentioned effects were already observed in MS patients receiving LA [50–52]. Given the impact on inflammation and oxidative processes, it is not surprising that EAE experiments demonstrated the efficacy of LA in neuroinflammation and neurodegeneration [47, 48].
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3.1.2.2 Studies
Completed in 2016, a double-blind, placebo-controlled, randomized Phase I trial analyzed the effects of LA in 31 patients diagnosed with experimental AON [53]. Treatment with 1200 mg/day LA or placebo was initiated within 14 days after diagnosis and lasted 6 weeks, followed by an 18-week observation period. The pSE evaluated the influence of LA on retinal nerve fiber layer (RNFL) thickness after 24 weeks compared to baseline. LA failed to improve RNFL regeneration. Strikingly, the LA group even displayed a trend of enhanced RNFL loss compared to placebo. On top of that, the secondary outcome criteria (e.g., RNFL thickness after 12 weeks, changes in low- and high- contrast visual acuity [VA]) were not met.
3.1.2.3 Comment The results from the reviewed study need to be interpreted with caution due to the small number of participants and the short follow-up period [53]. Nevertheless, the disenchanting outcome of this trial is contrary to the promising pre-clinical results. Of the many EAE studies conducted, however, only two investigated effects on AON. First, Chaudhary et al demonstrated alleviation of axonal injury both after prophylactic and therapeutic treatment with LA [54]. Protection of retinal ganglion cells (RGCs) was also observed by Dietrich et al [55]. However, the latter study reported that this protection only improved visual functions when LA was administered before clinical manifestation but failed to do so when applied after disease onset. Further, unlike prophylactic treatment, therapeutic administration also failed to protect the inner retinal layers from degeneration and ameliorate inflammatory infiltration into the optic nerve. Therefore, LA treatment in AON might only be suitable when administered immediately after symptom onset. Unfortunately, data on the mean duration of the first LA dose are not given in the reviewed trial [53].
Next, the pharmacokinetic characteristics of LA also impede its use in clinical care. Not only a short half-life but, even more important, strong interindividual variabilities in the concentration maximum and bioavailability complicate therapy management [52, 56, 57]. Apart from interindividual variations, the application of different formulations of LA results in variable responses [50, 56]. Unfortunately, no data are given concerning the formulation used in the described study [53].
Beyond pharmacokinetics, the consideration of pharmacodynamics is relevant in this context. The response to LA administration seems to vary between the different types of MS [51]. An increased cAMP response of peripheral blood mononuclear cells (PBMCs) upon LA administration was observed in SPMS patients. However, the treatment of RRMS patients led to reduced cAMP levels. The response of patients with AON to LA in terms of cAMP is unknown. Given the LA-induced increase of cAMP in SPMS, it should be mentioned that LA application reduced the annualized percentage brain volume change (PBVC) by 68% in patients with SPMS compared to placebo in a double-blind trial [58]. Consequently, a Phase II study in patients with progressive MS was started in 2018 [59]. Thus, research on LA seems to be more encouraging in patients suffering from progressive MS rather than AON.
3.1.3 Flupirtine Maleate
3.1.3.1 Background
Flupirtine maleate (here referred to as flirting) is a non-opioid analgesic drug [60]. Results were obtained from animal models of Parkinson’s disease (PD) [61] and Alzheimer’s disease (AD) [62] as well as a clinical trial in patients diagnosed with Creutzfeldt-Jakob dis-ease [63] indicate neuroprotective properties of this agent. Flupirtine seems to stabilize the membrane potential and reduce excitability by activation of KCNQ-type- [64] and G protein-coupled inwardly rectifying potassium channels [65]. Indirect antagonistic effects on N-Methyl-D-aspartate (NMDA) receptor activity [66] and gating of γ-aminobutyric acid (GABA)A receptors [64] seem to further contribute to neuroprotection. Last, flupirtine-induced neuroprotection was also demonstrated in animal models. During experimental AON, for instance, flupirtine treatment alleviated degeneration of RGCs and improved visual functions when used as an add-on medication to interferon-β1a [67].
3.1.3.2 Studies Including 30 RRMS patients, the FLO- RIMS study was conducted as a double-blind Phase II trial [60]. Participants received either flupirtine at a daily dosage of 300 mg or placebo for 1 year, both as an add-on treatment to interferon-β1b. While the trial was originally planned with 80 patients, only 30 patients were randomized due to unspecified recruitment problems. Finally, only 12 patients per group completed the trial. Due to the exploratory character of this study, no pSE was defined. Treatment with flupirtine resulted in advantages over placebo concerning clinical parameters such as the number of relapses or EDSS progression. Moreover, the active treatment group also displayed a reduced occurrence of GELs. However, none of these observations was significant. With regard to the safety profile, five patients in the flupirtine group displayed elevated liver enzymes leading to discontinuation in two cases.
3.1.3.3 Comment Although some non-significant trends indicated a mild benefit, the FLORIMS trial did not add reliable insights into the neuroprotective potential of flupirtine due to the small number of patients attending. This shortage of participants was caused by problems of patient recruitment and reduced the power to only 59% in terms of changes in EDSS [60]. Unfortunately, reasons underlying hampered recruitment were not further defined but may be related to reports of severe cases of flupirtine-induced hepatotoxicity [68]. The drug-induced liver injury resulted in an endorsed withdrawal of flupirtine by the EMA in 2018. The mechanisms of flupirtine-mediated hepatic damage are not completely understood. A genetic association with a class II human leukocyte antigen haplotype was observed, suggesting an inappropriate T cell response [69]. On the other hand, the formation of highly reactive and hepatotoxic metabolites has been discussed [70].
Moreover, there are still uncertainties regarding the role of flupirtine in neuroinflammation. Additional treatment with flupirtine during EAE, for instance, did not show any advantage compared to sole interferon-β1a administration [67]. In line with this observation, flupirtine did not ameliorate EAE symptoms when applied alone. Also, the mentioned protective impact of flupirtine treatment on RGCs during experimental AON was mediated independently from inflammatory processes. Thus, the selection of an RRMS cohort for the reviewed trial needs to be scrutinized. How- ever, following the endorsed withdrawal by EMA, further research on flupirtine treatment in MS is impeded.
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3.1.4 Uric Acid—Inosine
3.1.4.1 Background
Uric acid (UA) is a purine metabolite with anti-oxidative capacities [71]. Since oral supplementation does not increase serum concentrations of UA adequately, its precursor inosine can be used to raise UA levels [72]. UA-mediated anti-oxidation is mainly attributed to the scavenging of peroxynitrite [73]. Peroxynitrite is a neuro-toxic agent that causes damage to oligodendrocytes [74] and contributes to BBB disruption [75]. Moreover, peroxynitrite is associated with plaque formation in MS [76]. Of note, inosine and UA proved efficacy in several EAE experiments, as treatment alleviated disease severity [77, 78].
Also, clinical data suggest an involvement of UA in MS. First, even though there are contradictory reports, a meta-analysis showed that serum levels of UA are reduced in MS [79]. Interestingly, the lowest concentrations were found in SPMS patients. Second, twin studies demonstrated decreased UA levels in twins diagnosed with MS [80]. Last, an analysis of more than 20 million patient records revealed that the number of patients suffering both from MS and gout, which is characterized by an increased concentration of UA, was much lower than assumed, indicating a preventive role of UA in neuroinflammation [77].
3.1.4.2 Studies
The safety and efficacy of inosine were evaluated in 36 patients with RRMS in a Phase II trial [71]. Ino- sine at a daily dose of 3 g or placebo was administered in a double-blind manner as an add-on medication to interferon- β1a. Patients were followed up for 12 months. The pSE evaluated AEs and laboratory results related to UA. Unsurprisingly, inosine treatment elevated UA levels compared to placebo. Two patients treated with inosine experienced UA concentrations above 10 mg/dL while suffering from renal colic. Ten additional patients showed asymptomatic hyperuricemia. Secondary objectives analyzed clinical (e.g., number of relapses) and radiological disease activity (e.g., number of new MRI lesions). However, inosine failed to show any impact on these criteria.
3.1.4.3 Comment
Besides the mentioned study, a benefit of inosine treatment in MS must be further doubted, given the results from a double-blind, placebo-controlled trial in
159 RRMS patients [81]. There, inosine failed to ameliorate the relapse rate and disability progression. Strikingly, both this and the reviewed study evaluated inosine as an add-on treatment to interferon-β, although the latter is known to increase UA levels by itself [82]. Therefore, the simultaneous interferon-β application might have covered the beneficial effects of inosine.
Further, the selection of RRMS for evaluating inosine treatment remains questionable. Inosine might be more efficient in an SPMS cohort, given that UA levels are most depressed in these patients [79]. Also, considering the neuroprotective capabilities as an anti-oxidant and the lack of immunomodulation by UA in EAE [83], patients with progressive MS might profit more from inosine treatment than RRMS patients.
Since higher dosages than applied might be necessary to induce significant neuroprotection, side effects interfere with the clinical use of inosine. For instance, increased risks for cardiovascular events [84] and nephrolithiasis [85] associated with higher UA concentrations limit the dosage of inosine.
Outside the study design, the propagated mode of action also needs to be questioned. In light of the complexity of MS pathology, addressing a single anti-oxidant might not meet the needs required for manifest clinical improvement. Therefore, it seems unlikely that inosine monotherapy can effectively benefit RRMS patients.
