From therapeutic mechanisms to unraveling the pathophysiology of MS

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At MS inception, relapsing-remitting MS (RRMS) patients form focal lesions, quantified as contrast-enhancing lesions (CELs) on MRI. CELs are associated with the influx of immune cells from peripheral blood and opening of blood brain barrier (BBB) endothelial tight junctions. This focal inflammation is destructive to resident central nervous system (CNS) cells, resulting in focal demyelination and axonal transection. This leads to acute development of neurological disability presenting as MS relapse. Every CEL/relapse leads to permanent damage because human CNS transected axons do not regenerate, even if we cannot always measure this as permanent disability. Current FDA-approved immunomodulatory disease-modifying treatments (DMTs) are highly effective in inhibiting CELs and MS relapses, which we call in this review MS lesional activity. However, CELs and MS relapses diminishes as patients age and are rare after the 5th decade, even in untreated subjects. The mechanisms of MS progression that are not associated with the formation of new MS lesions well call here non-lesional MS activity. While the most efficacious DMTs inhibit CELs by more than 90% and do so equally in patients of all ages, the efficacy of DMTs on disability progression is much lower and decreases linearly with advancing patient age. Indeed, the meta-analysis of MS clinical trials shows that when initiated after age of 53 years, DMTs do not have measurable effect on disability progression. Based on age-prevalence of MS, this means that over 40% of patients living with MS have no effective treatment options and another 40% have sub-optimal benefit from current FDA-approved treatments. Therefore there remains very strong societal need for developing new treatments for MS that target non-lesional MS activity. What mechanisms drive MS progression once MS lesions stop forming is not known, although candidate mechanisms were identified in MS pathology studies and more recently from CSF biomarker studies (see AI001242-05). Causality of any candidate pathogenic process can be validated only in a successful interventional clinical trial. Currently, even the most sensitive disability outcomes require prohibitively large sample sizes (1000 patients treated for 2-3 years) to demonstrate efficacy of a single drug on inhibiting MS disability progression. This effectively permits testing only 1 drug for progressive MS worldwide at any given time, yielding unacceptably slow therapeutic progress for this underserved patient population. This project and associated adaptive, platform Phase II clinical trial TRAP-MS: Targeting Residual Activity by Precision, biomarker-guided combination therapies of Multiple Sclerosis (clinicaltrials.gov identifier NCT03109288) explores different paradigm of drug development and target(s) validation in MS patients with measurable disability progression despite current FDA-approved treatments. In concordance with systemic polygenic diseases such as cardiovascular diseases (CVD), we hypothesize that CNS damage in longstanding (progressive) MS is molecularly complex and intra-individually heterogeneous. Additionally, it is likely that disability accumulates after most endogenous regulatory and repair mechanisms have been exhausted. This leads us to hypothesize that if progressive accumulation of disability can be successfully inhibited (this remains unknown), it will require comprehensive measurements of patient-specific pathogenic mechanism(s) and attempts to limit them simultanously by using rational drug combinations. CVD-contributing mechanisms, such as hypertension, disorders of lipid and glucose metabolism, hypercoagulable state, and injuries to systems components (i.e., heart and vessels) must be analyzed in individual patients and if found abnormal, must be treated simultaneously. Without such patient-specific targeting, any cardiovascular DMT administered as monotherapy to all CVD patients would have marginal efficacy that would be very difficult to prove during drug development. The same is true for any chronic polygenic disorder, including MS. Thus, TRAP-MS trial aims to study CSF biomarkers in patients with advanced (progressive) MS who accumulate measurable disability progression while on FDA-approved treatments and without forming new MS lesions. Thus, these patients are progressing by non-lesional MS activity. CSF biomarker measurements in these patients are used to enhance knowledge about candidate disease mechanisms and to simultanously measure intrathecal pharmacodynamic (IPD) effects of tested drugs on such candidate pathogenic processes. We initiated TRAP-MS trial in 2017 lacking ability to comprehensively measure candidate pathogenic mechanisms in living people, lacking knowledge which of these may correlate with MS severity (i.e., speed of disability accumulation) or what are the IPD effects of drugs proposed for testing. Acknowledging this uncertainty in scientific and regulatory approvals, this protocol effectively aimed at filling this essential knowledge gap. As TRAP-MS recruits people progressing despite current DMTs (mostly older, moderately to severely disabled subjects excluded from pharma-sponsored trials) it targets understudied patient population with the greatest therapeutic need. The rationale for testing specific drugs, defined in the protocol, is beyond the limits of this report. Based on protocol-defined criteria, we have dropped from further testing 3 initial drugs because their IPD effects were insufficient: montelukast (showed no discernable IPD), losartan (showed intrathecal inhibition of ACE, but no desirable effects on MS severity) and hydroxychloroquine (showed expected decrease in intrathecal inflammation measured by CSF sCD27, but the decrease was <10% and remaining IPD were unfavorable). We also stopped clemastine fumarate when 3/8 patients triggered safety stopping criteria. Clemastine was studied as remyelinating agent based on positive animal experiments and Phase II MS trial, to define IPD signature of remyelination. Instead, the CSF data showed that clemastine activated ATP binding and purinergic signaling pathways. This review period we significantly expanded mechanistic understanding of this toxicity by demonstrating that clemastine, as an allosteric modulator of P2RX7 purinergic receptor, sensitizes P2RX7 to open P2RX7-associated pore with lower extracellular ATP concentrations. This led to activation of innate immune cells of myeloid lineage, resulting in inflammasome in clemastine-treated patients. We showed that this prolonged P2RX7 signaling leads to pyroptotic, inflammatory cell death in primary human cells of myeloid lineage, but also in human oligodendrocytes. Furthermore, we identified pyroptosis signature from CSF proteomic biomarkers that is significantly increased in MS patients compared to healthy controls and that significantly correlates with MS severity (i.e., with the rates of accumulation of clinical disability). We showed that clemastine treatment significantly increases this pyroptosis signature in-vivo. These observations provide strong evidence that pyroptosis is one of the leading mechanisms of CNS tissue destruction in most (but not all) MS patients. The CSF-biomarker-based pyroptosis signature could be used as tool to pre-select patients and to confirm desired IPD effect of future treatments that target intrathecal pyroptosis. Longitudinal CSF biomarker data collected under TRAP-MS protocol allowed us to identify and validate biomarker-based models that reproducibly correlate with MS severity. We used this knowledge to apply bioinformatics approaches for in-silico prediction of drugs that might inhibit candidate pathogenic mechanisms if they can cross the blood brain barrier (which is usually unknown). The 4 drugs we already dropped from testing were not predicted to have d