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 cant always measure this as permanent disability. Current FDA-approved immunomodulatory disease-modifying treatments (DMTs) are highly effective in inhibiting CELs and MS relapses. However, CELs and MS relapses diminishes as patients age and are rare after the 5th decade, even in untreated subjects. 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. 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. 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. Thus, TRAP-MS trial aims to study CSF biomarkers in patients with advanced (progressive) MS who accumulate measurable disability progression while maximizing their benefit from FDA-approved treatments. These measurements 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). This review period we 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. These results corroborate published report that clemastine is an allosteric modulator of P2RX7 purinergic receptor in-vitro. Clemastine sensitizes P2RX7 to open P2RX7-associated pore with lower extracellular ATP concentrations. P2RX7 is highly expressed in CNS, has been linked to neurodegeneration and its genetic polymorphism is associated with MS. Extracellular ATP induces inflammasome activation and pyroptosis, the latter identified by our CSF studies as putative MS pathogenic mechanism. Thus, clemastine facilitates the ATP-mediated opening of P2RX7 pore, likely leading to accelerated death of CNS cells, including neurons. CSF consistent with activation of pyroptosis may identify patients at risk of clemastine toxicity. 6/8 MS patients had CSF pyroptosis signature before starting clemastine treatment, including 3/3 patients who triggered safety stopping criteria. This highlights the value of CSF biomarkers, as they were able to pinpoint this off-target mechanism in only 8 MS patients treated for 6 months. 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 desired IPD effects based on our most recent in-silico models, whereas 3 drugs that we are currently testing, which are dantrolene (tested as inhibitor of toxic astrogliosis), pioglitazone (tested as modulator of myeloid lineage thanks to PPAR-agonistic activity) and pirfenidone (tested as antifibrotic agent) are predicted to have beneficial IPD effects, with sizeable proportion of screened MS patients (20-50%, depend ending on the drug) predicted to have the therapeutic target. We expect that biomarker and mechanistic studies that accompany TRAP-MS trial will provide missing knowledge necessary for application of precision neurology in broad MS practice.