Why conventional MS therapies miss the smoldering lesion problem

Progressive multiple sclerosis is not simply a more advanced form of relapsing disease — it is driven by a distinct, chronic neuroinflammatory state sustained within the CNS parenchyma itself, largely decoupled from the peripheral immune infiltration that current anti-relapse therapies target. The core pathological substrate is the slowly expanding lesion (SEL), also termed a smoldering lesion: an MRI-visible lesion that persists and enlarges at its rim over months to years, populated predominantly by activated microglia and macrophages rather than acutely infiltrating lymphocytes. Conventional disease-modifying therapies, including the highly effective anti-CD20 agents, largely fail to reach or suppress this compartmentalized inflammatory activity, which explains why disability continues to accumulate even in patients with no clinical relapses.

According to research published in The Lancet Neurology, smoldering neuroinflammation — characterised by SELs and chronic microglial activation — is now recognised as a defining feature of progressive MS that requires pharmacological strategies capable of penetrating the blood-brain barrier (BBB) and engaging CNS-resident immune cells directly. This recognition has catalysed a new generation of CNS-penetrant BTK inhibitors designed not merely to deplete peripheral B cells but to simultaneously suppress microglial activity within the brain parenchyma.

The cellular biology underpinning this therapeutic gap is well-characterised. Activated microglia at the SEL rim express BTK — Bruton’s tyrosine kinase — and signal through Fc-gamma receptor and pattern recognition receptor pathways to produce inflammatory mediators including TNF-α, IL-6, and reactive oxygen species. BTK also mediates phagocytosis of myelin debris by these cells. In parallel, meningeal ectopic lymphoid follicles — aggregates of B cells at the brain surface — drive subpial cortical demyelination through intrathecal immunoglobulin production, providing a second mechanistic rationale for BTK inhibition in the same disease context.

BTK’s dual role: B cells at the meninges and microglia at the lesion rim

BTK (Bruton’s tyrosine kinase), encoded by the BTK gene, is the critical signalling node linking B cell receptor (BCR) activation and Fc receptor (FcR) signalling in both peripheral B cells and CNS-resident microglia — which is precisely why it has become the most intensively pursued target in progressive MS drug development. Histopathological studies of human MS lesion tissue have confirmed BTK mRNA and protein expression in brain microglia, providing direct neuropathological rationale for CNS-penetrant BTK inhibitor strategies beyond what oncology-derived BTK inhibitors can offer.

The dual-mechanism hypothesis that underpins both fenebrutinib and tolebrutinib can be decomposed into two distinct but complementary pathways. First, at the meningeal level, BTK inhibition suppresses B cell maturation, plasmablast differentiation, and intrathecal immunoglobulin production within ectopic lymphoid follicles — directly targeting the meningeal inflammatory microenvironment that drives subpial cortical demyelination. Second, within the white matter parenchyma, BTK inhibition suppresses microglial activation at the SEL rim by blocking downstream NF-κB and PI3K-δ signalling cascades activated through Fc-gamma receptors and pattern recognition receptors.

This dual-mechanism rationale is what distinguishes CNS-penetrant BTK inhibitors from both the first-generation oncology BTK inhibitors (ibrutinib, acalabrutinib) — which were not designed for BBB penetrance — and from anti-CD20 therapies, which deplete peripheral B cells but cannot reach CNS-compartmentalised disease activity. According to research published in Nature Reviews Neurology, the direct link between BTK inhibition-mediated microglial suppression and clinical disability outcomes remains a Phase III hypothesis rather than a demonstrated clinical finding — representing both the core investment thesis and the primary risk in this programme class.

Downstream of BTK, the NF-κB and PI3K-δ signalling cascades are identified in patent filings from both Roche/Genentech and Sanofi as the relevant inflammatory output pathways in both lymphocyte and myeloid cell compartments. Fenebrutinib’s patent filings describe selectivity profiling against kinase panels including TEC family kinases, with particular attention to selectivity over ITK (interleukin-2-inducible T cell kinase) as a design criterion to avoid T cell immunosuppression — a documented differentiation strategy that distinguishes fenebrutinib from less selective BTK inhibitors.

Fenebrutinib vs. tolebrutinib: covalent, non-covalent, and the DILI divide

The most consequential mechanistic distinction between the two leading CNS-penetrant BTK inhibitors in progressive MS is their binding chemistry — and it has acquired clinical significance that extends well beyond pharmacology textbooks. Tolebrutinib (SAR442168, Sanofi) is an irreversible, covalent BTK inhibitor that forms a covalent bond with Cys481 on BTK, achieving prolonged target occupancy that has been proposed as advantageous for sustained microglial suppression. Fenebrutinib (GDC-0853, Roche/Genentech) is a non-covalent, reversible BTK inhibitor whose chemical scaffold — described in Genentech/Roche patent filings — was optimised for CNS penetrance and selectivity while avoiding the permanent kinase modification that characterises covalent agents.

The hepatotoxicity dimension is not merely a safety footnote. The FDA’s 2022 clinical hold on tolebrutinib — driven by drug-induced liver injury (DILI) signals — was subsequently modified to allow continued trials under enhanced monitoring, with Sanofi implementing mandatory liver function monitoring protocols and revised patient selection criteria. This regulatory event has introduced a competitive asymmetry: if fenebrutinib’s non-covalent mechanism yields a cleaner hepatic safety profile in Phase III data, this could substantially influence prescriber preference, label language, and commercial positioning even if both agents demonstrate comparable efficacy on disability progression endpoints.

“The direct link between BTK inhibition-mediated microglial suppression and clinical disability outcomes remains a Phase III hypothesis rather than a demonstrated clinical finding — representing both the core investment thesis and the primary risk in this programme class.”

Fenebrutinib’s selectivity profile adds a further dimension to the competitive picture. Patent filings from Genentech describe fenebrutinib’s selectivity over ITK (interleukin-2-inducible T cell kinase) as an explicit design criterion, aimed at avoiding T cell immunosuppression that could increase infection risk. This selectivity engineering — documented in patent records including US10213433B2 and US20220193048A1 — represents an IP-protected pharmacological differentiation strategy. Both compounds have demonstrated measurable concentrations in cerebrospinal fluid (CSF) in Phase I/II studies, with CSF:plasma ratios cited as evidence of CNS target engagement sufficient to inhibit microglial BTK.

Phase III trial designs and the smoldering lesion endpoint race

The Phase III programmes for fenebrutinib and tolebrutinib represent the most advanced clinical tests of the smoldering lesion hypothesis in progressive MS, with trial designs that reflect both the maturing science of SEL biology and the regulatory need for validated endpoints. Fenebrutinib’s Phase III programme, including the FENopta trial in PPMS and SPMS cohort designs, uses 12-week confirmed disability progression (CDP) as the primary endpoint alongside SEL volume change as a key secondary or co-primary measure. Tolebrutinib’s Phase III portfolio is broader: the HERCULES trial targets non-relapsing SPMS, GEMINI 1 and 2 target relapsing MS, and the DAYBREAK open-label extension provides long-term safety and durability data.

SEL metrics have been analytically validated in observational cohorts, with research published in the Annals of Neurology describing SEL volume and count on 3T and 7T MRI as imaging endpoints with sufficient reproducibility and sensitivity to detect treatment effects. The gene expression signatures of SEL-specific biology — enriched for microglial activation markers including TREM2, TMEM119, CX3CR1, and complement cascade components — provide a molecular rationale for why BTK inhibition might specifically attenuate SEL expansion even in patients with no active gadolinium-enhancing lesions.

The translational endpoint architecture in these trials reflects a broader trend documented across retrieved results: patient stratification by baseline SEL burden, microglial activation on PET, or CSF NfL levels is being explored as a mechanism to enrich trial populations for the subset of progressive MS patients with the highest compartmentalised inflammatory activity. According to research published in the Multiple Sclerosis Journal, this biomarker-stratified approach could enable more efficient Phase III designs and potentially support companion diagnostic development — a commercially significant consideration given the cost of large progressive MS trials. Standards for biomarker validation in neurological drug development are informed by frameworks from organisations such as NIH and EMA.

No retrieved result provides confirmed Phase III primary endpoint readout data or regulatory approval decisions for either fenebrutinib or tolebrutinib in MS — both programmes remain active at the time of writing. This means the smoldering lesion endpoint race is genuinely unresolved: the field is waiting to learn whether BTK inhibition can translate its mechanistically compelling preclinical and early-clinical rationale into statistically significant and clinically meaningful disability outcomes in the hardest-to-treat MS population.

Portfolio strategy, IP territory, and the next generation of BTK targeting

Roche’s strategic position in progressive MS is unusually strong because it combines two mechanistically complementary assets within a single portfolio. Ocrelizumab — the company’s anti-CD20 antibody and the first approved therapy for PPMS — depletes peripheral B cells but cannot reach CNS-compartmentalised disease. Fenebrutinib, if Phase III data are positive, would address precisely the disease biology that ocrelizumab cannot. Patent filings from Roche cover both ocrelizumab and fenebrutinib, and retrieved academic literature documents the biological rationale for sequential or combination regimens — positioning Roche to control the standard-of-care narrative across the progressive MS continuum in a way that no single-asset competitor can replicate.

The IP landscape around BTK inhibition in MS extends beyond the compounds themselves. Retrieved results signal that SEL-based MRI endpoints are being simultaneously developed as clinical trial tools and as IP-protectable diagnostic and stratification methods. Companies establishing SEL biomarker claims in patents alongside therapeutic filings may create integrated IP estates spanning treatment and companion diagnostics — a strategy that mirrors precedents in oncology where companion diagnostic co-development has become a standard commercial architecture. Patent filings from Genentech (including US20220193048A1, covering methods of treating MS using BTK inhibitors with improved CNS bioavailability) illustrate this integrated approach.

Beyond the current covalent/non-covalent inhibitor generation, retrieved patent data reveal an emerging next-generation direction: BTK-targeting PROTAC (proteolysis targeting chimera) molecules designed for CNS application. Patent filings from Nurix Therapeutics (WO2022256723A1) describe BTK PROTAC degrader compounds for use in neuroinflammatory CNS disease. These remain exclusively at the preclinical patent filing stage with no clinical translation signal in the current dataset, but they represent a monitored direction for post-inhibitor innovation — particularly relevant if resistance mechanisms or incomplete target suppression emerge as limitations of first-generation BTK inhibitors in long-duration progressive MS treatment.

The combination strategy landscape is also evolving. Academic literature documents the biological rationale for combining BTK inhibitors with pro-remyelination agents — such as anti-LINGO-1 antibodies — to simultaneously arrest SEL expansion and promote myelin repair, though no patent has explicitly claimed this combination. Retrieved results also discuss the theoretical possibility of sequencing anti-CD20 therapies with BTK inhibitors to address both peripheral B cell replenishment dynamics and compartmentalised CNS inflammation. Global innovation frameworks for neurological drug development, as monitored by organisations including WHO, increasingly recognise the need for combination approaches in progressive neurological diseases where single-mechanism therapies have shown limited impact.

CNS penetrance — measured as CSF:plasma ratio — has become the primary pharmacological criterion distinguishing MS-relevant BTK inhibitors from oncology-derived agents. Both Roche and Sanofi have engineered their compounds specifically on this criterion, creating a high barrier for late-entering competitors without equivalent CNS pharmacokinetic data. This pharmacological moat, combined with the extensive Phase III investment already made by both companies, means that meaningful new entrants in CNS-penetrant BTK inhibition for progressive MS will need to demonstrate not merely BTK inhibitory potency but differentiated CNS exposure, a clean safety profile, and a credible endpoint strategy — a combination of requirements that substantially raises the development cost and risk for any programme entering the field today.