Why Progressive MS Demands a New Mechanism

Progressive multiple sclerosis — encompassing primary progressive MS (PPMS) and non-relapsing secondary progressive MS (nrSPMS) — remains one of neurology’s most treatment-resistant conditions because the dominant driver of disability is not acute relapse, but chronic, smoldering neuroinflammation occurring behind the blood-brain barrier. Existing approved therapies reduce peripheral immune activity effectively, yet fail to penetrate the central nervous system in concentrations sufficient to suppress the microglial activation and compartmentalised B-cell pathology that sustain progressive tissue loss.

This biological reality has redirected drug development attention toward a class of agents that can act both peripherally and centrally: CNS-penetrant Bruton’s tyrosine kinase (BTK) inhibitors. BTK is expressed in B cells and in myeloid cells including microglia — the resident immune effectors of the brain — making it a dual-access target for the two cell populations most implicated in progressive MS pathology. According to patent filings reviewed in PatSnap’s life sciences intelligence platform, Genentech (a Roche subsidiary) and Merck KGaA have each built substantial IP portfolios staking out this therapeutic territory, with fenebrutinib and tolebrutinib respectively as their lead assets.

The scientific rationale is well-supported by emerging research. As documented in pharmacometric analyses of BTK inhibitors in neuroinflammatory disease, CNS penetration, target occupancy, and exposure-response relationships differ substantially between covalent and non-covalent BTK inhibitors — differences that have direct implications for both clinical efficacy and the IP strategies companies are pursuing. According to NIH-registered trial databases, both fenebrutinib and tolebrutinib are currently enrolling patients in Phase III studies specifically designed around progressive MS endpoints, including disability progression independent of relapse activity (PIRA).

Fenebrutinib’s CNS Penetration: The Science Behind the Patent Portfolio

Fenebrutinib — chemically designated as (S)-2-(3-(4-acryloyl-3-methylpiperazin-1-yl)-6-(1-(methylsulfonyl)-1H-pyrazolo[3,4-b]pyridine-5-yl)pyrazolo[1,5-a]pyrimidine-7-yl)-2-methylpropan-1-ol — is a non-covalent BTK inhibitor, distinguishing it from covalent agents like ibrutinib or tolebrutinib. Its non-covalent binding mechanism allows for reversible, highly selective BTK inhibition, which has implications for both safety profile and the duration of CNS target engagement.

Genentech’s patent strategy around fenebrutinib is notably granular in its coverage of CNS-specific mechanisms. The company has filed patents covering BTK occupancy assays in CNS tissue — methods for quantifying how much of the BTK target in the brain and spinal cord is actually engaged by the drug (WO2024102575A1, US20230309895A1, US20240285652A1). This is not a trivial distinction: demonstrating CNS target occupancy is both a regulatory and competitive differentiator, providing evidence that the drug is pharmacologically active where the pathology resides, not merely in the peripheral blood.

Beyond the occupancy assay patents, Genentech has protected the solid and crystalline forms of the fenebrutinib compound (US20230270733A1, US20210246139A1), establishing formulation-level IP that would be required for any generic challenger to design around. The company has also filed patents covering biomarkers for BTK inhibitor therapy in MS (WO2022150494A1, WO2024102576A1), including markers of B-cell activity and neuroinflammation — positioning fenebrutinib not just as a drug but as part of a companion diagnostic ecosystem that could reinforce its market exclusivity well beyond the compound patent itself.

Smoldering Lesions: The Battleground That Defines the Race

Smoldering lesions represent the most scientifically significant and commercially consequential frontier in progressive MS drug development. These are areas of chronic, low-grade neuroinflammation within the MS brain that persist even when relapses are fully suppressed — and they are now understood to be a primary driver of the insidious disability accumulation that characterises progressive disease.

The filing of WO2024072763A1 in April 2024 is strategically significant. By staking out smoldering lesion detection and treatment as a specific patent claim, Genentech is not merely protecting a drug — it is attempting to define an entirely new disease paradigm and own the IP around its clinical measurement and treatment. This mirrors the approach taken in oncology, where companion diagnostic patents have proven as commercially durable as the therapy patents themselves.

“By patenting smoldering lesion detection and treatment as a specific claim, Genentech is attempting to define a new MS disease paradigm — and own the IP around its clinical measurement.”

The biological rationale for targeting smoldering lesions with a CNS-penetrant BTK inhibitor is compelling. Microglia — the brain’s resident macrophage-like cells — are chronically activated at the rim of smoldering lesions, sustaining a low-grade but destructive inflammatory environment. Because microglia express BTK, a drug that penetrates the CNS and inhibits BTK in microglial cells can, in principle, suppress this chronic activation directly. This is precisely the mechanism that fenebrutinib’s patent filings, including the neurological disease treatment methods (WO2021183632A1, US20230372325A1), are designed to protect.

The clinical relevance of smoldering lesions is increasingly recognised by regulatory agencies. As documented by the European Medicines Agency in its evolving guidance on progressive MS trial design, endpoints that capture disability progression independent of relapse activity (PIRA) — which is the clinical manifestation of smoldering pathology — are now considered acceptable primary endpoints for progressive MS registration trials. This regulatory shift has accelerated investment in the CNS-penetrant BTK inhibitor class.

Fenebrutinib vs. Tolebrutinib: How the Patent Strategies Diverge

Tolebrutinib, developed by Merck KGaA (licensed to Sanofi for commercialisation), is the most direct competitive threat to fenebrutinib in the progressive MS space. Both are CNS-penetrant BTK inhibitors in Phase III clinical development, but their underlying chemistry, patent strategies, and clinical positioning differ in ways that matter for IP analysts and R&D leaders assessing the competitive landscape.

The covalent versus non-covalent distinction is not merely chemical: it has direct implications for patent strategy. Tolebrutinib forms an irreversible covalent bond with BTK, which produces prolonged target inhibition but also raises questions about selectivity and adverse events. Merck KGaA has filed patents covering covalent BTK inhibitors specifically for their CNS penetration and microglial inhibition properties (US20240024309A1), and separately filed patents covering non-covalent BTK inhibitors for neurological diseases (US20230338373A1) — suggesting the company is hedging its IP position across both binding modalities.

Merck KGaA’s tolebrutinib filings are clinically focused: the company has filed patents covering nrSPMS and PPMS treatment (WO2023031210A1, US20230381174A1), disability progression independent of relapse activity (US20230277560A1), and PPMS-specific BTK inhibitor therapy (US20230330084A1). What is notable by comparison is that Genentech has extended its fenebrutinib portfolio into mechanistic and diagnostic territory — CNS occupancy assays, smoldering lesion detection, biomarkers — that Merck KGaA’s tolebrutinib filings do not yet appear to cover. This asymmetry could prove strategically significant if smoldering lesion pathology becomes a regulatory label claim or a reimbursement-relevant endpoint.

According to WIPO‘s patent filing data, both companies accelerated their progressive MS-related BTK inhibitor filings between 2021 and 2024, reflecting the convergence of clinical readouts and regulatory clarity around PIRA as an acceptable endpoint. The competitive filing timeline suggests that both organisations are aware of the other’s IP activity and are filing defensively as well as offensively.

Patent Landscape Implications for R&D Leaders

The fenebrutinib and tolebrutinib patent race in progressive MS offers several lessons for R&D strategy teams and IP professionals tracking the CNS-penetrant BTK inhibitor space. The most important is that therapeutic IP in this area is no longer simply about protecting a molecule — it is about building a layered ecosystem of method, diagnostic, and biomarker patents that can sustain commercial exclusivity across the full clinical lifecycle of the drug.

Genentech’s strategy is instructive. The company has filed patents covering: the compound itself in solid and crystalline forms; methods of treating relapsing and progressive MS; BTK occupancy assays in CNS tissue; biomarkers predicting response to fenebrutinib; smoldering lesion detection and treatment; combination with anti-CD20 antibodies (WO2021026257A1); and dosing regimens (WO2019213268A1). This layered approach means that even if a competitor were to design around the core compound patent, they would face a thicket of method and diagnostic patents protecting the clinical use of the molecule in its most commercially valuable indications.

The combination therapy angle is particularly notable. Genentech has filed patents covering fenebrutinib in combination with anti-CD20 antibodies (WO2021026257A1, US20230041614A1), and separately with anti-CD20/anti-CD3 bispecific antibodies (US20230331763A1). Given that Roche’s ocrelizumab (an anti-CD20 antibody) is already approved for PPMS, these combination patents create a potential intra-portfolio synergy — and a patent-protected clinical development pathway — that Merck KGaA cannot easily replicate with tolebrutinib alone.

For R&D leaders and IP professionals, the key monitoring priorities in this space are: the outcome of Phase III trials for both fenebrutinib and tolebrutinib on PIRA endpoints; regulatory agency guidance on smoldering lesion imaging as a trial endpoint; and any patent oppositions or inter partes review proceedings targeting the core method patents in either portfolio. As noted by the World Health Organization in its reports on neurological disease burden, progressive MS affects hundreds of thousands of patients globally who currently have no approved therapy proven to slow disability accumulation — making the commercial stakes of this patent race substantial.

The broader lesson for drug development strategy is that the CNS-penetrant BTK inhibitor class in progressive MS illustrates how a mechanistic insight — that microglia express BTK and drive smoldering neuroinflammation — can be translated into a multi-layered IP position that extends from the molecule through to the diagnostic ecosystem. Companies that file early and broadly across the mechanistic, diagnostic, and combination therapy dimensions of a new therapeutic paradigm will be better positioned to sustain exclusivity even as the core compound patents mature. According to PatSnap’s IP analytics platform, this pattern of layered patent filing is increasingly characteristic of the most competitive therapeutic areas in neurology and immunology.