Nurix's BTK degrader NX-5948 scores just 2.2 on Pfizer's CNS Multi-Parameter Optimization scale, well below the 4-point threshold considered necessary for a drug to reach the brain, yet the report says it shows evidence of CNS exposure anyway.
That contradiction sits at the center of a new report examining whether brain-penetrant PROTACs, a class of drugs that degrade entire target proteins rather than simply blocking them, can succeed in neurodegenerative disease and CNS-involved cancers despite violating the design rules that have long governed CNS drug development. Two Natural Capital, whose author discloses a small long position in Arvinas, lays out early clinical evidence from Arvinas' ARV-102 and Nurix's NX-5948 while cautioning that neither program's ultimate therapeutic advantage is proven.
Ticker: NRIX (Nurix Therapeutics, Inc.); ARVN (Arvinas, Inc.)
Research Firm: Two Natural Capital
Report URL: https://www.twonaturalcap.com/p/brain-penetrant-protacs-arvinas-and?ref=shortreport.fyi
Position Disclosure: The author discloses a small long position in Arvinas.
Thesis
Two Natural Capital argues that brain-penetrant PROTACs may open up neurodegenerative and CNS-cancer targets that conventional small molecules cannot reach, using Arvinas' ARV-102 and Nurix's NX-5948 as early test cases.
- Whole-Protein Elimination: PROTACs bind both a target protein and an E3 ligase, triggering ubiquitination and full proteasomal breakdown, described by science writer Derek Lowe as making "the entire target protein disappear from the cell," a mechanism the report says could address transcription factors, scaffolding proteins, and intrinsically disordered proteins like alpha-synuclein and tau.
- Undruggable Target List: Mutant huntingtin, alpha-synuclein, and tau, the disease-driving proteins in Huntington's, Parkinson's, and Alzheimer's, have historically resisted conventional drugging because of their intrinsic disorder, positioning degradation as a potential alternate route.
- Catalytic Low-Dose Mechanism: Because one PROTAC molecule can degrade multiple copies of a target protein before exiting the cell, rather than needing to continuously occupy a single binding site, the report infers that less drug may need to cross the blood-brain barrier to work.
- Failed CNS Scorecard: Per Nurix's own investor presentation, NX-5948 scores 2.2 on Pfizer's CNS Multi-Parameter Optimization scale, where 4 or higher signals likely brain penetration, a shortfall attributed to excessive molecular weight, polar surface area, and hydrogen-bond donors.
- Brain Exposure Anyway: Despite that failing score, NX-5948, ARV-102, and C4 Therapeutics' CFT1946 all show evidence of CNS exposure, an outcome the report says standard rules like Lipinski's Rule of 5 do not fully explain.
- Whole-LRRK2 Bet: ARV-102 targets degradation of the entire LRRK2 protein, both its kinase and scaffolding functions, in contrast to Denali/Biogen's discontinued kinase-only inhibitor DNL151, which cut phosphorylated Rab10 in cerebrospinal fluid by 30% but still failed to slow Parkinson's progression in Phase 2b.
- CNS-Engineered BTK Degrader: NX-5948 degrades BTK and shows CNS exposure, potentially distinguishing it from existing BTK inhibitors like ibrutinib that cross the blood-brain barrier without being specifically engineered to do so; Nurix reports no liver issues so far, unlike Sanofi's tolebrutinib, which won European approval but was rejected in the U.S. for liver toxicity.
- Roche Validation: Nurix announced a partnership with Roche on NX-5948 in early June, the report's only cited external validation of the program.
Catalysts
- ARV-102 Phase 1 data in Parkinson's disease and progressive supranuclear palsy: Next reported readout would show whether whole-protein LRRK2 degradation succeeds where kinase-only inhibition failed.
- NX-5948 Phase 3 results in second-line-and-later chronic lymphocytic leukemia: Pending trial progress, would determine efficacy against existing BTK inhibitors.
- NX-5948 Phase 2 results in triple-exposed CLL and clinical updates in non-Hodgkin's lymphoma: Ongoing, would broaden or narrow the degrader's addressable market.
- Further NX-5948 data on CNS exposure and liver safety: Future disclosures, would confirm or undercut its safety edge over tolebrutinib.
- Developments from the Nurix-Roche partnership: No date given, would signal whether the collaboration advances toward commercialization.
- A development-partnering decision for C4 Therapeutics' CFT1946: Currently paused pending a partner, would determine whether a third brain-penetrant PROTAC advances.
Company Response
Not addressed in the source report.
Notable Details
- The blood-brain barrier's P-glycoprotein pump actively recognizes and ejects larger, more polar molecules, the exact profile PROTACs are typically expected to have, making brain penetration a double hurdle of size and active exclusion.
- NX-5948 spans an unusually wide ambition for a single degrader: Phase 3 in blood cancer, evidence of CNS exposure, and potential relevance to multiple sclerosis.
- C4 Therapeutics has paused development of its brain-penetrant BRAFV600-targeting PROTAC, CFT1946, while it seeks a partner, a reminder that scientific promise doesn't guarantee continued funding.
- FDA-approved drugs already exist that fall outside Lipinski's Rule of 5, the report notes, meaning the empirical CNS design rules PROTACs are violating were never absolute in the first place.
"These MPO guidelines should mean that PROTACs are definitively out when it comes to CNS-penetrance."
The author writes this just before presenting evidence that several PROTACs, including NX-5948 and ARV-102, have shown CNS exposure anyway, capturing the report's central scientific contradiction.
FAQs
What is Nurix's stock ticker NRIX tied to in this report?
NRIX is the ticker for Nurix Therapeutics, developer of the BTK degrader NX-5948, which is in Phase 3 trials as a second-line-and-later monotherapy for chronic lymphocytic leukemia and in Phase 2 for triple-exposed CLL. The report examines NX-5948's CNS exposure and its early-June partnership with Roche.
What is Arvinas, Inc. and what does it make?
Arvinas, Inc. is a biopharmaceutical company developing ARV-102, a Phase 1 candidate for Parkinson's disease and progressive supranuclear palsy that targets the LRRK2 protein. The report frames ARV-102 as a test of whether degrading LRRK2's full range of functions can succeed where a kinase-only inhibitor from Denali/Biogen failed.
Who is Two Natural Capital, the research firm behind this report?
Two Natural Capital is the Substack publication that authored this analysis of brain-penetrant PROTACs. Its author discloses a small long position in Arvinas and says the publication's readership includes investors from Baillie Gifford, GIC, and Susquehanna.
What is a PROTAC and how is it different from a normal drug?
A PROTAC is a molecule made of two small molecules connected by a linker, one that binds a target protein and one that binds an E3 ligase, which together trigger the target protein's ubiquitination and destruction by the cell's proteasome. Unlike conventional inhibitors that block one function of a protein, PROTACs eliminate the entire protein, which the report says could matter for scaffolding proteins like BTK and transcription factors like STAT6.
What is Pfizer's CNS MPO score and why does it matter here?
Pfizer's CNS Multi-Parameter Optimization tool scores drug candidates on properties like molecular weight, polar surface area, and hydrogen-bond donors, with each parameter rated 0 to 1 and a total of 4 or higher considered likely to indicate good brain penetration. NX-5948 scored 2.2 per Nurix's own presentation, which the report says should predict poor CNS access, yet the compound reportedly shows CNS exposure regardless.
Why did Denali and Biogen discontinue their Parkinson's drug?
Denali and Biogen discontinued their LRRK2-targeting candidate, DNL151, after Phase 2b results failed to slow Parkinson's disease progression, even though the drug showed peripheral LRRK2 kinase inhibition and a 30% reduction of phosphorylated Rab10 in cerebrospinal fluid. The report contrasts this kinase-only failure with Arvinas' ARV-102, which aims to degrade the entire LRRK2 protein rather than just inhibit its kinase activity.
What is the Nurix-Roche partnership and when was it announced?
Nurix announced a partnership with Roche involving NX-5948 in early June, though the report does not disclose financial terms. It is cited as the report's clearest example of external validation for Nurix's brain-penetrant BTK degrader program.
Why was Sanofi's tolebrutinib rejected in the United States?
Sanofi's BTK inhibitor tolebrutinib was approved in Europe but rejected in the U.S. because of liver toxicity concerns. The report uses this comparison to underline the safety stakes for BTK-directed CNS drugs, noting that Nurix says it has not observed liver issues with NX-5948 so far.
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