STRC Research

STRC Repurposed FDA Chaperone Branch

8 min read

STRC Repurposed FDA Chaperone Branch

Status: KILLED 2026-04-22

Mechanism disqualified. 4PBA is an ER chaperone. STRC is an extracellular structural protein. It doesn’t accumulate in the ER, so 4PBA has no target to act on. The only clinical data supporting cochlear 4PBA bioactivity (Li 2019, PMID 30516593) used a CDH23 model where ER stress is the actual mechanism — that mechanism does not transfer to STRC.

Kill summary:

  • 0 clinical trials on 4PBA for any structural extracellular protein
  • 0 human data on 4PBA for hearing loss
  • Li 2019 is the sole supporting paper: n=6 per group, no power analysis, dB values not reported in text, preventive design only, 0 replications in 6+ years, not cited in 2024 Frontiers ER stress reviews
  • Even if 4PBA reaches cochlear fluid at the concentrations Li 2019 implies, there’s no ER-trapped STRC protein for it to rescue

Lesson learned: Cochlear bioactivity (does the drug get there?) is a separate question from mechanism fit (does the drug have anything to do there?). Both must be answered. Li 2019 partially answered the first for CDH23. It doesn’t answer either for STRC.

What the research surfaced instead: Dual-AAV STRC gene therapy is the real path. Three active programs: Iranfar / Institut Pasteur (CTM 2026), Jeffrey Holt / BCH (Science Advances 2021), Regeneron AAV.104 (ex-Decibel, $109M acquisition 2023). See STRC Gene Therapy Landscape 2026.

Historical record preserved below for reference.

Parallel therapeutic path under STRC Pharmacochaperone Virtual Screen E1659A. Instead of developing a de-novo lead compound (5-10 years to clinic), screen already-FDA-approved chemical chaperones for E1659A-rescue activity. First green-light candidate takes an off-label / compassionate-use path to a Misha trial in months, not years.

Trigger

STRC Pharmacochaperone Phase 4c WT Decoy FAILED — all 5 Phase 3C de-novo top leads prefer WT over E1659A by mean +0.455 kcal/mol. The carboxylate-replaces-E1659 sub-mechanism is flagged. Continuing de-novo screening from the same chemotype is low-yield.

STRC Pharmacochaperone Phase 4g Repurpose Screen (2026-04-22) flipped the approach: dock known clinical chaperones instead. 4PBA (FDA-approved Buphenyl, since 1996) is the first and only compound in the entire Phase 4 series with mut-preferring selectivity (−0.32 kcal/mol mut−wt). IP-045 binds strongly but inherits the indole-3-AA WT-preference. TMAO below Phase 4b gate.

Why repurposing might beat de-novo

AxisDe-novo leadRepurposed FDA drug
Time to Misha5-10 years (IND → Ph 1-3 → approval)months (off-label via ENT or compassionate-use exemption)
Regulatory riskHigh (novel entity)Known safety profile (30 yr for Buphenyl)
Cost$50M-500M pipelinePharmacy cost of existing drug (~$1-5k/mo for Buphenyl)
PK dataZero humanDecades of pediatric dosing records
Efficacy in our indicationHigher possible ceilingBounded by what existing drug can do
Dose flexibilityFormulation-optimisableFixed to approved doses
Proof burden before trialFull INDCase report + ENT + parental consent (off-label) OR compassionate-use (IND-exempt)

Candidate pool (by priority)

Tier 1 — FDA-approved, chaperone mechanism validated

  1. 4-phenylbutyrate (4PBA, sodium or glycerol ester) — Buphenyl, Ravicti. Approved for urea cycle disorder, used in pediatrics. Known UPR modulator. Docking mut-preference in our screen. See STRC Pharmacochaperone Phase 4g Repurpose Screen.

  2. Tauroursodeoxycholic acid (TUDCA) — FDA-approved bile acid derivative in combination drugs; direct OTC in EU. Known chaperone, reduces ER stress, studied in ALS/Alzheimer’s/retinitis pigmentosa. Not yet in our Phase 4g — next extension.

  3. Sodium phenylbutyrate — closely related to 4PBA. Pediatric and adult dosing history.

Tier 2 — clinical / preclinical chaperones

  1. Trehalose — autophagy inducer, protein aggregation inhibitor. OTC in some regions. Oral bioavailability variable.

  2. Salubrinal — eIF2α phosphatase inhibitor, UPR-adjacent. Preclinical only.

  3. Curcumin — autophagy/UPR modulator. OTC, but PK/bioavailability weak.

Tier 3 — experimental / not pursued (too narrow)

  1. Ivacaftor / lumacaftor / elexacaftor — CFTR-specific, unlikely to bind STRC.

  2. VX-809 derivatives — same as above.

Key unknowns (the killer questions)

Gating whether this branch lives or dies:

  1. Does 4PBA reach outer hair cells at chaperone-active concentration after oral dosing? — PARTIALLY ANSWERED 2026-04-22 via web literature scan. Verdict: YELLOW leaning GREEN.

    • Li et al. 2019 (PMID 30516593, Neuroreport): IP 4PBA 50 mg/kg/day significantly protected ABR thresholds + DPOAE amplitudes in Cdh23^erl/erl mice (progressive OHC loss) over 12 weeks. Direct in-vivo proof of cochlear 4PBA bioactivity. Different mutation/mechanism, but chaperone action is generic on misfolded proteins.
    • Shimozono et al. 1998 (PMID 9447934): MCT1/MCT2 (4PBA’s primary BBB transporter) expressed in stria vascularis → mechanistic entry route to cochlea.
    • Berg et al. 2001 (PMID 11391852): CSF:plasma AUC ratio 41% in primates → implies cochlear fluid compartment reaches ~0.3-0.5 mM at pediatric Cmax 0.7-1.2 mM.
    • Samuels et al. 2016 (PMID 26748055): TUDCA IP prevents OHC death in same Cdh23 model. Sets precedent for systemic FDA-approved chaperone reaching cochlea.
    • Caveat: no study has directly measured 4PBA in perilymph. In-vitro threshold 1 mM vs estimated cochlear 0.3-0.5 mM is borderline, but Li 2019 + hayase 2026 in-vivo efficacy suggests in-vitro threshold overestimates requirement.
    • Path to GREEN: single LC-MS/MS perilymph measurement in mouse post-IP dose (weeks-scale, ~$5-10k).

  2. Does the E1659A mut-preference (from rigid docking) survive MM-GBSA rescoring? Docking’s −0.32 kcal/mol is within noise floor; MM-GBSA or short MD gives the authoritative number. Queued: Phase 4f modified script PHASE4F_INCLUDE_REPURPOSE=1 adds 4PBA to the MM-GBSA roster on next run.

  3. Is 4PBA’s chaperone action SPECIFIC to E1659A or GENERIC across missense mutations? Hayase 2026 used a different protein (DSCAML1 A2105T). If specific — docking mut-preference is real. If generic — 4PBA rescues by bulk stabilisation regardless of mutation identity; docking doesn’t measure that.

  4. Taste compliance. Buphenyl is notoriously bad-tasting; glycerol ester form (Ravicti) is tolerable for pediatric use. For Misha at age 4, formulation choice matters.

  5. Dose interaction with normal development. 450-600 mg/kg/day is not small. 4PBA is an HDAC inhibitor at high doses — long-term pediatric effect on cognition, growth? Needs literature check beyond urea cycle cohort.

Decision tree (if MM-GBSA confirms)

MM-GBSA ΔΔG_rescue for 4PBA on E1659A interface
  │
  ├─ >2.5 kcal/mol (clear pass)         → Cell-based test: transfect HEK293 with STRC E1659A,
  │                                       treat with 4PBA at 0.5/1/5 mM (cell-culture range),
  │                                       measure surface-expression by flow cytometry + IF
  │                                       COST: ~$3-5k reagents, 6-8 weeks
  │                                       │
  │                                       ├─ rescue observed → mouse model
  │                                       │   (DFNB16 mouse exists; dose 4PBA oral; ABR test)
  │                                       │   COST: ~$15-30k, 3-4 months
  │                                       │   │
  │                                       │   ├─ hearing improved → Misha trial via off-label
  │                                       │   │   prescription or compassionate-use IND
  │                                       │   │
  │                                       │   └─ no hearing effect → investigate PK, stop
  │                                       │
  │                                       └─ no rescue → this specific missense doesn't respond
  │                                           (maybe try TUDCA / salubrinal)
  │
  ├─ 0.5-2.5 kcal/mol (ambiguous)       → short MD + ensemble MM-GBSA (Phase 5)
  │
  └─ <0.5 kcal/mol (fail)                → drop 4PBA; escalate to TUDCA or extended Phase 4g roster

Off-label vs compassionate-use — fastest path to Misha

Off-label prescription (fastest, if ENT willing):

  • No regulatory filing needed
  • ENT writes Rx for Buphenyl/Ravicti indicating off-label DFNB16 use
  • Parental informed consent
  • Dosing by weight per Buphenyl label
  • Risk: no insurance coverage, cost ~$1-5k/month; ENT liability; without cell-based evidence, medically questionable

Compassionate-use (expanded access IND) — FDA-route:

  • File expanded-access IND (single-patient)
  • Requires minimal clinical rationale (which we’d have from cell-based + mouse test)
  • FDA responds in 30 days
  • Drug access at approved formulation
  • Insurance more likely to cover

Best path: do cell-based test first (6-8 weeks, $5k), get data → compassionate-use IND with ENT+institutional support → parallel off-label if ENT willing and family can pay bridge cost.

Scope — what this note is NOT

This is not a replacement for the Mini-STRC AAV S-tier path. That remains the primary therapeutic bet. 4PBA (if it works) is complementary: rescues existing misfolded maternal-allele protein while AAV delivers paternal-allele construct. Combination may raise total functional STRC above rescue threshold faster than either alone.

This is not a guarantee. Docking mut-preference is one data point. The hayase protein (DSCAML1) is different from STRC. 4PBA dose may not reach cochlea. Any single kill criterion fails → this branch dies.

Status

Active, upgraded to YELLOW-leaning-GREEN 2026-04-22 after cochlear PK scan returned strong precedents (Li 2019 ABR rescue + MCT1 in stria vascularis + TUDCA same-model precedent).

Gated on:

  • Cochlear PK literature scan → YELLOW-GREEN verdict 2026-04-22 (dedicated agent report)
  • Phase 4f MM-GBSA with 4PBA added to roster (PHASE4F_INCLUDE_REPURPOSE=1, pending current SMOKE run completion)
  • Extend Phase 4g with TUDCA + sodium phenylbutyrate + glycerol phenylbutyrate (Phase 4g v2)
  • LC-MS/MS perilymph measurement in mouse (the single experiment that resolves YELLOW→GREEN/RED)
  • If compute + PK green: design HEK293 E1659A cell-based 4PBA rescue experiment (wet-lab gate)
  • Reach out to Li et al. 2019 lab or STRC mouse model labs (Holt, Derstroff groups) about 4PBA protocol transfer

Precedent anchor papers (added 2026-04-22)

Connections

Graph View

Backlinks