# Compound Dive: TDP-43 Proteostasis, Autophagy, and Stress Granule Dynamics in ALS

**Date:** 2026-05-04  
**Prepared for:** Repurposing and translational strategy for ALS therapeutics targeting TDP-43–mediated pathology.

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## 1. Scope and Methodology

This review focuses on **approved or late-stage clinical compounds** that modulate three interconnected pathways implicated in TDP-43–associated ALS pathology:

1. **Proteostasis / TDP-43 aggregate clearance** — mechanisms that restore chaperone capacity and reduce TDP-43 misfolding/aggregation.
2. **Autophagy / lysosomal clearance** — pathways that remove TDP-43–containing cytoplasmic inclusions and damaged organelles.
3. **Stress granule (SG) dynamics** — processes governing reversible cytoplasmic RNA–protein foci that can seed persistent TDP-43 pathology.

**Note:** Real-time PubMed and web search returned no results during this session; the evidence base below synthesizes well-established knowledge curated from the clinical and open literature. Regulatory status descriptions reflect publicly known filings as of mid-2026 and should be verified against the latest FDA/EMA records.

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## 2. Compounds Targeting TDP-43 Proteostasis

### 2.1 Arimoclomol
| Attribute | Detail |
|---|---|
| **Class** | Hydroxylamine derivative; co-inducer of heat-shock proteins (HSPs) |
| **Primary Indication** | Niemann-Pick type C (NPC) — granted orphan designation and **conditional/approved** in some jurisdictions; investigational in ALS and other proteinopathies |
| **Mechanism** | Up-regulates the heat-shock response without directly binding HSPs. Amplifies stress-induced expression of HSP70, HSP90, and HSP27 by sensitizing the heat-shock factor-1 (HSF1) transcriptional response. Enhances refolding of misfolded TDP-43 and promotes clearance of detergent-insoluble aggregates. |
| **Key ALS Evidence** | Preclinical work in TDP-43 transgenic mice demonstrated reduced TDP-43 aggregation, improved motor function, and extended survival. Human ALS clinical trials (Phase II/III era by Orphazyme/Rhodes Pharmaceuticals) evaluated oral arimoclomol in patients with rapidly progressing ALS; the program was placed under regulatory review at multiple agencies, with mixed readouts. |
| **Translational Notes** | **Lipinski-compliant** (MW = 313.78, LogP = 1.2, HBD = 1, HBA = 5). Good oral bioavailability and CNS penetration profile. Pharmacodynamic biomarkers include serum HSP70 levels. |

### 2.2 AMX0035 (Sodium Phenylbutyrate + Tauroursodeoxycholic Acid)
| Attribute | Detail |
|---|---|
| **Class** | Fixed-dose combination of a histone deacetylase (HDAC) inhibitor / chemical chaperone (NaPB) and a taurine-conjugated bile acid (TUDCA) |
| **Primary Indication** | Amyotrophic lateral sclerosis — **conditionally approved** in Canada and the UK based on the Phase 2 CENTAUR trial; US regulatory pathway has been complex |
| **Mechanism** | NaPB acts as a histone deacetylase inhibitor at higher doses and a chemical chaperone/molecular stabilizer at lower doses, reducing ER stress and promoting upregulation of chaperones (e.g., HSP70). TUDCA is an endoplasmic reticulum stress attenuator and mitochondrial stabilizer. Together, they reduce neuronal apoptosis in the face of misfolded protein accumulation, including TDP-43 oligomers. |
| **Key ALS Evidence** | CENTAUR: statistically significant benefit in functional decline (ALSFRS-R total score) and trends in survival extension. Some post-hoc and biomarker analyses noted reductions in plasma neurofilament light chain (NfL), a marker of axonal degeneration correlated with TDP-43 burden in sporadic ALS. |
| **Translational Notes** | Both components have extensive human safety records (NaPB is approved for urea-cycle disorders; TUDCA has regulatory use in biliary disorders in some regions). The combination is already the standard of care for ALS in some jurisdictions, making it the *de facto* proteostasis reference standard in current ALS practice. |

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## 3. Compounds Modulating Autophagy for ALS

### 3.1 Sirolimus (Rapamycin)
| Attribute | Detail |
|---|---|
| **Class** | Macrocyclic lactone / mTORC1 inhibitor |
| **Primary Indication** | **Approved** as an immunosuppressant (solid organ transplant); also approved for lymphangioleiomyomatosis and some vascular malformations |
| **Mechanism** | Binds FKBP12 and inhibits the kinase activity of mTORC1. Suppression of mTORC1 disinhibits ULK1 and triggers macroautophagy initiation. In ALS models, enhanced autophagy accelerates clearance of TDP-43ΔNLS and C-terminal TDP-43 fragments that accumulate in the cytoplasm. Can also stimulate mitophagy, improving mitochondrial quality control in vulnerable motor neurons. |
| **Key ALS Evidence** | Multiple preclinical studies in SOD1 and TDP-43 mouse models show delayed disease onset, reduced aggregate load, and modest survival extension. A handful of small human trials and compassionate-use reports describe variable tolerability at immunosuppressive doses. No large Phase 3 has been completed in ALS. |
| **Translational Notes** | **Lipinski-violating** (MW = 914, LogP = 6.0), but this is expected for a natural-product–derived macrolide administered orally with excellent bioavailability. Chronic use carries significant risks (immunosuppression, stomatitis, impaired wound healing) that complicate dosing in ALS. |

### 3.2 Everolimus
| Attribute | Detail |
|---|---|
| **Class** | Rapamycin analog (mTORC1 inhibitor) |
| **Primary Indication** | **Approved** for renal cell carcinoma, breast cancer, neuroendocrine tumors, tuberous sclerosis complex, and transplant rejection prophylaxis |
| **Mechanism** | Same pathway as sirolimus but with a 2-hydroxyethyl substitution that improves water solubility and oral pharmacokinetics. Inhibits mTORC1 → stimulates autophagic flux. |
| **Key ALS Evidence** | Less ALS-specific literature than sirolimus, but mechanistically equivalent. Some preclinical labs have used everolimus in TDP-43 cell models with comparable aggregate-clearance phenotypes. |
| **Translational Notes** | Similar safety profile to sirolimus. The rapamycin analog class as a whole faces barriers in ALS due to infection-risk concerns in a physically declining patient population, but intermittent or low-dose schedules may retain autophagy benefits while avoiding full immunosuppression. |

### 3.3 Metformin
| Attribute | Detail |
|---|---|
| **Class** | Biguanide / antihyperglycemic |
| **Primary Indication** | **Approved** globally for Type 2 diabetes mellitus; off-label use in metabolic syndrome, longevity research, and some neurodegenerative conditions |
| **Mechanism** | Activates AMP-activated protein kinase (AMPK), which negatively regulates mTORC1 via TSC1/2 and Raptor phosphorylation, thereby *indirectly* promoting autophagy. Metformin also modulates mitochondrial complex I, reduces reactive oxygen species (ROS), and exerts neuroprotective effects independent of autophagy. |
| **Key ALS Evidence** | Robust preclinical data in toxin-induced and genetic ALS models. Metformin reduces neuroinflammation, delays motor decline, and in some TDP-43 models lowers phosphorylated TDP-43 burden. Human Phase 2 trials (e.g., NCT04220021, NCT04490510) have investigated metformin alone and with riluzole in ALS. |
| **Translational Notes** | **Lipinski-passing** (MW = 129.16, LogP = –1.3, HBD = 3, HBA = 1). Phenomenal safety record, inexpensive, and widely available. The main translational question is whether AMPK-mediated autophagy is sufficient to clear established TDP-43 aggregates at clinically achievable CNS concentrations, or whether it primarily prevents new aggregate formation. |

### 3.4 Trehalose
| Attribute | Detail |
|---|---|
| **Class** | Non-reducing disaccharide (α-D-glucopyranosyl α-D-glucopyranoside) |
| **Primary Indication** | Approved food ingredient/excipient in multiple jurisdictions; investigational as a therapeutic in neurodegeneration |
| **Mechanism** | Activates TFEB (transcription factor EB) and triggers autophagy–lysosomal biogenesis independently of mTOR. Also acts as a chemical chaperone, stabilizing native protein conformations against misfolding. Trehalose crosses cell membranes poorly, and its autophagic effects are often linked to intracellular conversion mediated by trehalase-enzyme activity in relevant tissues. |
| **Key ALS Evidence** | Demonstrated clearance of TDP-43 aggregates in cellular models and mouse models of TDP-43 proteinopathy. Oral trehalose delayed motor symptoms and reduced cortical TDP-43 pathology in transgenic mice. Human intranasal or oral trehalose formulations have been explored in early-stage safety studies. |
| **Translational Notes** | **Lipinski-violates** (HBD = 8, HBA = 11) because it is a highly polar disaccharide. Oral bioavailability is low (~0.5%) in humans due to gut trehalase. Intranasal delivery or analogs with improved brain penetration are under investigation. Tolerability is excellent; the pharmacokinetic barrier is the main hurdle. |

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## 4. Compounds Targeting Stress Granule Dynamics

### 4.1 ISRIB Class (Integrated Stress Response Inhibitors)
| Attribute | Detail |
|---|---|
| **Class** | Small-molecule activator of eIF2B; ISR inhibitor |
| **Primary Indication** | Investigational only — no approval as of 2026. Lead program (ISRIB analogs) explored for traumatic brain injury (TBI) and neurodegeneration, including ALS. |
| **Mechanism** | ISRIB binds eIF2B dimerization motifs and restores its guanine-nucleotide exchange activity, overriding the eIF2α phosphorylation blockade that promotes stress granule assembly. By relieving translational repression downstream of eIF2α kinases (PERK, GCN2, PKR, HRI), ISRIB prevents persistent stress granule accumulation and the subsequent conversion of TDP-43 into insoluble aggregates. |
| **Key ALS Evidence** | In preclinical TDP-43 models, ISRIB treatment reduced SG persistence, decreased phospho-TDP-43 accumulation, and improved survival. The mechanism directly links SG disassembly to TDP-43 pathology because chronic SGs recruit and trap TDP-43 oligomers. |
| **Translational Notes** | **Not yet approved.** ISRIB analogs have advanced through safety/tolerability Phase 1 work by Calico (in partnership) for CNS indications. For ALS repurposing, the drug would need to be de-risked specifically in motor-neuron disease. The translational promise is high because it directly targets the SG-to-aggregate cascade rather than downstream clearance. |

### 4.2 DNL-343 (eIF2B Activator / ISR Modulator)
| Attribute | Detail |
|---|---|
| **Class** | Brain-penetrant small-molecule eIF2B activator |
| **Primary Indication** | Investigational — ALS and frontotemporal dementia (FTD) pipeline |
| **Mechanism** | Analogous to ISRIB: restores eIF2B activity to normalize translational control after stress. By reducing chronic phosphorylated-eIF2α signaling, it suppresses maladaptive stress granule persistence and downstream TDP-43 hyperphosphorylation/aggregation. DNL-343 was engineered for superior CNS penetration versus first-generation ISRIB scaffolds. |
| **Key ALS Evidence** | Denali Therapeutics reported preclinical efficacy in TDP-43 models. Phase 1 data (SAD/MAD) indicated tolerability and CNS target engagement (CSF biomarker modulation). The program represents one of the most direct stress-granule targeting strategies in ALS clinical development. |
| **Translational Notes** | Closest-to-clinic SG-targeting asset for ALS. Because it is purpose-built for neurodegeneration, the translational path is clearer than for general immunosuppressants or diabetes drugs, but it remains experimental and not approved for any indication. |

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## 5. Summary Table

| Compound | Primary Approved Use | Target Pathway | ALS Clinical Stage | Key Advantage | Key Barrier |
|---|---|---|---|---|---|
| **Arimoclomol** | NPC (approved/orphan) | Proteostasis (HSP co-induction) | Phase II/III in ALS | Direct TDP-43 aggregate clearance; Lipinski-compliant; orphan experience | Mixed clinical readouts; dosing optimization needed |
| **AMX0035** | ALS (conditional in CA/UK) | Proteostasis / ER stress | **Approved** for ALS | Human efficacy signal in functional decline; two known drugs combined | Regulatory uncertainty in US; modest effect size |
| **Sirolimus** | Transplant (approved) | Autophagy (mTORC1 inhibition) | Early clinical / compassionate | Potent autophagy induction; extensive human PK data | Immunosuppression; PK burden at neuroprotective doses |
| **Everolimus** | Oncology / transplant | Autophagy (mTORC1 inhibition) | Preclinical / small case | Better PK than sirolimus; oral QD dosing | Same immunosuppression liability |
| **Metformin** | Type 2 DM (approved) | Autophagy (AMPK → mTOR) | Phase 2 in ALS | Unmatched safety/cost/access profile; anti-inflammatory bonus | CNS penetration modest; may prevent rather than clear aggregates |
| **Trehalose** | Food/excipient | Autophagy / chaperone | Early clinical (intranasal) | Excellent tolerability; direct TDP-43 clearance in models | Very low oral bioavailability; formulation-dependent |
| **ISRIB analogs** | None (investigational) | Stress granule disassembly | Preclinical to Phase 1 | Direct SG mechanism; prevents aggregate seeding | No approved indication yet; ALS-specific safety unknown |
| **DNL-343** | None (investigational) | Stress granule / ISR relief | Phase 1/2 in ALS | Designed for CNS; SG biology directly targeted | Experimental; no regulatory precedent |

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## 6. Most Promising Translational Profiles

### 🥇 **Metformin** — Autophagy / Proteostasis
**Why:** The combination of massive human safety data (decades, millions of patients), affordability, global availability, and mechanistic links to AMPK-autophagy makes metformin the lowest-risk repurposing candidate. Phase 2 trials are already underway.

**What would need to be true for a repurposing trial to be justified:**
1. **Pharmacodynamic confirmation:** Demonstrate that doses achievable in serum/CSF activate motor-neuron autophagy (e.g., LC3-II/p62 flux biomarkers in patient iPSC-derived motor neurons or accessible surrogate tissue).
2. **Phenotypic stratification:** Identify ALS patient subgroups most likely to benefit (e.g., those with higher baseline NfL, faster progression, or TDP-43–dominant pathology as inferred by SOD1-negative, C9orf72-non-expanded patients).
3. **Combination rationale:** Define whether metformin acts additively or synergistically with riluzole, edaravone, or AMX0035, given that ALS therapy is now combinatorial.

### 🥈 **Arimoclomol** — Proteostasis (HSP Induction)
**Why:** It is arguably the only HSP-targeting compound purpose-designed for a neurodegenerative proteinopathy that has already completed late-stage clinical work. Its orphan status in NPC yielded a regulatory approval framework that can be extended to ALS.

**What would need to be true:**
1. **Biomarker validation:** A robust, reproducible serum or CSF pharmacodynamic signal confirming target engagement (e.g., HSP70 up-regulation) correlated with clinical outcome in ALS patients.
2. **Confirmatory trial design:** A Phase 3 trial in a *specific TDP-43–associated* ALS population (e.g., sporadic, C9orf72-negative, SOD1-negative) with a well-powered functional or survival endpoint, ideally alongside NfL and pTDP-43 imaging/CSF biomarkers.
3. **Regulatory path clarity:** Agreement with agencies on whether the NPC dataset supports bridging to ALS, or whether a separate ALS-specific dossier is required.

### 🥉 **Stress-Granule–Targeting Compounds (ISRIB / DNL-343)**
**Why:** Of all mechanisms, stress granule disassembly is the most *proximal* to TDP-43 pathology. Unlike autophagy enhancers (which clear aggregates *after* they form) or HSP inducers (which prevent misfolding), SG inhibitors could stop the cascade at the earliest seeding event.

**What would need to be true:**
1. **Differentiation from downstream mechanisms:** Prove that SG modulation is the *dominant* driver of efficacy relative to general translational rescue, especially because ISR relief can broadly affect cellular proteome and stress responses.
2. **Safety in motor-neuron disease:** Efficacy trials must exclude unanticipated adverse events (e.g., immune modulation, altered viral response) given the delicate translational landscape in ALS.
3. **CSF target engagement:** A reliable method (e.g., phospho-eIF2α, ATF4, or SG component levels in CSF exosomes) to show dose-dependent pathway suppression in human motor-neuron tissue.

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## 7. Repurposing Decision Framework

For any of the above compounds to move from "interesting biology" to "justified ALS repurposing trial," the following conditions should be met simultaneously:

| Criterion | Rationale |
|---|---|
| **1. Human CNS target engagement** | Biomarker evidence (CSF, blood, or imaging) that the drug hits its intended pathway in the human nervous system at tolerated doses. |
| **2. Concordant preclinical efficacy** | Positive signals across ≥2 independent TDP-43 model systems (cellular aggregation, iPSC motor neurons, *and* rodent survival/behavior). |
| **3. Acceptable safety margin** | No additive or synergistic toxicity with standard ALS therapy (riluzole, edaravone, AMX0035 where available). |
| **4. Biomarker-stratified patient selection** | Ability to enrich for TDP-43 pathology (e.g., SOD1-negative, C9orf72-negative sporadic ALS) to maximize signal-to-noise in a trial. |
| **5. Feasible dosing and formulation** | Achievable and sustainable dosing in ALS patients with dysphagia, gastrostomy tube dependence, or limited functional reserve. |

**Summary recommendation:** Among approved drugs with the lowest barrier to a repurposing trial, **metformin** stands out. Among proteostasis-focused drugs already tested in ALS, **AMX0035** remains the reference standard despite regulatory complexity. Among the next-generation mechanistic strategies, **SG-targeting compounds** (ISRIB class / DNL-343) carry the highest *biological* rationale but also the highest translational uncertainty. The ideal trial strategy may ultimately be a **combinatorial approach** (e.g., metformin + arimoclomol, or AMX0035 + DNL-343), treating proteostasis and SG dynamics as complementary targets of TDP-43 pathology.

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*Document generated from structural scientific tool queries (PubChem, UniProt, Reactome) and domain knowledge synthesis. Verify current clinical trial status via ClinicalTrials.gov and regulatory filings.*