SBT-272

What is SBT-272?

SBT-272 is a mitochondria-targeting tetrapeptide developed by Stealth BioTherapeutics — the same company that developed elamipretide (SS-31, FORZINITY), the first FDA-approved mitochondrial peptide therapeutic. SBT-272 is engineered as the CNS-penetrant successor to elamipretide, designed to cross the blood-brain barrier and deliver mitochondrial protection to neurons in central nervous system diseases.

It has received FDA Orphan Drug Designation for the treatment of amyotrophic lateral sclerosis (ALS) and is the lead candidate in Stealth's CNS pipeline.

Background — Mitochondrial Peptides

The Szeto-Schiller (SS) peptides — including SS-31 (elamipretide) and SS-20 — are aromatic-cationic tetrapeptides that:

• Selectively concentrate in mitochondrial inner membrane
• Bind cardiolipin (the unique mitochondrial inner-membrane phospholipid)
• Stabilize the electron transport chain and ATP synthase organization
• Reduce mitochondrial reactive oxygen species
• Restore mitochondrial bioenergetics in dysfunctional cells

SS-31 (elamipretide) was developed primarily for cardiac, ophthalmologic, and rare metabolic diseases (Barth syndrome, dry AMD, primary mitochondrial myopathy). Its limited CNS penetration directed its indications away from neurodegeneration.

SBT-272 is engineered with a modified peptide structure that supports blood-brain barrier penetration while preserving the cardiolipin-targeting mitochondrial mechanism.

Mechanism of Action

• Cardiolipin binding — same fundamental mechanism as elamipretide
• Mitochondrial inner-membrane stabilization — preserves cristae structure and electron transport chain organization
• CNS-penetrant pharmacokinetics — tetrapeptide distribution profile reaches motor neurons in cervical and lumbar spinal cord, brainstem, and cortex
• Restored mitochondrial bioenergetics in upper and lower motor neurons in ALS models

In ALS, mitochondrial dysfunction is a prominent feature of motor neuron degeneration — including TDP-43-driven mitochondrial damage, fragmented cristae, and impaired oxidative phosphorylation. SBT-272 addresses this mitochondrial dysfunction directly.

Clinical Evidence

Preclinical:

• Restored mitochondrial bioenergetics in TDP-43 cellular models (Sci Reports 2023)
• Improved motor neuron survival in mouse ALS models
• Demonstrated CNS distribution after peripheral dosing

Phase 1 SAD/MAD:

• Single ascending dose and multiple ascending dose studies completed in healthy volunteers
• Safe and tolerable across the dose range
• Pharmacokinetic profile supporting daily dosing
• No dose-limiting toxicities reported

Pipeline Status

• Phase 1 complete in healthy volunteers
• FDA Orphan Drug Designation for ALS (2023)
• Phase 2 in ALS patients planned/initiating
• Additional CNS indications under exploration: frontotemporal lobar degeneration (FTLD), Leigh syndrome

Place in Future Therapy

If successful through later-stage trials, SBT-272 would be the first mitochondria-targeting peptide therapeutic developed specifically for ALS. The therapeutic landscape in ALS is sparse — only riluzole, edaravone, AMX0035 (Relyvrio, withdrawn 2024), and tofersen (for SOD1-ALS) are currently approved — and most provide modest efficacy. A mechanism-based mitochondrial intervention is mechanistically attractive but remains to be clinically validated.

Safety Profile

Phase 1 data show good safety and tolerability:

• No dose-limiting toxicities through SAD and MAD escalation
• Adverse events generally mild and transient
• Long-term safety in ALS patients to be established in Phase 2

Distinction from Elamipretide

The two molecules share a common scientific origin and mechanism but are deliberately differentiated by tissue distribution to address different therapeutic spaces.