Sonlicromanol (KH176)

Sonlicromanol (KH176) is an orally bioavailable chromanyl carboxamide and the most clinically advanced disease-modifying drug candidate for primary mitochondrial disease associated with the m.3243A>G mutation in the mitochondrial DNA tRNA-Leu(UUR) gene, encompassing the MELAS, MIDD, and related phenotypic spectrum. The compound was identified through a structure-activity relationship optimization of 226 Trolox-derived chromanyl analogs screened against patient-derived fibroblasts harboring oxidative phosphorylation defects at the Radboud Center for Mitochondrial Medicine in Nijmegen, the Netherlands, and was advanced into clinical development by Khondrion B.V. beginning with a first-in-human Phase 1 trial in 2015.

The pharmacology of sonlicromanol is distinguished by a triple mode of action mediated predominantly through its active metabolite KH176m (KH183), formed via cytochrome P450 3A4 biotransformation. KH176m functions as a direct antioxidant through interaction with the thioredoxin/peroxiredoxin enzyme system, binding peroxiredoxin-2 with a dissociation constant of approximately 0.305 micromolar and accelerating peroxiredoxin-mediated reduction of hydrogen peroxide. The metabolite simultaneously operates as a selective inhibitor of microsomal prostaglandin E synthase-1 (mPGES-1) with an IC50 of approximately 0.16 micromolar in purified microsomes, reducing prostaglandin E2 (PGE2) biosynthesis without affecting COX-dependent production of other prostanoids. This dual redox-modulating and anti-inflammatory mechanism is pharmacologically distinct from the electron carrier function of idebenone, the ferroptosis suppression of vatiquinone, the cardiolipin stabilization of elamipretide, and the NAD+ repletion strategy of niacin and nicotinamide riboside.

In preclinical studies, KH176 protected Complex I-deficient patient-derived fibroblasts from redox stress-induced cell death with EC50 values of 35 to 270 nanomolar (KH176) and 3.2 to 88 nanomolar (KH176m). In the Ndufs4 knockout mouse model of Leigh disease, long-term intraperitoneal administration at 10 mg/kg improved rotarod performance, gait parameters, brain microstructural coherence on diffusion tensor imaging, retinal ganglion cell survival, and normalized lipid peroxidation markers, though the compound did not prevent lethal brainstem lesion development or extend lifespan in this severe model. In iPSC-derived excitatory neurons carrying the m.3243A>G variant, sonlicromanol improved neuronal network activity and partially reversed transcriptomic dysregulation linked to high heteroplasmy. The active metabolite KH176m additionally demonstrated cardioprotective activity against short-duration ischemia-reperfusion injury in isolated mouse hearts at 10 micromolar, reducing infarct size from 31 to 15 percent.

Human pharmacokinetics following oral administration are characterized by rapid absorption (Tmax approximately 1 to 1.5 hours fasted), a plasma elimination half-life of approximately 10 hours for the parent compound and approximately 16 hours for KH176m, oral bioavailability of 68 to 74 percent in animal models, and a more than dose-proportional increase in exposure across the studied dose range. The Phase 1 trial in 32 healthy male volunteers established tolerability up to single doses of 800 mg and multiple doses of 400 mg in the published literature, with dose-limiting toxicity (nausea, vomiting, dizziness, QTc prolongation) observed at the supratherapeutic 2000 mg single dose. The 100 mg in the published literature regimen was identified as the optimal therapeutic dose.

The Phase 2a KHENERGY trial in 18 patients with m.3243A>G mitochondrial disease demonstrated safety and tolerability of 100 mg in the published literature for 28 days in a crossover design, with significant improvements in alertness and mood (Beck Depression Inventory reduction of 2.9 points, p = 0.042) but no significant improvement in gait parameters. The subsequent Phase 2b KHENERGYZE trial in 27 patients tested 50 mg and 100 mg in the published literature; the primary endpoint (Cogstate attention domain score) was not met in the unadjusted analysis but showed treatment effect in patients with greater baseline impairment. The 52-week open-label extension in 12 patients demonstrated significant improvements across multiple clinically relevant domains including alertness (TAP without alarm, change from baseline of minus 34.2 milliseconds, p = 0.0047), fatigue (Neuro-QoL Fatigue, change from baseline of minus 6.29 points, p = 0.0036), physical function (SF-12 Physical Component Score, change from baseline of plus 7.7 points, p = 0.0008), and pain (McGill Pain Questionnaire, change from baseline of minus 5.7 points, p = 0.0105).

Sonlicromanol holds orphan drug designations from the European Medicines Agency for MELAS, Leigh disease, and MIDD, and from the United States Food and Drug Administration for all inherited mitochondrial respiratory chain disorders. A Rare Pediatric Disease designation for MELAS has been granted by the FDA. The Phase 3 KHENERFIN pivotal trial in adults with the m.3243A>G variant received FDA IND clearance and is actively recruiting. A Phase 2 pediatric trial (KHENERGYC) and a Phase 2 proof-of-concept trial in post-COVID syndrome (SON4PEM) are ongoing. The compound is not approved in any jurisdiction. It is available as a research-grade preparation from multiple chemical suppliers; investigators should obtain analytical confirmation of identity and purity on every lot.