Levacetylleucine (Aqneursa™), an acetylated derivative and pro-drug of L-leucine, is the only FDA-approved monotherapy for Niemann-Pick disease type C (NPC). Its acetyl group enables transport via monocarboxylate transporters, supporting blood-brain barrier penetration and efficient cellular uptake. Inside cells, levacetylleucine is metabolised by acylases, generating elevated levels of L-leucine that enhance mitochondrial bioenergetics and is thought to ameliorate lysosomal dysfunction indirectly. Here, we describe a direct effect of levacetylleucine on lysosomal regulation through modulation of TFEB, the master transcription factor for lysosomal and autophagy genes. Levacetylleucine rapidly alters TFEB translocation between the cytoplasm and the nucleus in a biphasic, homeostasis-restoring manner. In wild-type HeLa cells, levacetylleucine promotes TFEB activation and nuclear localisation. However, in NPC1 disease models, where we show that TFEB is over-activated and enriched in the nucleus due to lysosomal stress, levacetylleucine reduces nuclear TFEB and restores a more normal cytoplasmic-to-nuclear balance. These effects occur at clinically relevant concentrations associated with lysosomal storage reduction. The effects of the drug are stereospecific: while the L-enantiomer is active, the D-enantiomer and racemate show no effect, revealing the antagonistic properties of the D-enantiomer. This bidirectional normalisation of TFEB activity highlights a direct mechanism through which levacetylleucine modulates lysosomal and autophagic pathways in the HeLa cell model, giving mechanistic insight into its therapeutic potential in NPC, and also across diverse neurological and neurodevelopmental disorders.

The GM2 gangliosidoses (GM2) are ultra-rare neurodegenerative disorders caused by deficient hexosaminidase A and/or B activity, leading to lysosomal GM2 ganglioside accumulation. Disease onset ranges from infancy to adulthood, with earlier onset associated with more rapid progression. Neurofilament light chain (NfL), a sensitive marker of axonal injury, has been extensively investigated as a biomarker for neurodegenerative disorders, including GM2. To evaluate its clinical utility as a biomarker for GM2, NfL was measured in patients with GM2 enrolled in a Phase 2b, multinational, rater-blinded study of levacetylleucine [NCT03759665], and in its open-label Extension Phase (EP). Nineteen participants had viable samples for NfL analysis at baseline, after six weeks of treatment, and after a six-week washout; 10 had samples in the long-term EP. After the initial 6-week treatment phase, NfL concentration declined a mean − 8.9

N-acetyl L-leucine (NALL, USAN or levacetylleucine, INN or trade name Aqneursa™) is an FDA-approved agent for the treatment of Niemann-Pick disease type C (NPC). The N-acetyl group renders the compound a prodrug of L-leucine, making it a substrate for membrane-spanning monocarboxylate transporters (MCTs), which are ubiquitously expressed delivering NALL to all tissues with high capacity, including to the central nervous system. NALL enters enzyme-controlled pathways that correct metabolic dysfunction and enhance mitochondrial bioenergetics. Because NALL improves energy production (adenosine triphosphate, ATP) and ameliorates lysosomal function, it is potentially a therapy for a broad range of neurodegenerative and neurodevelopmental disorders (in addition to lysosomal storage disorders) in which energy homeostasis and lysosomal function are impaired. Here, we have performed a series of in vitro studies which reveal an additional aspect of NALL’s polypharmacological mechanism of action. The studies demonstrate a direct lysosomal effect whereby NALL rapidly activates the translocation of the Transcription Factor EB (TFEB, a master regulator of lysosomal biogenesis and autophagy) from the cytoplasm to the nucleus in HeLa cells. The activation of TFEB is known to trigger the activation of specific genes that restore lysosomal biogenesis and function, as well as autophagy. Consistent with this, we show that NALL increases production of a TFEB target gene LAMP1, an integral lysosomal membrane protein responsible for maintaining lysosomal integrity, function and pH. This in vitro effect occurs at concentrations consistent with concentrations in plasma in humans after standard therapeutic dosing. We further demonstrated that acetylation is critical to this aspect of NALL’s mechanism of action, as L-leucine itself had no effect on the activation of TFEB. Consistent with previous studies N-acetyl-D-leucine was inactive and also had no effect. Similarly, N-acetyl-DL-leucine also had only a modest effect, providing further evidence that N-acetyl-D-leucine is even antagonistic and inhibits the effects of the active L-enantiomer. This mechanism of action of NALL to activate TFEB signalling, thereby enhancing lysosomal and autophagic function, further elucidates the ways by which this compound targets the fundamental etiology of rare and common neurodegenerative disorders, from Niemann-Pick disease type C to Parkinson’s disease. Based on its mechanism of action by improving the mitochondrial-lysosomal axis, NALL has the potential to be an effective therapy for a broad range of neurological and neurodevelopment conditions. ### Competing Interest Statement Grant Churchill, Antony Galione, Frances Platt and Michael Strupp are consultants for IntraBio, Inc. Mallory Factor, Taylor Fields and Marc Patterson are employees of IntraBio, Inc. All the employees and consultants listed above are also shareholders of IntraBio, Inc. * ATP : adenosine triphosphate CLEAR : network coordinated lysosomal expression and regulation network ER : endoplasmic reticulum FDA : Food and Drug Administration (USA) INN : International Nonproprietary Names LAT : L-type amino-acid transporter LSD : lysosomal storage disease MCT : monocarboxylate transporters mTORC1 : mammalian target of rapamycin complex 1 NPC : Niemann-Pick disease type C NALL : N-acetyl L-leucine MCS : membrane contact site(s) RBD : REM sleep behaviour disorder TFEB : Transcription Factor EB USAN : United States Adopted Name

Levacetylleucine (Aqneursa TM ), a chemically modified amino acid, is the only US Food and Drug Administration-approved monotherapy for the treatment of Niemann-Pick disease type C (NPC) (Beninger, 2024; Mullard, 2024; van Gool et al., 2025). This acetylated derivative of L-leucine functions as a pro-drug, with the acetyl group rendering it a substrate for the monocarboxylate transporter (MCT) family of transporters to allow appreciable penetration of the blood-brain barrier and its efficient uptake into cells (Churchill et al., 2021). Inside cells, levacetylleucine undergoes metabolism catalysed by acylases, and the resultant high quantities of L-leucine enter metabolic pathways which enhance mitochondrial bioenergetics and, as previously demonstrated, indirectly ameliorate lysosomal function (Kaya et al., 2020). Here, we show a novel aspect of levacetylleucine’s mechanism of action, demonstrating a direct effect on lysosomal function through its rapid modulation of the translocation of the transcription factor TFEB, a master regulator of lysosomal biogenic and autophagic genes (Napolitano and Ballabio, 2016), from cytoplasm to nucleus. Uniquely, we have demonstrated a biphasic action whereby levacetylleucine normalizes TFEB activity, consistent with levacetylleucine’s previously shown ability to regulate cellular homeostasis: in wild-type HeLa cells, levacetylleucine enhances and activates the translocation of TFEB to the nucleus. In contrast, in cellular models of NPC type 1 disease, where TFEB is already over-expressed in the nucleus (as the cell attempts to compensate for the primary defect by activating TFEB as a natural cellular response to the lysosomal substrate accumulation and associated cellular stress), treatment with levacetylleucine down-regulates and restores the distribution of TFEB to a more normalized cytoplasmic: nuclear ratio. Importantly, both effects of levacetylleucine occur at concentrations consistent with plasma concentrations in therapeutic dosing (Churchill et al., 2020). The effects were also confirmed to be stereospecific to the L-enantiomer, as neither the D-enantiomer (N-acetyl-D-leucine) or racemate (N-acetyl-DL-leucine) had any effect, The presence of the D-enantiomer in the racemic mixture inhibited the ability of levacetylleucine to promote TFEB bidirectional translocation, consistent with previous studies, which have established antagonism of N-acetyl-L-leucine by N-acetyl-D-leucine in the racemic mixture (rendering the racemic mixture without effect). This bidirectional mechanism of action of levacetylleucine to impact lysosomal function directly and normalize, either by activating basal TFEB signalling or reducing aberrant TFEB function in NPC1 knockout cells, thereby modulating lysosomal and autophagic functions, lends itself to the treatment of a broad range of neurological and neurodevelopment disorders.

We conducted a Phase 3 trial to confirm the safety and efficacy of the active L-enantiomer, N-acetyl-L-leucine (NALL) for pediatric (≥ 4 years) and adult patients with NPC.

Mitochondrial DNA (mtDNA) depletion syndrome manifests as diverse early-onset diseases that affect skeletal muscle, brain and liver function. Mutations in several nuclear DNA-encoded genes cause mtDNA depletion. We report on a patient, a 3-month-old boy who presented with hepatic failure, and was found to have severe mtDNA depletion in liver and muscle. Whole-exome sequencing identified a homozygous missense variant (c.544C > T, p.R182W) in the accessory subunit of mitochondrial DNA polymerase gamma (POLG2), which is required for mitochondrial DNA replication. This variant is predicted to disrupt a critical region needed for homodimerization of the POLG2 protein and cause loss of processive DNA synthesis. Both parents were phenotypically normal and heterozygous for this variant. Heterozygous mutations in POLG2 were previously associated with progressive external ophthalmoplegia and mtDNA deletions. This is the first report of a patient with a homozygous mutation in POLG2 and with a clinical presentation of severe hepatic failure and mitochondrial depletion.

Niemann-Pick disease type C (NPC) is a rare autosomal recessive neurodegenerative lysosomal disease characterized by multiple symptoms such as progressive cerebellar ataxia and cognitive decline. The modified amino acid N-acetyl-leucine has been associated with positive symptomatic and neuroprotective, disease-modifying effects in various studies, including animal models of NPC, observational clinical case studies, and a multinational, rater-blinded phase IIb clinical trial. Here, we describe the development of a study protocol (Sponsor Code “IB1001-301”) for the chronic treatment of symptoms in adult and pediatric patients with NPC. This multinational double-blind randomized placebo-controlled crossover phase III study will enroll patients with a genetically confirmed diagnosis of NPC patients aged 4 years and older across 16 trial sites. Patients are assessed during a baseline period and then randomized (1:1) to one of two treatment sequences: IB1001 followed by placebo or vice versa. Each sequence consists of a 12-week treatment period. The primary efficacy endpoint is based on the Scale for the Assessment and Rating of Ataxia, and secondary outcomes include cerebellar functional rating scales, clinical global impression, and quality of life assessments. Pre-clinical as well as observational and phase IIb clinical trials have previously demonstrated that IB1001 rapidly improved symptoms, functioning, and quality of life for pediatric and adult NPC patients and is safe and well tolerated. In this placebo-controlled cross-over trial, the risk/benefit profile of IB1001 for NPC will be evaluated. It will also give information about the applicability of IB1001 as a therapeutic paradigm for other rare and common neurological disorders. The trial (IB1001-301) has been registered at www.clinicaltrials.gov (NCT05163288) and www.clinicaltrialsregister.eu (EudraCT: 2021–005356-10). Registered on 20 December 2021.

Background The lack of approved treatments for the majority of rare diseases is reflective of the unique challenges of orphan drug development. Novel methodologies, including new functionally relevant endpoints, are needed to render the development process more feasible and appropriate for these rare populations and thereby expedite the approval of promising treatments to address patients’ high unmet medical need. Here, we describe the development of an innovative master protocol and primary outcome assessment to investigate the modified amino acid N-acetyl- l -leucine (Sponsor Code: IB1001) in three separate, multinational, phase II trials for three ultra-rare, autosomal-recessive, neurodegenerative disorders: Niemann-Pick disease type C (NPC), GM2 gangliosidoses (Tay-Sachs and Sandhoff disease; “GM2”), and ataxia telangiectasia (A-T). Methods/design The innovative IB1001 master protocol and novel CI-CS primary endpoints were developed through a close collaboration between the Industry Sponsor, Key Opinion Leaders, representatives of the Patient Communities, and National Regulatory Authorities. As a result, the open-label, rater-blinded study design is considerate of the practical limitations of recruitment and retention of subjects in these ultra-orphan populations. The novel primary endpoint, the Clinical Impression of Change in Severity© (CI-CS), accommodates the heterogenous clinical presentation of NPC, GM2, and A-T: at screening, the principal investigator appoints for each patient a primary anchor test (either the 8-m walk test (8MWT) or 9-hole peg test of the dominant hand (9HPT-D)) based on his/her unique clinical symptoms. The anchor tests are videoed in a standardized manner at each visit to capture all aspects related to the patient’s functional performance. The CI-CS assessment is ultimately performed by independent, blinded raters who compare videos of the primary anchor test from three periods: baseline, the end of treatment, and the end of a post-treatment washout. Blinded to the time point of each video, the raters make an objective comparison scored on a 7-point Likert scale of the change in the severity of the patient’s neurological signs and symptoms from video A to video B. To investigate both the symptomatic and disease-modifying effects of treatment, N-acetyl- l -leucine is assessed during two treatment sequences: a 6-week parent study and 1-year extension phase. Discussion The novel CI-CS assessment, developed through a collaboration of all stakeholders, is advantageous in that it better ensures the primary endpoint is functionally relevant for each patient, is able to capture small but meaningful clinical changes critical to the patients’ quality of life (fine-motor skills; gait), and blinds the primary outcome assessment. The results of these three trials will inform whether N-acetyl- l -leucine is an effective treatment for NPC, GM2, and A-T and can also serve as a new therapeutic paradigm for the development of future treatments for other orphan diseases. Trial registration The three trials (IB1001-201 for Niemann-Pick disease type C (NPC), IB1001-202 for GM2 gangliosidoses (Tay-Sachs and Sandhoff), IB1001-203 for ataxia telangiectasia (A-T)) have been registered at www.clinicaltrials.gov (NCT03759639; NCT03759665; NCT03759678), www.clinicaltrialsregister.eu (EudraCT: 2018-004331-71; 2018-004406-25; 2018-004407-39), and https://www.germanctr.de (DR KS-ID: DRKS00016567; DRKS00017539; DRKS00020511).