N-acetyl-L-leucine (Levacetylleucine) normalizes Transcription Factor EB (TFEB) activity by stereospecific bidirectional modulation

Levacetylleucine (AqneursaTM), 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 levacetylleucines 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 levacetylleucines 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.