Novel Corrector for Variants of SLC6A8: A Therapeutic Opportunity for Creatine Transporter Deficiency

Mutations in creatine
transporter SLC6A8 cause creatine
transporter
deficiency (CTD), which is responsible for 2% of all cases of X-linked
intellectual disability. CTD has no current treatments and has a high
unmet medical need. Inspired by the transformational therapeutic impact
of small molecule “correctors” for the treatment of
cystic fibrosis, which bind to mutated versions of the CFTR ion channel
to promote its trafficking to the cell surface, we sought to identify
small molecules that could stabilize SLC6A8 as a potential treatment
for CTD. We leveraged a novel chemoproteomic technology for ligand
discovery, reactive affinity probe interaction discovery, to identify
small-molecule fragments with photoaffinity handles that bind to SLC6A8
in a cellular environment. We synthesized a library of irreversible
covalent analogs of these molecules to characterize in functional
assays, which revealed molecules that could promote the trafficking
of mutant SLC6A8 variants to the cell surface. Further medicinal chemistry
was able to identify reversible drug-like small molecules that both
promoted trafficking of the transporter and also rescued creatine
uptake. When profiled across the 27 most prevalent SLC6A8 missense
variants, we found that 10–20% of patient mutations were amenable
to correction by our molecules. These results were verified in an
endogenous setting using the CRISPR knock-in of selected missense
alleles. We established in vivo proof-of-mechanism for correctors
in a novel CTD mouse model with the P544L patient-defined variant
knocked in to the SLC6A8 locus, where treatment with our orally bioavailable
and brain penetrant tool corrector increased brain creatine levels
in heterozygous female mice, validating correctors as a potential
therapeutic approach for CTD.