Construction of a self-amplifying mRNA-LNP complex for phenylalanine hydroxylase and its efficacy evaluation in gene therapy for phenylketonuria.

Phenylketonuria (PKU) is an autosomal recessive disorder caused by deficient phenylalanine hydroxylase (PAH) activity, with current pharmacotherapies showing limited response rates and age-dependent restrictions. We compared two liver-targeted lipid nanoparticle formulations delivering PAH-encoding mRNA-a self-amplifying construct (saRNA) and a conventional non-replicating counterpart (nr-mRNA)-in Pah-knockout mice. Both constructs reduced circulating phenylalanine (Phe) after a single intravenous injection. The saRNA formulation achieved comparable efficacy at one-sixth the nr-mRNA dose and maintained physiological Phe levels for 14 days, whereas nr-mRNA produced only transient reductions. Repeated saRNA administration was well tolerated, with no detectable immunogenicity or hepatic toxicity. These results establish that saRNA-mediated PAH replacement provides sustained activity with substantial dose reduction, overcoming the short-lived effect of non-replicating mRNA. This platform may inform broader strategies for durable enzyme therapy in inherited metabolic diseases requiring liver-restored function.