| Summary: | © 2020 The American Society of Gene and Cell Therapy Synthetic mRNA therapeutics have the potential to revolutionize healthcare, as they enable patients to produce therapeutic proteins inside their own bodies. However, convenient methods that allow external control over the timing and magnitude of protein production after in vivo delivery of synthetic mRNA are lacking. In this study, we validate the in vivo utility of a synthetic self-amplifying mRNA (RNA replicon) whose expression can be turned off using a genetic switch that responds to oral administration of trimethoprim (TMP), a US Food and Drug Administration (FDA)-approved small-molecule drug. After intramuscular electroporation, the engineered RNA replicon exhibited dose-dependent and reversible expression of its encoded protein upon TMP administration. The TMP serum level needed for maximal downregulation of protein translation was approximately 45-fold below that used in humans for therapeutic purposes. To demonstrate the therapeutic potential of the technology, we injected mice with a TMP-responsive RNA replicon encoding erythropoietin (EPO) and successfully controlled the timing and magnitude of EPO production as well as changes in hematocrit. This work demonstrates the feasibility of controlling mRNA kinetics in vivo, thereby broadly expanding the clinical versatility of mRNA therapeutics. Synthetic mRNAs are gaining attention as a safe alternative to viral and DNA-based gene delivery for therapeutic purposes. Recently, self-amplifying mRNAs whose translation can be controlled by the small-molecule drug trimethoprim were described but their utility had not yet been evaluated in mice. Mc Cafferty et al. demonstrate effective modulation of protein expression from intramuscularly electroporated self-amplifying mRNA by oral administration of trimethoprim.
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