Formulation and administration timing of lipid derivatized JAK-inhibitor and corticosteroid modulate saRNA-LNP inflammation and expression

Self-amplifying RNA (saRNA)-lipid nanoparticle (LNP) therapeutics represent a transformative advance in nucleic acid medicine, enabling prolonged protein expression at significantly lower doses compared to traditional mRNA-LNP systems. However, their broader clinical translation is hindered by pronounced innate immune activation elicited by saRNA replication intermediates and LNP components. To address this challenge, we investigated lipid-derivatized prodrugs of peficitinib (a pan-JAK inhibitor) and dexamethasone (a corticosteroid) for their ability to modulate saRNA-LNP induced inflammation. Using both in vitro and in vivo models, we optimized the formulation strategy, including evaluating co-formulation within the same LNPs and independent formulation into separate LNPs, as well as varied timing of administration. Our findings demonstrate that co-formulation of peficitinib exacerbated the endosomal damage signaling and downstream defence pathways, abrogating saRNA replication and expression despite suppressing innate immune activation. By contrast, independent delivery delayed these responses by a few hours, preserving saRNA replication and expression while still mitigating inflammation. Collectively, these results emphasise that the formulation strategy, and timing - not drug identity alone, influence the trade-off between controlling reactogenicity and maintaining transgene expression, offering a refined pathway toward reducing reactogenicity while maximizing the therapeutic potential of saRNA-LNP therapeutics.