Extracellular Vesicle–Mediated <i>In Vitro</i> Transcribed mRNA Delivery for Treatment of HER2+ Breast Cancer Xenografts in Mice by Prodrug CB1954 without General Toxicity

<jats:title>Abstract</jats:title> <jats:p>Prodrugs are harmless until activated by a bacterial or viral gene product; they constitute the basis of gene-delivered prodrug therapies called GDEPT, which can kill tumors without major side effects. Previously, we utilized the prodrug CNOB (C16H7CIN2O4; not clinically tested) and enzyme HChrR6 in GDEPT to generate the drug MCHB (C16H9CIN2O2) in tumors. Extracellular vesicles (EVs) were used for directed gene delivery and HChrR6 mRNA as gene. Here, the clinical transfer of this approach is enhanced by: (i) use of CB1954 (tretazicar) for which safe human dose is established; HChrR6 can activate this prodrug. (ii) EVs delivered in vitro transcribed (IVT) HChrR6 mRNA, eliminating the potentially harmful plasmid transfection of EV producer cells we utilized previously; this has not been done before. IVT mRNA loading of EVs required several steps. Naked mRNA being unstable, we ensured its prodrug activating functionality at each step. This was not possible using tretazicar itself; we relied instead on HChrR6′s ability to convert CNOB into MCHB, whose fluorescence is easily visualizable. HChrR6 mRNA-translated product's ability to generate fluorescence from CNOB vicariously indicated its competence for tretazicar activation. (iii) Systemic IVT mRNA–loaded EVs displaying an anti-HER2 single-chain variable fragment (“IVT EXO-DEPTs”) and tretazicar caused growth arrest of human HER2+ breast cancer xenografts in athymic mice. As this occurred without injury to other tissues, absence of off-target mRNA delivery is strongly indicated. Many cancer sites are not amenable for direct gene injection, but current GDEPTs require this. In circumventing this need, a major advance in GDEPT applicability has been accomplished.</jats:p>