As a Gene/Cell Therapy Deliver, mRNA Is a More Efficient Coding Language

Two chimeric antigen receptor (CAR) T cell therapies, Kymriah™ and Yescarta™ were approved by the FDA in 2017. Kymriah™ is for the treatment of pediatric patients and young adults with refractory or relapse (R/R) B cell precursor acute lymphoblastic leukemia (ALL), whereas Yescarta™ is for the treatment of adult patients with R/R large B cell lymphoma. They are both genetically modified autologous T cells expressing a CD19-specific CAR. Regarding to the vector used for these therapies, Kymriah™ used lentiviral vector to deliver the CAR sequence while Yescarta™ used retroviral vector.

Besides viral vectors, there are a few other ways of gene therapy based on RNA. The transient nature of RNA-based drugs can be advantageous in several settings. Qasim’s group has done electroporation of mRNA encoding TALENs to disrupt the expression of CD52 and TCRs. This strategy is applied in clinical trials. There is another approach that based on the use of mRNA encoding Cas9 nucleases to disrupt the expression of the endogenous TCR and β2-microglobulin to avoid graft-versus-host disease and to evade host- mediated immunity, respectively.

In vivo use of RNAs to express immunomodulatory proteins is still in a very early stage. For example siRNA technology has also been used in mouse models to demonstrate the role of master regulator genes of immune responses. MicroRNAs (miRs) are another attractive target for improving CAR-T therapy, as these non-coding RNAs are involved in the physiological regulation of T-cell development and effector function. MiRNA target sequences can be engineered to provide tissue-restricted expression profiles, taking advantage of the spectrum of miRNA expressed by each given cell type.

In this one-arm, open-ended phase I clinical study, 74% of participating RRMM patients ranging in age between 27-72 years old, were diagnosed as stage III based on the Durie-Salmon criteria. According to the report, the overall response rate to the drug was 88% (95% confidence interval, 76-95) with 74% of patients achieving complete remission (CR) (95% confidence interval, 60-85), 4% good partial remission (VGPR) and 11% partial remission (PR). In particular, in 42 patients who achieved complete remission, bone marrow flow cytometry showed that 39 patients (68%) had negative residual lesions (MRD). The median duration of remission (DOR) was 16 months (95% confidence interval: 12-not reached [NR]), and the median progression-free survival (PFS) of all admitted patients was 15 months. The median progression-free survival of patients who achieved complete remission was 24 months along with a median PFS of 24 months for patients who received an MRD-negative CR.

Figure 1. Schematic representation of the different RNA subtypes and their functions.
Pastor F, Melero I et al. An RNA toolbox for cancer immunotherapy. Nat Rev Drug Discov. 2018 Oct;17(10):751-767.

RNA drug is obviously more efficient than other types of vectors like virus vectors, from both gene expression and economic aspects. However, the stability is a big concern from time to time. Many technological advances have been made to help overcome prior limitations for the platform regarding stability. In the case of siRNA, 2ʹ-O-methyl or 2ʹ-fluoro substitution and phosphorothioate bonds are used. In the case of mRNA, each structural element of the mRNA must be carefully optimized. The cap structure that protects eukaryotic mRNA is essential for efficient translation and can be inserted by adding cap analogues that protect mRNA from decapping during in vitro transcription or by using capping enzymes in a second step.

New technologies and approaches are keeping on showing up. A new study suggests that RNA-based gene therapy may become more controllable through the introduction of RNA circuits. According to this study, which comes from scientists based at MIT, RNA transcripts that encode for therapeutic proteins can be delivered along with RNA transcripts that encode for RNA-binding proteins. These RNA-binding proteins can be designed to respond to small-molecule drugs and favor or disfavor, as needed, the expression of the protein-encoding RNA transcripts.


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