Metabolism of mRNA Therapeutics: Insights into Pharmacokinetics and Toxicology

INTRODUCTION

mRNA therapies are transforming healthcare by enabling cells to produce disease-fighting proteins. Unlike traditional medications, they allow the body to generate therapeutic proteins useful for vaccines, cancer treatment, and genetic disorders. COVID-19 vaccines use mRNA to trigger immune responses, while in cancer, mRNA targets tumor antigens, and for genetic conditions, it aids in restoring essential proteins. Understanding the absorption, distribution, metabolism, and elimination of mRNA is vital for ensuring its safety and effectiveness (Karikó et al., 2008).

mRNA metabolism: an overview

mRNA consists of key components: the 5′ cap, UTRs, ORF, and poly(A) tail, which affect its stability, translation, and which influence its stability, translation efficiency, and therapeutic use, as illustrated in Figure 1 (Mockey 2006; Henderson, 2021; Grier, 2016). The 5′cap, a modified guanine, prevents degradation and aids translation; improved analogs can enhance capping efficiency to 95%, boosting stability and protein production, as demonstrated in Figure 2. The poly(A) tail, consisting of approximately 100–300 adenine residues, also increases stability and can be optimized for better performance (Holtkamp, 2006; Schlake 2012). UTRs regulate translation and can be tailored for better efficiency and tissue-specific expression. While many mRNAs use α- or β-globin UTRs, further optimization can increase yield (Holtkamp, 2006; Vaidyanathan 2018; Ross and Sullivan, 1985).

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Modified nucleosides such as 1-methylpseudouridine and N1-methyladenosine boost mRNA stability and lower immune activation. N1-methyladenosine aids in evading RIG-I, which enhances delivery and diminishes the immune response-crucial for progressing mRNA therapies for infections, cancer, and genetic disorders (Pascolo, 2021; Sahin et al., 2014; Kowalski, 2019).

Degradation pathways

Endogenous enzymes control mRNA stability and degradation. Deadenylases shorten the poly(A) tail to start decay, while decapping enzymes DCP1 and DCP2 remove the 5' cap, resulting in degradation by the exosome complex, represented in Figure 3 (Karikó, 2011). Eukaryotic mRNAs degrade from the 3' to 5' end after the poly(A) tail is removed, which is crucial for mRNA stability and translation (Muhlrad et al., 1994; Muhlrad et al., 1995).

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In yeast, blocking 5' to 3' decay causes accumulation of 3' PGK1 mRNA fragments, suggesting a shift to 3' to 5' decay. Similar results in oat phytochrome A mRNA indicate this process is conserved (Higgs and Colbert, 1994). The exact roles of exonucleases and endonucleases remain unclear. Notably, some mRNAs display stability without a poly(A) tail, indicating that other factors contribute to stability (Decker and Parker, 1993; Dujon, 1994).

Pharmacokinetics of mRNA therapeutics

Delivery systems and tissue targeting

mRNA delivery efficiency is greatly influenced by the method used, with Lipid Nanoparticles (LNPs) being the most common. LNPs enter cells via endocytosis, releasing mRNA for protein synthesis. Advanced systems like polymeric and hybrid lipid-polymer nanoparticles enhance targeting, while polymeric carriers show promise for lung treatments in asthma and COPD (Hou, 2021; Riley, 2021).

Modern RNA-loaded Lipid Nanoparticles (LNPs), evolved from 1960s liposomes, consist of cholesterol, ionizable lipids, and stealth lipids. Their core features structural lipids, while neutral phospholipids stabilize the bilayer and cholesterol enhances RNA delivery efficiency (Riley, 2021; Ad, 1965; Briuglia, 2015; Gregoriadis and Davis, 1979; Kirby et al., 1980; Malone et al., 1989).

Stability and protein translation

mRNA stability is crucial for its longevity and protein production, especially in therapies. Innovations like Clean Cap, a 5' cap analog, enhance stability, translation efficiency, and degradation resistance, leading to a longer half-life and increased protein synthesis (Wu, 2024; Rouf, 2022). Modifying UTRs boosts stability, aiding single-dose vaccines like Zika by enhancing immune responses and reducing dosage (Richner, 2017; Hengelbrock, 2023). PERSIST-seq involves introducing mRNA pools into HEK293T cells, separating translating from non-translating mRNAs with sucrose gradients. RT-PCR and Illumina sequencing assess translation efficiency, with polysome-associated mRNAs indicating active translation (Kozak, 1994; Babendure, 2006). Early results showed clear translation differences between ActB mRNA and scrambled UTR controls (Osuna, 2017).

Clearance pathways

Post-translation, mRNA breaks down into nucleotides, regulating gene expression and preventing protein accumulation. These nucleotides can be reused or converted into essential metabolites, with excess eliminated to maintain balance (Hoecke and Roose, 2019; Stephenson and Zamecnik, 1978). Clearance mainly involves the kidneys and liver. The kidneys filter waste and salts into urine, while the liver transforms substances into less harmful or water-soluble forms (Sharma and Watts, 2015; Dammes and Peer, 2020; Holland, 2024). The liver produces proteins, regulates glucose storage, and secretes bile for fat digestion, all vital for drug development (Holland, 2024). Understanding nucleotide metabolism aids drug optimization, improving efficacy and safety by balancing clearance rates to reduce dosing frequency and toxicity, while enhancing stability and bioavailability (Watson and Crick, 1953).

Toxicological implications

Immunogenic responses

Unaltered mRNA activates receptors like TLR3, TLR7, TLR8, and RIG-I, causing inflammation or autoimmune responses. Modified nucleosides, such as 5-methylcytidine and pseudouridine, enhance stability, reduce immune detection, and improve translation (Moles, 2019; Mu and Hur, 2021). Enhanced delivery boosts safety and effectiveness in vaccines, cancer treatments, and genetic therapies (Haque, 2020; Heine et al., 2021). Continuous research focuses on improving carriers and minimizing immune reactions (Corbett, 2020; Maruggi, 2019).

Cumulative toxicity

The use of mRNA treatments, like vaccines and gene therapy, raises concerns about Lipid Nanoparticles (LNPs) accumulating in the liver and spleen, which are crucial for detoxification and immune function (Fang, 2022; Hu, 2020; Janas, 2018; Lv, 2006; Monnery, 2017). LNP accumulation can lead to lasting toxicity. To mitigate this, researchers are creating biodegradable carriers that decompose into safe by products, enhancing safety and delivery (Fischer, 2003; Moghimi, 2005; Kedmi, 2010; Soenen, 2009).

siRNAs can also induce immune responses and toxicity, complicating their clinical use (Dokka, 2000). Pattern Recognition Receptors (PRRs), such as Toll-like Receptors (TLRs), trigger cytokine release, inflammation, and reduced effectiveness (Dokka, 2000). Unmodified siRNAs can cause unintended effects, stress, and cell death, and certain delivery methods may heighten immune activation (Toh, 2009; Horie, 2013).

Potential integration risks

mRNA plays a crucial role in protein synthesis and does not change the host genome, making it suitable for vaccines and treatments (Wolff, 1990). Its stability and translation are influenced by the sequence, cell type, and surrounding environment (Boczkowski, 1996). Since 1990, the use of exogenous mRNA has led to significant advancements in therapeutic and preventive molecular medicine (Hoerr, 2000).

RNA technologies currently facilitate cancer immunotherapy, protein replacement, genome editing, and vaccine development (Hu, 2020). mRNA is responsible for encoding proteins that can fix mutations or enhance immune responses, while siRNA is used to silence detrimental genes associated with diseases (Chavda, 2021). To increase safety, current designs incorporate degradation signals to manage the lifespan of mRNA (Sahin, 2014), striking a balance between effectiveness and risk (Kreiter, 2016). Ongoing studies are focused on improving the performance of mRNA therapies (Zheng, 2000; Vader, 2016).

Emerging strategies to optimize mRNA therapeutics

Advanced delivery systems

Exosomes are natural vesicles that carry biomolecules such as mRNA, providing biocompatibility and targeted gene delivery for cancer with little effect on healthy cells, as depicted in Figure 4 (Sedic, 2018). Dendritic polymers, also known as dendrimers, are branched nanocarriers that can deliver small molecules, proteins, and nucleic acids with great accuracy and absorption. Merging these two systems could improve the effectiveness and safety of therapies, especially in cancer treatment (Li, 2014; Funabiki, 2014).

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Bioconjugation technologies

Attaching specific molecules to mRNA delivery systems enhances accuracy by focusing on particular tissues and minimizing systemic side effects. Lipid Nanoparticles (LNPs), which are frequently utilized for mRNA delivery, can be altered with ligands such as galactose to specifically target liver cells through receptor recognition. This approach is particularly beneficial for addressing rare liver-related metabolic disorders by delivering mRNA that encodes functional proteins directly to hepatocytes, thereby restoring function and alleviating symptoms along with off-target effects (Ramírez-Cortés and Ménová, 2025).

Crispr-driven optimization

Attaching specific molecules to mRNA delivery systems enhances the accuracy and effectiveness of mRNA therapeutics by allowing targeted delivery to specific tissues, which helps to minimize side effects from systemic distribution. Lipid Nanoparticles (LNPs) are a prominent delivery method, encapsulating mRNA and facilitating cellular entry. When modified with targeting ligands such as galactose-recognized by liver cell receptors-LNPs improve liver-specific delivery. This is particularly beneficial for treating liver-related metabolic disorders resulting from genetic mutations. The targeted delivery of mRNA that encodes functional proteins can restore liver function, alleviate symptoms, and reduce necessary dosages, thereby minimizing off-target effects (Ramírez-Cortés and Ménová, 2025).

CONCLUSION

mRNA therapies mark a significant breakthrough in the treatment of infectious diseases and genetic disorders. Their effectiveness depends on a solid grasp of pharmacokinetics, processing, and possible toxicity. Progress in chemical modifications, delivery methods, and translation efficiency has enhanced stability, minimized immune responses, and allowed for targeted actions. Their applications include vaccines, cancer therapies, and rare genetic disorders, although concerns regarding long-term safety and immune activation remain and immune activation. Emerging tools such as exosome-based delivery and CRISPR systems further expand therapeutic potential. Ongoing collaboration across disciplines is crucial to ensure a balance between safety and effectiveness in future mRNA therapies.