Polymeric Delivery System for mRNA Therapeutics: Design Principles and Recent Advances

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The review submitted by Bao et al. discuss about the latest research studies concerning the use of polymeric systems for the mRNA. The manuscript is clear, well written and structured. However, some corrections are needed in order to increase the overall quality of the paper:

1. the evolution of the research papers on this subject in the last 5 year could be of interest for the readers.
2. some personal critical remarks should also be added
3. there are systems in clinical trials?
4. which are the future perspectives of these systems?

Author Response

Comments 1: The evolution of the research papers on this subject in the last 5 years could be of interest to the readers.

Response 1: Thanks for the reviewer's suggestions. We have included a short discussion in the DISCUSSION section. 

Over the past five years, research on polymeric vectors for mRNA delivery has evolved from proof-of-concept polyplex formation and in vitro transfection toward rationally engineered, biodegradable, organ-selective, and cell-targeted nanocarriers. The field has moved through several stages: first responding to the limitations of LNP-dominated mRNA delivery, then developing chemically diverse polymer architectures, followed by high-throughput screening for tissue tropism, structure–activity-guided optimization of PBAE/HPAE and CART systems, and most recently, translational applications such as inhaled lung delivery, mRNA cancer vaccination, and in vivo immune-cell engineering. This evolution suggests that polymeric vectors are emerging not merely as backup systems to LNPs, but as a distinct and highly tunable platform for next-generation mRNA therapeutics.

Comments 2: Some personal critical remarks should also be added.

Response 2: We thank the reviewer for this helpful suggestion. We agree that, in addition to summarising recent advances, the manuscript should provide a more critical perspective on the current limitations and future needs of polymeric mRNA delivery systems. In the revised Discussion, we have added several critical remarks highlighting that many polymeric systems still rely heavily on reporter-gene studies, short-term expression readouts, and limited in vivo validation. We also emphasise that claims of superiority over LNPs should be interpreted cautiously unless supported by direct side-by-side comparisons using the same mRNA cargo, dose, administration route, and disease model. In addition, we now discuss that the high chemical tunability of polymers, while advantageous, also creates challenges for reproducibility, structure–function interpretation, and GMP translation. We further highlight the need for more systematic assessment of polymer degradation products, repeat-dose safety, complement activation, biodistribution, and long-term tolerability. These critical remarks have been incorporated into the revised Discussion to provide a more balanced and forward-looking assessment of the field.

From a critical perspective, the promise of polymeric mRNA delivery should be interpreted with caution. Many reported systems remain at the proof-of-concept stage and are evaluated mainly by short-term reporter-gene expression, with limited evidence of therapeutic efficacy, repeat-dose safety, or scalable manufacturing. In addition, claims of superiority over LNPs are often based on indirect comparisons rather than standardised head-to-head studies. The same chemical tunability that makes polymers attractive also increases the complexity of synthesis, characterisation, reproducibility, and regulatory control. Therefore, future work should focus less on generating additional polymer variants and more on establishing robust design rules, standardised benchmarking against LNPs, clinically relevant disease models, and GMP-compatible manufacturing strategies. In our view, polymeric vectors should not be considered universal replacements for LNPs, but rather complementary and application-specific platforms whose value will depend on whether they can solve delivery problems that lipid-based systems cannot adequately address.

Comments 3: There are systems in clinical trials?

Response 3: Thanks for the comment from the reviewer. We have gone through the clinical trial database, and only found one related trial, which has been added in the DISCUSSION section.

Although most of the polymeric mRNA delivery techniques are still in R&D and pre-clinical studies, early clinical translation of the hybrid system is now underway. A phase II trial (NCT07321301) is currently recruiting participants to evaluate polymer-lipid nanoparticle delivery of CD19/CD20 dual-targeting mRNA in CAR-T cell therapy for relapsed/refractory B-cell lymphoma and leukemia — representing a first-in-class demonstration of the combinatorial potential of polymeric and lipid-based platforms.

Comments 4: Which are the future perspectives of these systems?

Response 4: We thank the reviewer for this valuable comment. We have revised the Discussion to more clearly describe the future perspectives of polymeric mRNA delivery systems. These systems are expected to progress from proof-of-concept delivery platforms toward rationally engineered, biodegradable, tissue-selective, and clinically translatable carriers. Future development will likely focus on improving extrahepatic and cell-specific delivery, reducing toxicity through degradable or stimuli-responsive polymer designs, integrating polymer chemistry with RNA chemistry, and using high-throughput screening and machine learning to establish structure–function relationships. We have also highlighted the importance of lipid–polymer hybrid systems, ligand-functionalized carriers, and co-delivery strategies for mRNA vaccines, protein replacement, gene editing, and in vivo cell engineering. In addition, we now discuss the translational requirements for these systems, including GMP-compatible synthesis, batch reproducibility, scalable nanoparticle manufacturing, in vivo safety, biodistribution, clearance, and repeat-dose tolerability. These revisions have been incorporated into the Discussion section.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript provides a general overview in the field of polymeric delivery systems for the delivery of mRNA. 

The manuscript is readable and well-constructed.

The auhors should better stress advantages of using polymeric systems over lipid ones. 

The authors should also pointed out which is the scientific relevance to provide a review on the topic relating to the polymeric delivery systems.

The authors did not argumented on natural cationic polymers such as chitosan that could employed for mRNA delivery.

Author Response

Thanks for all the comments from the reviewer. We have given responses to each comment in the attachment, and the revision has been marked in red. 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors present a concise, but rather comprehensive review on polymeric delivery systems for mRNA Therapeutics as promising alternatives to LNPs and viral vectors. The manuscript is a timely contribution and of potential interest to the readership of Genes, in particular, as cytotoxic effects arising both from LNPs (for delivery) as well as from modified RNA bases as building-blocks of the mRNA therapeutics themselves - as crucial limitations of mRNA LNPs - are mentioned. 

Some amendments are recommended:

1. So far, RNA base modifications (modified RNA, modRNA) are a common (and likely an indispensable) ingredient of many mRNA therapeutics as described by the authors. Importantly, the detrimental effects of modified mRNA bases (like m1Ψ mentioned in l. 148), such as their cancerogenic potential are long known (e.g. Rubio-Casillas et al. (2024) Review: N1-methyl-pseudouridine (m1Ψ): Friend or foe of cancer? Int J Biol Macromol. 270(Pt 2):132447. doi: 10.1016/j.ijbiomac.2024.132447. Mulroney et al. (2024) N1-methylpseudouridylation of mRNA causes +1 ribosomal frameshifting. Nature 2024 Jan;625(7993):189-194. doi: 10.1038/s41586-023-06800-3. Morais et al. (2021) The critical contribution of pseudouridine to mRNA COVID-19 vaccines. Frontiers in cell and developmental biology, 9: 3187. Pepini et al. (2017). Induction of an IFN-mediated antiviral response by a self-amplifying RNA vaccine: implications for vaccine design. The journal of immunology 198(10): 4012-4024.)

Some of the major AEs from the mRNA COVID-19 vaccination campaign are related to that and meanwhile well documented. Therefore, it is important to know which mRNA therapeutics contain modRNA (thus actually rather represent modRNA therapeutics) and which not. The above mentioned dangers need to be briefly specified and indicated which therapeutics (model systems) presented in the manuscript contain modRNA as far as possible.

2. In this respect, it will be of substantial interest to explore/discuss the potential of the presented polymer species to (i) reduce the necessary content of modRNA and/or (ii) reduce the cytotoxic effects of modRNA in the packed mRNA therapeutic, in comparison to LNPs. This might be accomplished throughout the text and/or in a specific (sub)section and supplemented by a table.

3. In Figure 2, the letter labelling is missing. 

Author Response

Thanks for the comments from the reviewer. We have responded to each comment in the attachment, and the revision has been marked in red.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript is suitable for publication