Review of siRNA cancer delivery systems outlines challenges in tumor targeting, cytoplasmic release, and safety
From gene silencing to anticancer effects, delivery is key
In siRNA therapies that suppress cancer-related gene expression, nanoparticles can help reduce delivery barriers, but tumor targeting, cytoplasmic release, and safety remain central challenges. A review published in International Journal of Molecular Sciences summarized a range of siRNA delivery strategies and the results and limitations of cancer clinical trials. It is a review of prior research, not a single experiment or clinical trial that directly tested a new therapeutic effect.
siRNA induces degradation of target-gene messenger RNA (mRNA) through the RNA-induced silencing complex (RISC). But the ability to suppress a specific gene is separate from delivering a sufficient amount to a patient’s tumor. siRNA must avoid degradation in the blood, renal excretion, and clearance by the mononuclear phagocyte system; then it must move out of the vasculature and through tumor tissue. Even after entering a cell, it must escape endosomes—intracellular vesicles—into the cytoplasm. Endosomal escape is an intracellular barrier distinct from clearance in the bloodstream.
Design challenges between delivery efficiency and toxicity
The review covered lipid, polymer, inorganic, and protein nanoparticles; hybrid carriers combining multiple materials; and siRNA conjugates. Conjugation involves chemically linking siRNA to a molecule that helps with target recognition or delivery. These approaches aim to protect siRNA and control its cellular uptake and release, but the same performance is not guaranteed across all carrier types.
Carrier design involves competing requirements. Positive charge can help bind negatively charged siRNA, but excessive charge may increase the risk of cytotoxicity or immune activation. Polyethylene glycol (PEG) on the surface is used to reduce protein adsorption in the blood and rapid clearance. Particle accumulation that relies on leaky tumor vessels may be uneven because of heterogeneous vascular permeability and pressure inside tumors. This is why assessment must consider not only particle uptake by cells but also distribution in the body and cytoplasmic release.
Results in mouse models and the distinction from patient evidence
A separate preclinical study cited in the review delivered VEGF siRNA using lipid nanoparticles linked to a PD-L1-targeting antibody through an antibody-binding peptide. In a mouse glioblastoma model, final tumor weight was reported to be 63% lower than in the VEGF siRNA group, and no weight loss was observed. These findings support the targeted-delivery strategy in that model, but do not establish improved survival in patients or safety in humans.
As a clinical example, the review described a combination trial of the PKN3-targeting lipid nanoparticle Atu027 and gemcitabine. Twenty-three patients with unresectable metastatic pancreatic adenocarcinoma were randomly assigned to two dosing schedules. Disease control was reported in 4 of 11 patients in the once-weekly Atu027 group and 7 of 12 in the twice-weekly group. Since both groups received the combination therapy, this comparison cannot determine how much benefit Atu027 added relative to gemcitabine alone. The review’s description also did not specify when disease control was assessed or how long it lasted.
Confirming gene silencing does not guarantee treatment success
The review noted that MYC mRNA reduction was confirmed by tumor biopsy for the MYC-targeting lipid nanoparticle DCR-MYC, but the suppression was insufficient to produce a meaningful clinical response, and development was discontinued. This case shows that even when delivery and target-gene suppression are confirmed, whether they translate into effective anticancer activity in patients must be verified separately.
The researchers proposed that future carrier development should reduce cytotoxicity and immune activation, and improve delivery to tissues beyond the liver, including tumors, as well as endosomal escape. They also identified manufacturing consistency, quality control, and the ability to scale up production as challenges for clinical translation. Entry into clinical trials should not itself be interpreted as establishing safety or efficacy.
Details of the delivery strategies and cited studies are available in the full review; the article’s study type and abstract can be found in the Europe PMC bibliographic record.
siRNA, as a precise, specific, and highly effective gene-silencing therapy, has been extensively studied. Before reaching tumor cell targets, siRNA formulations must overcome multiple extracellular barriers, including clearance from the bloodstream, membrane impermeability, capture by the mononuclear phagocyte system (MPS), rapid renal excretion, endosomal escape, and precise recognition of target cells. These challenges limit siRNA's clinical application. Consequently, various modifications have been applied to siRNA to enhance transfection efficiency, while researchers continue to pursue improved siRNA-targeting delivery systems. Nanotechnology offers a rational technical approach to address siRNA delivery. Nanoparticles can increase transfection efficiency while exhibiting lower cytotoxicity and reduced off-target effects. Various matrices have been employed to construct nanoparticles for targeted therapeutic delivery. This review briefly discusses siRNA nanoparticle delivery strategies, illustrates examples of various siRNA nanodelivery systems, such as lipid nanoparticles, polymeric siRNA nanoparticles, inorganic nanoparticles, hybrid nanoparticles, and conjugate-siRNA delivery systems, and introduces clinical trials of siRNA-loaded nanoparticles for cancer treatment, which can provide valuable references for further research and clinical application of siRNA nanoparticle delivery systems.
The review’s practical significance is that it presents siRNA cancer drug development as an interconnected problem involving blood stability, distribution within tumors, cytoplasmic release, toxicity, and manufacturing quality. High cellular uptake or gene-silencing rates alone are not enough to conclude that a treatment will benefit patients. Researchers should check the model and comparator used in preclinical studies, and separately assess whether target-gene suppression in clinical trials leads to actual tumor responses or improved survival. In particular, the comparison between Atu027 dosing schedules does not establish the drug’s added effect. Success with liver-targeted siRNA platforms also does not guarantee successful delivery to tumors, so evidence on tissue-specific delivery and safety is needed.