Dendrimers represent a class of highly branched, monodisperse polymeric nanostructures with well-defined architecture and functional surface groups. Their unique properties, including controllable size, multivalency, and the ability to encapsulate or conjugate therapeutic agents, make them particularly suitable for nucleic acid delivery. Among dendrimers, cationic variants such as polyamidoamine (PAMAM) and polypropyleneimine (PPI) have been extensively studied for gene and small interfering RNA (siRNA) delivery due to their ability to form stable complexes with negatively charged nucleic acids through electrostatic interactions.
**Nucleic Acid Complexation and Transfection Efficiency**
Cationic dendrimers interact with DNA or siRNA to form dendriplexes, which protect nucleic acids from enzymatic degradation and facilitate cellular uptake. The binding affinity and complex stability depend on dendrimer generation, surface charge density, and the nitrogen-to-phosphate (N/P) ratio. Higher-generation dendrimers (e.g., G4-G7 PAMAM) exhibit stronger nucleic acid condensation due to increased primary amine groups, but excessive positive charge can lead to cytotoxicity. Optimizing the N/P ratio is critical; an N/P ratio of 5-10 often balances complex stability with minimal toxicity.
Transfection efficiency is influenced by dendrimer structure and modifications. Unmodified PAMAM dendrimers show moderate transfection efficiency, often limited by poor endosomal escape and serum inhibition. Modifications such as PEGylation or acetylation can reduce nonspecific interactions with serum proteins, enhancing stability in biological fluids. For instance, acetylated G5 PAMAM dendrimers demonstrate improved siRNA delivery efficiency in serum-containing media compared to unmodified counterparts.
**Endosomal Escape Mechanisms**
A major barrier to effective nucleic acid delivery is endosomal entrapment and lysosomal degradation. Cationic dendrimers facilitate endosomal escape through the proton sponge effect. The buffering capacity of tertiary amines in dendrimers leads to proton influx during endosome acidification, causing osmotic swelling and membrane rupture. PAMAM dendrimers, with their high density of ionizable amines, are particularly effective in this regard. Studies show that G7 PAMAM dendrimers achieve endosomal escape rates exceeding 60% in certain cell lines, significantly higher than linear polyethylenimine (PEI) of comparable molecular weight.
Alternative strategies to enhance endosomal escape include conjugation with endosomolytic peptides or hydrophobic moieties. For example, histidine-modified PPI dendrimers exploit the imidazole group’s buffering capacity to improve siRNA release from endosomes. Similarly, lauroyl chains grafted onto PAMAM surfaces promote membrane destabilization, increasing cytoplasmic delivery of genetic material.
**Cytotoxicity and Mitigation Strategies**
The primary drawback of cationic dendrimers is their dose-dependent cytotoxicity, attributed to membrane disruption and reactive oxygen species (ROS) generation. High-generation dendrimers (e.g., G7 PAMAM) exhibit greater toxicity due to stronger electrostatic interactions with cell membranes. Surface modification is a key strategy to reduce cytotoxicity while maintaining transfection efficiency.
PEGylation shields positive charges, decreasing nonspecific interactions with cell membranes and serum proteins. Partial acetylation or carboxylation of surface amines also mitigates toxicity. For instance, 50% acetylated G5 PAMAM dendrimers show a 70% reduction in hemolytic activity compared to unmodified dendrimers while retaining siRNA delivery capability. Biodegradable dendrimers, such as those with ester or disulfide linkages, offer another solution by allowing controlled degradation into nontoxic fragments after delivery.
**Targeted Delivery and Functionalization**
To enhance specificity, dendrimers can be functionalized with targeting ligands such as folate, peptides, or antibodies. Folate-conjugated PAMAM dendrimers selectively deliver siRNA to cancer cells overexpressing folate receptors, improving transfection efficiency by 2-3 fold in folate receptor-positive cells compared to untargeted dendrimers. Similarly, RGD peptide-modified dendrimers enhance uptake in integrin-expressing tumor cells.
Stimuli-responsive dendrimers further refine delivery. pH-sensitive linkages (e.g., hydrazone or acetal bonds) enable cargo release in acidic tumor microenvironments or endosomes. Redox-sensitive dendrimers, incorporating disulfide bonds, exploit the high glutathione levels in cancer cells for selective intracellular disassembly.
**Comparative Performance and Challenges**
Compared to other nonviral vectors like cationic polymers or inorganic nanoparticles, dendrimers offer superior uniformity and tunability. PAMAM dendrimers achieve transfection efficiencies comparable to PEI but with lower cytotoxicity at optimized conditions. However, challenges remain in scaling up synthesis, ensuring batch-to-batch consistency, and achieving in vivo stability.
Recent advances focus on hybrid systems, such as dendrimer-gold nanoparticle conjugates, which combine the nucleic acid binding capacity of dendrimers with the optical properties of gold for theranostic applications. Another approach involves dendrimer-lipid hybrids, though these fall outside the scope of lipid-based systems.
**Conclusion**
Cationic dendrimers, particularly PAMAM and PPI, are promising platforms for gene and siRNA delivery due to their precise structure, high nucleic acid loading capacity, and modifiable surface properties. Strategies to enhance transfection efficiency—such as optimizing N/P ratios, engineering endosomal escape mechanisms, and reducing cytotoxicity through surface modifications—have significantly advanced their therapeutic potential. Future work must address scalability and in vivo performance to translate these nanostructures into clinical applications.