Bioreducible polymers for gene delivery

Purpose: Bioreducible polymers containing disulfide linkages are attractive gene-delivery carriers because they are stable in the oxidizing extracellular environment but degrade rapidly in the reducing intracellular cytosol via thiol–disulfide exchange, triggered by high glutathione (GSH) levels. This review addresses the need for carriers that combine extracellular stability, intracellular gene release, and reduced cytotoxicity, unlike hydrolytically degradable polymers that can degrade too rapidly in moisture.

Hypothesis: This is a review article and does not test a single formal hypothesis. Its central premise is that incorporating disulfide linkages into cationic polymers yields “bioreducible” vectors that remain stable in blood/extracellular conditions, degrade specifically inside reducing cells, release genetic cargo, and reduce cytotoxicity by avoiding accumulation of high-molecular-weight polycations.

Aims: Describe the initiation and recent advances in bioreducible polymers for gene delivery. - Review major classes: reducibly cross-linked PEIs, bioreducible polypeptides, bioreducible polyion complex (PIC) micelles, and bioreducible poly(amido amine)s (PAAs). - Highlight applications in cancer and ischemic heart disease gene therapy, where tumor and infarct tissues are hypoxic/reducing. - Emphasize remaining issues in pharmacokinetics and safety for rational design of bioreducible polymers for human gene delivery.

Delivery system: Polymer class: Bioreducible cationic polymers with disulfide linkages. - Subclasses: - Reducibly cross-linked PEIs: low-MW PEI cross-linked with DSP, DTBP, CBA, or thiolation/DMSO oxidation; linear poly(ethylenimine sulfide) (l-PEIS); branched b-PEIS; PEGylated and cNGR-targeted derivatives. - Bioreducible polypeptides: PEG-SS-CWK18, Cys-substituted lysine/histidine peptides, reductively degradable polycations (RPCs), reducible copolypeptides (rCPPs), cross-linked KALA (cl-KALA), Tat-10H, reducible poly(oligo-D-arginine) (rPOA). - Bioreducible PIC micelles: thiolated PEG-PLL, PEG-SS-asODN/PEI, VEGF siRNA-PEG/PEI, PEG-SS-P[Asp(DET)]. - Bioreducible PAAs: SS-PAAs, SS-PAEIs, DMC-based PAAs, poly(CBA-DAH), arginine-grafted bioreducible polymer (ABP), guanidylinated bioreducible polymer (GBP). - Payloads: Plasmid DNA (pDNA), siRNA, antisense oligonucleotide (asODN), mRNA, VEGF, IL-12, Fas siRNA, GLP-1 plasmid, etc. - Targeting/functional ligands: cNGR, PGE2, primary cardiomyocyte-specific peptide (PCM), arginine/guanidine, Tat, KALA, nona-arginine (R9).

Approach: Review of in vitro and in vivo literature, mostly from the 2000s. In vitro models include CHO, HeLa, COS-7, MDA-MB-435, PC-3, HepG2, H9C2, A7R5, RENCA, 293T, and others. In vivo models include mouse lung, tumor models, rabbit myocardial infarct model, and rat cardiomyocytes. No new primary experiments or group sizes/doses are reported.

Key methods: Polyplex characterization: Size, zeta potential, gel electrophoresis, DLS, electron microscopy. - Transfection/gene expression: Luciferase, GFP, VEGF, IL-12, GLP-1 expression. - Gene silencing/RNAi: siRNA-mediated knockdown (e.g., VEGF, Fas). - Intracellular trafficking/degradation: Confocal microscopy, BODIPY-FL dequenching, EMA-labeled pDNA. - Reductive degradation: DTT or GSH treatment; GSH depletion with buthionine sulfoximine (BSO). - Cytotoxicity/safety: Cell viability, hemolysis, histological analysis, inflammatory changes. - In vivo pharmacokinetics/biodistribution: Blood levels, organ accumulation, VEGF expression in infarct region.

Key results: Reducibly cross-linked low-MW PEI (800 Da) with DSP or DTBP improved gene delivery to CHO cells; DTBP conjugates gave higher gene expression than DSP conjugates at equal N/P ratios due to amidine bond formation maintaining net polymer charge. - Cross-linked bioreducible PEIs generally showed lower cytotoxicity and higher transfection efficiency than PEI25k. - b-PEIS degraded under 5 mM GSH and did not accumulate in cells after 48 h, suggesting complete excretion; transfection was highly enhanced versus l-PEIS. - PEG-SS-CWK18 with a disulfide cross-linker showed much higher gene transfer efficiency than non-reducible PEG-VS-CWK18. - RPC complexes were destabilized by 2.5 mM DTT and enhanced transgene expression versus non-reducible PLL complexes in human retinoblast 911 cells. - His-rich RPC with at least 70% histidine mediated efficient gene transfer without chloroquine. - cl-KALA/siRNA showed gene silencing comparable to PEI in MDA-MB-435 cells; PEG-conjugated cl-KALA/siRNA was superior in serum. - Tat-10H gave a 7,000-fold improvement in gene transfection efficiency over the original Tat peptide. - rPOA polyplexes gave higher gene expression than PEI, sustained for 1 week after intratracheal injection into mouse lung without significant toxicity. - SS-PAEIs achieved nearly 20-fold higher transfection efficiency than PEI25k in mammalian cells, maintained in 10% serum. - SS-PAED/RTP-VEGF plasmid produced up to 4-fold increase in VEGF expression in the infarct region compared with control in a rabbit myocardial infarct model. - Poly(TETA/CBA) released siRNA completely under 2.5 mM DTT and showed much higher RNAi activity against VEGF than linear PEI (25 kDa) in PC-3 cells. - Poly(CBA-DAH) with a hexaethylene spacer showed the highest transfection efficiency among poly(disulfide amine)s, with significantly less cytotoxicity than bPEI. - ABP and GBP improved transfection efficiency compared with poly(CBA-DAH) or bPEI25k, likely due to cell-penetrating/guanidine functionality and nuclear localization ability.

Interpretation: Bioreducible polymers are considered ideal gene-delivery carriers because they provide high extracellular stability and rapid intracellular disulfide cleavage, reducing cytotoxicity by avoiding accumulation of high-molecular-weight polycations and enabling controlled intracellular release of genetic materials. However, development is still at an initial stage; pharmacokinetics and biosafety must be thoroughly evaluated, and the exact mechanism and intracellular location of reductive cleavage need clarification for rational design of clinically viable bioreducible polymers.

Limitations: This is a review, not a primary study; no new experimental data or meta-analysis. - Pharmacokinetics and biosafety of bioreducible polymers are not yet thoroughly evaluated. - Exact mechanism and molecular biological effects of reductive disulfide cleavage remain unclear. - Most systems are preclinical; clinical translation is not yet established. - Long-term toxicity, large-animal validation, and detailed in vivo fate of degradation products are not addressed. - Several studies rely on in vitro transfection and limited in vivo models.