Process Overview
Plasmid DNA is the active pharmaceutical ingredient (API) for DNA vaccines and gene therapy vectors. The purification process must remove host cell genomic DNA, RNA, proteins, endotoxins, and open-circular or linear plasmid forms while maintaining supercoiled (sc) plasmid content above 90%. GMP manufacturing requires rigorous quality control at each step.
Process Steps
1
Cell Harvest
Bacterial Cell Harvest
Harvest E. coli cells from fermentation broth by disc-stack centrifugation or tangential flow filtration. Typical cell density: 20–50 g/L (wet cell weight). Cell paste can be stored at −20°C or below for up to 6 months before processing. Typical yield: 2–10 g plasmid per kg cell paste.
Yield: >98%
Cell density: 20–50 g/L
2
Lysis
Alkaline Lysis
Resuspend cell paste in ice-cold resuspension buffer (50 mM Tris, 10 mM EDTA, pH 8.0). Add alkaline solution (0.2 M NaOH with 1% SDS) and mix gently for 5–10 min. Neutralize with acidic potassium acetate (pH 5.1). This step lyses cells and denatures genomic DNA and proteins while keeping plasmid DNA in solution.
Yield: >90%
Key: Gentle mixing
3
Clarification
Removal of Cell Debris
Centrifuge (15,000–20,000 ×g) or use depth filtration (0.2–0.65 μm) to remove precipitated cell debris, genomic DNA-protein complexes, and SDS precipitates. Depth filtration is preferred for large-scale GMP manufacturing. Filter with 0.45 + 0.2 μm for final clarification.
Yield: 85–95%
Removes: Debris, gDNA
4
Capture
Anion Exchange Chromatography
Load clarified lysate onto strong anion exchange resin (Q Sepharose, GENEFLOW, or POROS) at pH 7–8. Plasmid DNA (negatively charged due to phosphate backbone) binds; RNA, proteins, and endotoxins flow through or weakly bind. Elute with 1.0–1.5 M NaCl gradient. This is the primary capture step with 10–50× purification.
Yield: 70–85%
Purity: 80–95%
5
Polishing
Hydrophobic Interaction Chromatography (HIC)
Add ammonium sulfate (0.5–1.0 M) to the AEX eluate and load onto HIC resin (Phenyl or Butyl Sepharose). Open-circular (oc) and linear plasmid forms bind more strongly than supercoiled (sc) plasmid due to exposed bases. Gradient elution separates oc from sc plasmid. Final sc content: >90%.
Yield: 60–80%
SC content: >90%
6
Endotoxin Removal
Polishing & Endotoxin Clearance
Perform additional ion exchange or tangential flow filtration to reduce endotoxin to <100 EU/mg (FDA requirement for injectables). Activated charcoal or Triton X-114 extraction can also reduce endotoxin. UF/DF into final buffer (TE or PBS). Sterile filter through 0.22 μm.
Yield: >90%
Endotoxin: <100 EU/mg
Plasmid Quality Specifications
| Parameter | GMP Requirement | Method |
|---|
| Supercoiled content | >90% | CE, HPLC, agarose gel |
| Purity (A260/A280) | 1.80–1.95 | UV spectrophotometry |
| Genomic DNA | <5% | Hybridization, qPCR |
| RNA | <1% | Agilent Bioanalyzer |
| Protein | <1% | BCA or Bradford assay |
| Endotoxin | <100 EU/mg | LAL assay |
| Sterility | Pass | Membrane filtration |
Cost Considerations
| Step | Key Cost Driver | Relative Cost |
|---|
| Cell Harvest | Centrifuge or TFF | Low |
| Alkaline Lysis | Buffers, tanks | Low |
| Clarification | Depth filters or centrifuge | Medium |
| AEX Chromatography | Resin, column, buffers | High |
| HIC Polishing | Resin, ammonium sulfate | Medium |
| Endotoxin Removal | Additional chromatography | Medium |
Chromatography dominates plasmid manufacturing cost. Plasmid size (3–15 kb) affects binding capacity. Larger plasmids have lower volumetric productivity. Continuous chromatography (simulated moving bed) is emerging for large-scale production. Use
untangle.bio to model costs at your specific scale.
Frequently Asked Questions
Why is alkaline lysis the standard method for plasmid extraction?
Alkaline lysis exploits the differential denaturation of circular (plasmid) vs. linear (genomic) DNA. At pH 12–12.5, both denature, but on neutralization with potassium acetate, genomic DNA precipitates as a tangled mesh structure while plasmid DNA renatures and stays soluble. The method is fast, scalable, and effective for plasmid sizes up to ~20 kb.
How do you ensure >90% supercoiled plasmid content?
Supercoiled (sc) plasmid is the biologically active form. Hydrophobic interaction chromatography (HIC) exploits the exposed hydrophobic bases in open-circular (oc) plasmids to separate them from sc forms. Oc plasmid binds more strongly to HIC resin due to less compact structure. Alternative: cesium chloride gradient ultracentrifugation (higher purity but less scalable).
What is the main source of endotoxin in plasmid preparations?
Endotoxin (lipopolysaccharide, LPS) from Gram-negative E. coli cell membranes is the main concern. It co-purifies with plasmid through the process. Multiple chromatography steps (AEX, HIC) reduce endotoxin. Final UF/DF and activated charcoal treatment can achieve <100 EU/mg required for injectable products.
Build Your Plasmid Purification Process
Drag-and-drop the full plasmid manufacturing train, simulate mass balance, and estimate GMP production costs.
Open untangle.bio