Caspase-2 is the most specific protease of all caspases and therefore highly suitable as tag removal enzyme creating an authentic N-terminus of overexpressed tagged proteins of interest. The wild type human caspase-2 is a dimer of heterodimers generated by autocatalytic processing which is required for its enzymatic activity. We designed a circularly permuted caspase-2 (cpCasp2) to overcome the drawback of complex recombinant expression, purification and activation, cpCasp2 was constitutively active and expressed as a single chain protein. A 22 amino acid solubility tag and an optimized fermentation strategy realized with a model-based control algorithm further improved expression in Escherichia coli and 5.3 g/L of cpCasp2 in soluble form were obtained. The generated protease cleaved peptide and protein substrates, regardless of N-terminal amino acid with high activity and specificity. Edman degradation confirmed the correct N-terminal amino acid after tag removal, using Ubiquitin-conjugating enzyme E2 L3 as model substrate. Moreover, the generated enzyme is highly stable at −20 °C for one year and can undergo 25 freeze/thaw cycles without loss of enzyme activity. The generated cpCasp2 possesses all biophysical and biochemical properties required for efficient and economic tag removal and is ready for a platform fusion protein process.

A two-step purification process for human basic fibroblast growth factor (FGF-2) from clarified E. coli homogenate has been developed in which the impurity level after the second step is below the limit of quantification. Endotoxin content is cleared to 0.02 EU/μg FGF-2 and the overall yield is 67%. The performance of the cation exchanger Carboxymethyl-Sepharose Fast Flow (CM-SFF) was compared to the affinity resin Heparin-SFF regarding the impurity profile and product quality in the elution peak. The CM-SFF eluate was further purified using hydrophobic interaction resin Toyopearl-Hexyl-650C. The relative amounts of target product, host cell proteins (HCPs), dsDNA, endotoxin, monomer content, and high molecular weight impurities differed along the elution peak depending on the applied method. The bioactive monomer (>99%) was obtained with a yield of 48% for CM-SFF and 68% for Heparin-SFF. A half-load reduction in CM-SFF increased the yield up to 67% without deterioration of the impurity content. Assuming a dose of 400 μg FGF-2, endotoxin was reduced to 188 EU/dose, dsDNA <10 ng/dose, and HCP <2 ppm/dose using the cation exchanger. In the pooled eluate fractions, dsDNA was removed 4-fold (291 ng/mL) and endotoxin 14-fold (0.47 EU/μg FGF-2) more efficiently by CM-SFF than by affinity chromatography. In contrast, HCP clearance was 3-fold (13 ppm) more efficient with Heparin-SFF than CM-SFF. In contrast to process monitoring by UV or SDS-PAGE, this characterization is the basis for a Process Analytical Technology attempt when correlated with online monitored signals, as it enables knowledge-based pooling according to defined quality criteria.

Host cell DNA is a critical impurity in downstream processing of enveloped viruses. Especially, DNA in the form of chromatin is often neglected. Endonuclease treatment is an almost mandatory step in manufacturing of viral vaccines. In order to find the optimal performer, four different endonucleases, two of them salt tolerant, were evaluated in downstream processing of recombinant measles virus. Endonuclease treatment was performed under optimal temperature conditions after clarification and before the purification by flow‐through chromatography with a core shell chromatography medium: Capto™ Core 700. Virus infectivity was measured by TCID50. DNA and histone presence in process and purified samples was determined using PicoGreen™ assay and Western blot analysis using an anti‐histone antibody, respectively. All tested endonucleases allowed the reduction of DNA content improving product purity. The salt‐tolerant endonucleases SAN and M‐SAN were more efficient in the removal of chromatin compared with the non‐salt‐tolerant endonucleases Benzonase® and DENARASE®. Removal of chromatin using M‐SAN was also possible without the addition of extra salt to the cell culture supernatant. The combination of the endonuclease treatment, using salt‐tolerant endonucleases with flow‐through chromatography, using core–shell particles, resulted in high purity and purification efficiency. This strategy has all features for a platform downstream process of recombinant measles virus and beyond.