Expression and purification of the recombinant diphtheria fusion toxin DT₃₈₈IL3 for phase I clinical trials

A genetically engineered fusion toxin targeted to acute myeloid leukemia (AML) blasts was designed with the first 388 amino acid residues of diphtheria toxin with an H-M linker fused to human interleukin-3. The cDNA was subcloned in the pRK bacterial expression plasmid and used to transform BLR (DE3) Escherichia coli. A single transformed colony was grown in Superbroth with ampicillin; bacteria were centrifuged at an OD₆₅₀ of 1.3; master cell bank aliquots of bacteria in 30% glycerol/Superbroth were frozen and stored at -80°C. Master cell bank bacteria were diluted 1500-fold into Superbroth and recombinant protein was induced with 1 mM IPTG at an OD ₆₅₀ of 0.6. After two additional hours of fermentation, inclusion bodies were isolated, washed, and denatured in guanidine hydrochloride and dithioerythritol. Recombinant protein was refolded by diluted 100-fold in cold buffer with arginine and oxidized glutathione. After dialysis, purified protein was obtained after anion-exchange, size exclusion on FPLC, and polymyxin B affinity chromatography. The final material was filter sterilized, aseptically vialed, and stored at -80°C. Seventy-five 3-L bacterial culture preparations were made and pooled for the AT-1 batch (568 mL) and twenty-four 3-L bacterial culture preparations were made and pooled for the AT-2 batch (169 mL). The final product was characterized by Coomassie Plus protein assay, Coomassie-stained SDS-PAGE, limulus amebocyte lysate endotoxin assay, human AML TF/H-ras cell cytotoxicity assay, sterility, tandem mass spectroscopy, IL3 receptor binding affinity, ADP ribosylation activity, inhibition of normal human CFU-GM, disulfide bond analysis, immunoblots, peptide mapping, stability, HPLC TSK3000, N-terminal sequencing, E. coli DNA contamination, C57BL/6 mouse toxicity, cynomolgus monkey toxicity, and immunohistochemistry. Yields were 25. 7 ± 5.6 mg/L bacterial culture of denatured fusion toxin. After refolding and chromatography, final yields were 20 ± 11% or 5 mg/L. Vialed product was sterile. Batches were in 0.25 M sodium chloride/5 mM Tris, pH 8, and had protein concentrations of 1.8-1.9 mg/mL. Purity by SDS-PAGE was 99 ± 1%. Aggregates by HPLC were < 1%. Potency revealed a 48 h IC ₅₀ of 6-8 pM on TF/H-ras cells. Endotoxin levels were 1 eu/mg. The remaining chemical and biologic assays confirmed the purity, composition, and functional activities of the molecule. The LD₁₀ in mice was 250 μg/kg/day every other day for six doses. The MTD in monkeys was 60 μg/kg/day every other day for six doses. Drug did not react with tested frozen human tissue sections by immunohistochemistry. There was no evidence of loss of solubility, proteolysis aggregation, or loss of potency over 6 months at -80 and -20°C. Further, the drug was stable at 4 and 25°C in the plastic syringe and administration tubing for 24 h and at 37°C in human serum for 24 h. The synthesis of this protein drug should be useful for production for clinical phase I/II clinical trials and may be suitable for other diphtheria fusion toxins indicated for clinical development.