Isolation and detection of a KDEL-tagged recombinant cholera toxin B subunit from Nicotiana benthamiana

Preclinical Long-Term Stability and Forced Degradation Assessment of EPICERTIN, a Mucosal Healing Biotherapeutic for Inflammatory Bowel Disease

Background/Objectives: EPICERTIN, a biotherapeutic candidate for mucosal healing in inflammatory bowel disease (IBD) and other mucosal disorders, was subjected to an extensive long-term stability program to evaluate its molecular stability and physicochemical properties. Additionally, a forced degradation assessment was conducted to identify EPICERTIN's degradation products under various conditions, including thermal stress, pH variations, agitation, and oxidation. Methods: The stability of EPICERTIN drug substance (DS), formulated in phosphate-buffered saline (PBS) at 1 mg/mL and stored at 5 °C and 25 °C/60% relative humidity (RH), was monitored over a 2-year period, referencing relevant regulatory guidelines. Evaluations of EPICERTIN DS over the 24-month period included assessment of purity by SDS-PAGE and size exclusion high performance liquid chromatography (SEC-HPLC), identity by electrospray ionization mass spectrometry (ESI-MS) intact mass analysis and Western blotting, and potency by GM1-binding KDEL-detection ELISA (GM1/KDEL ELISA). The forced degradation patterns were analyzed by assessing purity (using SEC-HPLC and SDS-PAGE), potency (via GM1/KDEL ELISA), and intact mass (via ESI-MS). Results: The results overall support that EPICERTIN DS remains stable for 2 years under the tested conditions. The forced degradation assessment effectively identified degradation products, particularly under conditions of high temperatures (above 40 °C for 24 h), low pH values (pH 1 and 4), and oxidation upon exposure to 2% H2O2. Conclusions: These findings highlight EPICERTIN's robust long-term stability in PBS formulation, reinforcing its potential as a viable drug candidate for the treatment of IBD.

Mechanism study of cross presentation of exogenous antigen induced by cholera toxin-like chimeric protein

Tumor subunit vaccines have great potential in personalized cancer immunotherapy. They are usually administered with adjuvant owing to their low immunogenicity. Cholera toxin (CT) is a biological adjuvant with diverse biological functions and a long history of use. Our earlier study revealed that a CT-like chimeric protein co-delivered with murine granulocyte-macrophage colony stimulating factor (mGM-CSF) and prostate cancer antigen epitope could co-stimulate dendritic cells (DCs) and enhance cross presentation of tumor epitope. To further study the molecular mechanism of CT-like chimeric protein in cross presentation, major histocompatibility complex class I (MHC I)-restricted epitope 257-264 of ovalbumin (OVAT) was used as a model antigen peptide in this study. Recombinant A subunit and pentameric B subunit of CT protein were respectively genetically constructed and purified. Then both assembled into AB5 chimeric protein in vitro. Three different chimeric biomacromolecules containing mGM-CSF and OVAT were constructed according to the different fusion sites and whether the endoplasmic reticulum (ER) retention sequence was included. It was found that A2 domain and B subunit of CT were both available for loading epitopes and retaining GM1 affinity. The binding activity of GM1 was positively correlated with antigen endocytosis. Once internalized, DCs became mature and cross-presented antigen. KDEL helped the whole molecule to be retained in the ER, and this improved the cross presentation of antigen on MHC I molecules. In conclusion, hexameric CT-like chimeric protein with dual effects of GM1 affinity and ER retention sequence were potential in improvement of cross presentation. The results laid a foundation for designing personalized tumor vaccine based on CT-like chimeric protein molecular structure.

Characterization and utility of two monoclonal antibodies to cholera toxin B subunit

Cholera toxin B subunit (CTB) is a potent immunomodulator exploitable in mucosal vaccine and immunotherapeutic development. To aid in the characterization of pleiotropic biological functions of CTB and its variants, we generated a panel of anti-CTB monoclonal antibodies (mAbs). By ELISA and surface plasmon resonance, two mAbs, 7A12B3 and 9F9C7, were analyzed for their binding affinities to cholera holotoxin (CTX), CTB, and EPICERTIN: a recombinant CTB variant possessing mucosal healing activity. Both 7A12B3 and 9F9C7 bound efficiently to CTX, CTB, and EPICERTIN with equilibrium dissociation constants at low to sub-nanomolar concentrations but bound weakly, if at all, to Escherichia coli heat-labile enterotoxin B subunit. In a cyclic adenosine monophosphate assay using Caco2 human colon epithelial cells, the 7A12B3 mAb was found to be a potent inhibitor of CTX, whereas 9F9C7 had relatively weak inhibitory activity. Meanwhile, the 9F9C7 mAb effectively detected CTB and EPICERTIN bound to the surface of Caco2 cells and mouse spleen leukocytes by flow cytometry. Using 9F9C7 in immunohistochemistry, we confirmed the preferential localization of EPICERTIN in colon crypts following oral administration of the protein in mice. Collectively, these mAbs provide valuable tools to investigate the biological functions and preclinical development of CTB variants.

Spray-Dried Formulation of Epicertin, a Recombinant Cholera Toxin B Subunit Variant That Induces Mucosal Healing

Epicertin (EPT) is a recombinant variant of the cholera toxin B subunit, modified with a C-terminal KDEL endoplasmic reticulum retention motif. EPT has therapeutic potential for ulcerative colitis treatment. Previously, orally administered EPT demonstrated colon epithelial repair activity in dextran sodium sulfate (DSS)-induced acute and chronic colitis in mice. However, the oral dosing requires cumbersome pretreatment with sodium bicarbonate to conserve the acid-labile drug substance while transit through the stomach, hampering its facile application in chronic disease treatment. Here, we developed a solid oral formulation of EPT that circumvents degradation in gastric acid. EPT was spray-dried and packed into enteric-coated capsules to allow for pH-dependent release in the colon. A GM1-capture KDEL-detection ELISA and size-exclusion HPLC indicated that EPT powder maintains activity and structural stability for up to 9 months. Capsule disintegration tests showed that EPT remained encapsulated at pH 1 but was released over 180 min at pH 6.8, the approximate pH of the proximal colon. An acute DSS colitis study confirmed the therapeutic efficacy of encapsulated EPT in C57BL/6 mice upon oral administration without gastric acid neutralization pretreatment compared to vehicle-treated mice (p < 0.05). These results provide a foundation for an enteric-coated oral formulation of spray-dried EPT.