The capillary-tube-evaporative-concentration (CTEC) method: A new protein-sparing approach to directly determine protein apparent solubility values for antibody developability assessments

Solubility behavior is an important attribute to rank-order monoclonal and bispecific antibody (mAb, biAb) candidates during developability assessments. Current approaches to directly measure antibody solubility without adding precipitating additives (e.g., centrifugal ultrafiltration, protein powder rehydration) have limitations including high protein consumption (∼tens of mgs) and protein instability (e.g., aggregation or gel formation). In this work, we describe the Capillary-Tube-Evaporative-Concentrating (CTEC) method, a new approach to directly measure apparent solubility values from solid-liquid phase separation with only ∼one mg of protein. Proof-of-concept was first established with l-Tryptophan by comparing CTEC measured apparent solubility values with literature reports and theoretical calculations. Second, using the model protein lysozyme at three different pHs, CTEC determined apparent solubility values agreed well with orthogonal solubility measurements and literature reports. Third, we obtained apparent solubility values by CTEC for an IgG1 mAb at different pHs. We then rank-ordered the solubility behavior of three mAbs and one biAb at three different pHs using the CTEC assay and the commonly-used polyethylene glycol precipitation assay. Overall, rank-ordering results correlated well, but in some cases, complemented each other. The potential use of the CTEC solubility assay to directly screen antibody candidates with improved solubility behavior during developability assessments is discussed.