Improving pH gradient cation-exchange chromatography of monoclonal antibodies by controlling ionic strength

Analytical ion exchange chromatography (IEC) is widely used to profile the charge heterogeneity of therapeutic monoclonal antibodies (mAbs). Since conventional salt gradient IEC methods are product-specific and time-consuming to develop, a previously reported alternative pH gradient IEC (pH-IEC) method using a cation-exchange column has been shown to be a multiproduct charge sensitive separation method for mAbs with isoelectric points between 7.3 and 9.0. In the work presented here, we have extended the application of that pH-IEC method to also profile the charge heterogeneity of mAbs with extreme pI values (e.g. acidic with pI<7 or basic with pI>9). A key observation of our work is that for the buffer systems used by Farnan and Moreno, the ionic strength of the mobile phase containing multiple polyamine buffers is pH and concentration dependent, and the ionic strength decreases when the pH increases. For the mobile phase with high buffer concentration the ionic strength is high at low pH values, leading to the flow through of acidic mAbs on the cation-exchange column. The basic mAbs may not have an optimal elution profile as the relatively low ionic strength of the mobile phase reduces the resolution of pH-IEC. To modulate the ionic strength, we introduced a salt gradient in addition to the pH gradient. Studies were performed to optimize the buffer and salt concentrations simultaneously to improve the retention of low pI mAbs and the resolution of high pI mAbs. The optimized salt-mediated pH-IEC method was not only applicable to mAbs over a broader pI range from 6.2 to 9.4, but also offered better resolution for mAbs with pI values between 7.3 and 9.0 than the previously reported pH-IEC method. This salt-mediated pH-IEC method was demonstrated to be robust at various chromatography conditions and capable of assessing manufacturing consistency and monitoring degradation of mAbs.