Most host cell impurities will be very similar to process impurities. By extending the platform to the upstream processes
of cell culture and cell clarification, even greater success rates can be achieved for the downstream process. Platform analytical
approaches are also being applied to the development of monoclonal antibodies, reducing the time to toxicological and first-in-human
(FIH) studies. Other important efficiency gains include fewer protocols and lower complexity of multisite operations, more
lenient preliminary specifications for investigational new drugs (INDs), and more robust FIH methods and testing.5
Platform technologies can also reduce validation costs. The use of platform technologies, combined with the precision and
sensitivity of quantitative Polymer Chain Reaction (Q-PCR) has helped the evaluation of the efficiency of generic and matrix
viral clearance. Anion exchange steps in the platform are used as polishing steps—i.e., to remove impurities such as DNA,
host cell proteins, and adventitious viruses.
Because anion exchange chromatography is commonly used as part of the platform purification process for monoclonal antibodies,
a great deal of knowledge has been gained in this area. Most monoclonal antibodies have basic isoelectric points, typically
greater than 8, so they do not bind under neutral pH and low conductivity conditions. Experience with fast-flow agarose anion
exchangers combined with Q-PCR analysis has shown that bracketing and generic approaches demonstrate efficient removal from
the process of a nonenveloped virus, SV40.6
Another potential improvement in validation is minimizing resin lifetime studies for Protein A and anion exchange chromatography.
Studies with anion exchangers in flow-through mode have demonstrated that surrogate measurements such as DNA clearance, back-pressure,
and band spreading can predict when columns will no longer provide sufficient virus clearance.7 It has been shown that when Protein A columns are multiply cycled, antibody step yield and breakthrough, but not eluate impurity
content, are performance quality attributes that decay before a decrease in retrovirus log reduction values (LRV).8
Today some chromatography resins can achieve binding capacities greater than 250 grams of protein per liter. Fast mass transfer
rates enable the use of more than 80% of the capacity after only one minute, and flow velocities can now exceed 1,000 cm/h.
For ion exchangers, the most commonly used type of resin, these improvements have been achieved through a greater understanding
of resin structure and surface chemistry that enables bead design with appropriate mechanical properties, porosity, and distribution
of charged functional groups.
Binding capacity can influence process economics in several ways. High capacity can reduce column size requirements, which
in turn can reduce space requirements and water consumption for buffers and cleaning solutions. Binding kinetics also influence
raw material costs per gram of product, though this area is rather complex. In the example shown in Figure 2, resin A has
a high total capacity, slow kinetics, and low maximum flow velocity compared to resin B. Resin B has low total capacity, fast
kinetics, and a high maximum flow velocity. At very short residence times, resin B gave higher productivity than resin A;
but when residence time was optimized for resin A, it provided a more economical solution. The example also points out the
importance of process development for each individual biotherapeutic.
Figure 2. A comparison of two resins with different capacity and flow properties