Among the various sensing modalities available for bioprocess operations, index of refraction (IoR) is intriguing for its simplicity and capability for sample-less, real-time readout. A group led by Steven Harris of Pall showed in a recent publication that continuous, in-line IoR monoclonal antibody concentration measurement was feasible and, if combined with adaptive control, “could be used to ensure a consistent purification process performance with a seamless advanced process control.”

GEN asked Harris, lead author and principal scientist, why bioprocessors may have overlooked IoR—a legacy analytical technology if there ever was one—until now.

“Refractometry is not commonly used in determining protein concentration in biomanufacturing because the signal is not specific for proteins (such as monoclonal antibodies), meaning that matrix components and host cell proteins all contribute to the signal,” he says. “We demonstrated that this limitation may be mitigated by measuring the refractive index in chemically defined media,  for example, the chromatography elution buffer, and by using refractometry after the Protein A affinity capture step, at which point nearly all of the other host cell proteins have been removed.”

Process analytic technology and quality by design were promulgated in the first decade of the 2000s. Why has adoption been so slow?

According to Harris, as with most innovations in biomanufacturing, developers prefer being a close second to being first. “The upfront investment in money, time, and manpower to adopt PAT and QbD is quite high. Although companies recognize their potential benefits, the risks involved in being first is not negligible. This risk is balanced by not moving too fast or too far ahead of the industry standard. This, along with validation requirements, results in a slower pace of adoption.”

The case for continuous measurement rests on the presumption that upstream protein output will change over the course of the production run. Harris notes that while textbook perfusion processes generate consistent titers, “even the most well-defined and controlled bioreactor is still a variable process of living organisms. The goal of control platforms is to minimize the impact of that variability and, especially in the perfusion paradigm, to keep the cells as happy as possible for a long as possible. Ideally the upstream titer output will be maintained within the downstream operating titer range.”