If you’re using process mass intensity (PMI) to gauge sustainability of continuous and batch manufacturing processes for biologics, you’re only seeing part of the picture.

Recent research shows that PMI is comparable for continuous manufacturing and batch processes. That’s because, “PMI, a sustainability metric commonly used in small molecule manufacturing, [primarily] only captures water usage for biologics manufacturing,” Sri Madabhushi, PhD, biologics process research and development, Merck & Co. tells GEN. Yet, while water usage is a primary driver of sustainability, other metrics also must be considered.

Madabhushi says energy consumption is a key missing factor. “Energy utilization to run the equipment, cleanroom requirements, and HVAC systems for biologics manufacturing plants is a primary contributor of overall sustainability of biologics manufacturing processes,” she explains. “Processes that optimize the space/time/yield in manufacturing plants and comprehensive metrics that capture these aspects will be more indicative of sustainability drivers for biologics manufacturing.”

Productivity of intensified continuous processing (measured as grams of drug substance per unit of time) can be many-fold higher than that of batch processing, yet the overall energy consumed may be lower, Madabhushi and colleagues point out.

Compared batch and continuous processes

For example, they compared a batch and continuous process using the same size bioreactors. The PMI of the batch process is about 1.6-fold lower than that of the continuous process. The continuous process, despite having a higher PMI, needed to run significantly fewer batches than the batch process to meet a typical demand. “Thus, PMI might not correlate with overall sustainability of the process in all cases,” according to Madabhushi, who adds that “a comprehensive benchmarking metric needs to be developed to quantify the impact to sustainability.”

She advocates a holistic view that includes such elements as how purportedly greener products are derived as well as the power needed to operate facilities, the plant footprint, process duration, and recovery and remediation strategies during manufacturing.

Currently, Merck & Co., is working with industry partners to develop such sustainability metrics, as well as to improve sustainability in terms of other relevant areas, such as water use, waste, greenhouse gas emissions, and packaging design.

“High-yielding continuous manufacturing might be more sustainable because of the more optimal space/time/yield of these processes,” Madabhushi says. While PMI may be suitable for certain biologics “it doesn’t capture many other products, such as classes of vaccines, where the final drug product is measured based on potency of the molecule and not on a mass basis.”