Evaluating the technology
In applying QbD to the lyophilization process, the first task is to define the parameters that have the potential to affect process performance and product-quality attributes (5). Key points include the freeze-drying process operating parameters, formulation parameters, equipment, and component preparation and devices (5). PAT may be applied through sensors at various stages in freeze-drying, which may include using temperature sensors, pressure-rise analysis, manometric temperature measurements, calorimetry, microscopy, and spectroscopic techniques, such as near-infrared (NIR), Raman, and infrared spectroscopy (6).Assessing the tools
An established approach for PAT in lyophilization is offered by SP Scientific's SMART freeze-dryer technology, which is used to optimize the freeze-drying cycle. The SMART technology was developed by the University of Connecticut and Purdue University through the Center for Pharmaceutical Processing Research and licensed to SP Scientific. The technology relies on the use of manometric temperature measurement, which calculates the product temperature at the sublimation interface without having to place thermocouples or other temperature sensors in product vials (7). The SMART freeze-dryer technology is used on SP Scientific's Lyostar 3 development freezer. The SMART technology uses information, such as the number of vials, fill volume, fill weight, freeze-dryer chamber volume, and critical formulation temperature to optimize a cycle (8). It contributes to several key points in lyophilization: selects an optimum freezing cycle based on whether the formulation is crystalline or amorphous; selects the optimum chamber pressure; determines the target temperature of the product; and adjusts the shelf drying during primary drying to keep the product at a predetermined target temperature (8).
SP Scientific has partnered with the industrial-gas company Praxair for another PAT-based tool for lyophilization, Praxair's ControLyo Nucleation on Demand Technology, used to control the nucleation of the product solution in the freeze dryer. The companies first partnered in 2010, which gave SP the exclusive, global rights to commercialize the technology on development lyophilzers. In 2012, the companies expanded their collaboration to allow SP Scientific to equip its clinical, pilot, and production dryers with theControLyo Technology and to transfer the technology to allow SP to retrofit existing pilot and production units.
IQ Mobil Solutions, based in Holzkirchen, Germany, offers wireless and battery-free temperature sensors (Temperature Remote Interrogation System, TEMPRIS) as a PAT tool for lyophilization. In a recent study, the TEMPRIS system was assessed for measurement accuracy, capability of accurate endpoint detection, and effect of positioning by using product runs with sucrose, mannitol and trehalose (9). Data were compared to measurements with 36-gauge thermocouples and to noninvasive temperature measurement from manometric temperature measurements. The results showed that the TEMPRIS temperature profiles agreed with thermocouple data when sensors were placed center bottom in a vial. In addition, TEMPRIS sensors revealed reliable temperature profiles and endpoint indications relative to thermocouple data when vials in the edge position were monitored (9).
Researchers at Ghent University in Belgium used Raman and NIR spectroscopy as PAT tools in a freeze-drying process (10). For the study, Raman and NIR probes were built in the freeze-dryer chamber to allow simultaneous process monitoring of a 5% (w/v) mannitol solution. Raman and NIR spectra were continuously collected during freeze-drying and analyzed using principal component analysis and multivariate curve resolution (10). Raman spectroscopy provided data about the mannitol solid state, the endpoint of freezing, and several physical and chemical conditions (e.g., onset of ice nucleation and onset of mannitol crystallisation). NIR spectroscopy monitored key points in drying, the endpoint of ice sublimation, and the release of hydrate water during storage (10). A later study further examined the use of in-line spectroscopic process analyzers (Raman, NIR, and plasma emission spectroscopy) (11).
Another recent study examined the use of tunable diode laser absorption spectroscopy (TDLAS) for monitoring secondary drying in laboratory-scale freeze-drying with the purpose of targeting intermediate moisture contents in the product (12). An earlier study examined TDLAS to determine the average product temperature in primary drying (13).