Political and economic pressures are creating profound changes in the biopharmaceutical industry, as rising healthcare costs put increased scrutiny and regulation on the industry, particularly in the United States. Companies are facing intense competition from low-cost manufacturers in other regions of the world. Generics and biosimilars add to this competition, diminishing market share for branded medications. These market and environmental conditions are increasing the need and focus on sustainable processes, with lower operating costs and minimal environmental impact.
Process Economics Drivers
These aforementioned factors require a re-imagining of biopharmaceutical processes. Processes can be improved in several different ways:
- increase process flexibility
- reduce the cost of raw materials
- decrease waste/disposal costs.
Fewer blockbuster drugs generally mean fewer continuous production processes. With shorter production campaigns, flexibility in unit operations is critical. Single-use products allow production facilities to accommodate multiple products, as well as reduce validation costs. Emphasis has been placed on upstream fermentation, however, downstream purification processes can not be ignored. Economical, large particle size polymeric resins make excellent single-use guard columns. These guard columns, used in a negative adsorption mode, can remove contaminants such as color bodies and host cell proteins thereby reducing the contaminant load on more expensive chromatographic media.
Process economics can also be improved by carefully controlling materials costs. Process costs include initial raw materials, regeneration and disposal costs. Downstream purification costs are reduced by protecting or substituting expensive chromatographic media with polymeric resin. Additionally, effective process development results in lower process cost from decreased solvent usage through higher mass concentration in product effluent, and efficient solvent recycling. This article will focus on these two applications, Dimer Removal and Solvent Recovery.
Low molecular weight (<1000 kDa) active pharmaceutical ingredients (APIs) are sometimes prone to dimerization. Product dimers are undesirable from the standpoint that they decrease product purity and potency. Typical approaches for dimer removal include: crystallization, solvent extraction or adsorption with diatomaceous earth. These methods are not very efficient, expensive, and can generate hazardous wastes. Adsorptive chromatography is an alternative which not only provides high-capacity dimer removal, but can also eliminate other contaminants as well.
In this example, the API is poorly soluble under aqueous conditions (<2g/L), unstable at both high pH and high temperature. Additionally, the process requires that the API be recovered at high mass concentration (>100g/L) or low liquid volume, and the process step needs to be conducted within the constraints of one work shift (<8 hours).
Process development is performed using a reversed-phase polystyrenic resin with an average pore size of 150 angstroms, high surface area (900m2/g) and an average particle size of 75 microns. Complicating the development is the poor aqueous solubility of the API. The solvent concentration in the loading mobile phase requires a balance between reasonable API solubility and sufficient retention of the API on the reversed phase polymer.
The key to the process development is dissolving the API at high concentration. While the water solubility of the API is poor, the API is very soluble in acetone (>100g/L). Therefore, initial dissolution of the API is performed in acetone and the feed is then mixed with 40 percent acetone so that the loaded feed concentration is 10g/L in 40 percent acetone. The schematic for the purification process is shown in Figure 1. This solvent concentration, combined with the relatively high feed concentration, provides high operational capacity of the API on the reversed phase column (>80g/L). The solvent conditions also allow high linear velocity during loading, thus decreasing the total operation time for this step to <6 hours. High mass loading and optimized elution conditions provide high mass concentration of the API in the column effluent, reducing overall solvent volume and the requirements for further filtration or evaporation. Most importantly, the API is recovered at high purity with high yield. A summary of the optimized process conditions and results is shown in Table 1.
Figure 1 – API Loading Process with Solvent Recycle
Table1: Summary of Dimer Removal Process
Some synthetic APIs required synthesis under totally anhydrous conditions so that the chemical reactions are most efficient. While anhydrous solvents are readily available, solvents may contain water after synthesis is complete. Rather than dispose of these solvents, they can be recovered, thereby reducing raw materials and disposal costs. To recover and re-use solvents, desiccation may be required.
Strong cation exchange resins, with a low moisture level, can be used as desiccants for organic solvents, similar to the use of silica gels and molecular sieves. However, these strong cation exchange resins have the advantage of being very easily regenerated at low temperatures. The resins are particularly useful for the removal of trace amounts of water from non-polar solvents, such as chlorinated hydrocarbons. Most non-polar materials are not imbibed into the resin structure, and therefore, many of the inhibitor systems used in these solvent systems are not taken from the solvent upon drying.
Solvents which are relatively non-ionic in character can be dried by the strong cation to moisture levels of less than 10ppm. Capacities of 20 pounds of water adsorbed per 10 pounds of dried resin desiccant are obtainable. Reversibility can be accomplished using temperatures as low as 300°F, a decided improvement over the 365 – 650°F required by molecular sieves, thereby decreasing energy costs and providing a safer process.
Water content in the organic liquid fed to the desiccant affects the capacity obtained, with higher water content yielding higher capacity as shown in Figure 2. This is typical isotherm behavior with small molecule adsorption. Similarly, flow rate through the desiccant affects the capacity, with lower flow rates yielding higher capacity as shown in Figure 3.
Figure 2: Effect of Water Content in Feed on Dessicant Capacity
Figure 3: Effect of Flow Rate on Dessicant Capacity
Ion exchange resins used for desiccant applications are strong acid exchange resins based on polymers of styrene crosslinked with divinylbenzene, and functionalized by sulfonation. This yields a resin containing a sulfonate group to which an exchangeable counter-ion is attached. For desiccation, the sodium counter-ion is the most common. It is sometimes advantageous to use the resin in the potassium form to improve the kinetics of drying a specific solvent system.
The use of ion exchange resins as desiccants are most effective when using column operations. In this operation, the fluid to be dried is passed through the dry resin bed under controlled temperature and flow rate. Resin bed depths are typically between three and six feet, with column depths sufficient to contain the volume change which the ion exchange resin undergoes when water is imbibed during the drying process. The resin swells about 1 percent on a dry volume basis for each 1 percent of water imbibed on a weight basis. This expansion requires that the column dimensions are such that the volume change can occur vertically. Tall, narrow columns might restrain expansion and cause strong lateral pressure resulting in crushed resin or column rupture. Typical flow rates for non-polar solvents are in the range of 5 -10 gpm/ft2 based on the empty column cross-section. For polar liquids, the flow rates are generally lower (~1gpm/ft2) since the kinetics of drying decrease as the polarity of the liquid increases.
As shown in the two process examples, biopharmaceutical production economics are improved through the effective use of polymeric resins. These resins are not only employed in the purification of APIs, but in the recovery of valuable raw materials as well.
- API purification processes using polymeric resins can reduce solvent consumption while still delivering high purities, and high product yields. Reduced solvent consumption decreases overall production costs.
- Ion exchange resins are effective, high-capacity desiccants for solvent recovery and water removal. They have lower reactivation temperatures than molecular sieves resulting in lower energy costs.
- B.A. Perlmutter, Env. Progress 16 (2007) 132-136.
- M. Rios, Bioprocess Intl. (May 2010) 34-47.