By Paul A. Metz, Bioxel Pharma, Inc., and Olivier Dapremont, Aerojet Fine Chemicals LLC Simulated Moving Bed (SMB) chromatography is an important technology for separating chiral compounds, but it can also be used in challenging purifications to remove impurities that are structurally similar to target compounds. Bioxel Pharma, Inc. has developed and implemented a fully automated, high throughput SMB process to remove cephalomannine from paclitaxel, an active pharmaceutical ingredient used in cancer drugs. The novel purification process has been running for six months at the FDA inspected site of Aerojet Fine Chemicals in Rancho Cordova, Calif.Paclitaxel and cephalomannine are both natural products isolated from the Canada yew. Removing cephalomannine from paclitaxel is particularly challenging, since the molecular structures of the two compounds are very similar. The overall process, from starting biomass to finished product, is outlined in Figure 1.Bioxel has been producing paclitaxel since 1998 using single-column batch chromatography to remove cephalomannine. However, as demand for paclitaxel increased, the company sought to increase its production scale. While single-column chromatography was selective and produced a high purity product, it lacked the capacity, throughput and yield to meet growing demands.To improve its paclitaxel output and production economics, Bioxel recently installed an SMB purification process, using equipment built by Groupe NovaSep (Pompey, France). The process has been fully validated following the ICH Q7A cGMP guidance for active pharmaceutical ingredients. The facility now running at Aerojet Fine Chemicals can produce up to 120 kg/yr of USP-grade cGMP paclitaxel. U.S., Canadian and European Drug Master Files have been obtained for the new process. The large-scale separation of paclitaxel from cephalomannine poses several challenges, involving tradeoffs in equipment costs, separation efficiency, capacity, product yield and throughput. In addition, because paclitaxel is a potent cellular toxin, and separations involve the use of volatile organic solvents, any process has to be engineered to meet stringent safety and containment requirements.Thus far, Bioxel’s new SMB process has increased product yield and throughput, removed other trace impurities and has reduced or eliminated undesired solvents. This article will summarize the SMB project, highlighting the quality and economic results that have been achieved with the new process. It further compares these results with those expected using single-column batch chromatography.The processing steps used to make USP-grade paclitaxel are shown in Figure 1. The API is present in the yew biomass at less than 0.1%. The final product is purified to a concentration of greater than 98.5%. Cephalomannine content in the final product must be less than 0.5%. Because paclitaxel is cytotoxic, the final process must be run in a high containment environment.The Canada yew is harvested from forest lands throughout eastern Canada. Moisture is removed from the biomass, which is then ground into a powder. What follows is a series of fine chemical processing steps including liquid-solid extraction, solid phase extraction, adsorption and precipitation steps that remove paclitaxel and other taxanes from cellulose fiber, waxes, pigments and other materials in the biomass. These steps produce a concentrated intermediate extract powder containing paclitaxel, cephalomannine and other taxanes with side chain substitution on the C-13 position of the tricyclic baccatin ring system. Most of the processing involves proprietary technology using common engineering unit operations.Chromatography to Remove ImpuritiesWhile a few chemical processing approaches [1-2] have been applied to remove cephalomannine from paclitaxel, chromatography is most commonly used. Several chromatographic approaches have been reported for paclitaxel purification at various process points and at different production scales [3-6].After working with paclitaxel using single-column batch chromatography for several years, Bioxel found that the best strategy for purification was to push cephalomannine together with paclitaxel as far as possible through the process, favoring high paclitaxel yield along the way. Then, the goal was to identify the most efficient way to separate the unwanted impurity from paclitaxel at the end. The company settled on SMB for final purification following computer modeling and successful small-scale process demonstration runs.Figure 2 shows the SMB system, which is a continuous, closed-loop process. Paclitaxel intermediate extract is fed through a pre-filter and then into the SMB system. The intermediate is separated into two streams, paclitaxel (extract) and cephalomannine (raffinate). Two automated rotary evaporators are used to recycle the eluent collected in the extract and raffinate lines and to dry the isolated fractions. The unit is operated in the normal phase, with a proprietary column packing material.The SMB operation is followed by crystallization to remove remaining trace impurities, yielding a paclitaxel product that is greater than 98.5% pure. A very mild high-efficiency filter drying process removes residual solvents prior to packaging. The SMB, crystallizer and filter dryer are housed in a dedicated high containment facility. With low solvent utilization, the SMB is operated in a general-purpose area. The solvent tank farm, crystallization vessel and filter dryer are operated in a Class I, Division I, Group C&D area.Results Favor SMBBioxel carried out feasibility studies to assess the use of SMB on a commercial scale targeted at producing 100 kilograms of pure paclitaxel per year. Data from the studies along with historical chromatography data were used to model and compare an SMB process with a single column approach. Studies included column screening and mass overload experiments that were carried out using analytical HPLC and commercially available preparative chromatography packing materials. Sixteen stationary phases were screened with five organic solvents in varying ratios. System selection was based on the following criteria:
- Paclitaxel solubility in the mobile phase
- Paclitaxel and cephalomannine resolution
- Peak retention and elution order
- Cost and availability of the stationary phase
- Product stability
SMB is a type of multi-column chromatography that has been applied to difficult separations of chiral compounds . The basis for SMB is to simulate a counter-current contact between a solid stationary phase and a mobile liquid phase. Unlike single column batch chromatography, SMB is a continuous process in which the mixture to be separated is injected onto a ring of chromatographic columns at rotating points between the columns. At the same time, streams are withdrawn from the ring at rotating points, simulating the movement of the stationary bed. SMB has been applied to pharmaceutical separations [9-10] and is now in commercial use in FDA approved processes .
Varicol is the trade name for the patented process variation of SMB in which the injection and collection points are moved asynchronously across the ring of chromatographic columns. Normal SMB has four zones. The counter-current movement of the solid phase with respect to the liquid mobile phase is simulated by the periodic and simultaneous switching of the product feed and mobile phase make-up lines (inlets) and the extract and raffinate lines (outlets).
In SMB, the number of columns in each zone is constant and is an integer. Varicol uses an asynchronous shift of the inlet and outlet lines. In this way, the column distribution between zones varies with time, allowing for optimization of column distribution. Thus, the number of columns in each zone does not have to be an integer. This design promotes even more efficient use of the solid and liquid mobile phases as compared to SMB.
This results in lower operating costs. The unit is fully automated and computer controlled using validated software developed according to GAMP4 guidelines.
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