The Fifth International Symposium on Packed Column SFC (SFC 2011), which alternates between the United States and Europe, was held in New York City, July 20–22, 2011. Previous meetings were SFC 2007 in Pittsburgh, PA; SFC 2008 in Zurich, Switzerland; SFC 2009 in Philadelphia, PA; and SFC 2010 in Stockholm, Sweden. The symposium is dedicated to bringing primarily scientists in the pharmaceutical industry together to discuss the latest advances in supercritical fluid chromatography (SFC). The three-day conference included 17 oral presentations, 27 poster presentations, and two vendor workshops. An abbreviated short course dedicated to achiral, chiral, and preparative supercritical fluid chromatography was held before the conference. The conference was organized by the Green Chemistry Group. The complete program and many of the oral and poster presentations can be found in their entirety at www.greenchemistrygroup.org/index.html. This article presents selected highlights of the oral and poster programs.
The plenary lecture, “Development of Novel Stationary Phases for Achiral Packed Column Supercritical Fluid Chromatography (pcSFC),” given by Bill Farrell of Pfizer, Inc. (San Diego, CA), provided a short history of achiral phase development, which led to the currently popular silica bonded 2-ethylpyridine (2-EP) stationary phase. pcSFC separations of basic solutes can be achieved without utilizing a basic additive such as triethylamine to reduce peak tailing when 2-EP is employed. The 2-EP phase, however, does not exhibit the same compatibility with acidic solutes, which separate better on bare silica and 1,2-propanediol-bonded silica. The majority of the lecture discussed work performed at Pfizer in association with column manufacturers to develop improved phases for achiral pcSFC, such as commercially available morpholine, 4-ethylpyridine (4-EP), and benzamide.
The preparation and testing of several novel phases were also presented: hydroxyamino-dipyridinyl; 5-hydroxy-3-pyridinyl; 3-hydroxyphenyl; 3,4-dihydroxyphenyl; and 2-ethylpyrazinyl. A total of 12 stationary phases were used for this study. The tailing on each of the phases was deemed unacceptable if asymmetry was in the range of <0.8 or >1.2. Surprisingly, the worst phase for acceptable tailing was 2-ethylpyridine. For the last thousand achiral purifications performed in Dr. Farrell’s laboratory, only 13% used traditional (2-ethylpyridine; 4-ethylpyridine; and 1,2-propanediol) phases, whereas the vast majority of purifications utilized custom achiral phases. The lecture ended with a slide that provided recommendations for phase selection with acidic and basic analytes as well as ranking the phases based on retention characteristics.
The main use of SFC for the past 10 years has been preparative chiral resolution of enantiomers. While SFC is expanding into other areas, the preparative arena is expected to remain prominent due to the main advantages of SFC over HPLC, such as greater speed, lower costs, faster reequilibration, and reduced solvent usage. Eric Seest of Eli Lilly (Indianapolis, IN) lectured on “Green” Lab Scale Chiral Chromatography: Comparison of Purification Techniques for Cost Effective and Efficient Processing of Samples While Minimizing Environmental Impact.” He described the Global SFC Initiative at Lilly, which aims to move 80–85% of all purifications from LC to SFC. The company expects to see significant solvent reductions, greater efficiencies, and shorter timelines as well as reduced energy usage and significant time reduction for sample drying. The criteria utilized at Lilly to choose the most appropriate purification technique (HPLC, SFC, or SSR [steady state recycling]) were also discussed.
The presentation by Kyung Gahm of Amgen (Thousand Oaks, CA) addressed a solubility study in supercritical fluids to support drug discovery purification. He noted that not much was known about the compounds received and their components. Two major problems were mentioned: The system shuts down, and scaleup failure occurs, even when a successful analytical method is available. He described an instrument for measuring solubility in modified CO2. The solubility in CO2/modifier combinations was impossible to predict based on modifier solubility data alone. Experimental solubility measurements were thought to be necessary.
Successful supercritical fluid extraction (SFE) solubility screening with multiple vessels in diverse SFC conditions was described. The importance of these data in helping to choose appropriate conditions for developing optimized preparative conditions and avoiding system shutdown caused by sample precipitation in the SFC column was demonstrated.
Tony Yan of Pfizer, Inc. discussed the use of SFC for analyzing and identifying atropisomers in drug discovery projects. Axial rotational constraint results in chirality, which is termed atropisomerism. These stereoisomers result from hindered rotation around a single bond, and are recognized as separable species when at a given temperature they display a half-life of at least 1000 sec. SFC and HPLC were both useful for these separations, with SFC being faster and exhibiting greater selectivity, while HPLC offered better MS sensitivity. A technique suitable for measuring interconversion kinetics and calculating half-life was discussed. Human blood, human plasma, and human serum albumin were chosen to study the effects of proteins on interconversion. As a thermally sensitive class of chiral compounds, the presence of atropisomers was considered to be insufficiently studied in the medicinal chemistry community.
Later in the program, Fang Xia from Amgen compared productivity, solvent usage, and sample process time using batch, SSR, and SFC with a CO2 recycler. Steady-state recycling involves closed-loop recycling, and it was stated to work best for binary systems. The discussion was followed by the topic of impurity isolation via SFC wherein the impurities came from both active pharmaceutical ingredient (API) and from mother liquor. Several case studies were described: 1) purification of 5 kg of pharmaceutical starting materials, and 2) purification of 500 g of pharmaceutical intermediates. SFC demonstrated unique selectivity. It was shown to isolate low-level impurities that could not be isolated by HPLC due to close retention and insufficient resolution. SFC could also be combined with HPLC for fast impurity isolation.
Jeffrey Kiplinger from Averica Discovery Services (Worcester, MA) gave a talk entitled “Improving Productivity in Preparative SFC Systems.” He noted that in HPLC, the sample diluent and mobile phase are usually closely matched so that partitioning between mobile and stationary phases is constant. In SFC, a noncompressible liquid solvent is injected into a compressible fluid containing a large fraction of CO2, and the partition coefficient changes along the length of the column. This situation is known to impact chromatography in unpredictable ways, and modeling of the phenomenon has proven difficult. Practical solutions to this problem were presented.
While purification is still the primary use for SFC, activity in analytical SFC has been increasing. In this regard, Christine Aurigemma of Pfizer presented an interesting lecture entitled, “Expanding the Scope of SFC Beyond the Chromatography Experts: Utilization of a Walk-up Analytical SFC System.” She discussed 1) the rationale for and challenges to implementation of a walk-up or open-access SFC, 2) the configuration of a walk-up SFC/MS system, and 3) present and future applications. A walk-up LC/SFC-MS system was described that was simple, rugged, and designed to meet the comfort levels of nonexpert users. The system was stated to have applicability across medicinal chemistry, and it accelerated sample purification.
Mark Hayward from Lundbeck Research (Paramus, NJ) described interfacing MS detectors to SFC and his efforts to achieve SFC-MS responses comparable to LC-MS. Maximum sensitivity was achieved with the use of a preexpansion heater and replacement of the backpressure regulator (BPR) with a fixed restrictor.
The past three years have seen the migration of analytical SFC from mainly a discovery tool into a technique also used in pharmaceutical development. Development application of SFC was discussed by two presenters. Zhenyu Wang of Merck (Summit, NJ) presented the development of an SFC method for release of mometasone furoate. He was able to develop an SFC method that was comparable to the existing LC method but faster (12 vs 42 min), less expensive, and greener. The lecture finished with a discussion of sensitivity issues concerning SFC/UV. Excessive noise compared to HPLC/UV was attributed to electronic noise from the detector system, mechanical noise from the backpressure regulator and pumping system, and thermal noise arising from the endothermic process during decompression. In a related presentation, Eric Sun from Amgen gave a detailed report on the steps necessary to qualify an analytical SFC system for Good Manufacturing Practices (GMP) analysis.