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Sulfonate Ester Kinetic Study

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Organic Process Research & Development 2010, 14, 999–1007 A Detailed Study of Sulfonate Ester Formation and Solvolysis Reaction Rates and Application toward Establishing Sulfonate Ester Control in Pharmaceutical Manufacturing Processes Andrew Teasdale,*,† Edward J. Delaney,‡ Stephen C. Eyley,† Karine Jacq,§ Karen Taylor-Worth,⊥ Andrew Lipczynski,⊥ Wilfried Hoffmann,⊥ Van Reif,¶ David P. Elder,# Kevin L. Facchine,0 Simon Golec,9 Rolf Schulte Oestrich,4 Pat Sandra,§ and Frank David§ AstraZeneca, R&D Charnwood, Bakewell Road, Loughborough, Leicestershire LE11 5RH, United Kingdom; Reaction Science Consulting LLC, 11 Deer Park DriVe/Suite 202, Monmouth Junction, New Jersey 08852, U.S.A.; Research Institute for Chromatography, Pres. Kennedypark 26, B-8500, Kortrijk, Belgium; Pfizer Global Research and DeVelopment, Analytical R&D, Ramsgate Road, Sandwich, Kent CT13 9NJ, United Kingdom; Schering-Plough, 556 Morris AVenue, Summit, New Jersey 07901-1330, U.S.A.; GlaxoSmithKline, Park Road, Ware, Hertfordshire, SG12 0DP, United Kingdom; GlaxoSmithKline, FiVe Moore DriVe, Research Triangle Park, North Carolina 27709-3398, U.S.A.; Wyeth Research, 500 Arcola Road, CollegeVille, PennsylVania 19426, U.S.A.; and F. Hoffmann-La Roche Ltd., Grenzacher Strasse, 4070 Basel, Switzerland Abstract: presented that should be of value to process development scientists Sulfonate esters of lower alcohols possess the capacity to react with in designing appropriate controls in situations where risk for DNA and cause mutagenic events, which in turn may be cancer sulfonate ester formation does exist. inducing. Consequently, the control of residues of such substances in products that may be ingested by man (in food or pharmaceu- ticals) is of importance to both pharmaceutical producers and to Introduction regulatory agencies. Given that a detailed study of sulfonate ester Sulfonic acids and their derivatives have been important tools reaction dynamics (mechanism, rates, and equilibria) has not been to process development chemists since they were first discov- published to date, a detailed kinetic and mechanistic study was ered, and they continue to be of enormous value in the undertaken and is reported herein as a follow-up to our earlier manufacture of pharmaceuticals. In the pharmaceutical industry, communication in this journal. The study definitively demonstrates sulfonate salts of intermediates and active pharmaceutical that sulfonate esters cannot form even at trace level if any acid ingredients (APIs) are highly useful, and alcohols are frequently present is neutralized with even the slightest excess of base. A key employed as crystallization solvents in sulfonate salt isolation conclusion from this work is that the high level of regulatory processes. When a sulfonic acid and an alcohol are both present concern over the potential presence of sulfonate esters in API in a given process stream in any amount, there is at least a sulfonate salts is largely unwarranted and that sulfonate salts theoretical potential to form some level of an alkyl sulfonate should not be shunned by innovator pharmaceutical firms as a ester impurity, regardless of circumstance. The effects of potential API form. Other key findings are that (1) an extreme concentration, temperature, and pH may have profound impact set of conditions are needed to promote sulfonate ester formation, on the real potential to form traces of a sulfonate ester impurity requiring both sulfonic acid and alcohol to be present in high in any given situation, but unfortunately, there is no real concentrations with little or no water present; (2) sulfonate ester information published in the chemical literature to provide formation rates are exclusively dependent upon concentrations of guidance on the level that should be expected. In such situations sulfonate anion and protonated alcohol present in solution; and analytical chemists have traditionally been required to develop (3) acids that are weaker than sulfonic acids (including phosphoric assays with low limits of detection (ppm range) to determine acid) are ineffective in protonating alcohol to catalyze measurable the potential presence of sulfonate ester traces in the isolated sulfonate ester even when a high concentration of sulfonate anion intermediates or APIs in question. Further, when pharmaceutical is present and water is absent. Implications of the mechanistic and candidates do transition from R&D to production, the analytical kinetic findings are discussed under various situations where method may need to be transferred to a Quality Control lab if sulfonic acids and their salts are typically used in active pharma- the established method becomes a routine specification test, in ceutical ingredient (API) processing, and kinetic models are spite of the fact that the real potential for sulfonate ester formation had never been fully understood. * Author to whom correspondence may be sent. E-mail: andrew.teasdale@ With the 2007 adoption of an EMEA guidance limiting astrazeneca.com. † AstraZeneca, R&D. genotoxic impurities to exposure limits of not more than 1.5 ‡ Reaction Science Consulting LLC. µg/day, the potential for sulfonate ester residues to exist in APIs § Research Institute for Chromatography. has become a growing concern among regulators.1 Simulta- ⊥ Pfizer Global Research and Development, Analytical R&D. ¶ Schering-Plough. neously, with the publication of ICH guidelines Q8-Q10 and # GlaxoSmithKline UK. the pending Q11 (API Development), the pharmaceutical 0 GlaxoSmithKline US. 9 Wyeth Research. industry is being encouraged to adopt quality by design 4 F. Hoffmann-La Roche Ltd. principles that embrace the development of predictive scientific 10.1021/op900301n 2010 American Chemical Society Vol. 14, No. 4, 2010 / Organic Process Research & Development • 999 Published on Web 03/10/2010 sulfonate (EMS), pentafluorothiophenol (PFTP), dimethyl sul- foxide (DMSO), ethanol (absolute, EtOH), ethanol-d6 (EtOH- d6), isopropanol-d8 (iPrOH-d8), ethanol-d4 (MeOH-d4), and 2, 6-lutidine (ReagentPlus grade, 98%), and 2,5-dichloro-4-ni- troaniline were obtained from Sigma-Aldrich (Beerse, Belgium). 18O-Methanol (18O-MeOH) was obtained from Isotec (Mi- amiburg, Ohio, U.S.A.). Pentafluoroanisole (PFA), sodium sulfate (anhydrous) and sodium hydroxide were obtained from Acros Organics (Thermo Fisher, Geel, Belgium). Methanol (MeOH) and isopropanol (iPrOH) were obtained from Biosolve (Valkenswaard, NL). Karl Fischer Titration and Added Water Calculation. Alcohol/methanesulfonic acid mixtures used in measuring the rates of anhydrous forward reactions were confirmed to contain not more than 0.1% moisture by Karl Fischer titration, corre- Figure 1. Possible mechanistic pathways for sulfonate ester sponding to <0.04 mol equiv relative to methanesulfonic acid formation. inputs. In experiments where specific amounts of water were knowledge to guide the design of appropriate quality controls needed, actual amounts of alcohol, water, and methanesulfonic during the development phase of each API process. acid were measured by mass. This approach proved more During 2007, the Product Quality Research Institute (PQRI) consistent and accurate than using KF measurements to assess agreed to support a project proposal by a small group of water content when appreciable amounts of water were present. industrial pharmaceutical development leaders to commission GC/MS Method for Measuring Sulfonate Ester Levels a detailed study of the dynamics of sulfonate ester formation in Reaction Mixtures. Example Internal Standard Preparation. and degradation. This effort was undertaken with the goals of Methanesulfonyl chloride (1 g) was mixed with ethanol-d6 providing mechanistic knowledge, demonstrating appropriate (1 mL) in a reaction tube, closed with a Teflon lined screw analytical methodology, and establishing kinetic models. Hence, cap. The reaction mixture was heated for 72 h at 70 °C. After with the development of these tools, the group intended not cooling, water (2.5 mL) was added followed by diethyl ether just to qualify various processing situations with respect to risk, (2.5 mL, CAUTION: volatile acidic vapors). The formed ethyl but also to better enable industry chemists to develop appropriate methane sulfonate-d5 (EMS-d5) was extracted in the ether phase. control strategies when formation and carry-over risks do exist. This phase was separated, dried over anhydrous sodium sulfate, An initial product of this effort was a recently published concentrated under nitrogen, and diluted in acetonitrile (10 mL, communication that conclusively demonstrated the mechanism CAUTION: genotoxic material). The solution was stored at 4 for sulfonate ester formation as being Path B of Figure 1 when °C. The exact concentration of the internal standard in this solution methanol is reacted with methanesulfonic acid.2 In addition, the was checked by GC/MS using liquid injection and using EMS as methodologies for quantifying sulfonate esters in reaction an external standard. The analytical conditions were similar to the mixtures have been developed and published.3 In this full report, conditions used for headspace analysis (see below). the mechanistic findings are further confirmed by kinetic studies Solution Preparation for Analysis. The following solutions involving systems including methanol, ethanol, and isopropanol, were prepared: and representative alkyl and aryl sulfonic acids. A detailed • Reaction mixture: Methanesulfonic acid (or p-toluene- summary of all kinetic studies conducted within the design space
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