Novel Analytical Approach to Quantify Reactive Potentially Mutagenic Sulfonate Ester Impurities in Pharmaceutical Compounds and Its Application in the Development of a Control Strategy for Nonaflate Impurities in Belzutifan Synthesis

Sulfonate esters are encountered frequently as impurities that are potentially mutagenic in different manufacturing processes producing active pharmaceutical ingredients (APIs) due to the widespread use of sulfonyl halide/anhydride or sulfonic acid as reagents in the processes. While a scientific risk assessment, based on the formation and predicted purge of sulfonate esters, can justify the absence of these impurities in the final API for some cases, for other cases, analytical testing data showing the levels of sulfonate ester impurities in the API is required to support the risk assessment. The detection and quantitation of a subset of sulfonate esters that are reactive and unstable are challenging using the existing gas chromatography–mass spectrometry (GC–MS) or Liquid chromatography–MS (LC–MS) methods. We reported here a new approach to quantify reactive sulfonate esters at low levels in pharmaceutical compounds by taking advantage of the reactivity of the sulfonate ester. With this approach, an unstable sulfonate ester is intentionally hydrolyzed to the corresponding sulfonate anion and alcohol. The resulting stable sulfonate anion can then be measured readily using an LC–MS method as a way to quantify the sulfonate ester indirectly. The specificity, sensitivity, precision, and accuracy of the analytical method were validated with three stable sulfonate esters as the model compounds. The method was successfully applied to the belzutifan synthesis to analyze the amounts of several reactive nonaflate impurities formed in the final step of the synthesis. These data, in combination with a purge-based scientific risk assessment, supported a control strategy for these mutagenic impurities that centers around process control without the need to perform routine analytical testing.