Mutagenic Alkyl-Sulfonate Impurities in Sulfonic Acid Salts: Reviewing the Evidence and Challenging Regulatory Perceptions

Abstract

Alkyl sulfonates are direct-acting bacterial mutagens and are almost universally regarded by regulatory authorities to be potential impurities in drug substances presented as sulfonic acid salts, particularly if synthesized using an alcohol solvent. A detailed review of the available public domain data indicates that regulatory policy appears to be based on speculation, assumption, and assertion rather than actual evidence. Ester formation from a sulfonic acid in the presence of an alcohol is an extremely slow and thermodynamically unfavored reaction requiring strongly acidic conditions to produce even minimal conversion. Following addition of an equimolar amount of a sulfonic acid to a pharmaceutical base (the active), proton transfer to form an acid salt occurs instantaneously, thus neutralizing the acid and precluding any ester formation as a side reaction even if an alcoholic solvent is employed. Other possible routes to sulfonic acid ester formation by interaction of an alcohol with preformed sulfonic acid salt or via impurities in the sulfonic acid reagent can also be discounted based on mechanistic and experimental evidence. By contrast, C 1 −C 3 chloroalkanes are formed more rapidly in HCl/alcohol systems, but they can be readily purged from hydrochloride salts (if an excess of acid is used for synthesis) and are much weaker biological alkylators than alkyl sulfonates. Generic/compound-specific limits for alkyl sulfonates and considerably higher limits for chloroalkanes can be derived using the available toxicological data and provisions of the International Conference on Harmonization (ICH) M7 guideline. Since alkyl-sulfonate impurities can now be predicted with certainty to be absent at toxicologically significant levels, the current regulatory paradigm is no longer considered to be justified, thus removing the hypothetically based perception of potential risk associated with sulfonic acid salts.

International regulatory requirements for genotoxicity testing for pharmaceuticals used in human medicine, and their impurities and metabolites

“…A significant body of work now demonstrates that this is not routinely a concern. The sulfonate esters do not form easily and chemistry is well understood so that process controls prevent them [Snodin and Teasdale, ]; they are also easily removed by process chemistry, even if they were to form.…”

The cancer threshold of toxicological concern (TTC) in relation to foodstuffs and pharmaceuticals: A potentially useful concept compromised by a dubious derivation

“…This could be done for example by considering the margin between the appropriate TTC value (without any adjustment for duration of exposure) and the estimated dietary exposure.The above commentary is considered to be an inaccurate and incomplete description of the use of structural alerts in the context of pharmaceutical impurities, for the following reasons: There is no mention of the ICH M7 consensus guideline on mutagenic impurities 3 ; The EFSA statement mentions genotoxicity, whereas in ICH M7, structural alerts are interpreted in the context of mutagenicity; The determination of bacterial mutagenic potential in ICH M7 is a sophisticated and multi-layered procedure involving inter alia ; In silico evaluation using two complementary systems (rule- and statistical-based); Follow-up using expert assessment; Use of read-across from appropriate reference compounds (e.g. evaluating the drug substance and impurities for the presence of similar structural moieties); Performing an Ames’ assay, as necessary; For human pharmaceuticals data from mammalian-cell assays are discounted if the impurity is predicted with confidence to be non-mutagenic and/or is an experimental non-mutagen – not the case veterinary; If an impurity with a conventional structural alert for bacterial mutagenicity is shown by either in silico prediction or experimental means to be a non-mutagen, it is treated as a normal (non-mutagenic) impurity with no structural alert; Compound-specific limits can be determined for mutagenic impurities mainly based on the determination of an in vivo threshold for mutagenic effects 22 establishing that the mechanism of carcinogenicity is unrelated to genotoxicity, or linear extrapolation of carcinogenic potency data. 11 Some examples are shown in Table 1. …”

“…Compound-specific limits can be determined for mutagenic impurities mainly based on the determination of an in vivo threshold for mutagenic effects 22 establishing that the mechanism of carcinogenicity is unrelated to genotoxicity, or linear extrapolation of carcinogenic potency data. 11 Some examples are shown in Table 1.…”

Time-dependent selected reaction monitoring-based GC-MS/MS method for estimation of genotoxic impurities in new antibacterial agent: alalevonadifloxacin mesylate

“…Synthesis of ALA involves use of methanol, ethanol, and isopropanol as solvents at various stages of synthesis and during purification. Possibility of undesired reactions between the methane sulfonic acid and the alcohols exists resulting in the formation of AMs (Andrew et al 2010;Snodin and Teasdale 2015;Snodin 2019). These impurities are deoxyribo nucleic acid (DNA) reactive substances that have the potential to directly cause DNA damage, thereby leading to genetic mutations causing cancer.…”