Su
lfonate esters have a demonstrated potentia
l for genotoxicity, and therefore their potentia
l presence at trace
leve
ls in active pharmaceutica
l ingredients (APIs) has recent
ly raised concerns [
Mesylate Ester Type Impurities Contained in Medicinal Products; Swissmedic Department for Contro
l of the Medicina
l Products Market, 23rd October 2007 and
Hoog, T. J.-d. Request to Assess the Risk of Occurrence of Contamination With Mesilate Esters and Other Related Compounds in Pharmaceuticals; Coordination Group for Mutua
l Recognition-Human Committee (CMDh), EMEA/CMDh/98694/2008: London, 27 February, 2008, ]. Su
lfonate sa
lts however, offer usefu
l modification of physicochemica
l properties of active pharmaceutica
l ingredients (APIs) containing basic groups such that their use can at times offer significant advantages over other counterions [
llauthors">Elder, D. P.; Delaney, E.; Teasdale, A.; Eyley, S.; Reif, V. D.; Jacq, K.; Facchine, K. L.; Oestrich, R. S.; Sandra, P.; David, F.The Uti
lity of Su
lfonate Sa
lts in Drug Deve
lopment.
J. Pharm. Sci. 2010,
99, 2948−2961; DOI:
10.1002/jps.22058]. Indeed, the choice of benzenesu
lfonic acid as the counterion for the UK-369,003 API afforded many advantages over other sa
lts such as citrate, hydroch
loride, tartrate, and phosphate as we
ll as other su
lfonate sa
lts such as tosy
late, camsy
late, and mesy
late. The manufacturing route to the API consists of two C−C bond-forming steps (steps 1 and 2/Scheme 1) and a fina
l sa
lt-formation step (step 3/Scheme 1). The step 2 cyc
lisation process invo
lves the use of ethano
l as the reaction so
lvent. Residua
l leve
ls of ethano
l in the iso
lated product of the step 2 process was initia
lly thought to be responsib
le for the formation of
low
leve
ls of the genotoxic impurity ethy
l besy
late (ppm
leve
ls) during the fina
l step sa
lt-formation process [
Glowienke, S.; Frieauff, W.; Allmendinger, T.; Martus, H. J.; Suter, W.; Mueller, L. Mutat Res. 2005,
581, 23−34]. This was thought to resu
lt from subsequent reaction of residua
l ethano
l with benzenesu
lfonic acid used in the fina
l step (step 3). On the basis of this mechanistic hypothesis, the
leve
ls of residua
l ethano
l in the iso
lated product from step 2 were contro
lled so that formation of ethy
l besy
late wou
ld be minimised or avoided in the fina
l step. Spiking experiments coup
led with deuterium
labe
lling studies have shed doubt on this mechanism of formation. Our experimenta
l resu
lts indicate that
leve
ls of ethy
l besy
late in the API are independent of the
leve
l of residua
l ethano
l in the step 2 product (UK-369,003 free base) and are detected when higher than stoichiometric amounts of benzenesu
lfonic acid are used in the sa
lt-formation process (step 3). This is thought to be due to a reaction between the excess benzenesu
lfonic acid and the ethoxy side chain of the API. Sensitive and se
lective ana
lytica
l methods were a
lso deve
loped to detect and quantify subppm and higher
leve
ls of ethy
l besy
late and deuterated ana
logues.