Solubility needs to be
evaluated in any lead optimization campaign. Low solubility
can be detrimental in many respects as it can potentially impair
oral bioavailability or formulation of a suitable dosage form. In
consideration of the high polarity of this compound class, it was
not surprising that solubility turned out to not be an issue for
the different drug candidates (6, 7) as well as the simple ethyl
ester prodrugs (9, 10). But this fact was not readily predictable
for many of the other prodrug candidates. Our results showed,
however, solubility of >50 mM (in PBS) for all oseltamivir
derivatives presented herein (Table S2, Supporting Information).
The only exceptions were amidoxime ester 9 that
exhibited a solubility of 23 mM at pH 7.4 and 9.0 and guanidine
drug 7 with a solubility of 26 mM at at pH 7.4 and 9.0. Still,
solubility values in that range are far beyond doses required to
exert their pharmacological effect, that is, inhibition of viral
NAs.