Influence of pH on hydrolytic decomposition of diethylpropion hydrochloride: stability studies on drug substance and tablets using high-performance liquid chromatography

Characterization and demonstration of drug compound ring-chain tautomer formation and its impacts on quality control

Unknown chromatographic peaks, potential impurities, were observed in a series of related compounds. This led to the identification and characterization of tautomeric equilibria. Structural elucidation was required to understand the potential impurity profile, thus impacting method development for quality control. In this work, characterization of the chemical structures, AZ13581258 and AZD5718, and equilibria of the tautomeric forms was performed using a range of advanced analytical techniques such as preparative chromatography, nuclear magnetic resonance (NMR), chromatographic detection by mass spectrometry (MS), MSMS, and ultraviolet spectroscopy (UV). Predictions using density functional theory (DFT) further explains and confirms the tautomer equilibria through predictions of reaction barrier energies, UV-spectra and NMR data. These investigations led to fully understand the impurity profile and to the development of a quality control method for AZD5718 drug substance and drug product. In conclusion, ring-chain tautomeric structures are predominately formed under acidic conditions, and the additional peaks observed in LC during organic impurity determination were found to originate from ring-chain closed tautomers in equilibria with the parent open form compound. Hence, the closed and open tautomer forms should all be considered as the same compound.

Kinetics of racemization of (+)- and (-)-diethylpropion: studies in aqueous solution, with and without the addition of cyclodextrins, in organic solvents and in human plasma

The configurational stability of (+)- and (-)-diethylpropion [(+)- and (-)-2-(diethyl)-1-phenyl-1-propanone or (+)- and (-)-DEP] was investigated systematically from chemical, pharmaceutical, and pharmacological aspects. The enantiomeric ratio was monitored directly with a recently developed stability-indicating enantioselective HPLC method. In aqueous solutions, the rate of racemization increased non-linearly with increasing pH and with increasing phosphate buffer concentration. The racemization rate showed a positive slope with increasing temperature and decreasing ionic strength. The racemization rates of (+)- and (-)-DEP in the presence of cyclodextrins (CDs) did not differ significantly. CDs that were added to (+)- and (-)-DEP in a molar ratio 5:1 showed the following effects after dissolution in 10 mM phosphate buffer (final pH 6.7): sulfobutyl ether-beta-CD (SBE-beta-CD) and methylated-beta-CD (Me-beta-CD) retarded racemization; whereas hydroxypropyl-beta-CD (HP-beta-CD), acetyl-gamma-CD (Ac-gamma-CD), acetyl-beta-CD (Ac-beta-CD), gamma-CD, and beta-CD showed a weak destabilising effect. In contrast to the described CDs, alpha-CD distinctly accelerated the rate of racemization. The configurational stability of (+)- and (-)-DEP was also studied under physiological conditions. The half-life of racemization in heparinised human plasma was for both enantiomers determined to be approximately 23-25 min. In phosphate buffer (10 mM, pH 7.4), rac-DEP showed a high, but unselective affinity towards human alpha 1-acid glycoprotein (orosomucoid) immobilised on silica (Chiral AGP). The rate of racemization of the free base of (-)-DEP dissolved in organic solutions generally increases with the polarity of the solvating agent.