Development of Impurity Profiling Methods Using Modern Analytical Techniques.

This review gives a brief introduction about the process- and product-related impurities and emphasizes on the development of novel analytical methods for their determination. It describes the application of modern analytical techniques, particularly the ultra-performance liquid chromatography (UPLC), liquid chromatography-mass spectrometry (LC-MS), high-resolution mass spectrometry (HRMS), gas chromatography-mass spectrometry (GC-MS) and high-performance thin layer chromatography (HPTLC). In addition to that, the application of nuclear magnetic resonance (NMR) spectroscopy was also discussed for the characterization of impurities and degradation products. The significance of the quality, efficacy and safety of drug substances/products, including the source of impurities, kinds of impurities, adverse effects by the presence of impurities, quality control of impurities, necessity for the development of impurity profiling methods, identification of impurities and regulatory aspects has been discussed. Other important aspects that have been discussed are forced degradation studies and the development of stability indicating assay methods.

RP-HPLC separation and characterization of unknown impurities of a novel HIV-protease inhibitor Darunavir by ESI-MS and 2D NMR spectroscopy.

A selective RP-HPLC method for separation and determination of darunavir from its process related substances has been developed and validated. The separation was accomplished on a Hiber, LiChrospher 60, RP-select B, C8 (250 mm × 4.6 mm, 5 µm) column connected to a photo diode array detector (PDA) using 10 mM phosphate buffer with 0.1% of triethylamine (pH: 4.0 adjusted with orthophosphoric acid)/acetonitrile as a mobile phase under gradient elution. Two unknown impurities of darunavir were isolated and characterized by ¹H, ¹³C, 2D-NMR and mass spectrometry. The method was validated in terms of accuracy, precision, linearity, robustness, LOD and LOQ.

Evaluation of (R)-(-)-α-methoxy phenyl acetic acid as a chiral shift reagent for resolution and determination of R and S enantiomers of modafinil in bulk drugs and formulations by 1H NMR spectroscopy.

(R)-(-)-α-Methoxy phenyl acetic acid, (S)-(-)-1,1'-(2-naphthol), and (R)-(+)-α-methoxy-α-trifluoromethyl phenyl acetic acid were evaluated as chiral shift reagents (CSRs) for (1)H NMR spectroscopic resolution and determination of R and S enantiomers of modafinil (MDL) in bulk drugs and formulations. Effects of the nature of CSR and the weight ratio of substrate to shift reagent on enantiomeric discrimination were investigated. Intramolecular and intermolecular hydrogen bonding interactions between the drug and the CSR seem to be the driving force for desired resolution. A mechanism was proposed to explain the interactions between (R, S)-enantiomers of MDL and (R)-(-)-α-methoxy phenyl acetic acid. The method was validated and applied successfully to determine the enantiomeric purity of MDL in tablet formulations.

Rapid determination of rifaximin on dried blood spots by LC-ESI-MS.

The use of blood spot collection cards is a simple way to obtain specimens for therapeutic drug monitoring, assessing adherence to medications and preventing toxicity in a clinical setting. A high-throughput liquid chromatography-electrospray ionization mass spectrometric (LC-ESI-MS) method for determination of rifaximin on dried blood spots (DBS) was developed and validated. It involves solvent extraction of a punch of DBS followed by reversed-phase LC on a monolithic column consisting of a silica rod with bimodal pore structure and detection by ESI-MS. Rifampicin was used as an internal standard (IS). The run time was within 5.0 min with a very low back-pressure at a flow rate of 0.5 mL/min. The assay was linear from 0.1 to 10 ng/mL. The mean recovery was 98.42%. The developed method is very simple, rapid and useful for clinical applications.

Reversed-phase liquid chromatographic separation of antiretroviral drugs on a monolithic column using ionic liquids as mobile phase additives.

The use of 3-methylimidazolium cation-based ionic liquids (ILs) was evaluated as mobile phase additives for separation of antiretroviral drugs on a monolithic column by RP-HPLC. Separation of eight commonly used antiretroviral drugs was achieved on a Chromolith Flash, RP-18e column (25 × 4.6 mm, porous material) using water (pH 4.0 adjusted with acetic acid)/methanol v/v as a mobile phase containing ILs in a gradient elution mode. The effects of concentrations of ILs on retention, resolution and peak shape were studied and a regression equation correlating the interactions between stationary phase and the ILs was established. The retention of all the drugs was decreased notably by using 1-butyl-3-methylimidazolium tetrafluoroborate, while 1-ethyl-3-methylimidazolium methylsulfate reduced gradient drift drastically when compared to triethylamine.

Separation and characterization of forced degradation products of abacavir sulphate by LC-MS/MS.

Abacavir sulphate was subjected to forced degradation under the conditions of hydrolysis (acid, alkali and neutral), oxidation, photolysis and thermal stress as prescribed by ICH. Eight degradation products were formed and their separation was accomplished on Waters XTerra C18 (250 mm x 4.6 mm, 5 µm) column using 20 mM ammonium acetate:acetonitrile as a mobile phase in gradient elution mode by LC. The degradation products were characterized by LC-MS/MS and its fragmentation pathways were proposed. No previous reports were found in the literature regarding the degradation behavior of abacavir sulphate.