Enantiomeric separation and optical purity determination of thyroxine enantiomers in bulk and pharmaceutical formulations

Simultaneous enantioselective separation method for thyroid hormones using liquid chromatography-tandem mass spectrometry and its applications

An analytical method for the simultaneous determination of chiral thyroxine and the related iodinated chiral compounds using LC-MS/MS is introduced in this study. D-Thyroid hormones (THs), which are not commercially available, were produced through the racemization reaction of the L-THs in acetic acid solution containing salicylaldehyde. The solution containing D- and L-THs after the reaction was used for optimizing the chiral separation. The D- and L-THs were well separated enantiomerically under isocratic conditions in 70 % acetonitrile containing 0.1 % formic acid on a CROWNPAK® CR-I (+) column, but some peaks, such as those of diiodo-D-tyrosine (D-DIT)/monoiodo-L-tyrosine, diiodo-D-thyronine/diiodo-L-tyrosine and D-thyroxine/triiodo-L-thyronine, overlapped chromatographically, causing misinterpretation in impurity analysis. This was overcome by using the gradient condition providing the best chiral selectivity (α) and resolution (Rs) ranging from 1.14 to 1.37 and from 2.39 to 4.52, respectively. The linearity was above 0.999 and the detection limits ranged from 8.2 to 57.7 ng/mL by the separation method. This method was applied to identify and quantify chiral impurities in authentic standards and pharmaceuticals. As a result, D-enantiomers corresponding to the L-THs standards as well as L-DIT were commonly observed as impurities. In the stability test of DL-thyroxine under acidic conditions for identifying the distribution of chiral products, it was observed that the formation of DIT by hydrolysis increased over time. Additional products formed through esterification, including thyroxine methyl ester and diiodo-tyrosine methyl ester, were newly separated and identified using a C18 column.

A feasible method for indirect quantification of L-T4 in drugs by iodine determination

In this work, a method combining microwave-induced combustion (MIC) for sample preparation of commercial levothyroxine sodium (L-T₄) drugs (L-T₄: 25-200µg/tablet), and potentiometry with ion selective electrode (ISE) for iodine determination and subsequent indirect quantification of L-T₄ was proposed. The type and concentration of the absorbing solution were evaluated to select the most suitable conditions for this study. Using the MIC method, it was possible to use solutions as diluted as 150mmolL^-1 (NH₄)₂CO₃ (for samples containing 25-200µg of L-T₄/tablet) for I absorption. In these conditions, recoveries for L-T₄ were between 99% and 101%, and relative standard deviations were lower than 10%. The limit of detection for L-T₄ was 11.2µg/tablet, which is almost two times lower than the minimum concentration of L-T₄ in commercially available drugs. Thus, the MIC was suitable for the digestion of several L-T₄ drugs for subsequent I determination by ISE and indirect quantification of L-T₄. Furthermore, the proposed method presents high throughput with low reagent consumption and consequently lower waste generation, making it suitable for routine determination of L-T₄ in drugs. From the obtained results, it was possible to observe that one of the analyzed samples is not in agreement with the limits established by the United States Pharmacopeia, indicating the importance of the drug quality control. The United States Pharmacopeia establishes that each tablet must contain between 90% and 110% of the amount of active substance declared by the manufacturer.

Enantioselective resolution of thyroxine hormone by high-performance liquid chromatography utilizing a highly fluorescent chiral tagging reagent

A highly fluorescent chiral tagging reagent, 4-(3-isothiocyanatopyrrolidin-1-yl)-7-(N,N-dimethylaminosulfonyl-2,1,3-benzoxadiazole, [R(-)-DBD-PyNCS], was employed to develop an indirect resolution method for efficient separation of thyroxine enantiomers,D-T(4) and L-T(4). The reaction of R(-)-DBD-PyNCS with the thyroxine enantiomers proceeds effectively at 40 degrees C for 20 min in the presence of basic medium to produce the corresponding pair of diastereomers. No racemization occurs during the tagging reaction under the optimized conditions. Various experimental parameters for derivatization reaction including the species of catalyst, the concentration of tagging reagent and reaction temperatures, have been examined to get a highest yield for T(4) derivatives. The structure of T(4) derivatives was identified based on ESI-MS/MS measurements in negative mode. The efficient separation of D-, L-T(4) derivatives was achieved by isocratic elution with water-acetonitrile mobile phase containing 1% AcOH on a reversed phase column utilizing a conventional fluorescence detector. The resolution (Rs) value of the diastereomers derived from thyroxine was 5.1. The calibration curves of both the D-T(4) and L-T(4) were linear over the concentration range of 0.1-20 microg/ml. The limits of detection (S/N = 3) for both D-T(4) and L-T(4) were 0.2 ng per injection. The proposed method was applied to the determination of D-T(4) and L-T(4) in pharmaceutical formulations and human serum samples.

Direct separation and quantitative analysis of thyroxine and triiodothyronine enantiomers in pharmaceuticals by high-performance liquid chromatography

A rapid reversed-phase type HPLC method for the simultaneous separation and analysis of D- and L-thyroxine (D- and L-T(4)) and triiodothyronine (T(3)) was developed using a quinine-derived chiral stationary phase and applied for a quantitative assay of the enantiomeric impurity of the drugs in pharmaceutical formulations of levothyroxine. The influence of operating parameters has been studied for the optimization of the separation and also in order to gain an insight into the retention mechanism. Validation of the method included linearity, precision and accuracy which revealed R.S.D. values of <3.3% for intra-assay precision and percent error ranging from -6 to +2.1% for various defined validation samples, proving satisfactory accuracy. Quantitation was performed over the range of 0.5-500 microg ml(-1) with limits of detection and quantitation lower than 0.1 and 0.5 microg ml(-1), respectively, for both analytes. Further, the determination of 0.1% impurity, of D-T(4) as well as L- and D-T(3) in levothyroxine sodium tablets proved to be feasible.

Biosensor for the enantioselective analysis of the thyroid hormones (+)-3,3',5-triiodo-L-thyronine (T3) and (+)-3,3',5,5'-tetraiodo-L-thyronine (T4)

An amperometric biosensor based on L-aminoacid oxidase is proposed for enantioselective assay of (+)-3,3',5-triiodo-L-thyronine (L-T3) and (+)-3,3',5,5'-tetraiodo-L-thyronine (L-T4), due to the fact that only the L enantiomer has the hormonal activity. The construction of the amperometric biosensor is simple and reproducible. The analytical information obtained from enantioselective analysis are reliable. The RSD <1% assured by using the amperometric biosensors for L enantiomers assay as raw materials, and from tablets, demonstrated their suitability for the analysis of T3 and T4 at ppb concentration levels.

Recent applications of chromatographic resolution of enantiomers in pharmaceutical analysis

The principles and applications of chromatographic separation of enantiomers in pharmaceutical analysis have been reviewed. Several of recently reported enantioselective analysis of various racemic drugs using both the 'indirect' and 'direct' methods have been presented.