Method development in quantitative NMR towards metrologically traceable organic certified reference materials used as (31)P qNMR standards

Quantitative 31P NMR Spectroscopy Platform Method for the Assay of Oligonucleotides as Pure Drug Substances and in Drug Product Formulations Using the Internal Standard Method

One of the most widely used techniques for the quantification of small interfering ribonucleic acid (siRNA) is the ultraviolet (UV) spectroscopy method. However, due to uncertainties in the extinction coefficient affecting the accuracy of the method and a sample preparation including several dilution steps, the purpose of this study was to explore the possibility of determining the content of siRNA by a platform method using quantitative 31P nuclear magnetic resonance (31P-qNMR) and the internal standard method. In this paper, acquisition time, selection of a suitable internal certified reference material, signal selection used for quantification, relaxation delay, and precision are discussed. In addition, the robustness of the method and the ability to apply this platform method to both drug substance (DS) and drug product samples is also discussed. Quantifications of siRNA determined by the 31P-qNMR platform method were on average 98.5%w/w when adjusting for the sodium and water contents. The data confirmed the applicability of 31P-qNMR in siRNA content determinations. The quantifications were compared to quantifications determined by the traditional UV spectroscopy method by F- and t-tests. The statistical tests showed that the platform 31P-qNMR method provided more accurate results (mass balance close to 100% w/w) compared to the traditional UV spectroscopy method when analyzing DS samples.

Paramagnetic Relaxation Agents for Enhancing Temporal Resolution and Sensitivity in Multinuclear FlowNMR Spectroscopy

Sensitivity in FlowNMR spectroscopy for reaction monitoring often suffers from low levels of pre-magnetisation due to limited residence times of the sample in the magnetic field. While this in-flow effect is tolerable for high sensitivity nuclei such as 1 H and 19 F, it significantly reduces the signal-to-noise ratio in 31 P and 13 C spectra, making FlowNMR impractical for low sensititvity nuclei at low concentrations. Paramagnetic relaxation agents (PRAs), which enhance polarisation and spin-lattice relaxation, could eliminate the adverse in-flow effect and improve the signal-to-noise ratio. Herein, [Co(acac)3 ], [Mn(acac)3 ], [Fe(acac)3 ], [Cr(acac)3 ], [Ni(acac)2 ]3, [Gd(tmhd)3 ] and [Cr(tmhd)3 ] are investigated for their effectiveness in improving signal intensity per unit time in FlowNMR applications under the additional constraint of chemical inertness towards catalytically active transition metal complexes. High-spin Cr(III) acetylacetonates emerged as the most effective compounds, successfully reducing 31 P T1 values four- to five-fold at PRA concentrations as low as 10 mM without causing adverse line broadening. Whereas [Cr(acac)3 ] showed signs of chemical reactivity with a mixture of triphenylphosphine, triphenylphosphine oxide and triphenylphosphate over the course of several hours at 80° C, the bulkier [Cr(tmhd)3 ] was stable and equally effective as a PRA under these conditions. Compatibility with a range of representative transition metal complexes often used in homogeneous catalysis has been investigated, and application of [Cr(tmhd)3 ] in significantly improving 1 H and 31 P{1 H} FlowNMR data quality in a Rh-catalysed hydroformylation reaction has been demonstrated. With the PRA added, 13 C relaxation times were reduced more than six-fold, allowing quantitative reaction monitoring of substrate consumption and product formation by 13 C{1 H} FlowNMR spectroscopy at natural abundance.

Validated quantitative 31P NMR spectroscopy for positional isomeric impurity determination in L-α-glycerylphosphorylcholine (L-α-GPC)

In this study a quantitative ³¹P nuclear magnetic resonance (³¹P NMR) spectroscopy method was described to determine positional isomeric impurity β-GPC in commercial products of L-α-GPC. The samples were dissolved in D₂O and trimethyl phosphate (TMP) was selected as an internal calibrant. The measurements were performed on a Bruker 500 MHz spectrometer and the spectra were recorded under optimized process conditions. A good linear relationship was constructed for β-GPC in the range of 62.7-528.0 µg·mL^-1, i.e. 0.03-0.25 % (w/w %, in relative to L-α-GPC) with a correlative coefficient of 0.9996. The limit of quantification (LOQ) and limit of detection (LOD) were 62.7 µg·mL^-1 and 20.9 µg·mL^-1 with signal to noise of 3 and 10, respectively. The spiked recoveries were in the range of 98.17-99.78 % with the relative standard deviation (RSD %) less than 1.0 %. Therefore, it could be supposed that the ³¹P NMR was a promising alternative method for sensitive determination of β-GPC for strict quality control of L-α-GPC.

Purity Determination of Cyclophosphamide Hydrate by Quantitative 31P-NMR and Method Validation

Recently, quantitative NMR (qNMR), especially ¹H-qNMR, has been widely used to determine the absolute quantitative value of organic molecules. We previously reported an optimal and reproducible sample preparation method for ¹H-qNMR. In the present study, we focused on a ³¹P-qNMR absolute determination method. An organophosphorus compound, cyclophosphamide hydrate (CP), listed in the Japanese Pharmacopeia 17th edition was selected as the target compound, and the ³¹P-qNMR and ¹H-qNMR results were compared under three conditions with potassium dihydrogen phosphate (KH₂PO₄) or O-phosphorylethanolamine (PEA) as the reference standard for ³¹P-qNMR and sodium 4,4-dimethyl-4-silapentanesulfonate-d₆ (DSS-d₆) as the standard for ¹H-qNMR. Condition 1: separate sample containing CP and KH₂PO₄ for ³¹P-qNMR or CP and DSS-d₆ for ¹H-qNMR. Condition 2: mixed sample containing CP, DSS-d₆, and KH₂PO₄. Condition 3: mixed sample containing CP, DSS-d₆, and PEA. As conditions 1 and 3 provided good results, validation studies at multiple laboratories were further conducted. The purities of CP determined under condition 1 by ¹H-qNMR at 11 laboratories and ³¹P-qNMR at 10 laboratories were 99.76 ± 0.43 and 99.75 ± 0.53%, respectively, and those determined under condition 3 at five laboratories were 99.66 ± 0.08 and 99.61 ± 0.53%, respectively. These data suggested that the CP purities determined by ³¹P-qNMR are in good agreement with those determined by the established ¹H-qNMR method. Since the ³¹P-qNMR signals are less complicated than the ¹H-qNMR signals, ³¹P-qNMR would be useful for the absolute quantification of compounds that do not have a simple and separate ¹H-qNMR signal, such as a singlet or doublet, although further investigation with other compounds is needed.

Quantitative NMR as a Versatile Tool for the Reference Material Preparation

The assessment of primary calibrator purity is critical for establishing traceability to the International System of Units (SI). Recently, quantitative nuclear magnetic resonance (qNMR) has been used as a purity determination method for reference material development, and many related measurement techniques have been designed to acquire accurate and reliable results. This review introduces the recent advances in these techniques (including multidimensional methods), focusing on the application of qNMR to reference material preparation.

Purity determination of pyributicarb by internal standard correction-high-performance liquid chromatography-quantitative nuclear magnetic resonance

An internal standard correction-high-performance liquid chromatography-quantitative nuclear magnetic resonance (ISC-HPLC-qNMR) procedure was established as a reliable quantitative method for complex organic compounds with low purity in order to solve the risk of qNMR inaccuracy because of insufficient resolution of impurity peaks from the selected quantitative peak. This method collects a small quantity of target analyte from low-purity organics by LC. After drying and re-dissolving in deuterated solvent containing internal standard, the solution was analyzed by ¹H NMR and HPLC. Another solution prepared by accurately weighing unpurified low-purity substance and internal standard was analyzed by HPLC. Based on the theoretical derivation derived from the Beer-Lambert law, using the ratio of the HPLC peak areas of two solutions as correction, the purity was then calculated without the same reference as target analyte. Compared to previous methods with similar selectivity and accuracy, it has advantages such as a less purified sample is required, time for lyophilization is reduced by half, and sample preparation is more controllable. The proposed method was verified by analysis of a suite of six commercially available, high-purity compounds, and the difference of results between it and direct qNMR was within 0.1%. The result of pyributicarb using ISC-HPLC-qNMR was 97.6% (U = 0.5%; k = 2), and the reference value was 97.61% (U = 0.22%; k = 2). The results demonstrate that the proposed method provides a new way for reference material producers to calibrate lower-purity organics and has the potential advantage of accurate quantification of lower-purity organics.

Synthetic investigations of low-coordinate (N-phosphino-amidinate) nickel chemistry: agostic alkyl complexes and benzene insertion into Ni-H

Treatment of (PN)NiX (X = NHdipp or OtBu; PN = N-phosphinoamidinate ligand) with Me2PhSiH in benzene solvent afforded the crystallographically characterized, antifacial-coordinated, dinuclear species 1, the formation of which corresponds to the hitherto unknown net Ni-H addition of two equivalents of the putative (PN)NiH intermediate across C[double bond, length as m-dash]C units within a single benzene molecule. Computational analysis supports the view of 1 as being comprised of two cationic (PN)NiII fragments ligated by a substituted butadiene dianion μ2-η3:η3-C6H82- bridging group. Also described is the formation and characterization of three-coordinate (PN)Ni(alkyl) complexes stabilized by β-agostic (alkyl = Et, 2; n-Bu, 3; n-hexyl, 4) or γ-agostic (alkyl = neopentyl, 5) interactions, and our efforts to employ 2 and 3 as synthons for the generation of (PN)NiHvia β-hydride elimination. Notably, compound 5 represents both the first crystallographically characterized three-coordinate Ni-alkyl complex featuring a heterobidentate ligation, and the first neutral γ-agostic NiII-alkyl complex.

Quantitative 1 H NMR spectroscopy (qNMR) in the early process development of a new quorum sensing inhibitor

2-methyl-5,6,7,8-tetrahydro-2H-chromen-4(3H)-one (called 6-oxo) is presented as a new AI-1 quorum sensing inhibitor for Vibrio harveyi. The development of a chemical process to afford traceable materials for new biological assays demands the development of analytical methods to ensure their purity and quality. This work describes the use of quantitative ¹ H nuclear magnetic resonance (NMR) spectroscopy (qNMR) to assess the purity of a sample of 6-oxo (99.88%) and a sample of its major process impurity (E)-1-(2-hydroxycyclohex-2-en-1-yl)but-2-en-1-one (called HCB; 98.28%). To explore the scope of the use of qNMR to quantify the amount of low-content components in samples related to the chemical process for 6-oxo synthesis, this work also determined the amount of 6-oxo in two HCB samples: (a) the high-purity HCB sample described above and (b) a crude HCB sample collected during the chemical process. Despite the complexity of the crude sample, the amount of 6-oxo was readily assessed and could help to estimate the extent to which 6-oxo was already formed during the HCB synthesis. This information can help the understanding of how the process parameters can be modified to improve the performance of the whole process, by controlling the reaction mechanisms working at each step of this chemical process. In this context, our results reinforce qNMR as a complementary analytical tool for the quantification of the main component found in a sample, contributing to the standardization of reference materials and thus allowing the development of analytical methods for process control and traceability of the samples used for biological assays.

An isotope dilution LC-MS/MS based candidate reference method for the quantification of cyclosporine A, tacrolimus, sirolimus and everolimus in human whole blood

An isotope dilution LC-MS/MS based candidate reference measurement procedure for the quantification of cyclosporine A, tacrolimus, sirolimus and everolimus in human whole blood is presented to be used for evaluation and standardization of routine assays applied for therapeutic drug monitoring. The assay allows baseline separation of the four immunosuppressive drugs within a total runtime of 9 minutes using a C4 reversed phase column. Sample preparation is based on protein precipitation with zinc sulphate followed by purification with solid phase extraction. Reference materials used in this reference measurement procedure were characterized by qNMR and an absolute content of analytes calculated to guarantee traceability to SI units. As internal standards the corresponding deuterated and ¹³C-labelled analytes were used. The method allows the measurement of cyclosporine A in the range of 5 ng/mL to 2100 ng/mL; tacrolimus, sirolimus and everolimus were analysed in the range of 0.25 ng/mL to 50 ng/mL. Imprecision for inter-day measurements were found to be ≤3.5% for cyclosporine A and ≤4.4% for tacrolimus, sirolimus and everolimus. Accuracy was found to be within 101% and 108% for cyclosporine A and between 95% and 104% for the macrolide compounds. The uncertainty was evaluated according to the GUM. Expanded measurement uncertainties were found to be ≤7.2% for cyclosporine A, ≤6.8% for tacrolimus, ≤9.0% for sirolimus and ≤8.9% for everolimus (k = 2).

Insight into old and new pure shift nuclear magnetic resonance methods for enantiodiscrimination

Enantiodiscrimination and their quantification using nuclear magnetic resonance (NMR) spectroscopy has always been a subject of great interest. Proton is the nucleus of choice for enantiodiscrimination due to its high sensitivity and ubiquitous presence in nature. Despite its advantages, enantiodiscrimination suffers from extensive signal splitting by the proton-proton scalar couplings, which give complex multiplets that spread over a frequency range of some tens of hertz. These multiplets often overlap, further complicating interpretation of the spectra and quantifications. In the present review, we discuss some of the recent developments in the pure shift ¹ H NMR based methods for enantiomer resolution and enantiodiscrimination. We also compare various pure shift methods used for enantiodiscrimination and measurement of enantiomeric excess, considering the fact that conventional ¹ H NMR fails to provide any detailed insight.