Quantitative 31P-NMR for the Purity Determination of the Organophosphorus Compound Brigatinib and Its Method Validation.

The spectrum of 31P-NMR is fundamentally simpler than that of 1H-NMR; consequently identifying the target signal(s) for quantitation is simpler using quantitative 31P-NMR (31P-qNMR) than using quantitative 1H-NMR (1H-qNMR), which has been already established as an absolute determination method. We have previously reported a 31P-qNMR method for the absolute determination of cyclophosphamide hydrate and sofosbuvir as water-soluble and water-insoluble organophosphorus compounds, respectively. This study introduces the purity determination of brigatinib (BR), an organophosphorus compound with limited water solubility, using 31P-qNMR at multiple laboratories. Phosphonoacetic acid (PAA) and 1,4-BTMSB-d4 were selected as the reference standards (RSs) for 31P-qNMR and 1H-qNMR, respectively. The qNMR solvents were chosen based on the solubilities of BR and the RSs for qNMR. CD3OH was selected as the solvent for 31P-qNMR measurements to prevent the influence of deuterium exchange caused by the presence of exchangeable intramolecular protons of BR and PAA on the quantitative values, while CD3OD was the solvent of choice for the 1H-qNMR measurements to prevent the influence of water signals and the exchangeable intramolecular protons of BR and PAA. The mean purity of BR determined by 31P-qNMR was 97.94 ± 0.69%, which was in agreement with that determined by 1H-qNMR (97.26 ± 0.71%), thus indicating the feasibility of purity determination of BR by 31P-qNMR. Therefore, the findings of this study may provide an effective method that is simpler than conventional 1H-qNMR for the determination of organophosphorus compounds.

Quantitative 31P-NMR for Purity Determination of Sofosbuvir and Method Validation.

Quantitative 1H-NMR (1H-qNMR) is useful for determining the absolute purity of organic molecules; however, it is sometimes difficult to identify the target signal(s) for quantitation because of their overlap and complexity. Therefore, we focused on the 31P nucleus because of the simplicity of its signals and previously reported 31P-qNMR in D2O. Here we report 31P-qNMR of an organophosphorus compound, sofosbuvir (SOF), which is soluble in organic solvents. Phosphonoacetic acid (PAA) and 1,4-bis(trimethylsilyl)benzene-d4 (1,4-BTMSB-d4) were used as reference standards for 31P-qNMR and 1H-qNMR, respectively, in methanol-d4. The purity of SOF determined by 31P-qNMR was 100.63 ± 0.95%, whereas that determined by 1H-qNMR was 99.07 ± 0.50%. The average half bandwidths of the 31P signal of PAA and SOF were 3.38 ± 2.39 and 2.22 ± 0.19 Hz, respectively, suggesting that the T2 relaxation time of the PAA signal was shorter than that of SOF and varied among test laboratories. This difference most likely arose from the instability in the chemical shift due to the deuterium exchange of the acidic protons of PAA, which decreased the integrated intensity of the PAA signal. Next, an aprotic solvent, dimethyl sulfoxide-d6 (DMSO-d6), was used as the dissolving solvent with PAA and sodium 4,4-dimethyl-4-silapentanesulfonate-d6 (DSS-d6) as reference standards for 31P-qNMR and 1H-qNMR, respectively. SOF purities determined by 31P-qNMR and 1H-qNMR were 99.10 ± 0.30 and 99.44 ± 0.29%, respectively. SOF purities determined by 31P-qNMR agreed with the established 1H-qNMR values, suggesting that an aprotic solvent is preferable for 31P-qNMR because it is unnecessary to consider the effect of deuterium exchange.

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.