The aim of this study was to demonstrate the usefulness of the time domain nuclear magnetic resonance (TD-NMR) method to characterize the crystalline state of active pharmaceutical ingredients (APIs) containing solid dosage forms. In this study, carbamazepine and indomethacin are used as models for poorly water-soluble APIs. First, we measured the T 1 and T 2 relaxation behavior of crystalline and amorphous APIs. From the results, we were able to confirm that the T 1 relaxation time measured by TD-NMR is an effective parameter for distinguishing between crystalline and amorphous states in powdered APIs. We then examined physical mixtures of APIs with polyvinylpyrrolidone and their solid dispersion. The results indicated that TD-NMR allows the evaluation of not only the crystalline form of APIs but also the miscibility of APIs and polymers. In the final phase of the study, we conducted continuous monitoring of the crystalline state of APIs incorporated into physical mixtures during the thermal stress test. Conversion to crystalline forms of the APIs was successfully monitored based on the T 1 relaxation behavior. Our findings led us to conclude that TD-NMR is useful as a new approach to evaluate the crystalline state of APIs.

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The aim of this study was to demonstrate the usefulness of T 2 measurements conducted with a timedomain NMR (TD-NMR) for the characterization of active pharmaceutical ingredients (APIs) containing solid dosage forms. A solid dispersion (SD) and a physical mixture (PM) consisting of indomethacin (IMC) and polyvinylpyrrolidone (PVP) were prepared at different weight ratios as test samples, and then their T 2 relaxation curves were measured by TD-NMR. The T 2 relaxation curve of IMC was quite different from that of PVP by nature. T 2 values of the SD and PM samples became gradually shortened with increasing IMC content. No difference in T 2 relaxation curves was observed between SD and PM. By analyzing the T 2 relaxation curves in detail, we succeeded in precisely quantifying the IMC contents incorporated in the samples. Next, this study evaluated the T 2 relaxation curves of amorphous and crystalline states of powdered IMC. T 2 relaxation rate of crystalline IMC was slightly but significantly higher than that of amorphous IMC, proving that the T 2 measurement was sensitive enough to detect these differences. Finally, a thermal stress was imposed on SD and PM samples at 60°C for 7 d, and then an amorphous-to-crystalline transformation occurred in IMC in the PM sample and was successfully monitored by T 2 measurement. We believe that T 2 measurement by TD-NMR is a promising analysis for the characterization of APIs in solid dosage forms, including SD-based pharmaceuticals.

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