¹H NMR Relaxation Study to Evaluate the Crystalline State of Active Pharmaceutical Ingredients Containing Solid...

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 T₁ and T₂ relaxation behavior of crystalline and amorphous APIs. From the results, we were able to confirm that the T₁ 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 T₁ relaxation behavior. Our findings led us to conclude that TD-NMR is useful as a new approach to evaluate the crystalline state of APIs.