Nondestructive Investigation of the Agglomeration Process for Nanosuspensions via NMR Relaxation of Water Molecules

Time domain NMR for polymorphism characterization: Current status and future perspectives

Polymorphism is the ability of a compound to exist in multiple crystal forms while maintaining the same chemical composition. This phenomenon is reflected in different solid-state physicochemical properties due to variations in structural energy and the degree of lattice disorder. The pharmaceutical industry places significant emphasis on thoroughly characterizing polymorphism in Active Pharmaceutical Ingredients (APIs) because of its impact on the pharmacokinetic properties on the final medicine product. Standard characterization techniques are well documented in pharmacopeias and by international agencies. These techniques, whether applied individually or in combination, include crystallography (X-Ray Diffraction), thermal analysis (Differential Scanning Calorimetry), and various forms of spectroscopy, such as Near-Infrared, Raman, and solid-state Nuclear Magnetic Resonance (NMR). Analyzing NMR applications for solid-state characterization over the past five years, there has been a growing number of reports on the use of Time Domain NMR (TD-NMR) to evaluate polymorphism on APIs. Due to the increasing interest in this compelling technique, this study provides an overview of the current advancements in TD-NMR for polymorphism assessment in pharmaceutical products. Compared to high-field devices, TD-NMR has proven to be more convenient to industrial applications due to its smaller equipment size and shorter measurement times. This mini-review compares various applications of TD-NMR for API solid-state characterization and offer guidance for future research in this area.

Solid Fat Content and Relaxation Time Measurement of Petrolatum Using Time-Domain NMR, and the Correlation with Viscosity and Crystallinity

This study aimed to clarify the relationship between the NMR parameters of petrolatum obtained using the time-domain NMR (TD-NMR) technique and the physical properties obtained using conventional methods. Six commercially available drug-free petrolatums were used. First, the physical properties of these samples were recorded by conventional methods: polarized light microscopy, viscometry, and X-ray diffraction (XRD). The XRD pattern showed a characteristic diffraction pattern corresponding to the crystallization of paraffin wax. Next, the TD-NMR technique estimated the solid fat content (SFC) and T2 and T1 relaxation times as NMR parameters. The free induction decay of petrolatum showed the characteristic biphasic decay, while the SFC value was estimated from signal intensities. Finally, a scatterplot matrix was drawn to clarify the relationship between the NMR parameters and the physical properties. Using the Spearman rank-order correlation, the SFC showed a strong and positive correlation with the crystallinity (ρ = 0.855), and the T2 relaxation time showed a moderate and negative correlation with the viscosity (ρ = -0.707). In conclusion, this study clarified which NMR parameters correspond to the conventional physical properties: the SFC corresponded to the crystallinity and the T2 relaxation corresponded to the viscosity. Utilization of the TD-NMR technique to evaluate molecular mobility may be useful in terms of complementing the conventional physical characterization of petrolatum.

Synergistic quantification of mixed insulin preparations using time domain NMR techniques

Diabetes patients often rely on tailored insulin therapies, necessitating precise blends of various insulin types to achieve optimal pharmacokinetic profiles, including the quantity and action duration of insulin absorption into the bloodstream. This study aimed to develop an accurate quantification method for mixed insulin preparations, consisting of Insulin-NPH and Insulin Regular in ratios varying between 0:100-100:0. Time Domain NMR (TD-NMR) techniques, T2 relaxation times, and T1T2 maps were used to analyze the mixtures. Individually, neither technique provided a reliable determination of insulin ratios. However, the integration of both methods through chemometrics has been proven to be a synergistic approach, yielding a robust quantification technique suitable for quality control in the assessment of mixed insulin drugs. This innovative combined TD-NMR method is non-invasive, cost-effective, and user-friendly, offering at the same time a significant potential for preventing health complications associated with improper insulin dosing. Furthermore, our work elucidates the broader applicability of converging multiple TD-NMR techniques for analyzing intricate mixtures.

NMR Longitudinal Rotating Frame Relaxation Time (T1ρ) with a Weak Spin Locking Field as an Approach to Characterize Solid-State Active Pharmaceutical Ingredients: Proof of Concept

Nuclear magnetic resonance (NMR) longitudinal rotating frame relaxation time (T1ρ), rarely used in low-field NMR, can be more effective than conventional T1 and T2 relaxation times to differentiate polymorphic forms of solid pharmaceuticals. This could be attributed to T1ρ sensibility to structural and molecular dynamics that can be enhanced by changing the strength of the oscillating magnetic field (B1) of spinlock pulses. Here, we compared the capacity of T1, T2, and T1ρ to differentiate inactive (A) and active (C) crystalline forms of the World Health Organization essential drug Mebendazole. The results showed that T1 and T2 values of both forms were statistically identical at 0.47 T. Conversely, T1ρ of both forms measured with weak spinlock B1 fields, ranging from 0.08 to 0.80 mT were statistically different in the same spectrometer. The T1ρ also has the limit of detection to detect the presence of at least 10% of inactive A form in the active C form. Therefore, T1ρ, measured with weak spinlock B1 fields can be an effective, streamlined, and complementary approach for characterizing not only solid active pharmaceutical ingredients but other solid-state materials as well.

TGA and NMR relaxation measurement of nonmesoporous silica to investigate the amount of hydrolysis product in acetylsalicylic acid adsorbed on silica

This study investigated a crucial surface property of silica that contributes to the chemical stability of acetylsalicylic acid (ASA) physically adsorbed on silica. Hydrophilic nonmesoporous types of silica were selected, and the number of hydroxyl groups on silica (N(OH)) was evaluated using thermogravimetric analysis (TGA). The ASA-containing silica was stored at 40 °C in drying conditions, and the amount of ASA degradation was quantified based on salicylic acid. From the scatterplots between the number of hydroxyl groups per unit weight (specific surface area (SSA) × N(OH)) and the amount of ASA degradation, it was clarified that in ASA adsorbed on silica, the ASA chemical stability was determined by the formula (the SSA × N(OH)). In addition, a time-domain nuclear magnetic resonance measurement verified the N(OH) result by estimating the interaction between the silica surface and water in an aqueous silica suspension. The N(OH) result was found to be reasonable.

Use of Time-Domain NMR for 1H T1 Relaxation Measurement and Fitting Analysis in Homogeneity Evaluation of Amorphous Solid Dispersion

This study examined the usefulness of 1H T1 relaxation measurements for evaluating the homogeneity of amorphous solid dispersion (ASD). Indomethacin and polyvinylpyrrolidone were used to prepare two kinds of ASDs. One was inhomogeneous ASD (ASDmelt) prepared by a melt-quenching method, and the other was homogeneous ASD (ASDsolvent) prepared by a solvent evaporation method. The T1 relaxation was measured by the time-domain NMR (TD-NMR) technique using a low-field NMR system. Curve-fitting analysis of T1 relaxation plots was conducted using the Akaike information criterion. This fitting analysis revealed that the T1 relaxation of ASDmelt and ASDsolvent was biphasic and monophasic, respectively. ASDmelt and ASDsolvent were inhomogeneous and homogeneous on a nanometer scale, respectively, considering the spin diffusion of 1H nuclei. These T1 results were consistent with the Raman mapping of ASDs. From the fitting analysis of 1H T1 relaxation, we conclude that TD-NMR is a promising technique for evaluating ASD homogeneity.

Usefulness of Applying Partial Least Squares Regression to T2 Relaxation Curves for Predicting the Solid form Content in Binary Physical Mixtures

This study applied partial least squares (PLS) regression to nuclear magnetic resonance (NMR) relaxation curves to quantify the free base of an active pharmaceutical ingredient powder. We measured the T₂ relaxation of intact and moisture-absorbed physical mixtures of tetracaine free base (TC) and its hydrochloride salt (TC·HCl). The obtained T₂ relaxation curves were analyzed by two methods, one using a previously reported T₂ relaxation time (T₂), and the other using PLS regression. The accuracy of estimating TC was inadequate when using previous T₂ values because the moisture-absorbed physical mixtures showed biphasic T₂ relaxation curves. By contrast, the entire measured whole of the T₂ relaxation curves was used as input variables and analyzed by PLS regression to quantify the content of TC in the moisture-absorbed TC/TC·HCl. Based on scatterplots of theoretical versus predicted TC, the obtained PLS model exhibited acceptable coefficients of determination and relatively low root mean squared error values for calibration and validation data. The statistical values confirmed that an accurate and reliable PLS model was created to quantify TC in even moisture-absorbed TC/TC·HCl. The bench-top low-field NMR instrument used to apply PLS regression to the T₂ relaxation curve may be a promising tool in process analytical technology.