2023 年 71 巻 11 号 p. 838-842
This study determined the content of solid active pharmaceutical ingredient (API) powders dispersed in suspension-type pharmaceutical oral jellies using a low-field time-domain NMR (TD-NMR). The suspended jellies containing a designated API content were prepared and tested. Acetaminophen (APAP), indomethacin (IMC) and L-valine were used as test APIs. First, this study measured the T2 relaxation rate (the reciprocal of T2 relaxation time) by the Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence, and then evaluated whether the API content could be determined by the acquired T2 relaxation rate. The T2 relaxation rate negatively correlated with API content to a certain extent, but their correlation was not sufficient for achieving a precise determination. Subsequently, the solid–echo pulse sequence measurement was adopted for this study. We found that NMR signals corresponding to solid components strongly correlated with API content. Thus, this method achieved a precise determination of API contents in suspended jellies. In addition, this study confirmed the effect of API particle size on the T2 relaxation rate by using an L-valine-containing jelly: the T2 relaxation rate became faster when a smaller API size was incorporated into the suspended jelly, while there was no difference in terms of the NMR signals measured by solid–echo pulse sequence. From these findings, TD-NMR could be a powerful tool for evaluating the API dispersion state in suspended oral jellies.
Oral dosage forms that patients can easily swallow have been gaining much attention. Pharmaceutical oral jellies are considered as highly acceptable for patients having difficulty in swallowing medicines (e.g., young and elderly patients). These jellies can be taken easily without water and they also show a lower risk of accidental ingestion.1,2) In Japan, jellies for oral administration are listed in the Japanese Pharmacopeia since 2011 (16th edition, JPXVI). This dosage form has been expanding steadily to a wide variety of active pharmaceutical ingredients (APIs). The jelly-containing commercial pharmaceuticals released on the Japanese market are cited as alendronate sodium, acyclovir, cilostazol, amlodipine besylate, and donepezil hydrochloride.1,3)
In manufacturing pharmaceutical oral jellies, not all the API powders added are always completely dissolved in the formulation. This means that this dosage form can be divided into the “dissolution” and the “suspension” types. For instance, “Bonalon oral jelly 35mg (Teijin Pharm, Tokyo, Japan)” is a typical dissolution-type oral jelly: alendronate sodium is completely dissolved in the formulation. By contrast, a suspension-type oral jelly contains a considerable amount of dispersed solid API powders. Acyclovir and cilostazol-containing pharmaceutical jellies belong to the suspension-type.
The pharmaceutical oral jelly is an attractive new oral dosage form, but there is still much room for improvement regarding its pharmaceutical technology. In particular, for these jellies there is a strong demand for new characterization methods. In addition to conventional crucial quality attributes as oral dosage forms (i.e., API content uniformity, API stability, dissolution properties), some characteristics specific to jelly products are also crucial to develop pharmaceutical jellies. For example, the dispersion state of solid API particles is very important from the viewpoint of API content uniformity and drug dissolution property (i.e., it is very important to characterize how much solid APIs are dispersed and what is the degree of aggregation of the API particles in the jelly).
Against this background, the present study aimed to determine the amount of solid API powders in suspended pharmaceutical oral jellies. The key technology for this topic is low-field time-domain NMR (TD-NMR).4,5) TD-NMR is a technique specialized in the measurement of 1H-NMR relaxometry that enables rapid and easy measurement of the T1 and T2 relaxation times regardless of the physical state of the sample (the measurement can be performed on both liquid and solid samples). These NMR parameters are well known to reflect the molecular mobility of substances.6,7) TD-NMR has been used in various fields of research and industry (e.g., chemistry,8,9) food,10–12) and plants13,14)) to explore the physicochemical properties of samples in terms of molecular mobility. In relevant suspension studies, these NMR parameters were used not only for molecular mobility evaluation, but also for determining the solid particle concentration in the suspension.15–18) The present study prepared model suspension-type jellies containing three different APIs with distinct water solubility [i.e., acetaminophen (N-acetyl-p-aminophenol, APAP), indomethacin (IMC), and L-valine], and then the solid API content was determined using TD-NMR.
L-Valine, kappa carrageenan (κ-CG), locust bean gum (LBG), and D-sorbitol were purchased from FUJIFILM Wako (Osaka, Japan). IMC and iota carrageenan (ι-CG) was purchased from Tokyo Chemical Industry (Tokyo, Japan). APAP was purchased from Hachidai Pharmaceutical (Osaka, Japan). All other chemicals were of analytical grade and commercially available.
Preparation of Model-Suspended JelliesThe gel-forming agent mixture (LBG/κ-CG/ι-CG = 2/1/1) was dissolved in purified water at 90 °C. Next, D-sorbitol and KCl were added to the solution and stirred until these additives were completely dissolved. The final concentrations of the gel-forming agent, D-sorbitol, and KCl in the jelly base were adjusted to 1.0, 45.0, and 0.3% (w/w), respectively. Afterwards, the designated API amount was dispersed in the jelly base, and then the resultant solution was poured into a glass vial, which was cooled to room temperature to obtain the jelly.
TD-NMR MeasurementsThe TD-NMR measurements were performed using a Bruker minispec mq20 (Bruker BioSpin, Billerica, MA, U.S.A.) at a 1H frequency of 20 MHz at 25 °C. The Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence was used to measure the T2 relaxation rate. The acquisition parameters for the measurement were as follows: there were eight scans; the time between pulses (τ spacing) was 0.25 ms; the recycle delay was 10 s, and the echo number was 8000. The NMR signal was monitored for approx. 4 s. The T2 relaxation time was calculated using the TD-NMR Analyze software. Single-exponential curve fittings were performed using Eq. (1):
 | (1) |
where I(t) and I0 are the transverse magnetization at times t and 0 with exponential decay, t is the acquisition time, and T2 is the T2 relaxation time. The T2 relaxation rate is the reciprocal of T2 relaxation time.
Regarding the solid–echo pulse sequence, the acquisition parameters were as follows: there were four dummy scans and eight scans, and the acquisition scale was 0.1 ms. The recycle delay (scan interval) with a 90° pulse was set at the appropriate conditions in accordance with each API. The acquired T2 relaxation curves were analyzed with the TD-NMR Analyze software (Bruker Biospin GmbH, Rheinstetten, Germany) according to Eq. (2):
 | (2) |
where T2 is the relaxation time, t is the acquisition time, and I(t) and I0 are the signal intensities at times t and 0, respectively.
The solid–echo pulse sequence was also used to measure the T1 relaxation time. The T2 relaxation curves with different recycle delays were acquired, and then the T1 relaxation time was calculated according to the saturation recovery method. The T1 relaxation time was calculated using TD-NMR Analyze software according to Eq. (3):
 | (3) |
where t is the recycle delay; and T1, I(t), I0 are the T1 relaxation time and signal intensities at time t and equilibrium, respectively.
To determine the solid API contents in suspension-type pharmaceutical oral jellies, we first measured the T2 relaxation rate using the CPMG pulse sequence. This approach was based on relevant studies on titanium dioxide nanoparticle suspension.15–17) The NMR signal acquired by CPMG pulse sequence is mostly derived from liquid components (e.g., solvent molecules in the suspension), because this pulse sequence cannot detect the solid proton with an extremely rapid T2 relaxation decay (e.g., protons of rigid solid materials).6) In addition, the molecular mobilities of solvent molecules in suspension vary substantially according to their surroundings. The solvent molecules in the vicinity of the solid particles are substantially restricted due to the interactions with the particle surfaces, leading to a shorter T2 relaxation time compared with the bulk solvent. From this principle, the T2 relaxation rate should increase proportionally with increasing the particle contents in the suspension. There are numerous technical reports to determine the solid particle contents in suspension based on this principle.15–17)
In the present study, APAP, IMC, and L-valine were selected as test APIs by considering their different water solubility, and their suspended jellies were investigated by TD-NMR. The solubility of each API was preliminarily examined. API suspensions were prepared according to the same procedure as that of the test jellies except that no gel-forming agent was added; next, the concentration of the dissolved API in the sample suspensions was measured by a spectrophotometer (U-best 30, JASCO, Tokyo, Japan). The solubility values of APAP, IMC, and L-valine were 10.3, 0.053 and 35.7 mg/mL, respectively. The mean diameters (D50) were also measured by Zetasizer Nano-ZS90 (Malvern Instruments Ltd., Worcestershire, U.K.). The observed D50 values were 187.0 ± 5.9, 21.7 ± 0.2 and 157.0 ± 1.3 µm for APAP, IMC, and L-valine, respectively.
The scatterplots of the T2 relaxation rate versus API content are shown in Fig. 1. IMC and L-valine-containing jellies show positive relationships between T2 relaxation rates and API contents to a certain extent. However, the determination coefficients (R2) were lower than expected, 0.616 and 0.638 for IMC and L-valine, respectively. For the APAP-containing jelly, the R2 value was extremely low, 0.0101. From this, we thought it difficult to perform a precise determination of the solid API contents by using this method. The possible reasons for the undesirable results were as follows. First, unlike in relevant studies on titanium dioxide nanoparticles,15–17) the API powders partially dissolved in the jelly base, and then the dissolved APIs restricted the molecular mobility of the liquid components overall. As a result, the T2 relaxation rate of the bulk liquid component increased and the value approached that of the liquid component in the vicinity of the API powders. For another possible reason, there is a possibility that increasing API content increased the aggregation of API particles in the test jellies. A precise estimation of solid API content from T2 relaxation rate was accomplished under the assumption that the particle size kept constant regardless of API contents. If the aggregation status of the API particles varied with the API contents in the jelly, the specific surface area of solid API particles would differ, leading to difficulty in API content estimation from the T2 relaxation rate of liquid components.
The T2 relaxation rate was measured using a CPMG pulse sequence. Each value represents the mean ± standard deviation (n = 3).
The following study tried to determine the solid API contents by measuring the NMR signals of solid components. The solid–echo pulse sequence was used for this experiment. Unlike the CPMG pulse sequence, this pulse sequences could detect not only NMR signals derived from liquids, but also from solid materials having very short T2.6) Furthermore, because the NMR signal decays for solid and liquid components are distinct, their individual NMR signals can be identified by multicomponent curve-fitting analysis on the acquired T2 relation curve.19,20) The acquired NMR signals involve quantitative information about protons in the samples: thus, the solid–echo pulse sequence has been used for quantitative analysis as well as for the evaluation of molecular mobility of the solid samples.21–23) Our current study used this technique for continuous monitoring of the disappearance behavior of the nongelated core (solid component) in hydrophilic matrix tablets induced by immersion in water.19) In the present study, after acquisition of T2 relaxation curve, it was approximated by Eq. (2) in the same way as our current study. The first term in Eq. (2) is a Gaussian component that corresponds to the NMR signal decays of the solid API powders dispersing in the sample; the second term in Eq. (2), called “offset,” corresponds to the NMR signal derived from liquid components. A complete explanation of this technique is presented in our earlier report.19) After the curve-fitting analysis on the acquired T2 relaxation curve, the NMR signal intensity of Gaussian component was plotted against the API contents (Fig. 2). Regarding the jellies containing IMC and L-valine, the NMR signals were highly correlated with API contents: the R2 values of IMC and L-valine were 0.990 and 0.957, respectively. By contrast, no correlation was observed from the APAP-containing jelly. We assumed that the lack of correlation was attributed to an inadequate condition of recycle delay for the measurement. In general, it is recommended to set the recycle delay to be five times longer, or more than the T1 relaxation time of the sample. However, the measurement shown in Fig. 2 was conducted under the same conditions for all APIs with 10 s of the recycle delay. If the T1 relaxation time of the API was much longer than 10 s, a sufficient NMR signal could not be detected because of insufficient longitudinal magnetization recovery.
The NMR signals were measured by solid–echo pulse sequence with 10 s recycle delay.
We considered this issue and measured the T1 relaxation time of individual APIs to set adequate recycle delay conditions. The T1 relaxation curve of each jelly is shown in Fig. 3. T1 relaxation times of each API were calculated from the T1 relaxation curves with the saturation recovery method. As anticipated, the T1 relaxation time of APAP (70.6 s) was much longer that those of IMC (3.81 s) and L-valine (0.21 s).
The NMR signal acquisition was performed by a solid–echo pulse sequence.
From the result, the recycle delays for the measurement of APAP, IMC, and L-valine were set at 80, 20 and 1 s, respectively, and then NMR signals were acquired again (Fig. 4). According to the description mentioned above, a recycle delay of 80 s for APAP measurement might be too short for its T1 relaxation time, because the NMR instrument used in the present study had a maximum recycle delay setting limitation of 80 s. It was obvious that, in the case of the APAP-containing jelly, the correlation between the NMR signals and API contents was substantially improved. Therefore, for a precise estimation, adequate tuning of the recycle delay condition proved to be very important. From these findings, we concluded that direct measurement of NMR signals of a solid component using the solid–echo pulse sequence is an effective method to determine the solid component contents in suspended jellies.
The modified recycle delay for APAP, IMC, and L-valine-containing jellies was 80, 20 and 1 s, respectively. Each value represents the mean ± standard deviation (n = 3).
In the final phase of this study, we evaluated the effect of the API particle size on the T2 relaxation rate measured by the CPMG pulse sequence. As mentioned earlier, the CPMG pulse sequence failed to determine the solid API content in test jellies. However, in principle, the T2 relaxation rate is supposed to be sensitive to the specific surface area of the API powders. The T2 relaxation rate becomes faster with decreasing API powder size because of the larger specific surface area.17) In this experiment, L-valine was grounded in a mortar and then the tests on suspended jellies were prepared using grounded and nongrounded (the same powder as the above experiment) L-valine powders. For further information, the D50 values were 53.7 ± 1.2 and 157.0 ± 1.3 µm for grounded and nongrounded L-valine powders, respectively. The NMR signals of solid components measured by the solid–echo pulse sequence and T2 relaxation rate measured by CPMG pulse sequence are shown in Fig. 5. The NMR signals obtained from grounded L-valine jelly were completely consistent with those from nongrounded L-valine’s jelly (Fig. 5a), indicating no effect of API particle size on the measurement by the solid–echo pulse sequence. By contrast, the T2 relaxation rate substantially increased by using the grounded L-valine for jelly preparation (Fig. 5b). This result indicates that the T2 relaxation rate measured by the CPMG pulse sequence will also become an effective tool to evaluate the state of API powder dispersion in suspended pharmaceutical jellies: it can provide valuable information concerning particle size that relates to the aggregation.
L-Valine-containing jellies were used for this experiment. The NMR signals were measured by a solid–echo pulse sequence, whereas the T2 relaxation rate was measured by a CPMG pulse sequence. The data corresponding to nongrounded L-valine jelly are the same as those shown in Figs. 1 and 2. Each value represents the mean ± standard deviation (n = 3).
The present study successfully demonstrated the TD-NMR usefulness in characterizing the dispersion state of solid API powders in suspended oral jellies. First, it was confirmed that NMR signals corresponding to solid components and measured by a solid–echo pulse sequence could determine the solid API contents of the sample jellies. For precise determination, adequate tuning of recycle delay condition is very important. In addition, the T2 relaxation rate measured by a CPMG pulse sequence was found to be sensitive to the API powder size, indicating the possibility of evaluating API powder aggregation in the jellies by this method. This study provides valuable information for dispersion state characterization of suspended oral pharmaceutical jellies.
This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant number, 22K15261).
The authors declare that they have no financial or noncompeting interests concerning this manuscript. The Laboratory of Pharmaceutical Technology, University of Toyama, is an endowed laboratory, supported by an unrestricted grant from Nichi-Iko Pharmaceutical Co., Ltd. (Toyama, Japan).
