Article pubs.acs.org/ac
Proton NMR: A New Tool for Understanding Dissolution Steven R. Coombes, Leslie P. Hughes, Andrew R. Phillips,*,† and Stephen A. C. Wren Pharmaceutical Development, AstraZeneca, Macclesfield SK10 2NA, United Kingdom ABSTRACT: We present the use of 1H NMR as a new measurement approach for improving understanding of the dissolution of pharmaceutical tablets. NMR has benefits over the conventional UV measurement approach in respect to much greater analyte selectivity and the ability to detect nonUV-absorbing species such as sugars. We used an in-line flow cell and water suppression experiments to determine the release profiles of three drug substances and lactose from the same tablet. Dissolution was performed in a pharmacopieal dissolution system with a standard protic buffer. NMR was shown to give high selectivity with each analyte having a wellresolved signal and sufficient sensitivity to determine the full release profile of even a compound present at only 5 mg in the tablet. The in-line flow cell gives excellent quality NMR spectra having little impact on peak shape. Dissolution of all the drug substances and lactose were determined to proceed at the same relative rates.
D
1
H NMR is recognized as a key technique for structure elucidation, but conventional wisdom would suggest that it is not easily applied to rapidly changing systems such as dissolution. The situation is compounded by problems of low solution concentration, and hence limited sensitivity, and the desire to use deuterated solvents for frequency locking and sample shimming. In this paper, we show that these problems can be overcome by the use of modern NMR techniques and that NMR can provide a valuable insight into the behavior of oral immediate release tablets. Our approach builds on the experience gained from the use of 1H NMR for the monitoring of chemical reaction processes and may be more broadly applicable to the study of dynamic solution processes. Despite the fact that the mechanisms that underpin dissolution can be complex, pharmaceutical development still depends on UV-based dissolution to inform decision making about product performance. For example, dissolution of an active pharmaceutical ingredient can involve a simple erosion process or diffusion-controlled release of the API by its surrounding matrix as it passes through a hydrated polymer.2 The drug molecule may also undergo a form change during dissolution, and a simple assessment of the rate of release and the concentration of the drug in solution does not easily provide information about the mechanism of dissolution. In this respect, conventional dissolution testing does not provide any information about the role of excipients (for example, the filler, disintegrant, and lubricant present within tablets) in dissolution. Many excipients are transparent to UV, and for those that are visible, the UV spectrum is often
issolution testing is a key aspect of the analysis of pharmaceutical products and in particular oral immediate release dosage forms such as tablets. Dissolution testing demonstrates that the active pharmaceutical ingredient (API) will be released from the tablet into solution and so will be available for absorption in the gastro-intestinal tract. It also helps to ensure the quality and consistency of established commercial pharmaceutical products, and guidance is available from the regulatory authorities.1 The testing uses a standardized testing apparatus, and typically a simple measurement methodology (e.g., single wavelength UV) is used to determine the extent of drug release as a function of time. Dissolution testing also has a role to play in the development and optimization of new tablet formulations. For example, it is used to determine whether different tablet formulations, or different batches of the same formulation, give the same in vitro performance at a certain time point. Standard dissolution testing is however of relatively limited use for understanding the reasons for any differences in product performance. In particular, UV measurement tells us how much drug is in solution at a particular time point, but it gives us no information about the mechanism of release from a formulation. We believe that the design and optimization of new formulations can be improved by the judicious application of the sophisticated measurement tools that analytical chemists now have at their disposal. To that end, we are interested in developing and applying new measurement and modeling approaches which give information on the key processes involved in dissolution. In this paper, we present what we believe is the first use of 1H NMR spectroscopy (NMR) as an in-line dynamic measurement technique for dissolution under pharmacopieal conditions. © XXXX American Chemical Society
Received: October 21, 2013 Accepted: January 28, 2014
A
dx.doi.org/10.1021/ac403418w | Anal. Chem. XXXX, XXX, XXX−XXX
Analytical Chemistry
Article
Samples of guaifenesin, acetaminophen, and phenylephrine hydrochloride were sourced from Sigma-Aldrich Company Ltd. (Gillingham, United Kingdom). The dissolution medium was 900 mL pH 6.8 phosphate buffer (8.96 g L−1 NaOH, 77 g L−1 NaH2PO4·2H2O, then diluted 1:10). Dissolution Standards. For 1 H NMR, a solution containing standards of all of the drug substances (phenylephrine hydrochloride, guaifenesin, and acetoaminophen) and lactose monohydrate was prepared in 900 mL of the same phosphate buffer used for the dissolution experiments. The standard weights were approximately those expected from the tablet label claim, with the exception of phenylephrine hydrochloride, which was twice that expected to reduce weighing error. The weight of lactose monohydrate in the tablet was not stated, so 1H NMR was used to estimate the value on the basis of a preliminary dissolution experiment in comparison with the reference solution. This concluded that approximately 190 mg lactose was present in each tablet. For the UV measurements, individual standards of the drug substances were prepared in separate phosphate buffer solutions at appropriate concentrations. Dissolution Experiments. The dissolution experiments were performed in a USPII dissolution system using a paddle speed of 50 rpm and 900 mL of pH 6.8 phosphate buffer. The temperature was maintained at 37 °C using a thermostatted water bath. The dissolution medium was circulated continuously through both the 1H NMR and UV flow cells for in-line measurement. Flow to the NMR cell was at 2 mL min−1 using an Agilent HPLC pump with an in-line particulate filter and gave a measurement delay of approximately 90 s due to the length of the PEEK tubing. Flow to the UV cell was 14 mL min−1 using an Ismatec peristaltic pump and gave a measurement delay of 50 s. To give an indication of repeatability, the dissolution experiment was repeated twice in an identical manner denoted as runs (a) and (b). NMR Spectroscopy. All NMR measurements were performed on a Bruker Avance 600 MHz spectrometer (Bruker Biospin, Rheinstetten, Germany) equipped with a 5 mm TCI cryoprobe. The flow cell as described below was manually inserted into the system in exactly the same manner as a 5 mm tube. 1H NMR spectra were acquired at 300.0 K without sample rotation and with two prior dummy scans. Twenty thousand points were acquired with a spectral width of 10.6 kHz, constant receiver gain, an acquisition time of 1 s, and a recycle delay of 1 s. For run (a), sensitivity was the main consideration, and hence 32 scans were used per experiment with a total experiment time of 70 s. For run (b), sensitivity was sacrificed for increased time resolution, and hence 4 scans were used per experiment with a total experiment time of 12 s. Water suppression was achieved using the NOESY-presaturation pulse sequence (Bruker noesygppr1d pulse sequence) with irradiation at the water frequency during the recycle and mixing time delays. Data were processed in Bruker Topspin 2.1 software. Exponential line broadening was applied with the LB parameter set to 0.19, baseline correction was applied using the command “ABS” and integrals were directly compared across multiple spectra using the “Use lastscale for calibration” functionality. NMR Flow Cell. The flowcell8 is based on a standard 5 mm NMR tube sited in a PEEK headpiece, with PEEK HPLC tubing (i.d. 0.5 mm, o.d. 1.6 mm, total tubing length is approximately 6 m) delivering and returning the medium to and from the dissolution vessel. The HPLC tubing is held in the center of the NMR tube by PTFE spacers throughout the
characterized by broad superimposed peaks with no discrete maxima. For UV-absorbing components, chemometric techniques may be employed in an attempt to separate the spectra into individual components, but direct measurement of excipient solution concentration is easier and to be desired.3,4 Due to the complexity of the dissolution process and the limited mechanistic information provided by UV spectroscopy, a number of more selective measurement techniques have been utilized in order to try and better understand the controlling mechanisms.5,6 In previous work, we have used magnetic resonance imaging techniques combined with UV determination of API concentration in solution to provide mechanistic insights into the dissolution of spray-dried amorphous solid dispersions.7 In this current work using 1H NMR, we have examined the dissolution behavior of a model system using a flow-through cell, developed by Morris et al. for studying chemical reactions.8 This in-line flow cell can be placed within a 5 mm NMR probe, and when it is hyphenated with a USP2 dissolution system, it enables the measurement of both API and excipient concentrations. It also demonstrates that, in the case of combination products, the solution concentration of more than one active pharmaceutical ingredient (API) can be determined. We have used solvent suppression and shimming without a deuterium lock signal to overcome some of the dynamic range problems associated with using NMR with protic dissolution medium. Experiments were performed on a high field NMR cryoprobe system to maximize sensitivity and enable determination of drug and excipient concentrations at values typically seen with dissolution of pharmaceutical tablets, typically on the order of 100 μg mL−1. The dissolution of a model system has also been followed with in-line UV to highlight the advantages of using this NMR flow-cell approach.
■
EXPERIMENTAL SECTION Materials. A widely available over-the-counter combination product for the short-term symptomatic relief of colds, chills, influenza, and coughs was purchased from a local pharmacy. The product contains three active ingredients: (1) acetoaminophen (250 mg), (2) guaifenesin (100 mg), and (3) phenylephrine hydrochloride (5 mg), in addition to lactose monohydrate (4), the soluble filler shown in Scheme 1, plus
Scheme 1. Soluble Species Present in the Combination Tablet
other insoluble excipients. The product is an immediate release formulation with the active ingredients designed to relieve pain and fever (acetaminophen), act as an expectorant (guaifenesin), and act as a decongestant (phenylephrine hydrochloride). This type of combination product is sold under different brand names in different countries. B
dx.doi.org/10.1021/ac403418w | Anal. Chem. XXXX, XXX, XXX−XXX
Analytical Chemistry
Article
quantitatively compare intensities of signals to reference spectra.10,11 Again, this results in a clear advantage for monitoring dissolution as it should be possible to quantify NMR spectra generated throughout a dissolution experiment against an external reference spectra generated from a sample of known concentration. Conversely, however, is the issue of sensitivity, as due to the nature of NMR, detecting small amounts of material is always a challenge. Table 1 illustrates the quantities of material that need
length of the tube (apart from the active volume region). These spacers reduce the overall volume of medium within the NMR tube and serve to minimize any magnetic field inhomogeneity due to the PEEK tubing. The PTFE spacers are grooved to allow the flow to return back to the outlet port within the headpiece. The system is designed to pressure fail at the NMR tube interface with the PEEK headpiece. As such, the headpiece is housed within a glass cylinder, which seals around the NMR tube with an O-ring to collect any liquid upon overpressurization. A leak detector is in place inside the cylinder which is connected to an electronic interlock (into which the HPLC pump delivering the dissolution medium is plugged), and this switches off the pump in the event of a leak. UV Measurement. The UV measurements were recorded using a 1 mm quartz flow cell (Starna Scientific) with spectra recorded every 10 s between 200 and 500 nm (Agilent 8453 UV spectrophotometer). Reference spectra were recorded from flowing solutions of the individual standards.
Table 1. Illustrative Amounts of Material Present during the Dissolution Experiment species acetaminophen guaifenesin phenylephrine hydrochloride lactose
■
RESULTS AND DISCUSSION Measurement Considerations. Drug release during the dissolution of oral immediate release tablets can be measured by direct in-line approaches, or by sampling and subsequent offline analysis. Direct in-line methods such as UV absorbance are normally favored because of the speed, sensitivity, and simplicity of the approach. The UV absorbance data at a single or all wavelengths can be acquired using either fiber-optic probes located in the dissolution vessel3,9 or by pumping the dissolution medium through a UV flow cell. Off-line approaches such as HPLC analysis are used when higher selectivity is required, for example if a UV-absorbing excipient interferes or if there is more than one drug substance in the tablet. If there are several drug substances in the tablet then the UV absorbance of the dissolution medium at any one time will be a composite response. The response at a particular time will depend upon the chosen wavelength, the UV spectra of the individual drug substances, the levels of the substances within the tablet, and their rates of release. Under favorable circumstances, for example, where the substances have very different UV spectra, the composite response may be deconvoluted by collecting full spectral data and the application of multivariate curve resolution.3,4 We propose an alternative approach which is applicable for the dissolution of species lacking a UV chromophore, such as soluble fillers, or for multiple active components present at very different levels. 1 H NMR is the prime technique for the elucidation of chemical structure. This is in part due to the chemical shift: protons in different environments give rise to NMR signals at different frequencies. This results in a clear advantage for understanding dosage forms as it is likely that the individual chemically different components will have resolved signals at different chemical shifts, significantly reducing the chance of overlapping signals as compared to UV. These chemical species include soluble fillers such as lactose, which are missed by conventional approaches such as UV absorbance measurement. 1 H NMR is also routinely used for quantitative analysis as the intensity of a given signal is directly proportional to the number of nuclei contributing to that signal. Traditionally, quantitative analysis has been performed by the use of an internal reference standard, but more recently, it has been shown through techniques such as Eretic and Pulcon that it is possible to
amount in tablet (mg)
concentration at 10% of complete dissolution (mM)
concentration at complete dissolution (mM)
250 100 5
0.184 0.056 0.003
1.838 0.561 0.027
≈190
0.062
0.617
to be detected. For example, the combination tablet contains only 5 mg of phenylephrine hydrochloride. Therefore, even at the end of the dissolution, the system needs to be capable of detecting 5 mg dissolved in 900 mL of buffer, if it is assumed that approximately 400 μL is present within the active region of detection within the NMR spectrometerthis corresponds to needing to be able to detect 2 μg with a temporal resolution on the order of tens of seconds. In addition, the levels present within the active region at earlier time points will be significantly less. For this reason, all experiments were run on the highest sensivity system available to us, a 1H optimized cryoprobe on a 600 MHz spectrometer. To maximize sensitivity, the experimental aquisition time and relaxation delay between transients was kept as short as possiblethis has the effect of making integration within a single spectrum nonquantitative. However, because all spectra were recorded under identical conditions, it was still possible to quantitiatively compare the same signals across multiple spectra. A further consideration is that the solvent system is protonated. Normally, NMR experiments are performed using deuterated solvent both to remove intense solvent signals and provide a lock signal for stability, as well as to help optimize shimming (ensuring magnetic field homogeneity across the sample). Modern techniques using water suppression11 and gradient shimming (implemented through the Bruker topshim command) mean that high quality spectra can easily be obtained. NMR Dissolution System. The system we have developed for NMR dissolution is shown schematically in Figure 1. A standard dissolution vessel is placed in a temperature-controlled water bath and the USPII paddle driven by an overhead stirrer motor. The dissolution medium is recirculated continuously through the 1H NMR and UV flow cells. The NMR flow cell used was developed by Morris et al.8 for use in in-line reaction monitoring, an area where the use of NMR is well established.11 In all these cases, sample concentrations are high and so spectral quality and sensitivitiy are less important to ensure interpretable data are obtained. We believe that this work on the use of 1H NMR for dissolution to be the first example of using this approach on a cryoprobe system and attempting to observe such low concentrations. C
dx.doi.org/10.1021/ac403418w | Anal. Chem. XXXX, XXX, XXX−XXX
Analytical Chemistry
Article
dissolution of each species by simple integration. (c) The signal-to-noise ratio is more than sufficient to observe even a fraction of the amount present in the reference sample for three of the species. As phenylephrine hydrochloride is present at a much lower amount in the combination tablet (and the reference sample was made up at double the final concentration compared to that which will be present in the dissolution bath), it is likely that this component will be more difficult to detect and integrate, particularly at the earlier dissolution time points.
■
RESULTS The UV spectra of the individual drug substances, acetaminophen, guaifenesin, and phenylephrine hydrochloride at the concentrations which would arise from complete dissolution are shown in Figure 3. Figure 3 shows that there are significant differences between the spectra of acetaminophen and guaifenesin, which might be exploited by a multiwavelength measurement approach. The spectrum for phenylephrine hydrochloride is much more challenging, however, both because of the very weak overall response and the similarity of the wavelength maxima with those of guaifenesin. The dissolution profiles for the two runs at a wavelength of 227 nm are shown in Figure 4. As the response is measured every 10 s in the UV flow-cell system, it can be clearly seen that the dissolution profiles are sigmoidal in form with a rapid onset. Dissolution of the UV-absorbing species is rapid and smooth with the plateau obtained after about 15 min indicating
Figure 1. Schematic diagram showing the dissolution setup.
Figure 2 shows the quality of the spectra recorded using the flow cell compared with those obtained from a standard 5 mm tube. A mixture of the standards at the concentration expected from dissolution at a flow rate of 2 mL min−1 was compared with the individual components in standard 5 mm tubes. A number of observations can be made from these preliminary experiments: (a) The peak shape and widths, although not quite as good on flow, are still of remarkable quality. For example, the line width at half-height for the acetaminophen singlet at 2.5 ppm (region 7 in Figure 2) on flow is 2.1 Hz compared with 1.5 Hz for the reference solution in a tube. (b) There is at least one signal from each of the four species that is resolved, which allows an independent assessment of the rate of
Figure 2. (a) Reference solution containing all four species, acetaminophen, guaifenesin, phenylephrine hydrochloride, and lactose acquired in the flow cell flowing at 2 mL min−1. (b−e) Individual spectra of the four components, acetaminophen, guaifenesin, phenylephrine hydrochloride, and lactose, respectively, prepared in the same buffer solution at an appropriate concentration but acquired in a standard 5 mm NMR tube. Spectra (f) and (g) are comparisons showing the zoomed aromatic region for spectra (a) and (b). As can be seen, the loss of spectral resolution between a normal tube and a sample on flow is remarkably slight. Regions marked 1−7 are signals from individual species that are fully resolved and correspond to the regions that were integrated (1, 3, and 7, acetaminophen; 2, guaifenesin, 4 and 5, lactose; and 6, phenylephrine hydrochloride). All data were acquired with 32 transients. D
dx.doi.org/10.1021/ac403418w | Anal. Chem. XXXX, XXX, XXX−XXX
Analytical Chemistry
Article
on the bottom of the dissolution vessel and so experiencing different shear forces. Experimental results show that tablets placed in different regions on the bottom of a dissolution vessel give different dissolution rates, and this is consistent with the different shear rates determined using computational fluid dynamics calculations.13,14 The wavelength of 227 nm was used, as it corresponds to the approximate spectral maximum for guaifenesin and phenylephrine hydrochloride, and there is also a strong response from acetaminophen. From the discussion above, it is clear that the measured UV signal at 227 nm will be a composite response, which is a convolution of the different spectra, drug loadings, and release rates of the three active drug substances. Because of this degree of complexity, we felt that a multivariate curve resolution approach using UV spectroscopy would be rather challenging and so was not attempted. As discussed, 1H NMR gives much greater measurement selectivity than UV spectroscopy, as the analyte signals are much sharper relative to the spectral range. 1H NMR offers the additional benefit that the release profile of lactose, the soluble filler, may be determined in the same experiment. This approach means that we can use a single measurement to explore the relationship between the release rate of the soluble filler and the release rate of the active components. 1 H NMR experiments were performed on the two tablet replicates, runs (a) and (b). In run (a), the experimental time per experiment was 70 s, which provided an excellent signal-tonoise ratio, even for the low concentration species, phenylephrine hydrochloride. However, as can be seen from the UV data in Figure 4, the dissolution was almost complete within 10 min, and to increase the number of NMR data points for run (b), the experiment time was reduced to 12 s. This provided a sufficient signal-to-noise ratio for some signals but not the signals of lactose and phenylephrine hydrochloride (regions 4 and 6 in Figure 2). Figure 5 shows the temporal evolution of NMR spectra during dissolution. As can been seen, the data obtained are as expected, of a similar quality to that from the reference spectrum (Figure 2). For each spectrum, seven integrals were generated for the seven regions as labeled in Figure 2, and these were then scaled relative to integrals obtained from the
Figure 3. UV spectra of the individual components at their approximate respective concentrations in the combination tablet: acetaminophen, 1.64 mM; guaifenesin, 0.46 mM; and phenylephrine hydrochloride, 0.027 mM. No absorbance was observed above 350 nm.
Figure 4. UV absorbance at 227 nm for dissolution runs (a) and (b).
complete release. The two signal profiles for the two replicates are very similar, indicating good reproducibility in dosage form performance. We believe that the offset of approximately 1.0 min between replicates at 50% drug release in the dissolution profiles may be due to the tablets ending up in different regions
Figure 5. Portions of the 1H NMR spectra acquired during dissolution run (b). Four transients, taking 12 s, were acquired per experiment. Times relate from the point the buffer is in contact with the tablet, corrected for the 90 s lag between the USPII dissolution bath and the NMR flow cell. E
dx.doi.org/10.1021/ac403418w | Anal. Chem. XXXX, XXX, XXX−XXX
Analytical Chemistry
Article
run (b) are presented, because this has the greatest time resolution and so more fully represents the dissolution process. However, the phenylephrine hydrochloride data are from run (a), as increased sensitivity is required to get high-quality data (see Figure 7 for the data from both runs). Figure 6 shows that dissolution begins very quickly and reaches 100% after about 15 min. The dissolution rates of all the soluble components are very similar. The rapid dissolution measured by 1H NMR is in agreement with the generalized findings from the UV signal of the composite from the drug substances. The measured 1H NMR dissolution profiles are all sigmoidal in form, and the smoothness of the curves serves to illustrate that good quantification was obtained for all components. This smoothness in the dissolution curves was even seen with phenylephrine hydrochloride despite its low loading of only 5 mg in the tablet and hence maximum solution concentration of only 6 μg mL−1. Figure 6 also shows that the four components have similar dissolution profiles, highlighting that they are being released from the tablet at very similar rates. The similarity in the dissolution profiles was investigated by examining the extent of release for the individual drug substances relative to that of acetaminophen (region 1 in Figure 2). Table 2 shows the results of the linear regression, and it is clear that in all cases, there is a linear relationship between the dissolution rates of the three drug substances and that of lactose. The high degree of correlation in the dissolution rates suggests that the same underlying mechanism or mechanisms control the rate of release of all four components. This information can be used to build a more complete understanding of the fundamental tablet dissolution process. In this case, for example, one possible interpretation is that the tablet
reference solution of known concentration. The integrals from the reference solution were not obtained from a single spectrum but averaged over a number of experiments at the same experiment conditions (n = 7 for four scans taking 12 s each and n = 4 for 32 scans taking 70 s each). Finally, the data were converted to a percentage of the nominal concentration (e.g., 250 mg for acetaminophen, etc.). Figure 6 shows the dissolution profile obtained from selected integrals, corresponding to all four soluble species. Data from
Figure 6. Release profile of individual species obtained from the NMR spectra. All data are from run (b) apart from phenylephrine hydrochloride, which are obtained from run (a). Guaifenesin, phenylephrine hydrochloride, and lactose data obtained from single integrals regions at 7.3−7.5, 3.1, and 4.2−4.3 ppm, respectively (regions 2, 5, and 6 in Figure 2). Acetaminophen data are an average of three integrals at 7.5−7.6, 7.2−7.3, and 2.4−2.5 ppm (regions 1, 3, and 7 in Figure 2). Data are scaled using the reference standard solution as described in the text.
Figure 7. Release for the individual species: (i) acetaminophen, (ii) guaifenesin, (iii) phenylephrine hydrochloride, and (iv) lactose. The notation refers to either run (a) or (b), and the corresponding integral as labeled in Figure 2 (i.e., b2 are the data from run (b) integral 2). Data from b4 (lactose) have been omitted because of the large scatter due to the poor signal-to-noise ratio that has resulted from the increased time resolution. The scatter seen in b6 ((iii) phenylephrine hydrochloride) is also due to the poorer signal-to-noise ratio in run (b), as previously discussed. F
dx.doi.org/10.1021/ac403418w | Anal. Chem. XXXX, XXX, XXX−XXX
Analytical Chemistry
■
CONCLUSIONS H NMR with an in-line flow cell has been used to quantify the dissolution of a pharmaceutical tablet containing three active drug substances and a soluble filler. Water suppression enabled a standard protic dissolution buffer to be employed, and instrument stability meant that a lock signal was not required. The 1H NMR spectra obtained on-flow showed excellent line shape, and the excellent sensitivity obtained from a cryoprobe meant that normal pharmacopieal buffer volumes could be employed. This high sensitivity meant that good quantification could be obtained even for a drug substance which was present at only 5 mg in the tablet. 1H NMR enables us to determine the extents of release of three drug substances and lactose in a 12 s acquisition time. 1 H NMR offers a powerful new approach to determining the dissolution behavior of multicomponent pharmaceutical tablets, and although it is unlikely to replace UV for routine dissolution testing, it has great potential as a tool to improve the mechanistic understanding of dissolution processes.
Table 2. Correlation between Percentage Release as a Function of Time for Each Individual Species experiment a region 3 7 2 6 4 5
a
species acetaminophen acetaminophen guaifenesin phenylephrine hydrochloride lactose lactose
slope
b
R
2
Article
1
experiment b slopeb
R2
0.980 0.990 0.997 1.053
1.000 1.000 0.998 0.996
0.969 1.020 1.066 0.976
0.999 1.000 0.997 0.982c
1.047 0.988
0.994 0.995
0.869 1.121
0.969c 0.975c
a
Integral regions, as shown in Figure 2. bIntegrals for each region and time point were converted into percentage release by absolute comparison with the NMR data obtained from the standard solution. The linear regression was performed between region 1 (acetaminophen) and the specified region, with the intercept fixed at zero and only using data between 0 and 12 min. cThe relatively poorer correlation can be attributed to the low signal-to-noise ratio in these data.
■
disintegrates to produce small particles and that all the soluble components are then released rapidly from those particles. Figure 7 shows all the data obtained across different integrals and runs for each of the four species separately. It is clear that in all cases the data are again superimposable, highlighting the excellent reproducibility across experimental runs. The dissolution profiles determined using NMR, Figure 7, show similar offsets to that seen by UV, Figure 4. The UV offset at half dissolution is 1.0 min, with the corresponding NMR values for acetaminophen (region 1), guaifenesin (region 2), phenylephrine hydrochloride (region 6), and lactose (region 5) being 1.0, 0.6, 0.9, and 0.8 min, respectively. This suggests that the dissolution offset is not due to the measurement method but arises from the differences in the position of the tablet within the dissolution apparatus. Figure 7, however, does highlight the need for a good signal-to-noise ratio. In the case of run (b) and phenylephrine hydrochloride, though the release can be observed, the poor signal-to-noise ratio has resulted in very scattered data. Thus, for analytes present at low levels within the tablet, some temporal resolution may have to be sacrificed to achieve the sensitivity required for good quantification. In fact, from other data on different pharmaceutical products not presented, we have shown that the reproducibility obtained from the NMR (and UV) experiments is greater than the inbuilt variability between individual tablets (i.e., differences seen in release profiles obtained by NMR are due to sample variability and not experimental error). Finally a further option for consideration is the use of off-line sampling. While the in-line flow cell provides excellent data and is relatively easy to set up and use, we have also simply sampled the dissolution bath at regular time intervals and then run offline NMR experiments in a tube. For the combination tablet, this produced equivalent data, although without the same temporal resolution. In this work, we have shown that 1H NMR is a valuable new approach for improving the understanding of the dissolution of pharmaceutical products. It also seems likely that the same NMR techniques could also be beneficially applied to the study of other complex dynamic systems. New NMR techniques that offer high selectivity to mixtures of analytes and high sensitivity means that dilute systems which change on a time scale of tens of minutes are readily accessible.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +44 1625 519173. Present Address †
Oncology Chemistry, AstraZeneca, 33B135 Mereside, Alderley Park, Macclesfield, Cheshire SK10 4TG, United Kingdom Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Guidance for Industry: Dissolution Testing of Immediate Release Solid Oral Dosage Forms. U.S. Department of Health and Human Services, Food and Drug Administration, Center for Drug Evaluation and Research (CEDER): Rockville, MD, 1997. (2) Zuleger, S.; Fassihi, R.; Lippold, B. C. Int. J. Pharm. 2002, 247, 23−37. (3) Wiberg, K. H.; Hultin, U. K. Anal. Chem. 2006, 78, 5076−5085. (4) Ferraro, M. C. F.; Castellano, P. M.; Kaufman, T. S. Anal. Bioanal. Chem. 2003, 377, 1159−1164. (5) Abrahmsén-Alami, S.; Körner, A.; Nilsson, I.; Larsson, A. Int. J. Pharm. 2007, 342, 105−114. (6) Van der Weerd, J.; Kazarian, S. G. J. Pharm. Sci. 2005, 94, 2096− 2109. (7) Langham, Z. A.; Booth, J.; Hughes, L. P.; Reynolds, G. K.; Wren, S. A. C. J. Pharm. Sci. 2012, 101, 2798−2810. (8) Khajeh, M.; Bernstein, M. A.; Morris, G. A. Magn. Reson. Chem. 2010, 48, 516−522. (9) Johansson, J.; Cauchi, M.; Sundgren, M. J. Pharm. Biomed. Anal. 2002, 29, 469−476. (10) Bharti, S. K.; Roy, R. Trends Anal. Chem. 2012, 35, 5−26. (11) Dalitz, F.; Cudaj, M.; Maiwald, M.; Guthausen, G. Prog. Nucl. Magn. Reson. Spectrosc. 2012, 60, 52−70. (12) Zheng, G.; Price, W. S. Prog. Nucl. Magn. Reson. Spectrosc. 2010, 56, 267−288. (13) D’Arcy, D. M.; Corrigan, O. I.; Healy, A. M. J. Pharm. Pharmacol. 2005, 57, 1243−1250. (14) Bai, G.; Armenante, P. M. J. Pharm. Sci. 2009, 98, 1511−1531.
G
dx.doi.org/10.1021/ac403418w | Anal. Chem. XXXX, XXX, XXX−XXX
