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Solid State NMR Characterization of Ibuprofen:Nicotinamide Cocrystals and New Idea for Controlling of Release of the Drugs Embedded into Mesoporous Silica Particles Ewa Skorupska, Slawomir Kazmierski, and Marek J. Potrzebowski Mol. Pharmaceutics, Just Accepted Manuscript • DOI: 10.1021/acs.molpharmaceut.7b00092 • Publication Date (Web): 12 Apr 2017 Downloaded from http://pubs.acs.org on April 15, 2017

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Molecular Pharmaceutics

Solid State NMR Characterization of Ibuprofen:Nicotinamide Cocrystals and New Idea for Controlling of Release of the Drugs Embedded into Mesoporous Silica Particles by Ewa Skorupska, Sławomir Kaźmierski and Marek J. Potrzebowski Polish Academy of Sciences, Centre of Molecular and Macromolecular Studies, Sienkiewicza 112, 90-363 Łodz, Poland KEYWORDS Drug Delivery Systems, Cocrystal, Mesoporous Silica Nanoparticles, Solid State NMR, Multidirectional Treatment, Nonsteroidal Anti-inflammatory Drug

Abstract Grinding and melting methods were employed for synthesis of pharmaceutical co-crystals formed by racemic (R/S) and entiomeric (S) ibuprofen (IBU) and nicotinamide (NA) as coformer. Obtained (R/S)-IBU:NA and (S)-IBU:NA co-crystals were fully characterized by means of advanced 1D and 2D solid state NMR (SS NMR) techniques with very fast magic angle spinning (MAS) at 60 kHz. The distinction in molecular packing and specific hydrogen bonding pattern was clearly recognized by analysis of 1H, 13C and 15N spectra. It is concluded from these studies that both methods (grinding and melting) provide exactly the same, specific forms of co-crystals. Thermal solvent-free (TSF) approach was used for loading of (R/S)-IBU:NA and (S)-IBU:NA into the pores of MCM-41 mesoporous silica particle (MSP). The progress and efficiency of this process was analyzed by NMR spectroscopy. It has been confirmed that TSF method is an effective and safe technique of filling the MSP pores with active pharmaceutical ingredients (APIs). Analyzing the NMR results it has been further proved that excess of IBU and NA components which are not embedded into the pores during melting and cooling crystallize on the MCM-41 walls preserving very specific arrangement, characteristic for crystalline samples. Investigating kinetic of release for (R/S)-IBU /MCM41, (S)-IBU:NA/MCM-41 and (R/S)-IBU:NA/MCM-41 samples containing active components exclusively inside of the pores, it was revealed that release of IBU is much faster for the first of the samples compared to those containing IBU and NA inside the pores. The hypothesis that the rate of release of API can be controlled by specific composition of cocrystal embedded into the MSP pore was further supported by study of (R/S)-IBU:BA/MCM41 sample with benzoic acid (BA) as co-former.

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Introduction Pharmacological treatment is a complex process realized in a few stages.1,2,3 Among different factors, bioavailability4,5 and release rate6,7,8 of active pharmaceutical ingredients (API) are the most important properties which determine the usability of drugs. Under optimal conditions, the active substance should reach the target with a required concentration which should be preserved for the time needed to produce the therapeutic effect. In the perfect case, only the tissues where the pharmacological target is located should be exposed to the drug, maximizing the response and minimizing the collateral effects. Solving all or part of these problems is still a challenge for modern pharmacology. A number of approaches is currently tested as a prospective solutions of discussed supra issues.9,10,11 Pharmaceutical industry is mainly focused on increasing the bioavailability of API,12,13 its protection during transportation to desired localization in the body and its controlled release.14 Bioavailability depends mostly on water solubility of API and, in case of solid drugs, on their crystalline form.15 In the light of the fact that 40% of well-known API are poorly water soluble drugs,16 improvement of this parameter is very important. Such problem can be resolved by the so-called micronization or salt formation.17 Unfortunately, there are a lot of restrictions which preclude the application of these methods, e.g.: in the case of compounds chemically inert or non-ionized. Therefore, another way is use of drugs delivering systems (DDSs).18,19,20 Among them an important group of DDS are MSP such as MCM-41 (mobile crystalline mater) and SBA-15 (Santa Barbara amorphous No. 15).21 The MSP are characterized by a well-developed surface and a defined pore size with a low dispersion. Because of those unique physicochemical properties and additionally biocompatible and biodegradable properties,22,23,24 as well as biomedical applications, these materials have become one of the most interesting drug carriers. The DDSs are also needed in case of weak activity which is the result of low chemical stability and/or non-selective therapeutic effect.25 Using specially designed carriers, it is expected that encapsulation can provide the necessary protection of the APIs, higher selectivity, higher therapeutic effect and safety, particularly in the case of anticancer drugs, where a common problem is their high toxicity.26 An important advantage of DDSs is the ability to deliver a drug to a specific location in the body and release it at the appropriate time stimulated by external or internal factors (temperature, ultrasound, pH, redox processes, etc.).27,28,29,30


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An important issue constituting a challenge for pharmaceutical industry is the development of efficient methods of formation of API:DDS species. In the case of MSPs, the embedding of API into the pores is mainly based on wet methods using ethanol, methanol or hexane as solvents (“incipient wetness” or “long stirring”).31,32 According to our earlier work, wet methods have several drawbacks and limitations.33 One of them is their relatively low filling factor. Recently, we have developed an alternative method which we call Thermal Solvent-Free (TSF). This approach is based on melting of physical mixture of API and silica matrix. We showed the applicability of this method employing ibuprofen and MCM-41 as DDS.33 In the course of our studies, we concluded that the TSF is not a general approach and cannot be used for incorporation of API into MSP in the case of compounds degradable during heating process or with high melting point. In previous papers, we revealed that the solution to this problem is modification of the physicochemical properties of API by the formation of co-crystals.34 Co-crystals are solids composed of two or more different components in a specific stoichiometric ratio. In the case of pharmaceutical co-crystal at least one of the components belongs to an active pharmaceutical ingredient.35 The great advantage of such species is the possibility of multidirectional therapy, e.g. the analgesic and anti-tumor therapy or analgesic and regeneration therapy. Additionally, incorporation of such systems in the MSP may become a new tool for the delivery of multiple drugs to a specific location in the body at the same time reducing the amount of administered drugs, and reducing the side effects. In this work, we show and prove that by changing the composition of co-crystals it is possible to control the release rate of API embedded into the MSP pores. It is a new concept, not tested before for mesoporous materials. So far the release process was regulated by nature of drug delivery systems. In the case of MSP, mostly size and geometry of the pores, as well as chemical modifications of surface, are critical parameters controlling the release rate. We present the potential of the new strategy based on embedding two-component systems into the pores of MCM-41 by employing co-crystals formed by S and R/S ibuprofen and nicotinamide (pyridine-3-carboxamide) as co-former (see Scheme 1).



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Scheme 1. Chemical structure of ibuprofen (1) and nicotinamide (2) with numbering system. Solid State NMR (SS NMR) is used as the primary tool for structural characterization of synthesized (S)-IBU:NA and (R/S)-IBU:NA co-crystals. The difference in molecular packing for both crystalline samples exemplified by distinct 13C and 15N spectral pattern is shown. The SS NMR technique is also employed to follow the progress of incorporation of co-crystals into the pores using TSF method and further to figure out the filling factor. The distinction in rate of release is revealed by comparative analysis of (R/S)-IBU/MCM-41, (R/S)IBU:NA/MCM-41 and (S)-IBU:NA/MCM-41. Our observations are supported by inspection of IBU release for sample formed by ibuprofen and benzoic acid (IBU:BA co-crystal) embedded into the MCM-41 pores.

EXPERIMENTAL DETAILS Materials The silicon dioxide MCM-41 (hexagonal) was purchased from Sigma-Aldrich. The MSP was activated by calcination at 300°C for 1 h with a heating rate of 5°C min-1 to remove the water. The pore volume and pore size of MCM-41 were 0.86 cm3/g and 3.7 nm, respectively. The ibuprofen (2-(4-isobutylphenyl)propionic acid), (S)-(+)-ibuprofen, nicotinamide and benzoic acid were obtained from Tokyo Chemical Industry (TCI) or Sigma and were not purified before the use. The melting points of components were 78.6 °C, 90 °C and 130.5 °C, 122.9 °C respectively. Co-crystal preparation The co-crystal (R/S)- or (S)-IBU:NA and (R/S)-IBU:BA were prepared by employing two methods: in the first one, 1:1 molar mixture of the two components was shaken in a Mixer Mill MM 200 equipped with a 5-mL agate jar and 3 mm diameter balls for 60 min at


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oscillation rate of 20 Hz; the second is based on melting and cooling of 1:1 molar mixture of the two components at 140 °C. Loading procedure Thermal Solvent-Free (TSF) method: The loading of cocrystals (R/S)-IBU:NA, (S)-IBU:NA and (R/S)-IBU:BA was performed on calcined powders of MCM-41. This method is based on the mixing of the cocrystals with the mesoporous silica and heating the mixture to a temperature approximately 5 degrees above the melting point of the cocrystal (the heating temperature for (R/S)-IBU:NA, (S)-IBU:NA and (R/S)-IBU:BA were 100 °C, 90 °C, 65 °C, respectively). Diffusion procedure Samples of (R/S)-IBU:NA, (S)-IBU:NA and (R/S)-IBU:BA embedded in MCM-41 were placed in weighing bottles and put into a diffusion chamber with ethanol for 0.5 and 2 hours at ambient temperature (297 K ± 3 K). After the requisite diffusion times, all samples were immediately placed in 4 mm zirconia rotors. NMR measurements. SS NMR experiments were performed on a Bruker Avance III 400 spectrometer at 1H, and

15

N

13

C resonance frequencies of 400.152, 40.531 and 100.627 MHz, respectively.

Adamantane (resonances at 38.48 and 29.46 ppm) was used as a secondary 13C chemical-shift reference from external tetramethylsilane (TMS).36 The

13

C CPMAS (cross polarization

magic angle spinning) and SPE (single pulse experiment) spectra were recorded with a proton 90° pulse length of 4 µs. In typical CP MAS experiments, samples were packed into 4 mm ZrO2 rotor and spun at a spinning rate of 8 kHz. The 1024 transients were acquired. FIDs were accumulated using time domain size of 3.6 K data points and collected with a SPINAL decoupling sequence.37 A sample of 13C-labeled tyrosine (Tyr) was used to set the Hartmann– Hahn condition. Very-fast magic angle spinning (VF MAS) experiments were performed on a Bruker Avance III 600 spectrometer at 1H resonance frequency of 600.130 MHz employing a 1.3 mm MAS probe-head and ZrO2 rotors. The spinning rate of samples in the 1H experiments was in range of 50 - 60 kHz. The FIDs were accumulated using time domain size of 16 K data points. The temperature of sample measured under VF MAS conditions was found to be 313 ± 2K. The 1H−1H NOESY (Nuclear Overhauser Effect Spectroscopy) HR MAS NMR



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spectrum was recorded using a Bruker Avance III 400 spectrometer with a high resolution magic angle spinning (HR MAS) probehead and magic angle gradients. All measurements in the liquid phase were performed on a Bruker AV III 500 spectrometer, operating at 500.13 MHz for 1H, equipped with 5mm BBI probehead with z-gradients coil and GAB/2 gradient unit capable to produce B0 gradients with maximum strength of 50 G/cm. All measurements were carried at 295 K. The BCU-05 cooling unit, controlled by BVT3200 temperature regulation unit was used for temperature stabilization. Spectra were collected with original Bruker pulse program employing watergate W5 pulse sequence with gradients using double echo. The spectral data were processed using the TOPSPIN program.38 Differential scanning calorimetry (DSC) These measurements were performed using a DSC 2920, TA Instruments, the heating rate was 10K/min for all the samples. FTIR analysis Spectra of the solids were recorded at room temperature with the KBr pellet technique using a FT-IR Spectrometer with IR microscope Infinity ATI Mattson, resolution 1cm-1. In vitro release a) The UV spectrophotometric method. The release profiles of ibuprofen, nicotinamide and benzoic acid were obtained by (R/S)-Ibu/MCM-41, (R/S)-IBU:NA/MCM-41, NA/MCM-41 (1:4 by weight, 10 mg) samples suspended in 2ml PBS in a dialysis bag with an MWCO 12,000-14,000 (Serva). After that, the dialysis bag was immersed in 100 ml phosphate buffer solution (PBS) under stirring at 37 °C. 0.35 ml solution was collected for the release test at each given time intervals, and was returned immediately after the test. The amount of ibuprofen, nicotinamide released was analyzed by UV absorption spectroscopy on Shimadzu UV-2450. b) The NMR spectroscopy method. The release profiles of ibuprofen, nicotinamide and benzoic acid were obtained by (R/S)-IBU/MCM-41, (R/S)-IBU:NA/MCM-41, NA/MCM-41 and (R/S)-IBU:BA/MCM-41 (1:4 by weight) samples suspended in 2ml PBS in a dialysis bag with an MWCO 12,000-14,000 (Serva). After that, the dialysis bag was immersed in 100 mL phosphate buffer solution (PBS) under stirring at 37 °C. At each time interval, 0.35 ml of the


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solution was withdrawn to NMR tube and the 1H NMR spectrum with water suppression was carried out (with external solvent).39

RESULTS AND DISCUSSION i) Solid state NMR as a tool to study progress in synthesis of IBU:NA cocrystals. Literature describing the synthesis of IBU:NA cocrystals is rich and different methods of preparation have been reported.40,41 In our work we synthesized IBU:NA using IBU in racemic (R/S) form (cocrystal 3a) and pure S-enantiomer (cocrystal 3b) employing two solvent free approaches. First of them is based on the melting and cooling of equimolar physical mixture of both components and follows the procedure reported by Blagden and coworkers.42 Second is the classic mechanochemical approach based on the ball-mill grinding of IBU and the coformer.43 To date, numerous experimental techniques have been applied for monitoring the process of cocrystal formation and characterization of products including thermal analysis, X-ray powder diffraction, single crystal X-ray diffraction, FT-IR, Raman Spectroscopy and optical methods.40,41,42 It is rather surprising that so far the solid state NMR has not been used for analysis IBU:NA cocrystals. In our report we present detailed NMR analysis of both species.



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Figure 1.

13

C CP MAS NMR spectra of A)-nicotinamide, B)-racemic R/S ibuprofen, C)-

enantiomeric (S)-ibuprofen, recorded with spinning rate of 8 kHz D) (S)-IBU:NA (grinding), E)-(R/S)-IBU:NA (grinding) recorded with spinning rate of 12 kHz.


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13

Figure 1 shows the

C CP/MAS spectra of API and co-former used for synthesis of 3a

and 3b with assignments of signals of substrates. It is important to note that in order to obtain good quality

13

C CP MAS NMR spectrum of nicotinamide (2), the relaxation delays (RDs)

during the acquisition should be over 300 seconds (Supporting Information, Figure S1), because of very long

13

C and 1H relaxation times for this sample. Acquiring the

13

C NMR

spectra for (R/S)-IBU (1a) and (S)-IBU (1b) (Figure 1B and Figure 1C, respectively) was an easy process. Spectra recorded with RD equal 5 s are very similar. Figure 2 shows the

13

C CP MAS spectra of (R/S)-IBU:NA 3a and (S)-IBU:NA 3b

cocrystals recorded with spinning rate of 12 kHz and recycle delay 5s. Both samples were synthesized employing two approaches; grinding and melting. Bottom traces (C, D) represent sample 3a. From the inspection of spectra, it is clearly seen that a new solid state form is created. Resonances representing IBU and NA components are seen even for relatively short RD, which means that 13C and 1H relaxation times in the new systems are average and much shorter, compared to the starting material. It is indirect evidence confirming the synthesis of cocrystals. In the course of our studies, we have noted that the molten samples became completely amorphous and soft just after the synthesis, but their solidification and selforganization follows after a few hours at ambient temperature. Moreover, the final forms of cocrystals prepared using grinding and melting method are similar and we did not observe changes in the spectral patterns. This clearly suggests that both approaches lead to the formation of the same samples. The melting point of 3a and 3b are approximately 92.3 °C and 83.3 °C respectively, which is different for pure components (see Supporting Information Figure S2). It is consistent with literature data.42 The spectral pattern for 3a and 3b cocrystals is different. In the case of (R/S)-IBU, only one carboxyl (δC=OIBU=175.3 ppm) and one carbonyl (δC=ONA=163.7 ppm) signals have been observed. For 3b carbonyl signal from ibuprofen is splited (δC=OIBU=176.6 ppm; δC=OIBU=175.6 ppm). The doubling of signals is also observed both in aromatic and aliphatic regions (see Supporting Information, Figure S4).



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Figure 2.

13

C CP MAS NMR spectra of A)-(S)-IBU:NA grinding, B)-(S)-IBU:NA melting,

C)-(R/S)- IBU:NA grinding, D)-(R/S)-IBU:NA melting recorded with spinning rate of 12kHz.

Figure 3

15

N CP MAS NMR spectra of A)-(S)-IBU:NA, B)-(R/S)-IBU:NA recorded with

spinning rate of 12kHz. The difference in molecular packing for (S)-IBU:NA and (R/S)-IBU:NA cocrystals is also apparent from analysis of 15N CP MAS spectra (Figure 3). The splitting of nitrogen signals for 3b is observed.


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Trying to answer the question of what is the origin of distinction in the NMR spectra pattern for 3a and 3b, we assume that it is due to the difference in the crystal structure with one molecule for the former and two independent molecules in the asymmetric unit for the latter sample and/or distinction in hydrogen interactions involving the carboxylic groups. Figure 4 shows the different hydrogen bonding modes which are typical for the co-former used in this project.

Figure 4 Possible modes of intermolecular hydrogen bonding ii) Structural analysis of ibuprofen:nicotinamide cocrystals employing advanced 1D and 2D solid state NMR techniques. In this section we present the potential of 1D and 2D NMR techniques in structural analysis of pharmaceutical co-crystals based on applications of VF MAS with sample rotation at 60 kHz. Such high spinning rate has influence on the proton spectra resolution and brings out new mechanisms for nuclear spin interactions. In general, assignment of the 1H resonances and quantitative analysis of spectra using SS NMR spectroscopy is still challenging due to the extremely strong homonuclear dipolar couplings, which in many cases exceed the range of chemical shifts for protons. For true solids, the broadening of the proton lines is not removed by slow or medium MAS without the application of complex pulse sequences. Spectra recorded under slow MAS conditions are therefore difficult to analyze, and they usually do not contain subtle structural information. Figure 5 displays 1H VF MAS NMR spectra of API, co-former and powdered samples of 3a, 3b recorded under MAS of 60 kHz.



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Figure 5 1H VF MAS NMR spectrum of A)-nicotinamide; B)-(R/S)-ibuprofen, C)cocrystals 3a and D)- 3b recorded with a spinning rate of 60kHz.

The C and D spectra in Figure 5 show different NMR patterns compared to pure components 1 and 2 (Figure 5A and Figure 5B). The formation of a new solid forms is obvious. As we established, the most diagnostic is the carboxylic proton region. The C(O)OH 1

H δiso values for cocrystals 3a and 3b are downfield shifted compared to pure ibuprofen ca 2

ppm. It suggests that IBU as co-former contributes in hydrogen bonding and these bonds are stronger then in pure IBU. In the next step we carried out 1H-13C HETCOR experiments, which provide information about connectivity between proton and carbons. These experiments enable us to assign the resonances of IBU and NA and finally conclude the nature of hydrogen bonding. Depending on the applying contact time in 2D sequence, we can observe direct C-H bond (short contact time) or long range C-H interactions. High spinning rate of sample (vr = 60 kHz) gives opportunity to obtain proton spectra with good resolution. In order to increase the sensitivity of detection, we have carried out measurement with proton detection (Inv-HETCOR). The full


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1

H-13C inv-HETCOR spectra for 3a and 3b cocrystals are attached as supplementary

information (Figure S5). Figure 6 shows expanded diagnostic aliphatic, aromatic and carbonyl/carboxyl regions. Inspection of carbonyl/carboxylic region immediately eliminates from discussion the hydrogen bonding defined in Figure 4 as Mode IV. It is proved by the lack of correlation peak between carboxylic proton of IBU and the carbonyl group of NA. This conclusion is valid for both cocrystals. The splitting of carboxyl resonances for sample 3b is apparent. The ibuprofen and nicotinamide molecules can be clearly distinguished. This effect is perfectly observed for methine CH and methyl CH3 groups of IBU. These correlations reflect intra-molecular contacts of IBU.



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Figure 6 1H-13C inv-HETCOR correlation of cocrystal 3a (left column) and 3b (right column) A)-aliphatic region, B)-aromatic region, C)-carboxyl/carbonyl regions recorded with spinning rate of 60 kHz. Spectra were acquired with first contact time of 2 ms and second contact time of 1ms.


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Further information about the hydrogen bonding pattern was obtained from analysis of 1

15

H− N inv-HETCOR spectra. As in the previous case, the spectra were recorded with

spinning rate of 60 kHz. The 2D 1H−15N inv-HETCOR experiment allowed us not only to assign position of protons directly bonded to NH2 group located as well in the neighborhood of nitrogen in aromatic ring of NA, but also to conclude the mode of hydrogen bonding. The correlation peak between carboxylic proton of IBU and aromatic nitrogen of NA (δ15N =284 ppm) is a proof confirming the Mode II and/or III for both cocrystals. The discussion about the Mode I is vague because 2D 1H−15N inv-HETCOR experiment cannot confirm or exclude such bonding.

Figure 7 1H−15N inv-HETCOR spectra of A)-3a, B)-3b cocrystal recorded with a spinning rate of 60 kHz. Spectra were acquired with first contact time of 2 ms and second contact time of 2 ms. Finally, having full assignment of proton positions from heteronuclear correlations, we have employed 1H-1H 2D a single quantum−double quantum (SQ-DQ) experiment. Figure 8 shows homonuclear 1H−H back-to-back (BaBa) correlation.44,45 The connectivity between spins is labeled by red lines.



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Figure 8 1H−1H SQ-DQ Back-to-Back (BaBa) experiment of A)- 3a and B)-3b recorded with a spinning rate of 60 kHz. The length of the recoupling period was 33.3 µs. C) Correlations labeled as α, β, γ represent inter-molecular interactions between IBU and NA. From BaBa experiment, by inspection of DQ coherences between pairs of through space dipolar coupled protons we obtained reliable evidence confirming the connectivity between IBU and NA molecules in cocrystals. Figure 8a and Figure 8b show the noncovalent interactions between both components for 3A and 3B cocrystals. In both cases, the hydroxyl group is correlated with aromatic protons of NA and IBU. Our NMR assignments and proposed hydrogen bonding pattern are consistent with X-ray data.42 The 3a crystallizes in orthorhombic system with Pca21 space group. Sample 3b is obtained as monoclinic crystal with P21 space group (see Supporting Information, Figure S3).


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ii) Analysis of ibuprofen:nicotinamide cocrystals embedded into MSP pores Our previous studies of cocrystal/MSP systems show that two-component species can be easily incorporated into MCM-41 or SBA-15 pores using thermal solvent-free method.46 In some cases, the trapped cocrystal retains its arrangement inside the matrix pores. Searching the NPX:PA we have found that during melting, the NPX:PA is embedded into the pores of SBA-15, while MCM-41 acts rather as a separation medium.34 As a preliminary test for inspection whether IBU:NA co-crystal is embedded into the pores and/or located outside on the walls of MSP, we carried out 1H NMR measurements with sample spinning rate 60 kHz. Figure 9 shows the spectra with different ratio of co-crystal and silica mesoporous particles. It is worthy to express that spectral pattern and high resolution enable us to assign resonances of IBU and the co-former. The narrowing of the proton signals of both components results from the higher molecular dynamic of molecules embedded into matrix pore what was observed for ibuprofen embedded into MCM-41.33 According to our experience the carboxylic region is the most diagnostic.33 For API’s possessing the carboxylic group, when the drug is trapped into the pore, the acidic proton is transferred on the MSP wall and in consequence is not observed in the indicative region near 15 ppm. This observation is consistent with our previous studies33 and results coming from other laboratories.47



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Figure 9. 1H MAS spectra of IBU:NA into MCM-41 with different weight ratios A) 1:3; B) 1:1; C) 2:1, recorded with spinning rate of 60 kHz at ambient temperature.

For sample with IBU:NA/MCM-41 ratio 1:3 (Figure 9A) the characteristic peak at 14.8 ppm on 1H MAS spectrum is not observed even when the baseline in this region is amplified four-times (left column). Figure 9B shows 1H MAS spectrum for sample with 1:1 ratio of components. The carboxylic proton is clearly seen, in particular in enhanced part of the spectrum shown in the left column. This signal is significantly increased for sample with ratio of 2:1 (Fig. 9C). The apparent differences in intensity of carboxylic proton in relation to composition of samples allow us to conclude that in the case when IBU:NA/MCM-41 ratio is 1:3 the whole IBU is inside of MSP. With increase of the ratio (1:1 and 2:1) the significant part of cocrystal is outside of the pores and is located on the walls. Extending this interpretation, we can assume that from comparative analysis of difference spectra for pure crystal of IBU and IBU:NA embedded into the pores, we can determine the filling factor for IBU. This approach in graphical form is shown in Figure 10.

Figure 10 1H MAS spectra of IBU:NA (red line) and IBU:NA in MCM-41 with ratio 1:1 by weight (black line) recorded with spinning rate of 60 kHz at ambient temperature.

The drawback of methodology based on 1H NMR is a fact that we have a tool which precisely describes behavior of IBU, while the behavior and localization of NA is not reported


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with sufficient accuracy. The full picture showing the performance of both components and co-crystals during and after incorporation into the pores is 13C NMR. We present the power of this tool using IBU:NA/MCM-41 sample with ratio of components 1:1. We have chosen cocrystal with S-IBU which is represented by specific

13

C NMR pattern with clearly seen split

resonance lines in diagnostic regions (see Figure 2A and B). We further assumed that with such proportion of components part of co-crystals will be outside of the pore while part will be inside. According to literature and our own experience, the cross polarization (CP) experiment, which is based on 1H to

13

C magnetization transfer driven by 1H-13C heteronuclear dipolar

interactions, is not able to detect species encapsulated in the pore because of its high mobility.48 Fast molecular motion averages out this key interaction, and therefore, the CP technique is only sensitive to rigid molecules. The Single Pulse Experiment (SPE) is a more efficient technique for detection of components inside the pores. Figure 11A shows the 13C CP/MAS spectrum which represents the rigid fraction localized outside of pores. Clearly seen pattern, with splitting of NMR signals characteristic for (S)IBU:NA cocrystal, proves that during melting and cooling the excess of co-formers not trapped inside the pores crystallizes on the MCM-41’s walls with the same crystal packing as pure mixture (S)-IBU and NA. Shown in Figure 11B is 13C SPE spectrum which reflects the fraction embedded into the pores. The line shape analysis of spectrum clearly shows that aliphatic residues of IBU are under fast exchange regime, while aromatic groups undergo molecular motion in the intermediate region of NMR time scale. In consequence, the broad 13

C aromatic resonances are seen. The short diffusion of solvent vapors significantly changes

the molecular dynamics of components. The 13C SPE spectra (Figure 11C) of the same sample after methanol-d4 diffusion show very mobile species located inside the MCM-41 pores.

Figure 11. 13C MAS NMR spectra of IBU:NA embedded into MCM-41 (1:1 by weight) A) CP/MAS NMR before ethanol diffusion B) SPE MAS NMR before ethanol diffusion; C) SPE MAS after ethanol diffusion. All spectra were recorded with spinning rate of 12 kHz at ambient temperature.



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Figure 12 displays 2D 1H-1H NOESY correlation for sample shown in Figure 11C employing HR MAS probehead. The inspection of cross peaks proves that both components are separated and there is no contact between them. The interesting discovery which we made researching sample after the diffusion of methanol-d4 is presence of traces of water, which is very likely introduced together with solvent into the pores of MCM-41. Analysis of spectrum unambiguously shows the interaction between the nicotinamide and water. It means that hydrophilicity of NA is much higher compared to IBU. In the next section, we show that this feature is an important factor influencing the kinetic of API release.

Figure 12 1H−1H 2D HR MAS NOESY of IBU:NA/MCM-41 (1:4 by weight) after methanold4 diffusion recorded at spinning rate of 10 kHz and mixing time of 50 ms.

iii) Release analysis of ibuprofen:nicotinamide co-formers from MSP pores In a paper published elsewhere, examining ibuprofen trapped inside of MCM-41 pores we revealed that during diffusion of solvent vapors (ethanol, water) the IBU is easy removed from pores and located on the MSP surface.33 Testing the behavior of IBU:NA co-crystals embedded into MCM-41 and employing the identical methodology as reported in ref. [33], we


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found that system under investigation is very stable and IBU is not removed. This preliminary results prompted us to more detailed studies of IBU rate release. In standard and most common approach the kinetic of API release is investigated employing the UV methods due to excellent sensitivity and straightforward interpretation of the data.49,50 Unfortunately, this method is useless for complex mixtures in particular in the case when UV bands representing API and co-former overlap. It is a situation which we met examining release of IBU and NA. Hence, in this project we employed another method based on analysis of NMR data in the liquid PBS buffer. IBU:NA/MCM-41 samples (1:4 by weight) were suspended in 2ml PBS in a dialysis bag with an MWCO 12,000-14,000 (Serva). After that, the dialysis bag was immersed in 100 mL PBS and stirred at 37°C. At each time interval, 0.35 ml of the solution was withdrawn to NMR tube and the 1H NMR spectrum with water suppression was carried out. The representative spectrum in full range is attached as supplementary information (Figure S6). Figure 13 shows the stack plot for both components in diagnostic aromatic region. The intensity of peaks was calibrated on acetone signal kept in capillary during NMR measurement and used as an external reference. Analysis of data enables us to conclude that signals representing the NA component (left column) reach maximal intensity after 200 min of dialysis while the intensity of signals for IBU (right column) increases even after 1300 min of dialysis. This observation in pictorial form is shown in Figure 13.



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Figure 13. 1H NMR spectra of (R/S)-IBU:NA/MCM-41 (1:4 by weight) sample after release process in PBS with external solvent (acetone-d6).

The influence of the composition of embedded sample on the kinetic of IBU release from the pores of MCM-41 is shown in Figure 14. From the analysis of obtained data it is obvious that pure IBU trapped inside the pores is easy and quickly released from MCM-41. The process of release of IBU is greatly slowed when IBU:NA components are loaded into the pores. It is interesting to learn that rate of release for (R/S)-IBU and S-IBU is very similar. In order to verify whether this observation is unique and restricted only to IBU:NA species we carried out additional experiment using co-crystal synthesized employing (R/S)-IBU and BA (benzoic acid) as co-formers. The full NMR characterization of (R/S)-IBU:BA 4 embedded into the MCM-41 pores is attached as Supporting Information (Figure S7). The analysis of profile for (R/S)-IBU:BA/MCM-41 clearly shows that for the short time the rate of IBU release for all three species 3a, 3b and 4 is similar. For longer time IBU is faster released for sample 4 compared to 3a and 3b (full release profiles are presented in Supporting Information, Figure S8). It is interesting to note that NA and BA conformers can be very quickly removed from the pores of MCM-41 and the kinetic of release is comparable. Trying to explain this phenomenon and keeping in mind the results displayed in Figure 14, we can


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assume that process of controlling the release depends on hydrophilic affinity of co-former. We assume that strong NA or BA interaction with water in the pores is a factor determining the kinetic of API. Going further with interpretation of the data and comparing the profiles for IBU:NA and IBU:BA we can conclude that hydrophilicity is only one of the aspects and the mechanisms of release of individual components embedded into MSP pores that requires additional studies.

Figure 14. The release profile performed for A) ibuprofen and B) co-formers from MCM-41 (1:4 by weight) in different samples. Conclusion Continuing our interest in applications of Drug Delivery Systems (DDSs) in modern therapies, in this work we present further examples showing the power of Thermal Solvent Free (TSF) method as a simple, efficient and yielding approach which can be used for loading of Active Pharmaceutical Ingredients (APIs) into the pores of Mesoporous Silica Particles (MSP). Employing the SS NMR techniques we revealed that both components of pharmaceutical co-crystals can be fully loaded into the pores when proper weight ratio of cocrystal and MSP is chose. SS NMR is able to recognize components outside and inside of the pores. The most important part of our project is the unique observation that by the change of composition of embedded co-crystals it is possible to control the release rate of API. It has



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been proved that the process of release of trapped components is not synchronized and hydrophilic co-former is removed first from the pores of MSP. This conclusion is important not only from intellectual but also from practical points of view. It can open new possibilities in medical treatments, in particular in those cases when therapeutic synergism of both components in function of time is required.

ASSOCIATED CONTENT Supporting Information Corresponding DSC curves, FT-IR data, release profile, X-ray data and NMR spectra are attached. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Marek J. Potrzebowski, Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences,

Sienkiewicza

112,

90-363

Lodz,

Poland.

E-mail

addresses:

[email protected], Tel: +48 426803240, Fax: +48 426803261 Notes The authors declare no competing ﬁnancial interest ACKNOWLEDGMENT The authors are grateful to the Polish National Center of Sciences (NCN) for financial support, Grant No. 2014/13/N/ST5/03444. Authors are in debt to Dr. Agata Jeziorna and Mr. Piotr Paluch for technical assistance.


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For Table of Contents Use Only Solid State NMR Characterization of Ibuprofen:Nicotinamide Cocrystals and New Idea for Controlling of Release of the Drugs Embedded into Mesoporous Silica Particles. by Ewa Skorupska, Slawomir Kazmierski and Marek J. Potrzebowski. Polish Academy of Sciences, Centre of Molecular and Macromolecular Studies, Sienkiewicza 112, 90-363 Lodz, Poland

Scheme 1. Chemical structure of ibuprofen (1) and nicotinamide (2) with numbering system.

Figure 1.

13

C CP MAS NMR spectra of A)-nicotinamide, B)-racemic R/S ibuprofen, C)-

enantiomeric (S)-ibuprofen, recorded with spinning rate of 8 kHz D) (S)-IBU:NA (grinding), E)-(R/S)-IBU:NA (grinding) recorded with spinning rate of 12 kHz.


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Figure 2.

13

C CP MAS NMR spectra of A)-(S)-IBU:NA grinding, B)-(S)-IBU:NA melting,

C)-(R/S)- IBU:NA grinding, D)- (R/S)-IBU:NA melting recorded with spinning rate of 12kHz.

Figure 3

15

N CP MAS NMR spectra of A-(S)-IBU:NA, B-(R/S)- IBU:NA recorded with

spinning rate of 12kHz.

Figure 4 Possible modes of intermolecular hydrogen bonding

Figure 5 1H VF MAS NMR spectrum of A)-nicotinamide; B)-(R/S)-ibuprofen, C)-cocrystals 3a and D)- 3b recorded with a spinning rate of 60kHz.



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Figure 6 1H-13C inv-HETCOR correlation of cocrystal 3a (left column) and 3b (right column) A)- aliphatic region, B)-aromatic region, C)- carboxyl regions recorded with a spinning rate of 60 kHz.

Figure 7 1H−15N inv-HETCOR of A)- 3a, B)- 3b cocrystal recorded with a spinning rate of 60 kHz.

Figure 8 1H−1H SQ-DQ Back-to-Back (BaBa) experiment of A)- 3a and B)- 3b recorded with a spinning rate 60kHz. Correlations labeled as A, B, C represent inter-molecular interactions between IBU and NA.


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Figure 9. 1H MAS spectra of IBU:NA into MCM-41 with different weight ratios A) 1:3; B) 1:1; C) 2:1, recorded with spinning rate of 60 kHz at ambient temperature.

Figure 10 1H MAS spectra of IBU:NA (red line) and IBU:NA in MCM-41 with ratio 1:1 by weight (black line) recorded with spinning rate of 60 kHz at ambient temperature.

Figure 11. 13C MAS spectra of IBU:NA into MCM-41 (1:1 by weight) A) CP before ethanol diffusion B) SPE before ethanol diffusion; C) SPE after ethanol diffusion recorded with spinning rate of 12 kHz at ambient temperature.

Figure 12 1H−1H 2D HR MAS NOESY of IBU:NA/MCM-41 (1:4 by weight) after methanold4 diffusion recorded at spinning rate of 10 kHz and mixing time of 50 ms.

Figure 13. 1H NMR spectra of (R/S)-IBU:NA/MCM-41 (1:4 by weight) sample after release process in PBS with external solvent (acetone-d6).



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Figure 14. The release profile performed for A) ibuprofen and B) co-formers from MCM-41 (1:4 by weight) in different sample


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Solid State NMR Characterization of Ibuprofen ... - ACS Publications

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