J Therm Anal Calorim DOI 10.1007/s10973-017-6274-8
Physicochemical characterization and compatibility study of roflumilast with various pharmaceutical excipients Faraat Ali1 • Robin Kumar1 • Puran Lal Sahu1 • Gyanendra Nath Singh1
Received: 29 August 2016 / Accepted: 6 March 2017 Akade´miai Kiado´, Budapest, Hungary 2017
Abstract Roflumilast (RFL), a newly approved and highly selective phosphodiesterase 4 inhibitor for the treatment of severe chronic obstructive pulmonary disease, associated with chronic bronchitis and a history of numerous exacerbations. The main objective of this work was to evaluate the simultaneous (TG/DSC) thermoanalytical characterization and compatibility of roflumilast with the widely used excipients for solid dosage form employing differential scanning calorimetry (DSC), thermogravimetric analysis (TG), optical microscopy, isothermal stress testing (IST) by HPLC and liquid chromatography–mass spectrometer techniques with Fourier transform infrared spectroscopy (FT-IR) as a complimentary technique to contribute in the interpretation of results. The selected excipients were pregelatinized starch (PS), magnesium stearate, croscarmellose sodium (CCS), microcrystalline cellulose (MCC) and sodium starch glycolate (SSG). The DSC curve showed a sharp endothermic melting peak at 160.43 C for roflumilast. On the basis of the DSC results, some interactions were found with RFL–CCS, RFL–MCC, RFL–SSG and RFL–PS. However, during IST studies less than 2% change in roflumilast content was observed in all stressed physical mixtures except RFL–SSG (\14%) which showed incompatibility with roflumilast. These results would be suitable for formulation development of the filmcoated tablets of roflumilast.
& Faraat Ali
[email protected] 1
Pharmaceutical Chemistry Division, Indian Pharmacopoeia Commission, Ministry of Health and Family Welfare, Government of India, Sector-23, Rajnagar, Ghaziabad, Uttar Pradesh 201002, India
Keywords Compatibility HPLC LC–MS TG FT-IR DSC
Introduction COPD stands for chronic obstructive pulmonary disease, and it is a treatable and preventable disease, characterized by airflow, dyspnea and chronic bronchitis. COPD is not completely reversible. Although various experimental phosphodiesterase 4 inhibitors are under clinical development, roflumilast acts as a highly selective inhibitor of the enzyme phosphodiesterase 4 (PDE4) and has been recently approved in various countries for the treatment of severe COPD. It is administered orally and belongs to BCS (biopharmaceutics classification) class II drugs which have high permeability and low solubility [1, 2]. Roflumilast chemically is 3-(cyclopropylmethoxy)N-(3,5-dichloro-4-pyridinyl)-4-(difluoromethoxy)-benzamide. It occurs as a white to off-white powder. It is practically insoluble in water, heptane and hexane, sparingly soluble in ethanol and freely soluble in acetone, having a molecular formula of C17H14Cl2F2N2O3 and a molecular weight of 403.21 g mol-1 [3–5]. Pharmaceutical dosage forms are made up of a combination of drug or active pharmaceutical ingredients (APIs) and various excipients. Excipients are included in dosage forms to support manufacture, administration or absorption [6]. The best excipients must be able to fulfill the important functions, i.e., dose, stability and discharge of API from the formulation. Although excipients considered pharmacologically inert, excipients can participate, initiate and propagate in chemical or physical interactions with the API, which may compromise the efficiency of a medication [7, 8]. It is not exquisitely pure. Even for the most
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commonly used excipients, it is required to understand the milieu of their manufacture in order to identify potential API interactions with trace components. Excipients also contain functional groups that interact directly with active pharmaceutical ingredients. Alternatively, they may contain drug related impurities or residues, or form degradation products in turn cause decomposition of the drug substance [9–11]. Preformulation study is a primary investigation of the physical–chemical properties of the drug substance for product development and formulation, alone and in combination with excipients to predict the stability, safety and efficacy of drug formulation throughout the shelf life, a paramount importance during the development of a solid dosage form of an active pharmaceutical ingredient (API) by careful selection of excipients [12]. The potential physicochemical interactions between API and excipients can affect the dissolution rate, stability, chemical nature and bioavailability of drugs and, consequently, their therapeutic safety and efficacy. The assessment of possible incompatibilities requires a novel experimental design that provides the required information with the minimum of experimental effort [13, 14]. Furthermore, it must affirm the well-defined level of safety, efficacy and stability. However, it is difficult to attain required level of stability, because the APIs may show some chemical or physical interactions with the excipients in the intact formulation. These excipients, in general, do not have a specific pharmacological activity. However, excipients have tremendous formulation properties that impart positive attributes to a quality product [15–17]. The pharmaceutical excipients are generally considered as inactive and pharmacologically inert. However, no universally accepted protocol is available for evaluating the physicochemical compatibility of drug or active pharmaceutical ingredient (API) with different excipients [18]. Thermal analysis provides useful data to predict the physical and chemical interactions between drugs and excipients. Simultaneous TG/DSC and DSC are the extensively used thermal analytical methods for the detection of interaction by comparing the thermal curves of drug, excipients and their physical mixtures [19]. DSC can show changes in the appearance, shift or disappearance of melting endotherms and exotherms and/or variations in the corresponding enthalpy value of a particular reaction. DSC allows the rapid evaluation of possible incompatibilities; however, there are certain limitations also. This is because of exposure of drug–excipient mixture to high temperatures (up to 350 C or more), which, in actual situations, is not experienced by the dosage form [20, 21]. Hence, the DSC results must be
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interpreted based on the enthalpy change carefully to avoid distorted conclusions and some complementary techniques, such as Raman spectroscopy, Fourier transform infrared (FT-IR) spectroscopy and powder X-ray diffractometry (pXRD) can be used to support the result obtained from the thermal analysis to draw accurate conclusions [22]. FT-IR does not alter the physicochemical characteristics of the solid state as materials are not subject to thermal, chemical or mechanical stress. The appearance of new bands, expansion of the bands and alteration of the intensity are the specific characteristics to determine the interactions between API and excipients [23]. To the best of our knowledge, there are no data available in the literature for the drug–excipient compatibility of roflumilast till now. Hence, the objective of the present study is to evaluate the compatibility of roflumilast with the selected excipients used commonly in solid dosage form by thermoanalytical (TG/DSC) investigation and spectroscopic (FT-IR) techniques.
Materials and methods Materials Roflumilast (active ingredient having purity [99.5%) was obtained from Sigma-Aldrich Corporation, Bangalore, India (Fig. 1). The pharmaceutical-grade excipients croscarmellose sodium (disintegrant), microcrystalline cellulose (diluent, disintegrant, binder), magnesium stearate (lubricant), sodium starch glycolate (disintegrant), pregelatinized starch (diluent, binder) were purchased from S.D. Fine Chemicals, Mumbai, India. Sodium perchlorate, perchloric acid (analytical reagent grade), acetonitrile and methanol (HPLC grade) were obtained from Merck Pvt. Ltd. (Mumbai, India). HPLC grade water having resistivity of 18.2 MX cm (Milli Q water purification system, Elix, Milli Q A10 Academic, Molsheim, France) was used during the analysis.
F
F O
Cl H N
O O
N Cl
Fig. 1 Chemical structure of roflumilast
Physicochemical characterization and compatibility study of roflumilast with various…
Methods
collected in the range of 400–4000 cm-1 with an average of 32 scans at spectral resolution of 2 cm-1.
Preparation of physical mixture Optical microscopy At first, drug and excipients were passed through 40-mesh size sieve and then physical mixture of roflumilast, and each selected excipients was prepared by simple mixing in a glass mortar for 5 min in the ratio of 1:1 (w/w). The samples were filled in an amber-colored glass vials for further use. A 1:1 (w/w) physical mixture of drug and excipients was chosen for carrying out the compatibility study to maximize the probability of observing any interaction. Differential scanning calorimetry DSC analysis of drug, individual excipients and their physical mixtures was performed on DSC instrument (Make: PerkinElmer, USA; Model: DSC 6000). The instrument was calibrated for temperature with indium (156.6 C) and zinc (419.7 C) as the melting point standards. Using Sartorius balance (Germany), 1–2 mg of sample (excipients and physical mixtures) was weighed directly in an aluminum pan and sealed followed by scanning in the temperature range of 30–375 C at a heating rate of 10 C min-1 under nitrogen atmosphere with the flow rate of 20 mL min-1. DSC curves obtained were analyzed with the aid of Pyris software to observe any interaction. Thermogravimetric analysis TG instrument (Make: Mettler Toledo, Switzerland; Model: TG/DSC1) was used to obtain TG curves of drug, individual excipients and their physical mixtures by putting approximately 4–5 mg samples in alumina crucible at a heating rate of 10 C min-1 in the range of 30–600 C under atmosphere of nitrogen at the flow rate of 50 mL min-1. TG curves were analyzed with the aid of Stare software. TG cell was calibrated with indium (156.6 C) and aluminum (660.3 C) standards melting point.
Fourier transform infrared spectroscopy (FT-IR) The FT-IR spectra of drug, individual excipients and their physical mixtures were obtained using the FT-IR spectrophotometer instrument (Make: PerkinElmer, USA; Model: Frontier Optica with spectrum software). About 5 mg sample was mixed with about 500 mg potassium bromide by triturating in a mortar and then compressed into KBr disk in a hydraulic press. The disk was placed in a cell with an atmosphere of dry nitrogen. The spectrum was
Optical micrographs of roflumilast, roflumilast–excipient physical mixtures were scanned using oil immersion objective at 1009 magnification by optical microscope. The microscope was fitted with built-in camera (Eclipse TiS, Nikon Instruments Inc., Tokyo, Japan). Isothermal stress testing (IST) studies by HPLC For IST studies by method previously reported by Nath and Singh with slight modification [24], drug and the different excipients of interest were weighed directly in 5-mL glass vials (n = 2) and the vials were mixed on a vortex mixer for 3 min. To each vial containing the drug– excipient blend, approximately 10% w/w water was added and mixed further with a glass capillary (both ends which were heat sealed), and the capillary was left inside the vial to prevent any loss of material. All the vials were sealed using Teflon-lined screw caps and stored at 50 C in a hot air oven. Two set of vials were prepared as per the procedure outlined above. One set of vials were control samples without added water and stored at 2–8 C in refrigerator. The second sets of samples were stored at 50 C and removed after 3 weeks of storage at the above conditions. The samplers were quantitatively analyzed by HPLC. For the analysis of RFL and its physical mixtures, chromatography separation was performed on Waters alliance 2695 liquid chromatography system. The instrument was equipped with Waters 2996 photodiode array detector and variable UV/visible detector, and autosampler injector was utilized. Empower 3 software was used for data acquisition and integration. The chromatography separations were carried out on a X Select CSH C18 (150 mm 9 4.6 mm, 3.5 lm; Waters, Ireland) column. Isocratic elution was achieved with simple mobile phase consisting of 0.015 M sodium perchlorate, adjusting the pH 2.90 with perchloric acid (buffer) mixed with acetonitrile in the ratio of 45:55. The retention time of roflumilast was about 8 min, and the total run time was 12 min. The flow rate was 0.9 mL min-1 with detection wavelength at 213 nm, and the injection volume was 6 lL. The calibration curve was prepared from stock solution by serial dilution with water/methanol (1:1) to achieve solutions containing RFL in the range of 10–150 lg mL-1. For sample preparation, 2 mL of methanol was added into each vial. The mixture was vortexed for 2 min and transferred to 50-mL volumetric flask. Vials were rinsed thrice with
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F. Ali et al. Fig. 2 DSC (a) and TG (b) curves of roflumilast
14.11
(a)
16
RFL
Heat flow/mw
18 20 22 24 26 28 30 31.15 30
50
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Temperature/°C
(b) 100
Mass loss/%
RFL
0
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methanol, and the volumes were made up with the same solvent. The samples were centrifuged, and the supernatant liquid was filtered through 0.45-lm nylon membrane filters. After appropriate dilutions, samples were analyzed using HPLC. Validation of the method was done according to the ICH guidelines. Liquid chromatography coupled to a mass spectrometer (LC–MS) Mass analysis was conducted on the Agilent technologies 6520 quadrupole–time of flight (Q-TOF) mass system at dual electrospray ionization (ESI) mode, negative ionization, source temperature (80 C), desolvation temperature (300 C), desolvation gas flow (480 L h-1) and cone gas flow (80 L h-1). High-purity nitrogen was used as the auxiliary and nebulizer gas. The sample after IST studies transferred and injected to mass system. The mobile phase composition was methanol and water (50:50), flow rate was 0.3 mL min-1, injection volume was 1 lL, and 1- to 3-min intervals maintained between injections for mass system stabilization.
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Results and discussion Characterization The data show that roflumilast is a crystalline solid substance with a well-defined fusion peak in the DSC curve. DSC curve of roflumilast showed three transitions, initially an endothermic event between 50.36 and 53.80 C, another sharp melting endothermic event between 159.16 and 161.48 C with a melting temperature of 160.43 C and a sharp exothermic curve between 285.92 and 308.92 C with a peak observed at 300. 57 C. The heat of fusion observed during these endothermic and exothermic events was 3.62, 94.08 and -262.91 J g-1, respectively. TG curve exhibited 114.39% of mass loss between 104.40 and 328.40 C due to the sharp decomposition of roflumilast. The thermoanalytical curves of roflumilast are shown in Fig. 2. The onset temperature (Tonset/C), peak temperature (Tpeak/C) and heat of fusion (DHf/Jg-1) for pure drug, pure excipient and physical mixture of roflumilast with different excipients are tabulated in Table 1.
Physicochemical characterization and compatibility study of roflumilast with various… Table 1 Peak temperature and enthalpy values of RFL, excipient and RFL–excipient physical mixtures (n = 3) Entries RFL
CCS MCC MS
Tonset/C 50.36 ± 0.1
DHf/Jg-1
Tpeak/C 51.97 ± 0.2
3.62 ± 2
159.16 ± 0.2
160.43 ± 0.3
94.08 ± 1
285.92 ± 0.3
300.57 ± 0.6
-262.91 ± 3
39.04 ± 0.2
55.12 ± 0.4
275.75 ± 0.8
298.38 ± 0.2
-261.50 ± 2
42.04 ± 0.7
322.04 ± 0.4
346.34 ± 0.5
461.96 ± 4
71.95 ± 0.1 92.80 ± 0.6
82.47 ± 0.1 112.12 ± 0.6
9.51 ± 2 27.98 ± 3
SSG
37.05 ± 0.3
60.62 ± 0.7
85.52 ± 4
RFL–CCS
40.09 ± 0.7
53.64 ± 0.2
337.46 ± 1
75.29 ± 0.3
80.24 ± 0.1
-326.99 ± 2
158.99 ± 0.7
160.15 ± 0.9
49.83 ± 0.4
52.29 ± 0.7
1.69 ± 2
158.98 ± 0.4
159.91 ± 0.6
41.69 ± 1
307.57 ± 0.2
313.85 ± 0.1
74.86 ± 0.7
50.31 ± 0.9
52.14 ± 0.9
158.38 ± 0.4
159.71 ± 0.3
314.59 ± 0.1
330.68 ± 0.6
41.73 ± 2
65.13 ± 0.7
70.97 ± 0.9
-411.53 ± 3
RFL–MCC
RFL–MS
RFL–SSG
RFL–PS
37.67 ± 0.8
1.62 ± 0.3 48.16 ± 1
100.02 ± 0.4
102.90 ± 0.1
-179.74 ± 0.9
159.00 ± 0.8
159.89 ± 0.7
29.63 ± 2
49.52 ± 0.6 158.66 ± 0.7
51.97 ± 0.3 159.92 ± 0.1
1.91 ± 1 34.15 ± 0.5
285.06 ± 0.1
292.52 ± 0.9
74.83 ± 0.8
RFL
RFL–CCS
RFL–MCC
RFL–MS
RFL–SSG
RFL–PS
and MS drug is substantially adhered to the MS particles after grinding, indicating a high degree of homogeneity (Fig. 3). In RFL–SSG mixture minute particles of drug are attached to SSG particles while larger remains segregated which might be due to inhomogeneity of drug and SSG (Fig. 3). However, in RFL–PS mixture drug is considerably mixed with large and irregular particles of PS. The subsequent step of the present study was to analyze the FT-IR spectra of roflumilast, used pharmaceutical excipients and their binary mixtures in order to identify a possible physicochemical interaction between them. In this study, this technique was used as a complementary tool to assist in the interpretation of the DSC and TG results. Table 4 enlists the principle peaks for roflumilast observed at wave numbers N–H stretching; 3254 cm-1, C–H stretching in aromatic group and cyclopropyl; 3134-3011(3094,3028) cm-1, -CH2- aliphatic symmetrical, asymmetrical stretching vibration; 2939-2878(2925,2878) cm-1, C=O stretching vibration in the amide moiety; 1654(1654) cm-1, C=C aromatic stretching vibration; 1595 ? 1502(1596,1502) cm-1, N–C=O symmetrical stretching; 1558(1558) cm-1, -CH2deformation vibration; 1464(1466) cm-1, C–O–C symmetrical stretching in cyclopropyl moiety; 1197(1197) cm-1, -CF2- asymmetric stretching; 1156(1156) cm-1, C–H aromatic out of plane vibration; 870-748(870, 851, 808, 764, 748) cm-1, [25, 26]. FT-IR spectra of roflumilast, used excipients and their binary mixtures with drug are presented in Figs. 8 and 9, respectively. FT-IR spectra of drug with excipients shown absence of chemical interaction as the binary mixtures showed all the characteristic peaks of the drug and excipients without any shifting of the peaks. LC–MS is a powerful tool that has very selectivity and sensitivity and is useful in pharmaceutical analysis. Mass spectra of RFL and its physical mixtures after IST studies are represented in Fig. 10. The full-scan negative-ion electrospray substance ion mass spectra showed a similar mass pattern to roflumilast and its physical mixtures (Fig. 10).
Fig. 3 Optical microscopic images of RFL, RFL–excipients physical mixtures at 1009
Compatibility study The optical microscopic images of RFL and RFL–excipient mixtures are shown in Fig. 3. In this study, pure drug, RFL shows presence of irregular lamellar crystals. Further, after grinding the drug with excipients drug did not retain the native crystal habit. In a 1:1 mixture of RFL and CCS, drug is adhered on the surface of CCS particles (Fig. 3). In RFL and MCC mixture ground particles of drug are adhered to the MCC particles mainly although some grounded larger particles are not adhered to the MCC particles (Fig. 3), whereas in the mixture of RFL
Evaluation of interactions between drug and excipient for stable dosage forms by a fast and accurate method to select the appropriate excipients is considered a real achievement in the preformulation stage. Therefore, DSC and TG curves of roflumilast and each of the used excipients were compared with those obtained from their physical mixtures at the first stage. Figures 4 and 5 illustrate the DSC and TG curves of RFL and investigated excipients, respectively, and
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0
Endo down
Fig. 4 DSC curves of RFL and excipients
RFL
Head flow/mW
5
10
CCS 15
MCC
MS 20
SSG
PS 25 26.86 30
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50
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100
C
80 60
Mass/%
Fig. 5 TG curves of RFL and excipients: croscarmellose sodium (A), microcrystalline cellulose (B), magnesium stearate (C), sodium starch glycolate (D) and pregelatinized starch (E)
E D A B RFL
40 20 0 –20 –40 50
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Tables 1 and 2 summarize the DSC and TG data of RFL, excipients and its physical mixtures, respectively, used in this study. In the DSC curve of excipients, these exhibited a
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shallow broad endothermic hump in the temperature range of 50–100 C due to heat of dehydration. An exothermic peak at around 276 C in croscarmellose sodium was
Physicochemical characterization and compatibility study of roflumilast with various… Table 2 TG data of RFL, excipient and RFL–excipient physical mixtures Samples
Mass loss/%
Onset/C
End/C
Inflection point/C
Midpoint/C
RFL
114.39
104.40
328.40
291.21
276.25
CCS
44.68
211.42
395.38
295.31
295.55
MCC
83.95
269.21
435.04
347.78
346.28
MS
87.04
271.42
450.57
350.80
356.91
SSG
55.74
210.29
411.69
279.53
285.14
PS
71.55
235.07
397.56
317.09
312.91
RFL–CCS
73.40
196.93
395.34
285.39
280.37
RFL–MCC
50.54
184.08
289.72
267.54
257.79
60.71
295.05
398.12
332.93
331.08
RFL–MS
95.76
201.77
396.97
299.99
285.63
RFL–SSG RFL–PS
70.43 115.17
155.28 151.10
356.81 401.02
287.45 277.46
279.29 279.43
observed. DSC curve of magnesium stearate showed two endothermic events in the 90–120 C temperature range followed by a small shoulder toward higher temperature which is characteristic for the dehydration process and probably due to the presence of the corresponding palmitate salt impurity [27]. TG data of excipients (Fig. 5) showed that maximum mass loss of individual excipient was occurred in the temperature range of 200–450 C. In majority of the cases, melting endotherm of pure drug was well preserved with slight changes in terms of broadening or shifting toward the lower temperature. It has been reported that the quantity of material used, especially in drug–excipient mixtures, affects the peak shape and enthalpy value. Thus, these minor changes in the melting endotherm of drug could be due to the mixing of pure drug and excipient, which lowers the purity of each component in the mixture and may not necessarily indicate potential incompatibility [28]. DSC curve of physical mixture of roflumilast and croscarmellose sodium (CCS) presented three events, one an endothermic broader hump at a temperature 53.64 C due to melting of croscarmellose sodium, second exothermic sharp peak at 80.24 C and third endothermic peak corresponding to the melting process of roflumilast at a temperature between 158.99 and 161.35 C with an enthalpy of 37.67 J g-1, because the reduction in enthalpy of melting endothermic peak observed was in the normal range due to the 1:1 (w/w) mixture of pure drug with excipients. The TG curve of RFL ? CCS showed 73.40% mass loss was observed between 196.93 and 395.34 C due to the slow decomposition of roflumilast. The DSC and TG curves of the physical mixtures demonstrated that there are some interactions in the thermoanalytical profiles of the drug (Figs. 6, 7).
DSC curve of roflumilast and microcrystalline cellulose (MCC) depicted an endothermic peak at a temperature 52.29 C with a heat of fusion 1.69 J g-1 and other endothermic peak corresponding to the melting of roflumilast between 158.98 and 160.84 C with peak at 159.91 C with an enthalpy value 41.69 J g-1. The decreased enthalpy value of roflumilast pure drug from 94.08 to 41.69 J g-1, because the reduction in enthalpy of melting endothermic peak observed was in the normal range due to the 1:1 (w/w) physical mixture of pure drug with excipients. TG curve of the physical mixture shows two mass loss patterns. The first mass loss exhibited 50.54% between 184.08 and 289.72 C due to the generation of stable volatile degradation product of roflumilast, and second mass loss was observed 60.71% from 295.05 to 398.12 C corresponding to the stable degradation of microcrystalline cellulose. This binary mixture showed physical interaction between these substances under condition of testing (Figs. 6, 7). The DSC curve of roflumilast and magnesium stearate presented endothermic events at 50.31–54.16 C temperature range with an enthalpy value of 1.62 J g-1 which is characteristic of roflumilast, followed by the endothermic event of roflumilast melting at 158.38–160.87 C and peak observed at 159.71 C which shows enthalpy value 48.16 J g-1, this enthalpy value increases comparative to other excipients like CCS and MCC, the upshift enthalpy value indicated a high degree of homogeneity which is shown in optical image (Fig. 2), another endothermic peak observed at 330.68 C due to decomposition of magnesium stearate. TG curve showed 95.76% mass loss observed between 201.77 and 396.97 C due to stable decomposition of drug and excipient. DSC and TG data suggested a lack of incompatibility between pure drug and magnesium stearate (Figs. 6, 7) [29].
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F. Ali et al. Fig. 6 DSC curves of RFL and its 1:1 (w/w) physical mixtures
–4.459
0
RFL 5
10
RFL + CCS
Head flow/mW
15
20
RFL + MCC
25
RFL + MS 30
40
Endo down
35
RFL + SSG
RFL + PS 45
50 51.29 30
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Temperature/°C
100
A
80
E RFL D C
60 40
Mass/%
Fig. 7 TG curves of RFL and its 1:1 (w/w) physical mixtures with croscarmellose sodium (A), microcrystalline cellulose (B), magnesium stearate (C), sodium starch glycolate (D), pregelatinized starch (E)
20 0
B
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Temperature/°C
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Physicochemical characterization and compatibility study of roflumilast with various…
Transmittance/%
100.0 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0.0 4000.0
RFL
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100.0 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0.0 4000.0
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Fig. 8 FT-IR spectra of RFL and excipients: croscarmellose sodium (CCS), microcrystalline cellulose (MCC), magnesium stearate (MS), sodium starch glycolate (SSG) and pregelatinized starch (PS)
PS
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F. Ali et al. 100.0 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0.0 4000.0
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Fig. 9 FT-IR spectra of RFL and its 1:1 (w/w) physical mixtures
RFL
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100.0 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0.0 4000.0
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100.0 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0.0 4000.0
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3600
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100.0 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0.0 4000.0
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3600
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100.0 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0.0 4000.0
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2000
1800
1600
Wavenumber/cm–1
Transmittance/%
100.0 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0.0 4000.0
RFL + PS
3600
3200
2800
2400
2000
1800
1600
Wavenumber/cm–1
123
x10 6
(a) 401.0277
3.5 3 2.5 2 1.5 1 0.5 0
(RFL)
150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950
Relative intensity/counts
Mass to charge ratios/m/z x10 6
(b) 401.0281 (RFL + CCS)
2.5 2 1.5 1 0.5 0
150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950
Relative intensity/counts
Mass to charge ratios/m/z x10 6
(c) 401.0283 (RFL + MCC)
2.5 2 1.5 1 0.5 0
150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950
Relative intensity/counts
Mass to charge ratios/m/z x10 6
(d)
2.5
401.0281
(RFL + SSG)
2 1.5 1 0.5 0 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950
Mass to charge ratios/m/z Relative intensity/counts
Fig. 10 Negative-ion mode electrospray mass spectrum of samples after IST a RFL; b RFL ? CCS; c RFL ? MCC; d RFL ? SSG e RFL ? PS
Relative intensity/counts
Physicochemical characterization and compatibility study of roflumilast with various…
x10 6
(e) 401.0284
2.5
(RFL + PS)
2 1.5 1 0.5 0 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950
Mass to charge ratios/m/z
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F. Ali et al. Table 3 Results of analysis of samples of drug content kept under IST after 3 weeks of storage Physical mixtures
Drug content/%a Control sample
Change in physical appearance Stressed samples
RFL–CCS
82.50 ± 0.012
81.57 ± 0.091
No
RFL–MCC
80.17 ± 0.052
78.31 ± 0.014
No
RFL–MS
98.72 ± 0.037
97.53 ± 0.130
No
RFL–SSG
77.34 ± 0.180
63.78 ± 0.026
No
RFL–PS
75.34 ± 0.064
73.43 ± 0.018
No
a
Value expressed as average ± standard deviation
In the DSC curve of roflumilast and sodium starch glycolate (SSG) physical mixture, a broad exothermic event at 65.13–74.99 C, peak observed at 70.97 C with an enthalpy value -411.53 J g-1, followed by exothermic peak at 102.90 C, enthalpy value was found -179.74 C, the melting endothermic event of roflumilast was observed between 159 and 160.85 C, peak observed at 159.89 C with heat of fusion 29.63 J g-1. TG curve showed 70.43% mass loss observed between 155.28 and 356.81 C due to stable decomposition, and the inflection point and midpoint were observed at 287.45 and 279.29 C, respectively. It can be stated that the sample was stable up to 155.28 C, followed by thermal decomposition which affects the thermal stability of roflumilast. Therefore, results indicate the presence of interaction between binary mixtures of drug and excipients (Figs. 6, 7). DSC curve of the physical mixture of roflumilast with pregelatinized starch exhibited the characteristic roflumilast fusion peak at 51.97 C with an enthalpy value of 1.91 J g-1, followed by sharp melting endothermic event between 158.66 and 160.89 C, peak observed at 159.92 C with an enthalpy of 34.15 J g-1 which shows that the melting behavior of drug was almost not affected in the presence of excipient, other broad endothermic peak at 292.52 C with accompanying enthalpy value of 74.83 J g-1 which shows the complete decomposition of physical mixtures. TG curve showed 115.17% mass loss observed between 151.10 and 401.02 C, these results affect the thermal stability of RFL, and thermoanalytical profiles of mixture can be considered as a proof of physical interaction between roflumilast with the used excipient (Figs. 6, 7). The thermoanalytical investigations (TG and DSC) did not reveal any interaction between roflumilast and magnesium stearate but presence of physical interactions between RFL–CCS, RFL–MCC, RFL–MS, RFL–PS. Physicochemical interactions between drugs and excipients do not necessarily indicate incompatibility, but overall
123
there was a change in DSC curves which is an undeniable evidence of interaction [29]. The obtained values from the heat of fusion were compared statistically by using an ANOVA (analysis of variance). The 1% significance test showed composition of mixtures influences the value of enthalpy changes (p \ 0.00011). The Scott–Knott test was performed to compare the mean values (data not shown), and a statistically change in the enthalpy was observed for physical mixtures, such as RFL–CCS, RFL–MCC, RFL–SSG and RFL–PS, which may indicate some interaction (Figs. 8–10). The data of DSC studies showed some proof of physical interaction due to changes in enthalpy value, peak shape and shifting toward lower temperature. These physical interactions that arise at higher temperature do not necessarily show incompatibility between the drug and inactive ingredients [30]. These physical mixtures should be further exposed to IST studies to confirm the results obtained in DSC studies. The data of IST studies are tabulated in Table 3. Less than 2% loss of roflumilast content was observed for all the stressed physical mixtures except RFL– SSG which showed 14%, indicating incompatibility between them due to the appearance of new peak at 4.092 min with 3.5% area of the RFL standard. The RFL excipient physical mixtures did not show any change in color and physical appearance. The slight proof of interaction in case of RFL–CCS, RFL–MCC and RFL–PS physical mixtures due to DSC curve was found misleading after the IST studies as there was no change in physical appearance of the stressed samples and change in the roflumilast content values was also less than 2%. The HPLC chromatogram of RFL was observed at about 8 min as represented by Fig. 11. However, the HPLC chromatograms of RFL–CCS, RFL–MCC, RFL–SSG and RFL– PS showed unchanged retention time and peak shape of RFL. So, it can be concluded that there was no incompatibility between RFL and all excipients used in this study except with SSG.
(a)
7.956
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
(RFL)
8.00
900
Roflumilast
1.00
2.00
3.00
4.00
5.00
6.00
7.00
10.00
11.00
12.00
11.00
12.00
11.00
12.00
11.00
12.00
11.00
12.00
11.00
12.00
AU
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 –0.05
7.930
Time/min
(b)
(RFL + CCS)
Roflumilast
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
900
10.00
Time/min
AU
0.20
8.003
0.25
(c)
(RFL + MCC)
0.15
Roflumilast
0.10 0.05 0.00 1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
900
10.00
AU
0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 –0.05
7.932
Time/min
(d)
(RFL + MS)
Roflumilast
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
900
10.00
0.25
(e)
(RFL + SSG)
Roflumilast
0.15 Degradation product 1
0.10
4.092
AU
0.20
7.906
Time/min
0.05 0.00 1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
900
10.00
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
7.893
Time/min
AU
Fig. 11 Chromatogram of samples after IST: a only RFL; b RFL ? CCS; c RFL ? MCC; d RFL ? MS; e RFL ?SSG; f RFL ? PS
AU
Physicochemical characterization and compatibility study of roflumilast with various…
(f)
(RFL + PS)
Roflumilast
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
900
10.00
Time/min
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F. Ali et al. Table 4 Characteristic wave numbers of functional groups corresponding to roflumilast Wave number/cm-1 (roflumilast)
Peak type
Assignments
3254
m, s
N–H stretching vibrations in secondary amines
3134-3011
w, s
C–H stretch vibration in aromatic group and cyclopropyl
2939-2878
w, s
-CH2- aliphatic symmetrical ? asymmetrical stretching vibration
1654
s, s
C=O stretching vibration in the amide moiety
1595 ? 1502
m, s ? s, s
C=C aromatic stretching vibration
1558
m, s
N–C=O symmetrical stretching vibration in amide moiety
1464
w, s
-CH2- deformation vibration
1197
s, s
Symmetrical, stretching vibration of C–O–C in the cyclopropyl moiety
1156
s, s
-CF2- asymmetric, stretching vibration in the –OCHF2 moiety
870-748
m, s
C–H aromatic out of plane vibration
m, s medium, sharp; s, s strong, sharp; w, s weak, sharp; m, b medium, broad; s, b strong, broad; w, b weak, broad
Conclusions
References
Drug–excipient compatibility studies were conducted with some selected excipients in order to guide formulating tablet dosage form of roflumilast. DSC and TG were used as a fast technique to evaluate the compatibility of excipients with the drug. Roflumilast was fully characterized using thermal analysis (DSC and TG), IR, optical microscopy and LC–MS. The DSC analysis showed that roflumilast was compatible with magnesium stearate and that some physical interactions were found with croscarmellose sodium, microcrystalline cellulose, sodium starch glycolate and pregelatinized starch. On the other hand, results from IST showed no drug–excipient(s) interaction with a little loss in RFL content from stressed RFL–CCS, RFL–MCC, RFL–MS and RFL–PS physical mixtures was observed. Also results from LC–MS showed no extra peak observed. However, the incompatibility of RFL with sodium starch glycolate observed from DSC curve was confirmed by the HPLC. Results obtained from HPLC showed that a degradation product was formed and was represented by a peak observed at 4.09 min indicating a chemical interaction between RFL and SSG (Fig. 10). Based on our results, all excipients used in this study were found to be compatible with roflumilast except sodium starch glycolate (Table 4).
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Acknowledgements One of the authors Faraat Ali is grateful to Indian Pharmacopoeia Commission (IPC), Ministry of Health and Family Welfare, Government of India, India, for providing necessary instrumental facilities. All the authors have no conflict of interest regarding this publication. This article does not contain any studies with human and animal subjects implemented by any of the authors.
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