Thermoanalytic techniques, such as DTA, are widely used to detect incompatibilities in a short period of time, with the curve of the mixture being a result of the individual curves of each substance analyzed. The suppression, disappearance, appearance, or displacement of thermal events, as well as variation in the expected values of enthalpy, should be considered as possible incompatibility. These parameters were used to evaluate the binary mixtures described below [43
The DTA curves of the binary mixtures of the AMX plant extract and the pharmaceutical excipients are shown in Figure 2
, Figure 3
and Figure 4
, and the data are described in Appendix A
). Most of the mixtures showed significant variations of their thermal profiles. The aspartame thermoanalytical curves (Figure 2
a) recorded a shift of the melting temperature of the pharmaceutical excipient (249.17 °C) at all mixing ratios—1:1 (235.55 °C), 1:2 (237.58 °C), and 2:1 (234.78 °C)—suggestive of physical incompatibility. Variations of the energy involved in these thermal events were also observed and predicted according to the proportion of the mixture [29
Signs of AMX-pharmaceutical excipient incompatibility were observed with carbopol (Figure 2
b), in the curves of which a drastic reduction of the energy involved in the stages of the decomposition of CBP was observed, leading to the disappearance of these peaks. This can be considered as a physical incompatibility, despite the presence of AMX characteristics in the thermal profiles of the mixtures [44
The thermal curves of the mixtures with the carboxymethylcellulose (Figure 2
c) and the colloidal silicon dioxide (Figure 2
d) demonstrated the preservation of the characteristics of both AMX and the pharmaceutical excipients, thus indicating no apparent physical incompatibility between them.
According to Figure 3
, physical incompatibility was also possible in the AMX-fructose mixtures (Figure 3
a). It shows displacement of melting temperatures (107.92 °C) and decomposition (210.13/279.24 °C) of the FRU at the ratio 1:1 (126.09/195.05/222.28 °C), anticipation of thermal events at 1:2 (101.92/149.76/237.24 °C), and the junction of all events at 2:1, resulting in a single high-intensity endothermic peak (ΔH
= −294.63 J·g−1
) at 121.37 °C [29
For the AMX-hydroxyethylcellulose mixture (Figure 3
b), suppression of the second HEC decomposition event (364.60 °C/ΔH
= 10.15 J·g−1
) was observed at 1:1 and 1:2, and a temperature delay of this event was seen at 2:1 (383.22 °C) with suppression of the first HEC decomposition event (343.28 °C), suggesting possible incompatibility, despite the presence of thermal characteristics of the AMX in the mixtures. These characteristics were also represented in the binary mixtures with hydroxypropyl methylcellulose (Figure 3
c), in which no apparent alterations were observed in the thermal profiles that characterize incompatibility with the AMX.
Analyzing the AMX-lactose curves (Figure 3
d), we were able to verify some evidence of physical incompatibility. Thermal analysis of the 1:1 ratio mixture showed a change in the temperature and melting energy of the LAC in the mixture, from 219.34°C (ΔH
= −226.34 J·g−1
) to 207.32°C (ΔH
= −4.14 J·g−1
), as well as the disappearance of the second decomposition event of this excipient (304.85 °C/ΔH
= −137.36 J·g−1
]. Displacement at this temperature was also found in ratios 1:2 (211.31 °C) and 2:1 (210.83 °C), in addition to the suppression of decomposition events of the pharmaceutical excipient (241.72/304.85 °C).
Two endothermic events characterized the thermal profile of mannitol (Figure 3
e): melting (169.89 °C/ΔH
= −450.53 J·g−1
) and decomposition (321.83 °C/ΔH
= −812.74 J·g−1
). The melting point found is close to the one described in the literature, at 166–168 °C [45
]. Signals of AMX-MAN incompatibility were verified in the binary mixtures, with displacement of these events at 1:2 (165.63/304.15 °C) and 2:1 (162.94/325.73 °C). At the 1:1 ratio, the anticipated melting temperature was observed, along with a drastic energy decrease (163.10 °C/ΔH
= −50.80 J·g−1
) and the disappearance of the peak decomposition of the pharmaceutical excipient (321.83 °C).
For magnesium stearate (Figure 4
a), the endothermic AMX moisture loss peaks (98.67 °C/ΔH
= −283.69 J·g−1
) and melting point (129.00 °C/ΔH
= 255.25 J g−1
) melted, resulting in a single low energy peak in the 1:1 ratio mix (127.00 °C/ΔH
= −71.44 J g−1
). Commercial samples of magnesium stearate have a melting range of 117-150 °C [29
]. Still in the ratio 1:1, the magnesium stearate decomposition event (319.75 °C/ΔH
= −379.99 J·g−1
) was seen to be suppressed. In addition, displacement of this temperature was observed at the 1:2 mixture (341.03 °C/ΔH
= −204.32 J·g−1
), indicating physical incompatibility in the AMX-MST mix.
In the literature, a possible explanation put forth for incompatibility with magnesium stearate is its chemical nature, a mixture of organic salts formed by magnesium cations and anions from different fatty acids that can generate chemical reactions with the active compounds by forming new products of degradation [46
Saccharin also showed evidence of physical incompatibility with AMX (Figure 4
c). Saccharin salts have a melting temperature above 300 °C; the melting temperatures determined in the mixtures still remained close to that of the excipient (365.15 °C/ΔH
= −210 J·g−1
), but the energy spent in these thermal events was significantly reduced at 1:1 (356. 32 °C/ΔH
= −26.91 J·g−1
), 1:2 (357.94 °C/ΔH
= −65.39 J·g−1
), and 2:1 (356.49 ° C/ΔH
= −18.02 J·g−1
Polyvinylpyrrolidone K-30 (Figure 4
b) and talc (Figure 4
d) maintained their thermal characteristics in a mixture with AMX, demonstrating that no changes occurred that were not predicted.
In summary, according to the results of this study, CSD, HPMC, CMC, PVP, and TAL were the pharmaceutical excipients that showed no evidence of physical incompatibility with the AMX. The binary mixtures were then submitted to FTIR for confirmation of possible incompatibilities.
PAPI-excipient compatibility studies are important tools that predict possible reactions between the formulation components that may occur during the storage period under storage conditions. The combination of thermal techniques (such as DTA) and non-thermal techniques (such as FTIR) is successful in identifying and confirming incompatibilities. Since these techniques present different work fundamentals and vary the analysis time, amount of sample, and mechanical and/or thermal stress employed, the results obtained are complementary and provide different conclusions. However, in the absence of evidence of interaction, compatibility should be confirmed by FTIR [43
The FTIR spectra of the AMX extract and their binary mixtures are set forth in Figure 5
, Figure 6
and Figure 7
. According to the FTIR spectrum obtained from the AMX (Figure 5
), a wide and intense absorption band is visible between 3800 and 3000 cm−1
, suggestive of O–H stretching. These hydroxyls may be related to the phenolic compounds and moisture content present in the extract, since it is an amorphous and hygroscopic sample. In addition, the presence of organic compounds in the extract was characterized in the spectrum by the occurrence of a low-intensity peak in the range between 3000 and 2850 cm−1
, indicative of C–H stretching. Acute peaks observed between 1600 and 1475 cm−1
are possibly related to C=C bonds of aromatics. Near this region, characteristic peaks of C–H were observed for folding between 1450 and 1375 cm−1
, as well as peaks suggestive of C–O stretching in the range of 1300 to 1000 cm−1
. Between 788 and 674 cm−1
, narrow peaks of a low intensity were visualized, probably related to C–H bonds in substituted aromatics. These chemical bonds refer to a variety of functional groups—such as ethers, esters, and carboxylic acids—that accompany the chemical composition of flavonoids, tannins, anthraquinones, and other secondary metabolites present in the extract [16
Indices of AMX-pharmaceutical excipient chemical incompatibility were evaluated, taking into account the emergence of new bands, as well as changes in the absorption ranges and/or intensity of the characteristic peaks of the extract and excipient in the binary mixtures [27
The spectra of the binary mixtures of AMX with aspartame (Figure 5
a) did not show any apparent changes in the spectroscopic profile of the plant extract; however, the characteristic peaks of the pharmaceutical excipient were predominantly observed in all the analyzed spectra, even at a ratio with a higher proportion of the extract (2:1). The peaks related to the AMX were not very pronounced throughout the spectra, due to the overlap of absorption bands of the pharmaceutical ingredient. The overlapping bands are not considered a parameter that is indicative of chemical incompatibility [49
]. The decrease in peak intensity, observed in the 1:2 and 2:1 mixtures, may be attributed to possible changes in the geometric mixture of the components, since this reduction occurred on the spectrum as a whole. Therefore, there was no evidence of chemical incompatibility between AMX and ASP.
With respect to the mixtures of AMX and carbopol (Figure 5
b), representative absorption bands of the plant extract could be visualized in all mixing proportions, with overlapping rare peaks. As expected, the intensity of the bands was also found to vary between the mixtures, according to the proportion. The same behavior was observed in mixtures of AMX and carboxymethylcellulose (Figure 5
c). Therefore, it is clear that no evidence shows that AMX interacted chemically with these pharmaceutical excipients.
The binary mixtures of AMX-colloidal silicon dioxide (Figure 5
d) showed spectra consistent with the characteristics of each of the components of the mixture. However, the intensity of the peaks related to the extract was reduced. This can be explained by the presence of a broad and intense band at 1080 cm−1
, suggestive of Si–O–Si bonding of the siloxane groups of the pharmaceutical ingredient, which promotes overlapping and attenuation of the other spectral bands. A chemical compatibility can be seen between AMX and CSD.
Indices of chemical compatibility were also determined in spectra of the AMX mixtures with fructose (Figure 6
a), hydroxyethylcellulose (Figure 6
b), hydroxypropyl methylcellulose (Figure 6
c), lactose (Figure 6
d), and mannitol (Figure 6
e). The obtained spectra resulted from a sum of characteristics of the extract and the pharmaceutical ingredient evaluated.
For mixtures with FRU (Figure 6
a), the intensity of the AMX peaks was shown to be positively influenced by a greater amount of extract in the mixture (2:1), as predicted. In a similar way, the excipient at 1:2 was verified, in which the intensity of the peaks related to FRU increases. In mixtures with HEC (Figure 6
b), a different behavior was noted. The total peaks resulting from the mixture are reproducible between the different proportions, and their intensities increase (compared to 1:1 mixture) when the amount of the extract (2:1) or the excipient (1:2) is increased; in other words, 1:2 and 2:1 present similar bands of intensity, showing that both the AMX and the pharmaceutical excipient have an equal influence on the spectrum of the mixture.
The spectra of the AMX-hydroxypropylmethylcellulose mixture (Figure 6
c) expressed opposite-than-expected effects with the increasing proportions of extract and excipient in the binary mixtures: the 1:1 proportion mixture had higher peak intensities than those from mixtures with higher amounts of excipient (1:2) and extract (2:1). Band fusion may have occurred at the regions of absorption common to the components of the 1:1 mixture, which could not be seen at the other proportions.
A similar profile to that observed in the HPMC mixture was repeated for lactose (Figure 6
d), mannitol (Figure 6
e), polyvinylpyrrolidone K-30 (Figure 7
b), saccharin (Figure 7
c), and talc (Figure 7
d) in mixtures with AMX. Generally, the spectra retained the identity of the AMX, as well as those of the pharmaceutical excipients; no displacement of the characteristic absorption regions of the mixtures’ components was observed. Some bands related to the extract were superimposed because the region of spectral absorption was with the same as that for the excipients; however, the intensity of the absorption bands was verified between the mixtures within what was expected or could be explained. Thus, no evidence was shown for chemical incompatibility between these pharmaceutical excipients and AMX.
For mixtures with magnesium stearate (Figure 7
a), the intensity of the AMX and excipient peaks were shown to be directly related to the component concentration in the mixtures: AMX characteristic absorption bands were most evident at the 2:1 proportion mixture, and characteristic peaks of the MST were more intense at the 1:2 proportion mixture, as expected.
It is important to note that for the AMX-talc mixtures, the peak intensity of the extract significantly changed, as observed for the AMX-colloidal silicon dioxide mixtures. Once again, this phenomenon can be attributed to the siloxane groups in the pharmaceutical excipient, the presence of which causes an intense absorption at 1080 cm−1
, thus promoting attenuation of the other spectral bands [50
Therefore, the evaluation of the binary AMX-pharmaceutical excipient mixtures by FTIR made it possible to determine that all the excipients studied did not show evidence of chemical incompatibility with the AMX plant extract.