Figure 5.3: Erosion profiles of pure G50/13 and caffeine dispersions in G50/13 at different drug loadings
Chapter 6: Ageing studies with DSC
N. B Endothermie heat flow for all the Figures is in the downwards direction The thermal profiles are vertically offset for clarity.
Chapter 6: Ageing studies with DSC.... 180
20°C could have stabilised the polymorphic forms o f Peak 1 but accelerated the transformation o f certain Peaks 3 and 4 polymorphic forms from Peak 2 forms. The contradictory effects of storage on the polymorphic forms of various peaks could possibly be due to the difference in the gelucire components that constitute each peak. Certain polymorphic forms which are not the most stable form of a particular glyceride can be very unchanging under certain conditions, such as that shown by the P -2 in the HMF of milk fat which remained unaltered after storage at 40°C for 31 days even though the other polymorphic forms of the rest of the fractions were transformed (Timms, 1980). Polymorphic forms which make up Peak 1 were regarded as metastable as they had the lowest melting range but at 20°C, could be very stable and were not tending towards Peak 2. Another explanation for this effect of the reduction in Peak 2 heat of fusion value is that some o f the components making up the gelucire form that gave rise to that peak had segregated out and are now contributing to the composition o f the other peaks, possibly by forming solid solutions with them. The increase in the crystallinity of the gelucire forms that gave rise to Peaks 1, 3 and 4 could be an exclusive process that was not related to polymorphic changes, as had previously been shown for substances containing mixtures of glycerides (Yoshino et al, 1984). Therefore the increase in the heats of fusion was a total effect o f polymorphic transformation and elevation in crystallinity.
Storing the pure G50/13 samples at 37°C showed a drastic change in its thermal profile whereby Peak 1 corresponding to the freshly prepared sample had now fused with the next peak so that a peak temperature could only be observed as that for Peak 2 (Figure 6.2). Binary mixtures of triglycerides including tripalmitin and tristearin also demonstrated a reduction in the number o f transitions on the thermal profiles upon ageing with an eventual melting point emerging (Liversidge et al, 1981). The elevation o f temperatures for the other peaks after 30 days at 37°C suggests that the gelucire forms were becoming increasingly stable. The fusion of lower melting form to Peak 2 could mean that the polymorphic forms of Peak 1 had transformed to the higher melting, more stable forms o f Peak 2. This transformation could be of the melt type because the temperature of storage coincides with the melting point of Peak 1. Holding the sample at or near the melting temperature of a less stable polymorph will transform it to a more stable form and this is a concept capitalised by the tempering process.
C h a p ter 6: A geing stu d ies with D SC.... 181
Figure 6.2: Thermal profiles of pure G50/13 after ageing at 37°C
-0.5 f
I
;
- >- 5-
ê
fresh 7 days - - - 30 days 90 days 180 days -2.5 temperature (®C)Chapter 6: Ageing studies with DSC.... 182
Tempering had been reported to greatly affect the rate of phase change, crystal size and especially the rate of polymorphic transformation (Timms, 1984). The heat of fusion value for the merged form became increasingly higher over time compared to the combined heat of fusion values of Peaks 1 and 2 of the freshly prepared samples. Also, the percentage increases in the total heat of fusion values for samples aged at 37°C were higher than those aged at 20°C. In addition to the increase in crystallinity, the higher values for 37°C stored samples were likely due to the bigger amount of the more stable polymorphic forms as these forms have the higher heat of fusion values. A conversion to glycerides stable forms was obtained after storage at more elevated temperatures (Yoshino et al, 1984).
The storage of fatty suppositories also demonstrated similar small shifts in the position of the peak temperatures upon ageing of up to 6 months at room temperature, regardless of the initial peak positions (Coben and Lordi, 1980). In addition, there was sharpening of the peak and an increase in peak height throughout the duration o f the storage. Storing the samples at temperatures just below their melting ranges were also found to have accelerated the reaching of their equilibrium crystalline state.
Liversidge et al (1981) attributed the increase in the melting points of triglyceride suppositories upon ageing to the polymorphic transformation of metastable forms to stable forms. The most significant change to the melting profile when stored at room temperature occurred within the first day, followed by slower increases on the successive days until a final maximum value was achieved. Elevating the temperature of storage minimized the time taken to achieve this final value due to the energy supplied to the molecules aiding their orientation into the stable polymorphic forms. When this storage temperature exceeded the melting point o f the base, it was not able to stabilize itself in its most stable form in the time allocated until analysis so that the base would have a very low melting point when resolidified. It was important therefore, for the current study to put aside enough time (24 hours) from the time the samples were taken out from their storage environs to the time o f their analysis, in order to allow the samples to equilibrate at room temperature.
The thermal profiles of the pure G50/13 after being aged at 37°C were similar to the thermal profile o f non-heat treated sample o f G50/13, that is the sample in the condition
Chapter 6: Ageing studies with DSC.... 183
received from the manufacturer without first subjecting it to the heating and cooling protocol (Figure 6.3). Figure 6.3 was obtained from a comminuted whole batch of G50/13 and is different from Figure 2.3 which was obtained from different positions within a batch. The peak temperatures of the untreated G50/13 were close to those of the aged samples and a total heat of fusion value similar to the untreated sample was achieved by the samples stored at 37°C by 30 days of ageing. This gave an indication that pure G50/13 aged at 37°C was relatively stable.
F igure 6.3: T h erm al p rofile o f u n treated 0 5 0 /1 3
-
0.2
- - -0.4 - I ® -0.6 ■ ■I
tem peratu re (®C)The transition to more stable forms o f fat matrices was shown to be accelerated at higher temperatures o f storage (Yoshino et al, 1981). Furthermore, ageing at 37°C offered an advantage over untreated G50/13 in that the unstable form that gave rise to Peak 1 was no longer present in the former samples. This could ensure that no great changes in polymorphic forms or stable/unstable forms would occur further after storage at long periods o f time. Even though Peak 1 was not as distinctive in untreated G50/13 samples as heat treated ones, a clear shoulder on the lower temperature side of the main peak suggested that there still existed less stable forms which could be susceptible to modifications in the untreated samples. Therefore, even though storage at 37°C can be referred to as an accelerated ageing condition, the outcome after 180 days was not totally coincidental with the untreated sample but this minor difference may be beneficial to the matrices.
Chapter 6: Ageing studies with DSC.... 184
Work performed by Sutananta et al (1994b) postulated that the numerous endothermie peaks on a 050/13 thermal profile were due to the segregated gelucire components. The number o f peaks evolved was also dependent on the cooling rate used when setting the base with fast cooling (in liquid nitrogen) producing more peaks than slow cooling (10°C/hour). On storing the samples at room temperature, the thermal profiles changed even after only two weeks with new peaks emerging and the existing peaks becoming more distinct. After 280 days, the thermal profiles for both of the samples were still non-equivalent considered to be caused by the components being far too segregated to be recombined into an equilibrium structure. However, this current study showed that tempering the 050/13 at a temperature just at the first peak resulted in the structure that resembled more closely to the equilibrium structure (represented by the untreated 050/13) to be achieved more quickly. Changes as a result of tempering usually suggest that polymorphic transformation is involved. Moreover, bases which contain mixed glycerides and other constituents tended to be crystallised in a metastable form.
There was probably less of the a form of the triglycerides crystals than the p' form in this metastable crystal mixture as the gelucire is composed of previously hydrogenated oils. Hydrogenation increased the ordering of molecules and so, facilitated the crystallisation of the P' form over the less ordered CL as demonstrated by the DSC scans when the lower melting peak of palm stearine was gradually replaced to an extent by a slightly higher melting peak as the hydrogenation times increased (Busfield and Proschogo, 1990). On tempering at 37°C, sufficient energy was probably supplied in order for the transformation to the more stable p forms of the glycerides to occur. In a study with G43/Q1 which contained only triglycerides, the investigators concluded that in order for a stable form to be obtained, a tempering temperature that was approximately 9°C below the desired peak needed to be used (Sutananta et al, 1994b). However, on closer examination, this suggested temperature also happened to be in the vicinity o f the first endothermie peak, with the desired stable peak being the second endotherm. Therefore, for the most stable form to develop upon ageing, it would be necessary to hold the gelucires for a certain time period at a temperature which is close to the first endothermie peak o f the thermal profile, not just below the largest and presumed most stable peak. Achieving a desired form by tempering is not restricted to the carrier only but also to the drugs. Holding gepirone hydrochloride at
Chapter 6: Ageing studies with DSC.... 185
a temperature just below the first endotherm of its thermal profile transformed it to a higher melting, more stable form and holding at this temperature for a longer period finally transformed even this form to the most stable form (Behme et al, 1985).
6.3.2 10% paracetam ol dispersion in G50/13
No major changes could be seen in terms of the peak temperatures for most of the duration of the ageing. Only after 180 days storage at 20°C it could be distinguished that Peaks 1 and 2 had slight elevations of temperatures and heat of fusion values whilst Peak 3 had small declines, giving rise to the observation that the peaks were moving towards the centre of the thermal profile after a long period of ageing (Figure 6.4). The slight decline for Peak 3 could signal the exclusion of certain high melting components from the gelucire forms that gave rise to this peak after 180 days. The total heat o f fusion after various periods of ageing fluctuated but this value after 180 days was significantly higher (tested with Student’s two-sample t-test at 0.05 significance level) than the freshly prepared sample, which like the pure G50/13, suggests an increase in the crystallinity o f the sample.
Storing at 37°C changed the thermal profile significantly and also caused certain peaks to split further (Figure 6.5). After 7 days ageing, a higher melting form emerged to give a subpeak to Peak 1. It could be postulated that storage at this temperature had caused the transformation of many components within Peak 1 to their higher melting polymorphs and this change was big enough to not only push the temperature of Peak 1 to be higher but also to produce another form of this dispersion. The Peak 4 form could have developed a mixed crystals aggregate with the Peak 3 form since it had now been reduced to a shoulder of Peak 3. After 30 days ageing, the subpeak to Peak 1 seemed to have merged with Peak 2 and in addition, there now emerged a higher melting subpeak to Peak 2 which was then reduced to a shoulder after 180 days. Peak 1 became progressively less prominent until it fused with Peak 2 after 180 days.
The classification of subpeaks is only arbitrary as subpeak 1 and Peak 2 of 7 days aged sample could be Peak 2 and subpeak 2 o f 30 days aged samples respectively. They were labelled according to the main peak which the subpeak temperatures were closest to. The
Chapter 6: Ageing studies with DSC.... 186
Figure 6.4: Thermal profiles of 10% paracetamol dispersions in G50/13 after ageing at 20°C -0.5 - -2 - - fresh — - 7 days - 30 days — 90 days — 180 days -2.5 -- temperature (®C)
Chapter 6: Ageing studies with DSC.... 187
Figure 6.5: Thermal profile of 10% paracetamol dispersions in G50/13 after ageing at 37®C -0.5 f r"- JS -2
+
fresh 7 days - - - 30 days 90 days 180 days ■2.5 + temperature (®C)Chapter 6: Ageing studies with DSC.... 188
formation o f subpeaks was due to the storage temperature being 37°C because this occurrence was not observed for samples stored at 20°C. The increase in the total heat of fusion values to be higher than 20°C stored samples reflected the gain in crystallinity and the transformation to more stable forms. This transformation to higher stability forms was further evidenced by the movement of the subpeak towards the higher temperature ranges. The thermal profile o f milk fat showed that the melting peak o f the high melting fraction was very wide and thought to be partly due to this fraction crystallising out together with some lower melting glycerides in a semi-stable mixture (Timms, 1980). When the fat was tempered, the solid solutions were able to isolate out into their equilibrium compositions and proportions at this temperature. In G50/13, the metastable forms seen on the addition of paracetamol were eventually transformed closer to an equilibrium composition after storage at 37°C.
This modification to the higher melting, more stable forms occurred more gradually for the 10% paracetamol dispersion samples than for the pure 050/13. Peak 1 fused with Peak 2 after only 7 days ageing at 37°C for the pure 050/13 sample but Peak 1 o f the paracetamol dispersion sample decreased in amount slowly while at the same time giving rise to new, higher melting subpeaks. It could be stated that the presence of paracetamol caused a resistance to the transformation to more stable forms, even after a tempering process. It had been reported that during transformation, the glycerol group structure o f glycerides remain unaltered but the lateral arrangements of the hydrocarbon chains in the dimeric units become dissimilar (Timms, 1984). Thus, it could be possible that the paracetamol molecules were disrupting these lateral arrangements of the hydrocarbon chains.
6.3.3 10% caffeine dispersion in G50/13
A small increase in the peak temperatures and the total heat o f fusion values after being aged at 20°C indicated the gradual transformation to more stable forms as with previous samples (Figure 6.6). The rate at which the fusion value for Peak 1 was depreciating with the concurrent increase in the value of Peak 2 was higher than for the pure 050/13 samples. By 180 days ageing. Peak 1 of the caffeine dispersion samples had already been reduced to a shoulder on the main Peak 2, giving a profile which is similar to the untreated 050/13.
Chapter 6: Ageing studies with DSC....189
Figure 6.6: Thermal profiles of 10% caffeine dispersions in G50/13 after ageing at 20®C -0.5 f -1.5
I
I
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— fresh —• 7 days ' - 30 days — 90 days — 180 days -2.5 T temperature (®C)Chapter 6: Ageing studies with DSC.... 190
Storage at 37°C also caused an increase in the peak temperatures but to a bigger extent than storage at 20°C. An elevation in the melting ranges of drug incorporated fatty suppositories upon storage at a higher temperature than ambient had been demonstrated by de Blaey and Rutten-Kingma (1976). The rearrangement of components within the gelucire forms that gave rise to Peaks 3 and 4 could have lead to the formation of solid solutions within the structures, thus no longer presented as separate peaks but as a common, expanded endotherm after 180 days (Figure 6.7). As expected, the total heat of fusion values increased after ageing for the same reasons postulated for pure 050 /1 3 and 10% paracetamol dispersion samples. A decrease in Peak 1 heat o f fusion value together with an increase for the Peak 2 value was a similar event to the sample aged at 20°C, but at a slower rate than pure 050/13.
Overall, caffeine dispersions in 050/13 seemed to be the least affected samples after ageing, either at 20°C or at 37°C. Caffeine seemed to resist the transformation of the metastable forms to the more stable ones when held at 37°C more successfully than paracetamol as even after 180 days ageing. Peak 1 was still present and not fused to the bigger Peak 2. A possible explanation could be that the temperature o f Peak 1 of the freshly prepared 10% caffeine dispersion sample was the highest amongst all the freshly prepared samples and closest to the storage temperature o f 37°C, so that there was less Peak 1 melt to transform than the other samples. Transformation had to proceed through a limited melt- transformation process and the slower solid-solid transformation.
6,3,4 Solid Fat Content (SFC)
As previously stated in Chapter 2, there are three phases to the SFC profile o f pure G50/13 and can be described as Low Melting Fraction (LMF) from 28°C to about 37°C, Middle Melting Fraction (MMF) from about 37°C to 46°C and High Melting Fraction (HMF) from about 46°C to 60°C. It was observed that pure G50/13 samples which were aged at 20°C had a lower quantity of LMF but a higher quantity of HMF than freshly prepared samples (Figure 6.8). The quantity o f MMF was similar amongst the samples and since this fraction represented the majority o f the matrix, it suggested that storage at this temperature did not alter the melting behaviour to a great extent. At any given temperature, the amount of solid
Chapter 6: Ageing studies with DSC.... 191
Figure 6.7: Thermal profiles of 10% caffeine dispersions in G50/13 after ageing at 37®C -0.5 - ^ -1.5 -- -2 - - — fresh — - 7 days ' - 30 days — 90 days — 180 days -2.5 -- temperature (®C)
Chapter 6: Ageing studies with DSC.... 192
F igure 6.8: S olid F a t C on tent o f p u re G 50/13 a fter ag ein g a t 20®C 100 fresh 7 days 30 days 90 days — 180 days 90 - 80 - 70 - a 50 -- ^ 40 -- 30 -- 20 - 10 - - 25 30 35 40 45 50 55 60 tem peratu re (®C)
F igure 6.9: S olid F a t C on tent o f p u re G 50/13 a fter a g ein g a t 37®C
100 90 -- fresh 7 days 30 days — 90 days — 180 days 80 -- 70 -- i 60 - 3 5 0 - (A ^ 40 -- 30 -
20
- -10
- - 25 30 35 40 45 50 55 60 temperature (®C)Chapter 6: Ageing studies with DSC.... 193
fat was slightly higher for the aged samples than for the freshly prepared samples (with the exception of samples aged for 30 days between 36.5°C and 46.5°C). Ageing at 37°C decreased the LMF and increased the HMF vastly, with the percentage of the matrix remaining as solid at any particular temperature being elevated with the period of ageing (Figure 6.9). This suggests that ageing at this temperature increased the hardness of the