Real time PCR - European Pharmaceutical Review

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THERMAL ANALYSIS ISSUE 2009 system, TA Instruments LLC) was employed to quantify the extent of any disorder. The use of IGPC for amorphous content has become popular and the technique offers many advantages 6 . Briefly, in a gas-perfusion experiment the sample is held in an ampoule while a carrier gas flows over the sample. The relative vapour pressure (RVP) of the gas is controlled by proportional mixing of dry (0% RVP) and wet (100% RVP) gas streams. The system can thus be programmed to follow a specific RVP program. IGPC is convenient for quantifying amorphous material; the sample is held dry and allowed to reach thermal equilibrium. The RVP is then increased (usually to ca. 90%) which results in solvent uptake by the sample. This plasticises the sample, lowering the glass transition temperature and causing recrystallisation. The RVP is then returned to zero, which dries the sample. Subtraction of the areas under the wetting and drying peaks thus indicates the presence of any irreversible events. A calibration curve of amorphous content versus heat of crystallisation was prepared by mixing proportional mass ratios of the crystalline and amorphous SS samples. Full experimental details are available upon request. The calibration curve obtained of SS amorphous content versus measured heat was linear (see Figure 2 on page 29). It was thus possible to use the data to quantify extents of disorder in milled SS samples, although it should be noted that the use of this type of calibration curve has some limitations 6,7 . Principally, the material used to prepare the calibration curve (a mixture of wholly amorphous and wholly crystalline particles) is physically distinct from the study material, which in this case will have disordered regions (that is, a combination of crystal dislocations and amorphous areas) forming a Figure 3: Particle size and amorphous content data as a function of milling time for ball milled salbutamol sulphate corona around an otherwise crystalline substrate. The magnitude of this effect will vary according to the sample studied. In the case of lactose6 significant issues arise because of the existence of 2 anomers and 3 crystal forms and the effect of muta-rotation must be considered. For SS the case is much simpler and hence confidence in the correlation between the data sets is higher, since there are no isomeric forms. Since the calibration curve was prepared with totally amorphous material (because it was spray-dried), the values reported below for milled SS samples are expressed in amorphous content (% w/w) but it is noted that this term really means extent of disorder and accounts for crystal dislocations as well as true amorphous regions. SS samples were milled for various time periods up to one hour. Samples were subsequently removed from the mill and analysed for particle size and amorphous content immediately (to reduce the risk of relaxation or recrystallisation increasing the particle size or reducing amorphous content value). The average particle size and extent of disorder data are plotted in Figure 3. Over short milling times there is a sharp reduction in particle size, with no increase in extent of disorder detected with the calorimeter. After ca. five minute milling time, the particle size distribution remains constant and there is a concomitant increase in extent of disorder. The data clearly shows that the hypothesis that particle size reduction occurs before generation of amorphous content is correct. The immediate benefit of this is that it appears feasible to control the micronising process to produce a product with a defined particle size distribution and a crystalline surface. Although we did not attempt to make similar measurements on a larger scale, there would seem to be no reason why this outcome would not apply to larger ball milling apparatus; the break-point time at which maximum particle size reduction is achieved would simply increase. Although not explicitly explored in this study, IGPC analysis may have application to air-jet milling also. In this case, samples are ejected from the mill under centrifugal force, but changing the pressure of the input gas would modulate ejection time and attrition rate. One further conclusion can be drawn from the data in Figure 3. At longer milling times, the extent of disorder starts to reach a plateau while the particle size starts to increase. It is likely that at these longer milling times the conditions in the mill are such that the ambient RH or, more likely, temperature become 30 www.europeanpharmaceuticalreview.com
SUBSCRIBE ISSUE 2009 THERMAL ANALYSIS sufficiently high to cause some crystallisation of the amorphous regions. This in turn would tend to cause particle agglomeration. Since extended milling will result in continual formation of disordered regions, it seems likely that the system will reach a dynamic equilibrium, with both amorphous content and particle size remaining constant. This means that, for SS at least, it would not be possible to prepare a totally amorphous sample simply by extended ball-milling. Of course, these limiting values may not be ideal from the perspective of DPI product performance. Further study could correlate DPI product performance with extent of disorder and hence the calorimeter could be used to optimise the milling process of the drug prior to formulation. To summarise, the data shows that milling of a crystalline pharmaceutical results initially in a reduction in particle size with no measurable surface disorder. Continued milling does not reduce particle size further; rather, the surface of the material becomes disordered. This disorder can act to change the surface energy and, as a direct consequence, may alter DPI product performance. Controlling this process starts with being able to monitor the formation of these disordered regions. The study has shown that isothermal calorimetry has the potential to accomplish this. Acknowledgements I would like to thank Mansa Dennison and Matthew Jones from the School of Pharmacy, University of London and Mark Saunders from Synectix Pharmaceutical Solutions Ltd. References 1. Brodka-Pfeiffer, K., Langguth, P., Graß, P. and Hausler, H. (2003). Influence of mechanical activation on the physical stability of salbutamol sulphate. Eur. J. Pharm. Biopharm., Vol 56, pp 393-400. 2. Feeley, J.C, York, P., Sumby, B.S., Dicks, H., 1998. Determination of surface properties and flow characteristics of salbutamol sulphate, before and after micronisation. Int. J. Pharm. 172, 89-96. 3. Hogan, S.E., Buckton, G., 2000. The quantification of small degrees of disorder in lactose using solution calorimetry. Int. J. Pharm. 207, 57-64. 4. Newell, H.E., Buckton, G., Butler, D.A., Thielmann, F., Williams, D.R., 2001. Pharm. Res. 18, 662-666. 5. Hancock, B., Zografi, G., 1997. Characteristics and significance of the amorphous state in pharmaceutical systems. Int, J. Pharm. 86, 1-12. 6. Ramos, R., Gaisford, S. and Buckton, G. (2005). Calorimetric determination of amorphous content in lactose; A note on the preparation of calibration curves. Int. J. Pharm., Vol 300, pp 13-21. 7. Gaisford, S. and Ramos, R. (2007). Calorimetry for amorphous content quantification. Eur. Pharm. Rev., Issue 3, pp 46-52. Simon Gaisford Dr Gaisford is a Senior Lecturer in Pharmaceutics and runs a laboratory dedicated to the application of thermal analysis in the development of medicines. He is a founding partner and Director of Synectix Pharmaceutical Solutions Ltd, and Chair of the Thermal Methods Group (Royal Society of Chemistry). @ email the author Thermal Analysis Excellence METTLER TOLEDO sets the standards in thermal analysis as with its worldclass balances. Mettler-Toledo AG, CH-8603 Schwerzenbach Tel. +41-44-806 77 11 www.mt.com/ta
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