Chapter 2. Material and Methods
3.2. Materials and methods
In this study ‘PVP polymers’ or ‘PVP carriers’ represents both the homopolymer PVP and its derivative copolymer PVPVA 6:4 or otherwise specified.
3.2.1. Preparation of physical mixtures
Physical mixture (PM) of APIs (i.e., Paracetamol (PCM) and caffeine (CAF)) and PVP carriers were weighed according to the desired drug-polymer ratio and the mixtures were gently mixed in a mortar and pestle for approximately 2 minutes.
3.2.2. Estimation of drug-polymer miscibility
Prior to the preparation of solid dispersions, the miscibility of the drug-polymer systems was investigated. The Flory-Huggins theory has been used for calculating free energy mixing and estimating miscibility of drug-polymer components. Although limitations of this approach have been described (Zhao et al., 2011, Marsac et al., 2006, Tian et al., 2012), this method is still useful as a starting point for the understanding of drug-polymer thermodynamics.
In considering the mixing of a large molecular weight polymer and a low molecular weight API, the Flory Huggins theory defines a hypothetical “lattice” in space. It assumes that the probability of the solvent (in this case the API) making contact with the segment of polymer (in this case monomer) is equal to the volume fraction of the polymer segments, i.e. its monomer (Gong et al., 1989). Following the Flory-Huggins theory of polymer solution, with the description of interaction parameter, χ to account for the enthalpy of mixing, the equation for free energy mixing of an API-polymer system, ΔGm is given by Equation (3.1)
where is number of mole of the drug, is number of moles of polymer, is volume fraction of the drug, is the volume fraction of the polymer, is the interaction parameter between the drug and polymer, R is gas constant, and T is absolute temperature. By the knowledge of the interaction parameter, χ, one can estimate the mixing behaviour of an API to polymer system using the Flory-Huggins theory via estimation of the free
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energy of mixing which in turn indicates the driving energetics of the process. However the approach requires the interaction parameter χ to be known.
Earlier reports indicated that the solubility parameter and melting point depression approaches maybe used to estimate the interaction parameter of a blend and hence, possibly, the degree of miscibility and solid solubility (Marsac et al., 2009, Marsac et al., 2006, Zhao et al., 2011, Tian et al., 2012). Both of these approaches were tested in the current study in relation to the characteristics of both the raw materials and the prepared products.
3.2.2.1. Solubility parameter approach
The solubility parameter approach is a widely used method in estimating the miscibility and compatibility of a mixture system. The origin concept of this approach is described by Hildebrand (1961) who stated that solubility of a given solute in a solvent is determined by the cohesive energy density, i.e. cohesive energy per unit volume of the substance (Van Krevelen and Te Nijenhuis, 2009). This concept is developed to specify a parameter (the solubility parameter) that is defined to be the square root of the cohesive density energy. According to Van Krevelen and Te Nijenhuis (2009) the solubility of a given solute is largely determined by the chemical structure. As a general rule, similar chemical structure between the solute and solvent favours solubility, i.e. solubility is favoured when structures of solute and solvent possess similar solubility parameters (Van Krevelen and Te Nijenhuis, 2009).
In a low molecular weight liquid, the cohesive energy is closely related to the molar heat of evaporation ΔHvap, as presented in Equation (3.2)
where is cohesive energy, p is pressure, is volume changes, R is universal gas constant and T is temperature. Since it is not possible to obtain the vaporization energy of a polymer directly, an indirect method was developed to estimate the solubility parameter of polymer system i.e. via group contribution methods.
In the literature, two chemical group contribution methods are reported for the indirect prediction of solubility parameter of the API and polymer system, i.e. the Hoftyzer/ Van Krevalen method and the Hoy method (Van Krevelen and Te Nijenhuis, 2009). Based on these methods, the cohesive energy of a molecule is dependent on different forces in the chemical structure of the molecule which include dispersive force (Fdi) , hydrogen bond force (Ehi), as well as polar force (Fpi). The
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values of these forces are given as a reference table in Van Krevelen and Te Nijenhuis (2009).
With the known of these forces, the solubility parameter of a molecule can be estimated.
3.2.2.2. Melting point depression approach
Apart from the solubility parameter approach, the melting point depression approach is another method that can be used to estimate the miscibility in a mixture of different components. The miscibility of the components is presented by the negative value of a defined parameter, namely interaction parameter, as a function of melting point depression phenomenon.
Additionally, this approach is also introduced for the prediction of solid solubility of a drug in a polymer system which is strongly attributed to the drug-polymer interaction. When a drug interacts with a polymer system, the chemical potential of the mixture will reduced and thus the melting point of the drug would be reduced (Tian et al., 2012). Consequently, by accessing the degree of melting point depression as a function of polymer composition, the energy density of the interaction between the two systems could be anticipated (Marsac et al., 2009, Zhao et al., 2011).
This density of interaction is frequently represented by the interaction parameter, χ12, where subscription 1 denotes the first component and subscription 2 denotes the second component.
To predict the interaction parameter by using the melting point depression method, PM API-carriers were prepared in drug rich proportions (from 70 - 90% w/w drug loading) and scanned by modulated DSC with ± 0.212 oC every 40 s at 2 oC per minute to 200 oC using pin hole pan.
MTDSC was used to separate the Tg or relaxation endotherm of the polymer particularly, PVP K 29-32 from the melting endotherm of PCM. This is to avoid misinterpretation of the Tg related endotherm (particularly PVP K29-32) as a depressed melting point of PCM.
3.2.3. Preparation of hot melt extruded solid dispersions
HME sample of PCM PVP K29-32 ranging from 20%-70% of PCM loading was prepared. While 20%-50% of PCM loading were prepare in HME PCM PVPVA 6:4. For both HME system of CAF PVP K29-32 and CAF PVPVA 6:4, only 10% and 20% loading of CAF systems were prepared.
The discrepancy of the API loading is dependent on the experimental observation whereby the loading limit of preparation was extruded up to a point where the appearance of the extrudate was opaque. Please refer to Table 2.12 for the processing parameters used in the production of HME systems.
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3.2.4. Hot stage microscopy
The melting or Tg temperature of pure PCM, CAF, PVP polymers and fusion temperatures of the PM of drug and polymer were observed by hot stage microscopy (HSM). Samples were heated from room temperature up to 200 oC at a heating ramp of 10 oC per minute. Events that occurred in the samples while heating were recorded using a JVC colour video camera with studio capture software.
3.2.5. Thermogravimetric analysis
Thermogravimetric analysis (TGA) was used to measure the water content and detect the decomposition temperature of the raw materials and the prepared HME PVP-based formulations.
Raw powders or intact extrudates (3-4 mm) were heated from room temperature and isothermed at 100 oC for 15 minutes before further heating to 350 oC at a heating rate of 10 oC per minute.
Weight loss after 15 minutes of isotherm at 100 oC was taken as water content of the samples.
Dramatic weight loss at higher temperatures was regarded as being indicative of the decomposition temperature.
3.2.6. Modulated temperature differential scanning calorimetry
All samples, including the drugs, polymers, physical mixtures and HME PVP-based extrudates were analysed using a DSC with modulated mode (MTDSC). Pin-hole lids were used to allow removal of water, particularly given the hygroscopic nature of the PVP polymers (Callahan et al., 1982). Samples were heated from 0 oC in modulated mode (± 0.212 oC every 40 s) at 2 oC per minute to 200 oC. All experiments were run in triplicate.
3.2.7. Attenuated total reflectance -Fourier transform infrared spectroscopy
Attenuated total reflectance -Fourier transform infrared spectroscopy (ATR-FTIR) measurements were carried on freshly ground extrudates. The spectra were recorded over a wavenumber range of 500 cm-1 to 4000 cm-1 with a resolution of 2 cm-1 and 64 scans. To detect changes as a function of heating, the ATR crystal was heated to the desired temperature (ranging from room temperature to 200 oC) before scanning. Extrudates were scanned in powder form by gently grinding in a mortar and pestle.
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3.2.8. Powder X-ray diffraction
Powder X-ray diffraction (XRPD) experiments were performed on freshly prepared HME PVP-based extrudates. The extrudates were crushed into powder form and compacted into the sample holder of the XRPD. Measurements were performed from 10o to 30o (2θ) coupled with scanning speed of 0.01o / step and 1 second for every scan step to cover the characteristic peaks of the crystalline PCM and CAF.
To detect crystallinity of the HME PVP based extrudates using XRPD method, a calibration curve was constructed based on the relationship between the area under the characteristic peaks and the crystallinity of PM was performed. This method of quantifying PCM using XRPD was reported before by de Villiers et al., 1998 (de Villiers et al., 1998). PMs of 10% to 80% w/w were prepared by simple mixing in a mortar and pestle. The PMs were then compacted into the X-ray sample holder and scanned from 10o to 30o (2θ) to cover the characteristic peaks of the crystalline PCM and CAF. Calibration curves were constructed according to the intensity of the two sharp peaks at 23.4 and 24.5o 2θ Bragg diffraction peaks versus the known crystal content.
3.2.9. Scanning electron microscopy
The surface morphology of the fresh extrudates was investigated using scanning electron microscopy (SEM). The extrudates were cut through by cross section using a microtome. Both the cut extrudates and whole spaghetti–like extrudates were placed on a sample stub and sputtered with a thin layer of gold prior to imaging.