5.4 Drug loading and release into and from the thermosensitive PVCL materials (I, IV)
5.4.2 Drug loading and release into and from the physically stabilized PVCL particles (IV)33
At 37 °C, the concentrated polymer solution droplets were solidified due to the partial dehydration and phase transition of the polymer chains above their LCST; water was squeezed out and the polymer droplets were collapsed, thus creating a physical network.
At the same time, the drug was entrapped in the hydrogel. High concentration of the polymers was a prerequisite for the formation of the particles allowing the entanglement of the polymer chains already at lower temperatures. At high temperatures, the chains did not aggregate just by colliding to each other, but were already entangled and the whole droplet was held together forming a stable hydrogel (Kirsh, 1998). In pure water or HBSS-Hepes buffer (no cross-linking agent) the unstable granules were aggregated over time and dissolved completely when the temperature was decreased. Aggregation is proposed to be due to absence of the electrostatic charge on the particle surface because of the non-ionic PVCL and the presence of potential hydrophobic interactions between the aggregated macromolecules (Markvicheva et al., 1991).
Drug release from the particles was related to the concentration gradient. With all the drugs, the higher the concentration was in the drug-polymer loading solution, the higher was the release. The amount and rate of drug release were affected also by pH (drug ionization). At higher pH (7.4), more ketoprofen was released from both the particles than at pH 3.0 (Figure 13). The opposite effect could be detected in the case of propranolol base. The more lipophilic drugs, ketoprofen and propranolol, were released more efficiently from the PVCL and PVCL-graft-C11EO42 particles into the buffer solution at 37
°C compared to the hydrophilic nadolol. The released amount of the cationic propranolol from the particles at pH 7.4 was of the same order of magnitude than the corresponding release of the anionic ketoprofen, so the different charge of the model drugs did not seem to affect the total amount of drug release.
Figure 13. Ketoprofen release from the PVCL or PVCL-graft-C11EO42 particles at 37 °C at pH values of 7.4 and 3.0 (n = 3 ± SEM). Initial drug loading concentration was 0.25 mg/ml.
The drug release from the PVCL-graft-C11EO42 particles during the 6 h experiments was clearly attenuated when compared to the PVCL-particles with every drug studied. Also, the release rates of the drugs from the PVCL particles were faster with a burst-like effect, whereas the release rates of the PEO-grafted particles were relatively constant already from the beginning of the experiments, as can be seen in Figure 13. During the release experiments, also the cross-linker (salicylic acid) diffused out from the particles. Also the salicylic acid was released more efficiently from the PVCL particles in 6 hours than from the PVCL-graft-C11EO42 particles. The total amount of salicylic acid release from the particles was found to be less when the drugs were present. The release of the salicylic acid from the particles was found more efficient at pH 7.4 than at pH 3 due to higher ionization at the higher pH. If the particles were kept in the cross-linker solution, they remained intact also at room temperature. However, at 23 °C the release of the drugs to the buffer was instantaneous, as the stabilizing effect was not strong enough to prevent fast disintegration/dissolution of the particles below the phase transition temperature.
The amount of model drugs entrapped in the particles was found to be higher in the PVCL-particles (55-80% from the initial amount), than in the PEO-grafted particles (10-30%) in the cases of nadolol and propranolol. The particle-drug interaction was found to be stronger in the case of ketoprofen, as it was not released to the salicylic acid-solution from the particles within the two-hour stabilization. The recovery values (neutral pH) of propranolol and nadolol were from >70% up to 100%, whereas the recovery values for ketoprofen, especially in the cases of grafted particles and lower pH (3.0), were smaller.
Time (min.)
Hydrogen bonding and hydrophobic interactions are known to have an important role in stabilizing the structures of thermosensitive interpenetrating polymer networks (Aoki et al., 1994) and in the formation of microspheres composing of pH/temperature-sensitive polymer complexes (Kim et al., 2001). Association of carboxylic acids and non-ionic polymers via hydrogen bonding forms interpolymer complexes that are recently considered as potential materials for pharmaceutical applications (Khutoryanskiy, 2007).
It has been suggested that the carboxylic acids in different polymers form hydrogen bonded complexes with ether oxygens of PEO at acidic conditions (Miyoshi et al., 1996 and Wang et al., 2005). Therefore, salicylic acid can also form complexes with the PVCL via the H-bonding of the carboxylic acid and hydroxylic groups of the salicylic acid and the carbonyl groups of the lactam ring of PVCL (Figure 7). As a consequence, a tight crosslinked net is formed. Hydrogen bonds between the stabilizer and PVCL-graft-C11EO42 retarded the drug release even further. In that case the forming network was tighter, because the H-bonds could occur between the salicylic acid and PVCL, but also between the salicylic acid and ether oxygens of PEO, thus retarding the release. The dense structure of PVCL-graft-C11EO42 particles decreased also the released amount as compared to the more gel-like PVCL particles.
Large number of PEO-macromonomer chains is buried inside the core of the particles (Figure 7). These chains bound the drugs in the core with hydrogen bonds, but also with hydrophobic interactions resulting from the attraction between the hydrophobic molecules in water, and this phenomenon was increased with increasing temperature. Moreover, the alkyl segments of the PEO-macromonomers could create hydrophobic pockets for the lipophilic drugs in the PVCL-graft-C11EO42 structures that retarded their release (Laukkanen et al., 2005). The release of nadolol was assumed to be retarded even further because it was bound to the polymers via phenolic groups (Laukkanen et al., 2005), forming similar tight complexes like salicylic acid.
6 Conclusions
Various thermosensitive PVCL polymers were studied in the field of pharmaceutical and biomedical applications. The effect of PEO-macromonomer grafting on the properties of PVCL was evaluated and the PVCL was compared to another thermosensitive polymer, PNIPAM. The main conclusions drawn from the results of this study are:
1. Thermosensitive PVCL is non-toxic in cellular contact during short exposure times, and enhanced cellular attachment is achieved with the PVCL coated particles (compared to PNIPAM).
2. By grafting PVCL with the amphiphilic PEO-macromonomer, the cellular toxicity is further diminished, because the polymer-cell contact is inhibited due to the steric repulsion created by PEO-macromonomer chains at the surface of the PVCL particles.
3. Binding of various model drugs to the PVCL and PEO-macromonomer grafted PVCL polymers occurs via hydrogen bonding and via hydrophobic interactions.
Temperature dependence to inhibit the release of the drugs was clearly demonstrated. Physico-chemical properties of the drugs and the environment affect the loading and release into and from the PVCL.
4. Stable thermosensitive hydrogel particles of PVCL can be obtained by creating a tight net with physical cross-linking, thus affecting the release properties of the PVCL particles.
The thermosensitive nature and low cellular toxicity may open up several pharmaceutical applications for the PVCL and PVCL-graft-C11EO42 polymer materials in the forms of either enhanced cellular contact or stealth-carrier behaviour.
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