THERMOSENSITIVE INJECTABLE GELS AS MODIFIED DRUG RELEASE SYSTEMS FOR VETERINARY USE

BERMUDEZ JM; RAMIREZ RIGO MARIA VERONICA; GRAU R; GOTTIFREDI JC

Bahia Blanca

Simposio; IX Simposio Argentino de Polímeros (SAP2011); 2011

INTRODUCTION Fluid injection systems for in-situ formation of polymer matrices represent an attractive alternative for the development of drug delivery systems in the field of human and animal medicine. In particular, the utilization of polymer solutions capable of gelling in situ at temperatures close to the physiological, designed to assure long residence times and control drug release. Thus, the development of these applications in veterinary field has become of increasing commercial interest, generating intensive R&D work. The global market for advanced drug delivery systems amounted to $134.3 billion in 2008, and was forecast to grow to $139 billion in 2009. The estimate is $196.4 billion by 2014, for an annual growth rate of 7.2% in the 5-year period. The estimated sales for sustained-release systems are $ 36.1 billion in 2009 and $ 45.8 billion by 2014 for an annual growth rate of 4.9% (Advanced Drug Delivery Systems: New Developments, New Technologies). The poloxamers are thermosensitive materials more frequently used for their advantages such like easy availability, simple method for gel preparation, and good compatibility with various drugs and pharmaceutical excipients. Poloxamer gels are widely used in the pharmaceutical field, being generally considered as safe excipients (Mayol et al., 2008). It is well known that poloxamer solutions present the reverse thermal gelation phenomenon, behaving as solution at low temperature and gelling as temperature increases (Dumortier et al., 1991, Lenarets et al., 1987). The aim of the research work is to explore the potential of the combination of poloxamer 407, poloxamer 188, and biocompatible polymers for their application as a injectable depot controlled drug release platform for veterinary use. At this first stage carrageenan was used as a natural polymer. Progesterone was used as a model drug. The drug release and erosion were evaluated in vitro. It was also determined gelation temperature and gel strength of poloxamer gels. METHODOLOGY Materials and methods Materials used are: poloxamer 407 (P407), poloxamer 188 (P188) (trade name Lutrol® F127 and Lutrol® F68, respectively), kindly provided by BASF, κ-carrageenan (CA) (kindly provided by Soriano, Buenos Aires, Argentina), progesterone (Farmabase, Rovereto, Italy) and sodium chloride (Cicarelli, Santa Fe, Argentina). All reagents were used without any further purification process. Preparation of thermosensitive gel: Poloxamer gels were prepared by "cold method" described by Schmolka (1972). To prepare carrageenan solutions, appropriate amounts were weighed and dissolved in water (70-80 ° C). Finally, polymeric platforms are loaded with progesterone (1% w/w), by direct dispersion of the drug in the polymer blend. To homogenize the mixture, magnetic continuous agitation was used. Gel dissolution and drug release: To simulate physiological conditions sodium chloride 0.9% w/w was used. The release experiments were performed using the membraneless model. At predetermined intervals time, the release medium was completely replaced by fresh medium kept at 38 ºC and formulations were weighed to calculate the proportion of dissolved gel. Measurement of gel strength: 50 ml of poloxamer gel was placed in a 100 ml graduated cylinder and gelled in a thermostated bath at 37 °C. The apparatus for measuring gel strength (weight 33 g) was placed onto poloxamer gel. The gel strength was determined by the time that the apparatus took to sink 5 cm down through the poloxamer gel. If the time exceeds five minutes, several weights were placed on top of the apparatus Rheological tests Oscillatory rheometry was also performed for all evaluated formulations using a rheometer MCR301 controlled stress (Anton Paar, Germany). Experiments were performed to assess changes in the rheological parameters as function of temperature, by means of oscillatory measurements at a fixed frequency of 10 Hz and with stress amplitude to ensure linear viscoelasticity (nondestructive dynamic conditions). Gelation temperature was chosen as the temperature at which the values of both moduli were equal, reflecting similar elastic and viscous properties (“G´ G´´ crossover” criterion) RESULTS AND DISCUSSION Gel dissolution: To evaluate the influence of the concentration of poloxamer and carrageenan in the erosion of the release platform, gels of P407, P188 and CA in different proportions were prepared. The composition of P407/P188/CA formulations were: 28/10/0 % w/w (abbreviated 28/10); 28/15/0 % w/w (abbreviated 28/15); 28/10/0.1 % w/w (abbreviated 28/10/0.1) and 28/15/0.1% w/w (abbreviated 28/15/0.1). The experiments show that erosion is significantly influenced by the concentration of P188. An increase of 5% in the P188 concentration results in a erosion decrease of 12% in the gels without CA. The gels constituted by poloxamers as unique component, dissolves at a higher proportion, as reported in previous works (Liu et al., 2009, Bermudez and Grau, 2010). Also, Gel erosion behaviour was modified by carrageenan. Apparently, carrageenan would strengthen gel structure, exhibiting erosion between 35-40% at the 48 hours Drug release: Carrageenan decreases the release of progesterone from the gels containing 10% of P188, but this effect is not observed in the gels with 15% of P188. Kinetic data were processed using the power model. The values of n obtained are between 0.5 and 1, suggest that, besides the diffusion mechanism, the erosion of the gels is involved in the kinetic control of drug release. Gelation temperature (Tgel): Tgel is the temperature at which the liquid phase undergoes the transition from sol to gel. A proper rheological analysis is a valuable technique to investigate the gelling process and the viscoelastic properties of thermosensitive gels. The oscillatory measurements using low oscillating angle is appropriate because the gel structure remains intact during measurements. This gives information of dynamic properties, G´, the elasticity modulus, and G´´, the viscous modulus. In this study the moduli were measured within the linear region, as the shear force of gel depot is small, the results might be close to the in vivo situation. Tgel was identified as the temperature at which G´ and G´´ curves intersect each other. Tgel of poloxamer solution containing 28% of P407 alone was 18.2 °C. The results obtained indicated that P407 alone could not provide the suitable gelation temperature. Gelation temperature increased when increasing the concentration of P188. With the addition of 10.0% of P188, Tgel increases about 7 °C (25.5 °C) and add 15% of P188 the increase was 10 °C (28.2°C). The addition of 0.1% w/w of κ-carrageenan practically did not change the gelation temperature with respect the value of the 28/10 and 28/15 poloxamer solutions. Gel strength: Besides a suitable gelation temperature, injectable intended for both in situ gelification and controlled release should exhibit an adequate consistency and strength. The value of gel strength, which measures the viscosity of the poloxamer gel at physiological temperature, was determined using the apparatus described by Choi et al. (1998). The addition of P188 increases the strength of poloxamer gel. With the addition of 10% P188, the gel strength of P407 28% w/w increased from 2 to 13 s and adding 15% P188, the gel strength increased to 73 s. By incorporation of κ-carrageenan, the gel strength values increased considerably. In 28/15 formulation, the gel strength increased from 73 to 206 s with the addition of 0.1% carrageenan. Therefore, κ-carrageenan macromolecules might be able to interact with micelles through secondary bonds, such as hydrogen bond, reinforcing the structure and therefore the mechanical properties of the gel. These results correlate with the obtained dissolution data. CONCLUSIONS Poloxamer-carrageenan gels appear to be versatile release platforms that provide a range of release rates and erosion in function of their composition. Our results show that the addition carrageenan leads to an improvement of gel mechanical properties with an increase of gel strength and a decrease the erosion of the gels based in poloxamer, which would allow getting injectable platforms with optimal drug release properties and adequate gelation temperature.