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Sustained release injectables formed "in-situ" for veterinary use
Research Area: thesis Year: 2006
Type of Publication: Phd Thesis  
Authors:
  • Sautter, Caroline Anne Francoise
 
   
Abstract:
Repetitive oral administration of tablets to companion animals is particularly challenging and there is a continuing need for alternative options such as long acting injections or implants. Therefore, properties of sustained release injectables formed in-situ for use in dogs were investigated. These formulations comprise a biocompatible solvent in which the biodegradable PLA/PLGA polymers and the lipophilic anti-infective NOA449851, derivative of milbemycin against the parasite Dirofilaria immitis are dissolved. These formulations coagulate into solid implants on contact with aqueous fluids after i.m. or s.c. injection, thereby releasing the incorporated drug slowly over a period of weeks to months. This technology has several attractive features such as simplicity of concept, ease of manufacturing as well as use of FDA approved polymers. Dissolution tests were performed to investigate in-vitro drug release characteristics from injectable formulations varying in polymer type, polymer concentration, active ingredient concentration and solvent composition. At high drug loads, release properties were independent of polymer type. However, in case of very low drug loads, drug release was controlled by polymer properties. Major releasing mechanism was found to be drug diffusion and therefore was influenced by drug concentration. Significant reduction of initial burst was observed when polymer concentration was increased. Also the solvent composition influenced in-vitro drug release. Especially a significant reduction of the initial burst was observed when a fraction of the main solvent triacetin was substituted with hydrophilic co-solvents such as ethanol absolute or anhydrous glycerol, while a lipophilic co-solvent such as Miglyol 812 did increase the initial drug release. Solvent composition, depending on its affinity to the dissolution medium, influences the rate of fluid-convection, the hardening process of the polymers, the internal structure of the implant and therefore its drug release rate. Raman and IR spectroscopy revealed that the active ingredient was incorporated in the amorphous conformation in all investigated batches. No evidence of any interaction between the active ingredient and the polymeric matrix could be detected. Tolerability and pharmacokinetic properties of six sustained release injectables formed in-situ, varying in polymer concentration and solvent composition were explored after subcutaneous administration to Beagle dogs. The high viscosity of the formulations and consequently the poor syringeability turned out to be a critical issue. Viscosity of the formulations was decreased by reducing the polymer concentration and by varying the composition of the solvent mixture. All investigated formulations were very good tolerated by the animals. In agreement with in-vitro investigations, reduction of polymer concentration gave rise to increased initial drug release. Presence of hydrophilic co-solvents reduced maximum drug concentration in dog plasma profiles. The active ingredient NOA449851 was detectable in blood of experimental animals over 450 days after subcutaneous injection of sustained release formulations. However, very high inter-animal variations were found for some formulations and important differences in AUC values were calculated, despite the same amount of drug injected to each dog. These differences could be explained by possible encapsulation of the subcutaneous implant with connective tissue. The degree of correlation between the in-vitro dissolution parameters and the in-vivo pharmacokinetic data was investigated. Cmax was positively correlated to cumulative in-vitro drug release at Tmax, however not in a significant manner. In general, for this type of dosage form and drug, no satisfactory IVIVC are observed. The model used for in-vitro drug release testing neglect probably some crucial aspects of physiological conditions governing in-vivo release and cannot replace biological systems. Stability studies were performed for three sustained release injectables formed in-situ during six months storage at the four selected temperatures 5?C, 25?C, 30?C and 40?C. The formulations were based on PLA polymers, active ingredient NOA449851, solvent triacetin and in case of one formulation, co-solvent ethanol absolute. An HPLC-method was utilized for determination of the active ingredient content. No differences between the three formulations were observed. The content of active ingredient slightly decreased with time and temperature. Molecular weights of PLA polymers were determined with GPC. Decrease in molecular weight was significantly increased with storage temperature and time. These results are in agreement with the findings of Wang et al. [Wang et al., 2003]. No significant influence of co-solvent ethanol absolute on the PLA stability could be measured. However, presence of active ingredient seemed to decrease hydrolysis process of PLA polymer, probably by competitively attracting water molecules responsible for polymer degradation. NIR data analysis of solvent triacetin showed spectral changes for wavelengths at 1900 nm. These spectral changes were consistent in every analyzed spectra set as solvent triacetin was in excess in all investigated samples. Influence of solvent effect could not be removed by study design, as no specific wavelength could be attributed to PLA polymers. Surprisingly, in-vitro drug releases from a formulation tested directly after manufacturing and after six months storage at 40?C were found to be similar, despite the important reduction of the molecular weight of the PLA polymers. This confirms a drug release mechanism mainly controlled by drug diffusion through the matrix and not erosion controlled. Microspheres and sustained release injectables formed in-situ are both technologies intended for parenteral application, planned to achieve a long lasting drug release. In both technologies, the sustained effect is caused by biodegradable PLA/PLGA polymer matrix in which the active ingredient is embedded. In order to investigate the influence of the preparation method of the polymer matrix on the release of the drug substance, microparticles batches were prepared for comparison with regards to in-vitro release properties. For all tested microsphere batches, drug release was independent on type of biodegradable polymer. A bigger fraction of active ingredient was released from the microparticles at high drug loads. In every investigated case, drug release from sustained release injectables formed in-situ was faster and to a much larger extent than from related microparticles. As possible explanation of the slower release from microparticles may be the denser packing of the polymer matrix compared to the in-situ formed implants. The microspheres polymer matrix is solidified before injection by applying a much more efficient solvent extraction procedure then the implants which only solidifies slowly at the site of injection. For that reason, diffusion controlled drug release is slower from the more densely packed microsphere matrix. Sustained release injectables formed in-situ showed, under in-vitro as well as in-vivo conditions, a prolonged active ingredient release, confirming that this drug delivery technology is a suitable approach to achieve a controlled long term release of the lipophilic anti infective NOA449851. This technology fulfills, for this particular compound, some basic requirements such as a good tolerability, controlled release of the active ingredient over a long period of time as well as an acceptable stability of formulation during storage for several months at low temperature conditions. Release properties of the active ingredient could be modified by changing composition of the formulation and possible detrimental burst effects could be suppressed by careful selection of polymer concentration and solvent mixture. Especially, the latter finding, the suppression of a burst effect can be considered as a significant improvement of the in-situ implant technology. It is to be expected that in the future, development of new implantable systems will, increasingly, help reducing cost for drug therapy, potentate medical treatments and, simultaneously enhance patient compliance.
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