Sustained release injectables formed "in-situ" for veterinary use

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.