Biodegradable polyesters for controlled release of trypanocidal drugs: in vitro and in vivo studies

Copolymers of epsilon-caprolactone and L-lactide P(CL-LLA), epsilon-caprolactone and D,L-lactide P(CL-DLLA) and epsilon-caprolactone and trimethylene carbonate P(CL-TMC) were synthesized. The composition of comonomers and their sequence lengths were determined by means of 1H and 13C NMR measurements. The effect of the comonomer on the thermal properties was investigated by differential scanning calorimetry (DSC) analysis. The in vitro degradation of the rods obtained by melt extrusion of the synthesized copolymers and the commercial homopolymers poly(epsilon-caprolactone) P(CL) and poly(D,L-lactide) P(DLLA) was carried out in phosphate buffer (PB) pH 7.4 at 37 degrees C. The rate of degradation depends on comonomers and polymer composition. The in vitro release of the selected drugs, isometamidium chloride (IMM) and ethidium bromide (EtBr), from such devices was carried out under the same conditions as used for the in vitro degradation. The release experiments show that the release of IMM is faster than for EtBr. During the first stage, for IMM the release is governed by osmotic pressure whereas for EtBr the release is mainly diffusion-controlled. The in vitro release of these drugs is governed by polymer matrix degradation at the later stage of the release process. Comparative in vitro release study from the different polymers showed that the release depends mainly on the physical properties of the polymer. The in vivo experiments carried out in the field on cattle and in the laboratory on rabbits using the classical treatment (intramuscular injection) and the sustained release devices (SRD) subcutaneously implanted, showed that the prophylactic period is significantly enhanced in the case of SRD as compared to intramuscular injection. The comparative efficacy of SRD containing IMM and EtBr evaluated in the case of rabbits showed that, the SRD (IMM) prophylactic period is much longer than for SRD (EtBr).