How bulk fluid renewal can affect in vitro drug release from PLGA implants: Importance of the experimental set-up

Abstract

The aim of this study was to better understand the potential impact of partial vs. complete renewal of the bulk fluid during drug release measurements from poly (lactic-co-glycolic acid) (PLGA)-based implants. A "standard experimental set-up", in which the implants were directly exposed to well agitated phosphate buffer pH 7.4 was used, as well as set-ups, in which the implants were embedded within agarose hydrogels (mimicking living tissue). The gels were exposed to well agitated phosphate buffer pH 7.4. Ibuprofen-loaded implants were prepared by hot melt extrusion. The systems were thoroughly characterized before and during drug release by optical and scanning electron microscopy, gravimetric analysis, pH and solubility measurements as well as gel permeation chromatography. The bulk fluid was either completely or partially replaced by fresh medium at each sampling time point. In all cases, sink conditions were provided in the agitated bulk fluids throughout the experiments. Interestingly, the agarose set-ups did not show any noteworthy impact of the bulk fluid sampling volume on the observed drug release patterns, whereas complete fluid renewal in the "standard set-up" led to accelerated drug release. This could be explained by the considerable fragility of the implants once substantial polymer swelling set on, transforming them into PLGA gels: Complete fluid renewal caused partial disintegration and damage of the highly swollen systems, decreasing the lengths of the diffusion pathways for the drug. The mechanical stress is very much reduced at low sampling volumes, or if the implants are embedded within agarose gels. Thus, great care must be taken when defining the conditions for in vitro drug release measurements from PLGA-based implants: Once substantial system swelling sets on, the devices become highly fragile.

Keywords: Controlled drug delivery; Experimental set-up; Ibuprofen; Implant; PLGA.

Graphical abstract

Fig. 1

Schematic presentations of the experimental set-ups used to monitor drug release from the PLGA-based implants: (A) In well-agitated release medium in Eppendorf tubes, (B) In agarose gels in Eppendorf tubes, the gels being exposed to well-agitated release medium, (C) In agarose gels in transwell plates, the receptor compartment containing well-agitated release medium. Details are given in the text.

Fig. 2

Drug release, increase in volume and change in wet mass of ibuprofen-loaded PLGA implants upon exposure to phosphate buffer pH 7.4 using the 3 experimental set-ups. The implants were placed in well agitated bulk fluids in Eppendorf tubes, in agarose gels exposed to well agitated bulk fluid in Eppendorf tubes, or in agarose gels in transwell plates (the receptor compartment containing well-agitated bulk fluid). At pre-determined time points, the entire bulk fluid, or 3 or 1 mL thereof, was renewed (as indicated).

Fig. 3

Optical macroscopy pictures of ibuprofen-loaded PLGA implants after different exposure times to phosphate buffer pH 7.4 using the 3 experimental set-ups. The implants were placed in well agitated bulk fluids in Eppendorf tubes, in agarose gels exposed to well agitated bulk fluid in Eppendorf tubes, or in agarose gels in transwell plates (the receptor compartment containing well-agitated bulk fluid). At pre-determined time points, the entire bulk fluid, or 3 or 1 mL thereof, was renewed (as indicated).

Fig. 4

SEM pictures of surfaces and cross-sections of ibuprofen-loaded PLGA implants before and after 8 days exposure to phosphate buffer pH 7.4 using the 3 experimental set-ups. The implants were placed in well agitated bulk fluids in Eppendorf tubes, in agarose gels exposed to well agitated bulk fluid in Eppendorf tubes, or in agarose gels in transwell plates (the receptor compartment containing well-agitated bulk fluid). At pre-determined time points, the entire bulk fluid, or 3 or 1 mL thereof, was renewed (as indicated). Please note that after exposure to the release medium, the implants were freeze-dried prior to analysis. Thus, caution must be paid due to artefact creation.

Fig. 5

Dynamic changes in the pH of the well agitated bulk fluid, in the implants' dry mass and PLGA polymer molecular weight (Mw) upon exposure of ibuprofen-loaded implants to phosphate buffer pH 7.4 using the 3 experimental set-ups. The implants were placed in well agitated bulk fluids in Eppendorf tubes, in agarose gels exposed to well agitated bulk fluid in Eppendorf tubes, or in agarose gels in transwell plates (the receptor compartment containing well-agitated bulk fluid). At pre-determined time points, the entire bulk fluid, or 3 or 1 mL thereof, was renewed (as indicated).

Fig. 6

Dependence of the solubility of ibuprofen as a function of the pH at 37 °C. Phosphate buffer pH 7.4 USP 42 served as bulk fluid. Its pH was adjusted using aqueous 10 N lactic acid or 0.05 N NaOH solutions (final pH values are reported).