Advanced detection gel permeation chromatography (GPC) was employed for the quantification of bioabsorbable polymer degradation to assess the performance and safety of implantable bioresorbable metal coated drug eluting stents (DES). The method developed enable the extraction and analysis of trace amounts of polymer.
The in vitro degradation study was carried-out in phosphate buffer (PBS, pH 7.4) at 37°C whereas the in vivo degradation study was carried-out in miniature swine animal model. In the in vivo study, the polymeric material from coronary stented artery samples was successfully extracted without biological interferences thus assuring accurate quantification of polymer content and molecular properties.
The overlay plot of the GPC elution profiles obtained from the concentration detectors (i.e. refractive index (RI) detector and UV detector at 280 nm) of the polymer extracts at different in vivo degradation time points demonstrated the shifting of the polymer peak to later retention volumes (Figure 1). The different intensities observed in the RI peak arise from the injection of sample extracts that were pooled together at later time points (1, 2, 4, 6, and 8). The lack of UV absorbance at 280 nm of the polymer peak confirms that only the polymer was quantified and also added the ability to detect the co-extract drug in the DES and stented artery samples. Although not presented here, this method can be used to investigate pharmacokinetics of the DES in the stented artery samples.
The comparison of degradation results obtained included polymer molecular weight (Mw), molecular weight distribution (i.e. PDI, polydispersity index) and polymer mass content (Table 1, Figure 2). The degradation profile measured for the DES in vitro and in vivo correlated very well with similar Mw loss and residual mass detector at early time points. The differentiating factor between the two degradation studies was the accelerated polymer mass loss measured in vivo compared to in vitro. Interestingly, the polymer Mw decreased to a maximum of ~85% of the initial raw polymer Mw while the mass of the polymer at corresponding time points decreased to below detectable levels.