Correlation of tissue drug concentrations with in vivo magnetic resonance images of polymer drug depot around arteriovenous graft

Abstract

Sustained delivery of anti-proliferative drugs to the perivascular area using an injectable polymeric platform is a strategy to inhibit vascular hyperplasia and stenosis. In this study, the concentrations of sirolimus in vascular tissues were evaluated after delivery using an injectable platform made of poly(lactic-co-glycolic acid)-polyethylene glycol-poly(lactic-co-glycolic acid) (PLGA-PEG-PLGA). In order to optimize the drug release profile, the effect of two solvents or solid loading of the sirolimus into the polymer gel was first examined in vitro. The early release was slower with loading of dry drug into the polymer, compared to drug dissolution in solvents. Dry sirolimus was therefore used to load the polymer and applied to the perivascular surface of the graft-venous anastomosis at the time of surgical placement of a carotid-jugular synthetic hemodialysis graft in a porcine model. This was replenished by ultrasound-guided injection of additional drug-laden polymer at one, two and three weeks post-operatively. Magnetic resonance imaging (MRI) using pulse sequences specifically designed for optimal detection of the polymeric gel showed that the polymer injected post-operatively remained at the juxta-anastomotic perivascular site at two weeks. Sirolimus was extracted from various segments of the juxta-anastomotic tissues and the drug concentrations were determined using HPLC MS/MS. Tissue sirolimus concentrations at one and two weeks were highest near the venous anastomosis, which were approximately 100- to 500-fold greater than the concentrations necessary to inhibit vascular smooth muscle cell proliferation in vitro. Drug concentrations remained above the inhibitory concentrations for at least six weeks post-operatively. Thus, serial injections of sustained-delivery polymer gel loaded with sirolimus can provide high localized concentrations at target vascular tissues and thus may be useful for the prevention and treatment of vascular proliferative disorders such as hemodialysis graft stenosis. In addition, MRI is useful for the monitoring of the location of the drug depot.

Figure 1

Panel a) Surgical placement of spiral-enforced polytetrafluoroethylene (PTFE) graft between the common carotid artery and the ipsilateral external jugular vein. Panel b) Polymer gel loaded with sirolimus placed at the graft-venous anastomosis intraoperatively.

Figure 2

Cartoon of the explanted carotid-jugular graft and adjacent tissues. The tissues were cut transversely into multiple samples along the length and subjected to sirolimus extraction and analysis.

Figure 3

Accumulation of sirolimus in release media in vitro as a percent of the initial amount loaded (10 mg/mL). A more rapid initial release was observed when sirolimus was dissolved in either DMSO or methanol before mixing with polymer (*p<0.05 vs. solid loading). At two weeks, however, there was no statistically significant difference in cumulative drug released from polymer among the various formulations.

Figure 4

Sirolimus concentrations in venous and arterial tissues at various post-operative time points. Panel a) drug concentrations in tissue taken from one animal one week after graft placement. Two mL of polymer containing 5 mg of drug was applied to the graft-venous anastomosis at the time of surgery only; Panel b) Drug concentrations in tissue taken from one animal two weeks after graft placement. Two mL of polymer containing 5 mg of drug was applied to the graft-venous anastomosis at the time of surgery. At one week postoperatively, the drug depot was replenished by an ultrasound-guided injection of 2 mL of gel containing 5 mg of drug; Panel c) Drug concentrations in tissue taken from one animal six weeks after graft placement. The drug applications were the same as those described in panel b); in addition, a third injection of 2 mL of gel containing 5 mg of drug was applied at three weeks. This animal thus received 4 applications of polymer and drug. The error bars are derived from repeat analyses of the same extraction (each sample was analyzed twice) and by the propagation of error from the standard curve.

Figure 5

Three-dimensional (3D) reconstruction of a carotid-jugular graft and polymer gel from MRI and correlation to tissue drug concentrations. The polymer gel was applied to the graft-venous anastomosis at time of graft placement and then injected to the same region one week later. MRI was performed five days after the injection (12 days after graft placement). The polymer was reconstructed in pale green, while the blood vessels and graft were reconstructed in red, from the MR image. The sternocleidomastoid muscle was not included in the 3D reconstruction. Panel a) Dorsal view of the 3D reconstruction of the graft and polymer gel. Panel b) Ventral view of the same 3D reconstruction of the graft and polymer gel shown in panel a). Panel c) Graphic representation of measured sirolimus concentrations in various segments of tissues collected from the same animal described in panel a) and panel b). Each color bar in the linear concentration scale represents a 187.5 ng/g increment in concentration.

Figure 6

Three-dimensional (3D) reconstruction of polymer gel adjacent to external jugular vein without graft from MRI and correlation to tissue drug concentrations. The polymer was injected percutaneously to the perivascular area adjacent to the contralateral external jugular vein without graft at the time of graft placement on one side. The injection of polymer to the same region was repeated one week later. MRI was performed five days after the last injection (12 days after graft placement surgery). The polymer was reconstructed in pale green, while the blood vessels and graft were reconstructed in red, from the MR image. The sternocleidomastoid muscle was not included in the 3D reconstruction. Panel a) Dorsal view of the 3D reconstruction of the external jugular vein, adjacent polymer gel and common carotid artery. Panel b) Ventral view of the same 3D reconstruction of vein, polymer gel and artery shown in panel a). Panel c) Graphic representation of measured sirolimus concentrations in various segments of tissues collected from the same animal described in panel a) and panel b). Each color bar in the linear concentration scale represents a 187.5 ng/g increment in concentration.