Dose and dose rate effects of beta-particle emitting radioactive stents in a porcine model of restenosis

Preclinical restenosis models and drug-eluting stents: still important, still much to learn

Percutaneous coronary intervention continues to revolutionize the treatment of coronary atherosclerosis. Restenosis remains a significant problem but may at last be yielding to technologic advances. The examination of neointimal hyperplasia in injured animal artery models has helped in our understanding of angioplasty and stenting mechanisms, and as drug-eluting stent (DES) technologies have arrived, they too have been advanced through the study of animal models. These models are useful for predicting adverse clinical outcomes in patients with DESs because suboptimal animal model studies typically lead to problematic human trials. Similarly, stent thrombosis in animal models suggests stent thrombogenicity in human patients. Equivocal animal model results at six or nine months occasionally have been mirrored by excellent clinical outcomes in patients. The causes of such disparities are unclear but may result from differing methods, including less injury severity than originally described in the models. Ongoing research into animal models will reconcile apparent differences with clinical trials and advance our understanding of how to apply animal models to clinical stenting in the era of DESs.

Pathogenetic heterogeneity of in-stent lesion formation in human peripheral arterial disease

PURPOSE

Intimal hyperplasia is known to promote in-stent restenosis after vascular stent implantation. Although previous studies have presented a variety of evidence that suggests the mechanisms of intimal lesion formation, it is still controversial which factor(s) predominantly contribute to the development of in-stent restenosis. In this study, we hypothesized that heterogeneous mechanisms coexist in the same lesion and then assessed the validity with resected whole arteries with in-stent lesions.

METHODS

Whole arterial specimens with in-stent lesions were surgically resected from five patients who had undergone implantation of a Palmaz-Schatz stent 7 to 19 months previously and were histologically analyzed. For assessment of the pathogenetic heterogeneity of in-stent lesions, we divided each cross section into three parts: the inner intima within 250 microm from the luminal surface (zone A), the area surrounding the stent struts within 250 microm from the strut hole (zone B), and the remaining part of the intimal layer (zone C). We then evaluated cell density, cell replication, and cellular composition in each zone. Cell replication and cellular composition were analyzed with immunohistologic staining with antibodies against proliferating cell nuclear antigen and cell-specific antibodies. Each section was also stained with Alcian blue or Elastica van Gieson method for detection of matrix components.

RESULTS

In all samples, the cell density of zone A was significantly higher than that of zone B (P <.05). Proliferating cell nuclear antigen staining results showed significantly higher cell replication in zone A as compared with that in other zones (P <.05). To the contrary, cell-specific immunostaining results revealed marked accumulation of leukocytes, macrophages, and T lymphocytes in zone B (P <.05). Regarding matrix components, proteoglycan was predominantly stained around stent struts and in the inner intima.

CONCLUSION

The data of this study showed that two different pathogenetic processes in different zones possibly contributed to in-stent lesion formation at the same time. One process was an increase of cell number in the inner intima, which was the result of a prolonged increase of cell replication. The other process was accumulation of matrix around stent struts, which was suggested to be linked to infiltration of inflammatory cells in the same zone.

Vascular morphometric changes after radioactive stent implantation: a dose-response analysis

OBJECTIVES

The goal of this study was to evaluate the dose-dependency of morphometric changes in the coronary arterial wall after radioactive stenting.

BACKGROUND

Radioactive stents have been found to reduce intrastent intimal hyperplasia (IIH) but lead to a characteristic type of restenosis occurring predominantly at the stent edges.

METHODS

Fifteen patients underwent intravascular ultrasound (IVUS) examination after implantation of a P-32 radioactive stent and at the six-month follow-up. The post-stent IVUS measurements on seven predefined locations of each lesion were subjected to a computer algorithm for the development of dose-volume histograms (DVH). Thus, we derived the radiation doses delivered to at least 10% and 90% of the adventitia (DV10, DV90). The IIH and vascular remodeling at follow-up were correlated with the doses in each segment.

RESULTS

The IIH was most pronounced at the stent edges and lowest in the stent-body, whereas we detected a significant expansive remodeling within the stent body. The delivered doses correlated with a decreased IIH (r = 0.52, p < 0.001 for DV10 and r = 0.62, p < 0.001 for DV90) and with expansive remodeling (r = 0.48, p = 0.009 for DV10 and r = 0.50, p = 0.006 for DV90). A DV10 >90 Gy or a DV90 >15 Gy reduced IIH and induced expansive remodeling. Plaque growth was not reduced by radioactive stents.

CONCLUSIONS

The DVH analysis reveals a dose-dependent increase of external elastic lamina area behind radioactive stents, whereas plaque growth is not reduced but inverted into an outward direction from the stent. A DV10 >90 Gy or a DV90 >15 Gy results in a beneficial long-term outcome after radioactive stenting.

Dosimetry of a 188rhenium-labeled self-expanding stent for endovascular brachytherapy in peripheral arteries

PURPOSE

Radioactive stents have been proposed as endovascular irradiation device to prevent in-stent restenosis by inhibiting neointimal proliferation. 32P-stents have been used in several studies so far, but require large-scale labeling procedures and endovascular barotrauma for stent expansion supporting the development of edge restenosis. Purpose of this study was to establish dosimetry of a self-expanding nitinol stent for peripheral vascular disease, which was radiolabeled with 188rhenium (188Re) by a dip coating technique.

METHODS AND MATERIALS

The surface of nitinol Memotherm FLEXX stents was polymer-coated providing functional NH(2) groups for diethylenetriaminepentaacetic acid (DTPA) binding, providing the ligand for the complexation of 188Re onto the stent surface. Stability of radiolabeling was tested over 48 h using an in vitro blood circulation (Chandler Loop). Radial and longitudinal dose distributions of a radiolabeled stent were obtained with a plastic scintillator dosimetry system.

RESULTS

Stents with a length of 30 mm and a diameter of 8 mm were labeled with up to 33 MBq 188Re. A total of 69+/-4% of the labeled 188Re remained stable on the stent surface after 48 h. Ninety-five percent of the infinitely accumulated dose was supplied to the target tissue within 72 h. Including correction for radioactivity washout from the stent, the infinitely accumulated dose at 1 mm radial distance from the stent surface was 1.85+/-0.19 Gy/MBq 188Re/cm stent length.

CONCLUSIONS

We developed a technique for radiolabeling of self-expanding nitinol stents with 188Re by dip coating and formation of 188Re chelate complexes. We provide dosimetry data useful for application of this beta-emitting stent for endovascular brachytherapy in peripheral vascular occlusive disease.