Almost two years after implantation, Sirilimus-eluting coronary stents remain exposed to the circulation. August 11, 2007

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August 11, 2007 By Benjamin A. Olenchock, M.D. Ph.D. [1]

Japan A new study from investigators in Japan has used angioscopy to monitor the appearance of coronary stents over a two year follow-up period. They found that Sirilimus-eluting stents (SES) remained incompletely covered by endothelium, even two years after implantation. Their findings, published online in the journal Circulation, are sure to contribute to the ongoing discussion about the risk of very late stent thrombosis in patients who receive drug-eluting stents (DES).

Coronary stents are life-saving in patients with acute coronary syndromes. There are two consequences, however, of placing metal in the lumen of coronary arteries. First, stents are very thrombogenic, activating circulating platelets and clotting factors. This problem is ameliorated by dual anti-platelet therapy with aspirin and a thieopyridine (e.g. ticlopidine or clopidogrel). Over time, the hope is that the normal lining of the blood vessels, the endothelium, re-grows and covers the stents so that the risk of thrombosis is reduced and thienopyridines can be discontinued. The second problem is that coronary stents can activate a local inflammatory response, resulting in fibrosis and restenosis of the coronary artery. This second problem is greatly improved by DES, which elaborate anti-proliferative drugs to stop the inflammatory process. The current discussion is whether DES also prevent the protective endothelialization, with the consequence being that the risk of stent thrombosis is persistent and dual anti-platelet therapy is needed indefinitely.

Investigators in Japan followed 17 patients who received 28 coronary stents. They used angioscopy to directly visualize the coronary lesions where the stents were deployed, and asked very simple questions: how well were the stents covered with endothelium, where there any blood clots visualized within the stents, and what does the coronary plaque look like? First, their data verified that SES prevent re-stenosis. By 20-months follow up, the percent diameter of stenosis was less in SES vs. bare-metal stents (BMS; 26.4 vs. 16.0, p=0.1), and late loss was improved (0.6 vs. 0.2, p=0.01). Endothelialization, however, was markedly different. All of the 11 BMS were completely covered with endothelium by 3-6 months after implantation, while 0 of 14 SES were completely covered at 20-months of follow-up. Stents with less endothelialization were more likely to have a thrombus inside the stent (p=0.002) and a yellow coronary plaque visualized (p<0.001). No thrombi were observed in BMS, while thrombi were seen in 4 DES. The authors speculate that thrombus might represent part of the healing process, which appears to be delayed in the SES group.

This small, non-randomized, single-center study has obvious limitations. However, it is the longest angioscopy follow-up study published to date and it demonstrated a clear, statistically significant difference between SES and BMS with a very small sample size. Delayed endothelialization of DES has been documented in intravascular ultrasound studies as well. The clinical question remains whether the delayed healing process after implantation of DES requires prolonged, or indefinite dual anti-platelet therapy in order to prevent the infrequent but devastating occurrence of very late stent thrombosis.