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Long-term effects of biodegradable versus durable polymer-coated sirolimus-eluting stents on coronary arterial wall morphology assessed by virtual histology intravascular ultrasound

Hui-liang, LIU; Zhi-geng, JIN; Jian-ping, LUO; Dong-xing, MA; Sheng-li, YANG; Ying, LIU; Wei, HAN; Li-min, JING; Rong-ying, MENG; Jiao, ZHANG

Section Editor(s): WANG, Mou-yue; PAN, Cheng

doi: 10.3760/cma.j.issn.0366-6999.2011.06.007
Original article
Free
SDC

Background The durable presence of polymer coating on drug-eluting stent (DES) surface may be one of the principal reasons for stent thrombosis. The long-term coronary arterial response to biodegradable polymer-coated sirolimus-eluting stent (BSES) in vivo remained unclear.

Methods Forty-one patients were enrolled in this study and virtual histology intravascular ultrasound (VH-IVUS) was performed to assess the native artery vascular responses to BSES compared with durable polymer-coated SES (DSES) during long-term follow-up (median: 8 months). The incidence of necrotic core abutting to the lumen was evaluated at follow-up.

Results With similar in-stent late luminal loss (0.15 mm (0.06-0.30 mm) vs. 0.19 mm (0.03-0.30 mm), P=0.772), the overall incidence of necrotic core abutting to the lumen was significantly less in BSES group than in DSES group (44% vs. 63%, P <0.05) (proximal 18%, stented site 14% and distal 12% in BSES group, proximal 19%, stented site 28% and distal 16% in DSES group). The DSES-treated segments had a significant higher incidence of necrotic core abutting to the lumen through the stent struts (73% vs. 36%, P <0.01). In addition, more multiple necrotic core abutting to the lumen was observed in DSES group (overall: 63% vs. 36%, P <0.05). Furthermore, when the stented segments with necrotic core abutting to the lumen had been taken into account only, DSES-treated lesions tended to contain more multiple necrotic core abutting to the lumen through the stent struts than BSES-treated lesions (74% vs. 33%), although there was no statistically significant difference between them (P=0.06).

Conclusions By VH-IVUS analysis at follow-up, a greater frequency of stable lesion morphometry was shown in lesions treated with BSESs compared with lesions treated with DSESs. The major reason was BSES produced less toxicity to the arterial wall and facilitated neointimal healing as a result of polymer coating on DES surface biodegraded as time went by.

Edited by

Department of Cardiology, General Hospital of Chinese People's Armed Police Forces, Beijing 100039, China (Liu HL, Jin ZG, Luo JP, Ma DX, Yang SL, Liu Y, Han W, Jing LM, Meng RY and Zhang J)

Correspondence to: Dr. LIU Hui-liang, Department of Cardiology, General Hospital of Chinese People's Armed Police Forces, Beijing 100039, China (Email: lhl518@vip.sina.com)

(Received December 8, 2010)

It has been proved that the first-generation drug-eluting stents (DESs) are more effective than bare-metal stents (BMSs) in reducing the risk of in-stent restenosis (ISR) and target lesion revascularization mainly by suppressing in-stent neointima hyperplasia.1,2 Nevertheless, the long-term safety of the current DES arouses a lot of controversy as numerous late and very late stent thrombosis have been reported.3-6 Pathological autopsy study results indicate that the durable presence of polymer coating on DES surface may be one of the principal reasons for localized arterial wall inflammation, hypersensitivity reactions, delayed or incomplete neointimal healing, and induction of stent thrombosis.7,8

In recent years, biodegradable polymer-coated sirolimus-eluting stent (BSES) was developed to avoid the above adverse effects possibly caused by persistent residue of drug-carrier coating. Compared with the first-generation DESs and other types of BSESs, the EXCELTM SES (JWMS, China) uses a novel polylactic acid material, which is gradually biodegraded within approximately 6 months and eventually fully metabolized to water and carbon dioxide in vivo. Moreover, the drug is only coated at the side facing to the arterial wall. The biodegradable and unique abluminal coated polymer would have theoretic advantages of producing less toxicity to the arterial wall and facilitating neointimal healing, while remarkably reducing the incidence of ISR and major adverse cardiac events (MACE).9,10

Virtual histology intravascular ultrasound (VH-IVUS) has demonstrated its potential to assess coronary plaque composition objectively and accurately both in vitro and in vivo.11,12 Previous study using serial VH-IVUS analysis of lesions treated with durable polymer-coated SESs (DSESs) has demonstrated a greater incidence of unstable lesion morphometry at follow-up, compared with BMSs.13 However, the long-term coronary arterial response to the BSES in vivo remained unclear. The purpose of this study was to assess long-term native artery vascular responses after implantation of BSES compared with DSES using VH-IVUS analysis.

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METHODS

Sample

Between May and November 2010, we identified a total of 41 patients only implanted in de novo coronary lesions with BSESs (EXCELTM, JWMS) or/and DSESs (Cypher-SelectTM, Cordis, USA) in the percutaneous coronary intervention (PCI) database of our catheter center. Then, angiography and VH-IVUS were performed at follow-up. According to the different type of DES that had been implanted in prior intervention, they were divided into two groups: EXCEL group and CYPHER group. Exclusion criteria: age <18 years or >80 years; follow-up period <6 months or >24 months; chronic total occlusions; bifurcation lesions needing kissing balloon or with a diameter >2.5 mm side branch; left main lesions; coronary artery bypass graft history; target vessel treated with two types DES; previous PCI complication (presence of no-reflow or slow-reflow or dissection in procedure); unsuitable for VH-IVUS interrogation (presence of extensive angiographic calcification and/or severe vessel tortuosity). This study was approved by the institutional review boards of the institutions in which the procedures were performed, and all patients gave written informed consent before enrollment.

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Quantitave coronary angiography (QCA)

Prior angiographic data regarding pre-, peri-, and postprocedure were retrieved from CD stored on digitally after last procedure. Follow-up coronary angiography was performed and data were digitally recorded. The analysis was performed using automated edge-detection software (QAngio XA version 7.2, Medis Medical Imaging System, Netherlands) at a single projection showing the most severe stenosis. The same projection was used at follow-up. The proximal and distal edges were evaluated up to 5 mm from the stent.

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VH-IVUS acquisition and analysis

Before imaging, 200 μg of intracoronary nitroglycerin were administered. A 20 MHz, 2.9 Fr intravascular ultrasound catheter (Eagle Eye, Volcano Corp., USA) was advanced >10 mm beyond the distal stent edge, and automated pullback was performed at a speed of 0.5 mm/s using a mechanical pullback device (Volcano Corp.), ending at the coronary ostium. Virtual histology images were generated simultaneously during motorized pullback and acquired at every R-peak during continuous electrocardiogram (ECG) registration. Volumetric data were generated by software using the Simpson's method with a dedicated VH-IVUS console (IVG3, Volcano Corp.). Data were stored digitally on DVD for offline analysis. VH-IVUS analysis classified the color-coded tissue into four major components: green (fibrotic (FT)), yellow-green (fibrofatty (FF)), white (dense calcium (DC)), and red (necrotic core (NC)).11,12

The region of interest (ROI) defined as the stented segment and the stent edge (5 mm length adjacent to each stent edge) was assessed every 1 mm. The external elastic membrane (EEM), lumen, plaque and media (P&M, EEM minus lumen) and P&M components were measured at each stent edge. In-stent measurements were also obtained including EEM, lumen, plaque behind the struts (PBS, EEM minus stent).

It has been already reported that the struts of both DES and BMS are misinterpreted by VH-IVUS as areas of apparent “white surrounding red”. Moreover, there was no change between after stenting immediately and follow-up.13,14 In order to analyze the PBS tissue characteristics using VH-IVUS, drawing a contour, behind the struts of DES, delineated a region of PBS excluding the struts (Figure 1).

Figure 1.

Figure 1.

Repeated frames were caused by the catheter getting stuck during pullback. In the present study only a small number of such frames were observed. However, to minimize their influence on results for absolute plaque volume, repeated frames were ignored during this analysis. All above, VH-IVUS analysis results were reported as absolute and relative volume. All volumes calculated using Simpson's rule were normalized for analysis length (normalized volume).

In addition, we evaluated the presence of NC abutting to the lumen (Figure 2), defined as a confluent NC >10% of plaque area without evidence of overlying non-NC tissue in the stented site and in the each stent edge. Moreover, the NC abutting to the lumen was consistent with a VH-IVUS-identified thin-cap fibroatheroma (VH-TCFA).13 When there were more than one NC abutting to the lumen appeared at the same segment, we defined it as multiple NC abutting to the lumen. Importantly, as stent struts had a VH-IVUS appearance of “white surrounding red”, this surrounding NC identified with the help of the corresponding grey-scale image, was not interpreted as NC abutting to the lumen. The only things which we diagnosed as NC abutting to the lumen was the one independent with stent strut artifact. In other words, if there was a clear stent strut, we did not count as NC abutting to the lumen. The ring-down artifact (presented as a “flare” around the IVUS catheter) was excluded manually and had not been put into account also.

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Statistical analysis

SPSS 13.0 (SPSS Inc., USA) for windows was used for all analysis. Categorical variables were presented as numbers or percentages and compared using chi-square statistics or Fisher's exact test as appropriate. Continuous variables were presented as medians and interquartile ranges and compared using Student's t test. Moreover, if normality assumption was violated, Mann-Whitney U test or Wilcoxon signed ranks test would be used. For all comparisons, a P value of <0.05 (2-sided) was considered statistically significant.

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RESULTS

Baseline characteristics

As shown in Table 1, a total of 41 patients were enrolled in present study (21 in EXCEL and 20 in CYPHER). Most of patients had received PCI last time due to unstable angina pectoris and no stable angina pectoris. There were no differences in levels of lipid and fasting blood glucose between two groups preintervention and at follow-up. However, there were declines in both levels of total cholesterol and low-density lipoprotein (LDL) cholesterol from preintervention to follow-up in the two groups. The follow-up LDL levels in the two groups were both <2.59 mmol/L. Moreover, the fasting blood glucose levels decreased from preintervention to follow-up in CYPHER group, but not in EXCEL group. All patients had taken aspirin (100 mg/d) and statin (atorvastatin, 20 mg/d) after PCI without intermission and clopidogrel (75 mg/d) up to 12 months. No insulin had been used among them. The overall follow-up period was 8 months (6-12 months), and no differences between EXCEL and CYPHER groups (P=0.187).

Table 1

Table 1

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Characteristics of lesions and last procedure and results of QCA

Table 2 showed the characteristics of lesions and last procedure. QCA analysis of late luminal loss at follow-up was presented at Table 3. There were 28 lesions treated with 33 EXCELTM SESs and 22 lesions treated with 26 Cypher-SelectTM SESs. In addition, there was one ostium lesion in EXCEL, but two in CYPHER (data were not shown). As a result, QCA analysis had been done in a total of 47 proximal stent edges (27 in EXCEL and 20 in CYPHER), 50 distal stent edges (28 in EXCEL and 22 in CYPHER), and every stent. Both two groups had the similar characteristics of lesions and procedure. The in-stent late luminal loss was 0.15 mm in EXCEL group, and 0.19 mm in CYPHER group. No significant difference had been found between them (P=0.772). Moreover, there were no obvious differences of late luminal loss in stent edges between two groups.

Table 2

Table 2

Table 3

Table 3

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Virtual histology intravascular ultrasound analysis

The results of proximal and distal stent edges of VH-IVUS volumetric analysis at follow-up are shown in Tables 4 and 5. At the proximal stent edges, the absolute volumes of EEM, lumen and P&M were parallel between EXCEL and CYPHER groups. There were no differences in normalized volumes of EEM, lumen or P&M at proximal stent edges. The VH-IVUS assessments of the composition of PBS at proximal stent edges showed no differences among the four types of components between two groups. The main component at proximal stent edges appeared as fibrous tissue in both EXCEL and CYPHER groups. The distal stent edges had similar presentation with the proximal stent edges between EXCEL and CYPHER groups. No differences were found in absolute or normalized volumes of EEM, lumen, P&M or four types of components at distal stent edges.

Table 4

Table 4

Table 5

Table 5

At stented sites, as shown in Table 6, a total of 50 segments performed VH-IVUS analysis (28 in EXCEL and 22 in CYPHER). Between EXCEL and CYPHER groups, both data of absolute and normalized volumes of EEM, lumen and phosphate-buffered saline (PBS) made no change. Although, between the two groups, no differences existed in absolute and normalized volumes of each four component in PBS, EXCEL group had a larger proportion of NC than CYPHER group. Moreover, the proportion of other three types of components made no difference in evidence. Both of the PBS in EXCEL and CYPHER groups were composed mainly of fibrous tissue. Moreover, we noted that, expect six lesions, most of the DES-treated lesions showed uncovered with neointima in the VH-IVUS images.

Table 6

Table 6

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Incidence of NC abutting to the lumen

There were 156 segments used to analyze the incidence of NC abutting to the lumen. In EXCEL group, the overall incidence of NC abutting to the lumen was 44% (proximal 18%, stented site 14% and distal 12%). On the contrary, in CYPHER group, this overall incidence increased up to 63% (proximal 19%, stented site 28% and distal 16%) compared with EXCEL group (P <0.05, Figure 3A). No difference had been found in each group with regard to the distribution of NC abutting to the lumen among all three segments. There was no significant change regarding the incidence of NC abutting to the lumen either in proximal (Figure 3B) or distal (Figure 3D) stent edge between the two groups. However, when compared with stented segments each other, the CYPHER-treated segments had a significant higher incidence of NC abutting to the lumen through the stent struts (73% vs. 36%, P <0.01, Figure 3C). In addition, more multiple NC abutting to the lumen was observed in CYPHER group (overall: 63% vs. 36%, P <0.05, Figure 4A). No obvious difference among the distribution of multiple NC abutting to the lumen in all three segments was shown in each group (proximal 18%, stented site 10% and distal 8% in EXCEL; proximal 19%, stented site 32% and distal 12% in CYPHER. Furthermore, when the stented segments with NC abutting to the lumen had been taken into account only, CYPHER-treated lesions tented to contain more multiple NC abutting to the lumen through the stent struts than EXCEL-treated lesions (74% vs. 33%), although there was no statistically significant difference between them (P=0.06, Figure 4C). The same trend was observed at proximal (Figure 4B) and distal (Figure 4D) stent edges between the two groups.

Figure 3.

Figure 3.

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DISCUSSION

In this VH-IVUS study, it was mainly demonstrated that a greater frequency of stable lesion morphometry during follow-up was shown in lesions treated with BSESs compared with lesions treated with DSESs. Furthermore, it was similar that the stent struts were interpreted in VH-IVUS images as a red halo surrounding the white stent metal at follow-up in both BSES and DSES.

However, in present study, none of the patients had VH-IVUS pre- or post-intervention immediately, the results remained convincing to some extent. First, previous intravascular ultrasound study showed that LDL cholesterol level during follow-up was the only independent predictor of changes in coronary plaque size. When patients achieved a LDL cholesterol level <100 mg/dl (approximately equal to 2.59 mmol/L) during follow-up, regression or no progression of coronary plaque was expected.15 Other intravascular ultrasound studies showed that the DES did not affect plaque burden located behind the struts.13,16 In this study, the LDL levels during follow-up in the two groups were both <100 mg/dl. And that no differences had been found among the volumes of EEM, lumen or PBS between two groups, implied that with similar plaque burden preinterventian no differences had been detected during follow-up. These suggested that the unstable lesion morphometry observed in DSES-treated lesions during follow-up should not be the result of plaque progression.

Second, it has been already reported that the struts of DES are misinterpreted by VH-IVUS as areas of “DC surrounding NC”, and there is no change between postintervention immediately and follow-up.13,14 Moreover, this red “halo” is presumably an artifact and should not be interpreted as peristrut inflammation or NC when observed at follow-up.13,14 Although this artifact could led to a directly proportional increase in the area coded as NC in VH-IVUS, these increase strictly follow a linear, indicating that it may be amenable to mathematical correction.17 Compared with EXCELTM SES, Cypher-SelectTM SES has a thicker metal strut (140 μm vs. 119μm).10,18 It suggests that the PBS in lesions treated with Cypher-SelectTM SESs may have more NC area induced by the stent struts with the same stent size. In contrast, with similar diameter and length of the stent, the relative NC volume in PBS, in present study, was higher in BSES-treated lesions, which could not be caused by the increase of artifacts aroused from the addition of the struts.

Third, different plaque characteristics preintervention (stable or vulnerable plaque) have an important impact on the results in the present study. Characteristics of vulnerable plaque mainly include localized expansive enlargement of the vessel wall (“positive remodeling”), microcalcification within the plaque and VH-TCFA. No matter which type of vulnerable plaque mentioned above, all of them have a common feature containing a large amount of NC. Kim et al19 demonstrated that DES implantation into a TCFA results in NC contact with the lumen in almost 50% of lesions. Recently, Hong et al20 indicated that baseline relative NC at the minimum lumen area site was an independent predictor of plaque progression during follow-up. However, in this study, BSES-treated lesions had a larger proportion of NC than DSES-treated lesions. Even so, they still showed stable morphometry, no matter whether this greater proportion of NC was existed before or after intervention and during follow-up.

Finally, statin therapy also has an important impact on the results of present study due to the capacity to change the plaque composition during follow-up. Nasu et al21 reported that one-year lipid-lowering therapy by fluvastatin showed significant regression of plaque volume and alterations in atherosclerotic plaque composition with a significant reduction of fibro-fatty volume, however, NC and DC volumes remained unchanged. Another recent study suggested that the usual dose of statin cannot change NC or DC easily compared to fibrotic tissue or fibrofatty.20 Moreover, patients of both two groups received the same statin therapy with atorvastatin 20 mg/d after PCI without intermission. Consequently, the lower proportion of NC in DSES-treated lesions behind the struts was also not the result of statin therapy, but more likely due to the NC content existing before intervention or postintervention immediately.

The major reason for this stable morphometry of lesions treated with BSESs during follow-up was BSES produced less toxicity to the arterial wall and facilitated neointimal healing as a result of polymer coating on DES surface biodegraded as time went by. Because the durable presence of polymer surface coating may be one of the principal reasons for localized arterial wall inflammation, hypersensitivity reactions, delayed or incomplete neointimal healing, and induction of stent thrombosis.7,8 At present study, as two types of the stents used in present study have the similar properties in terms of drug and metal,10,22,23 it is deemed that the biodegradable polymer on BSES surface should be the major cause of this phenomenon. Moreover, the unique abluminal coating of the EXCEL stent, that is, only located on the outside surface facing vessel wall, can minimize the possible adverse effects of polymer on vascular healing while allowing controlled unidirectional drug release.10

Although in this study, except six lesions, most of the DES-treated lesions were uncovered with neointima in the VH-IVUS images, this stable morphometry in lesions treated with BSESs was still probably a consequence of facilitating neointimal healing as polymer coating on DES surface biodegraded. BSES and DSES used in present study have the similar in-stent late lumenal loss, compared with the previous studies (0.12 mm and 0.19 mm).10,24 It suggests that the in-stent neointima hyperplasia is parallel with the former reports. In addition, prior studies showed that VH-IVUS has an axial resolution of 150 μm and spatial accuracy of 240 μm,25 and most of the struts of DES with neointimal thickness <100 μm at 6-12 months follow-up.26 Moreover, 75% of the stented segments do not exhibit neointimal hyperplasia during follow-up.16 Accordingly, the VH-IVUS images of struts appearing uncovered with neointima were mainly due to the reasons mentioned above.

Pathologic studies have proposed that a thin-capped fibroatheroma (cap thickness <65 μm) is a precursor of plaque rupture.27 VH-TCFA is used to describe this vulnerable plaque in VH-IVUS images defined as focal, NC-rich (≥10% of the plaque cross-sectional area) plaques (≥40% plaque burden) in contact with the lumen.28 The NC abutting to the lumen through the struts could represent lesion vulnerability after stenting by VH-IVUS.13 The persistence of a thin-capped fibroatheroma behind stent struts might contribute to future coronary thrombosis. As reported in prior trials, EXCELTM SES has a lower cumulative rate of MACE than Cypher-SelectTM SES (range: 2.7%-4.8% vs. 5.8%-6.51%), and a lower incidence of late stent thrombosis (range: 0-0.34% vs. 0.19%-0.50%) at 9-12 months follow-up.10,23,24,29-31 This stable morphometry in lesions treated with BSESs could be one of the major causes for the above decrease. The results of Providing Regional Observations to Study Predictors of Events in the Coronary Tree (PROSPECT) trial32 has indicated that the combination of large plaque burden (>70%), small minimum lumen diameter (≤4 mm2) and a large NC without a visible cap (VH-TCFA) can identify the lesions which are at especially high risk for future adverse cardiovascular events (predictive incidence=18.2%). However, approximately 12% of patients develop MACE from non-culprit lesions during 3 years of follow-up, which was less than the expected result and likely attributed to the optimal medical therapy. Therefore, the effects of this stable morphometry on clinical prognosis still remain a controversy, and long-term clinical follow-up is needed in future.

This study had several limitations. First, the sample was relatively small. Second, VH-IVUS data pre- and post-intervention were not available. Third, VH-IVUS images were acquired at every R-peak during continuous ECG registration and some lesion information were lost as a result. Fourth, VH-IVUS has not been validated for neointimal hyperplasia. Finally, tissue structures beyond the resolution of VH-IVUS can not be identified with VH-IVUS.

In conclusion, by VH-IVUS analysis at follow-up, a greater frequency of stable lesion morphometry was shown in lesions treated with BSESs compared with lesions treated with DSESs. The major reason was BSES produced less toxicity to the arterial wall and facilitated neointimal healing as a result of polymer coating on DES surface biodegraded as time went by.

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Keywords:

biodegradable; polymers; drug-eluting stents; virtual histology; intravascular ultrasound

© 2011 Chinese Medical Association