Passive Coating: The Dream Does not Come True

Stainless steel and small amounts of nickel, chromium molybdenum and other contaminants tends to produce a foreign body reaction when implanted in human coronary arteries. The goal of “passive coatings” is to make the stent as neutral as possible to this recognition. The two most common approaches utilized in the clinical setting are to coat the stent with phosphorylcholine or with carbon ions. Carbon-coated stents are widely used in different clinical settings and several companies have designed and developed stents with different technology to coat the stainless steel. The initial enthusiasm for these new devices arose from several in vitro and animal studies. These studies emphasized the potential advantages of the amorphous coating in decreasing the risks of acute and subacute thrombosis and in reducing the incidence of restenosis.1–4 Namely, two pathophysiological hypothesis provided the rationale for carbon coating: 1) the metal compounds of the stainless steel alloy may trigger local immune responses and inflammatory reactions to foreign materials;1,2,5 and 2) the heavy metal ions released from the surface of the stainless steel may increase the circulating protein adhesion and thus enhancing platelet and inflammatory cells activation.2–4 Despite these theoretical advantages, the effectiveness of carbon coated coronary stents have never been fully demonstrated. The only information currently available is from the results of prospective series of lesions and patients treated in various clinical settings.6–8 In this issue, Haase et al. randomly compared the CarboStent (Sorin, Saluggia, Italy) with different bare metal stents slightly See Haase et al. on pages 562–565 differing in design and strut thickness (Tristar™, Tetra™, Penta™, Guidant, Santa Clara, California). The authors report similar incidence of binary angiographic restenosis at 6-month follow-up (18% in the CarboStent group, 21% in the stainless steel group; p = 0.56) as well as similar MACE rates, suggesting that carbon coating does not provide clinical advantages in comparison to bare metal stents. The comparison of stents which differ in thickness and design other than for the presence of carbon coating certainly represent a limitation which should have further helped the carbon-coated stent. It must be realized that the CarbonStent has thinner struts than the stents of the control group. It is likely that this manufacturing difference may have provided an additional benefit in terms of restenosis rather than reducing the potential advantages of carbon coating.9,10 Three other studies comparing carbon coated stents with bare metal have recently been completed. To overcome the issue of possible bias due to different stent designs, these three studies compared stents with same design, the only difference being the presence of carbon coating. The main difference between these studies is in the type of carbon coating as well as clinical and angiographic characteristics. However, they all reached the same finding of no significant difference in restenosis rates between carbon-coated stents and control stents (Table 1). Furthermore, there were no differences reported in the study by Haase et al. or in the other studies, in terms of overall clinical outcome. The only exception is represented by a sub-group analysis from the TRUST study (https://www.tctmd.com/expert-presentations/multi-slide.html?product_id=2272): in patients with Brawnwald III B class of unstable angina the rate of TLR at 6-months was significantly lower in lesions treated with the carbon coated stent (4.7%) than in the control group (15.3%; p = 0.023). Following the results of these studies, is it possible to give a final judgment on the role of carbon-coated stent in today’s clinical practice? The initial enthusiasm for these devices decreased as they progressed from the bench to the bedside. Certainly, the strategy of passive coating to reduce restenosis has already been outclassed by the new stents carrying and eluting drugs that actively inhibit neointimal formation.13,14 These findings are clearly not unexpected due to the fact that the foreign body reaction is only a minor element in the restenotic process. Healing following the inevitable trauma of stent dilatation and expansion against a plaque represents one of the strongest components of the hyperplastic process which can only be brought under control with an aggressive approach such as antiproliferative medications. The bastion of the lower thrombogenicity maintains an appeal especially in the setting of long stent implantation in small vessels. Unfortunately, we still need a clinical demonstration of this concept so far only supported by animal and in vitro models.1,2

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