The future of bioresorbable vascular scaffolds: niche or workhorse devices?
The main benefit of stenting compared to balloon angioplasty is the reduction of restenosis within the first year of the intervention, with a limited clinical need for vessel scaffolding after this period 1. Coronary stenting has witnessed several advances in the past three decades that include the use of high-pressure implantation (largely based on intravascular ultrasound use), the use of dual antiplatelet therapy, the change in stent geometry from simple (i.e. wire mesh/coil, tubular slotted design) to complex designs (i.e. modular stents), the thinning of the struts, the addition of antiproliferatives, and the development of fluorinated and biodegradable drug polymers. These advances have revolutionized the field of interventional cardiology, but have not led to an eradication of device-related complications.
Long-term follow-up data are now available for the first-generation (paclitaxel and sirolimus) drug-eluting stents (DES) introduced in 2003 and 2004. Their use resulted in a significant reduction of in-stent restenosis compared with bare metal stents, but at the cost of an increased risk of late stent thrombosis. As reported recently from the SORT OUT II trial, the long-term event rates are linear (Fig. 1a), with a steady 1.3% annual rate of stent thrombosis throughout 10 years 2. Second-generation DES have shown better efficacy and safety compared with first-generation DES, but continue to have a steady rate of target lesion failure (TLF) of ∼2% annual rate of events that carries a relatively similar slope to first-generation DES 4. The phenomena of incomplete endothelialization, polymer hypersensitivity, and neoatherosclerosis may still occur and may be related to the persistence of the metallic implant within the arterial wall. Permanent vessel scaffolding also carries distinct disadvantages including jailing of side branches, hindrance with placement of bypass grafts to the stented segment, and limiting the use of noninvasive imaging 5.
Current bioresorbable vascular scaffolds/stents (BRS) consist of a bioresorbable polymer [often poly L-lactide acid (PLLA)] or alloy (usually of magnesium), a drug-releasing coating, and a time-release antiproliferative agent. To attain radial strength similar to that of contemporary metallic DES, PLLA and metallic alloy-based BRS have required strut thicknesses of 120–160 μm, far in excess of the typical 70–80 μm DES. As this would be expected to induce flow turbulence-related excess platelet deposition and to delay healing (Fig. 1b), an early risk of device thrombosis might be anticipated. An important tenant of the BRS value proposition is that any early excess risk would be offset by long-term benefit originating from the absence of a permanent device and a reduction in device-related complications. The strut thickness differences between the current generation of BRS and DES help explain the differences in outcomes noted in trials conducted to date.
The Absorb of bioresorbable vascular scaffold (BVS) has been the most extensively studied BRS, and its efficacy and safety compared with the Xience (Abbott Vascular, Santa Clara, California, USA) everolimus-eluting stent has been studied in a series of randomized-controlled trials (RCTs). Absorb has a PLLA scaffold and elutes everolimus at a similar dose to the Xience cobalt-chromium metallic stent. The results of the ABSORB trials have been mixed, and have raised concern on the use of BVS as a workhorse device. We will briefly contrast the clinical results between the two devices in terms of TLF and device thrombosis.