Clinical Outcomes of Bioresorbable Scaffold in Coronary Artery Disease: A Systematic Literature Review

OBJECTIVES

We aim to perform a systematic literature review on all studies reporting the clinical outcomes of the use of bioresorbable scaffolds (BRS) in different settings of coronary artery disease (CAD).

BACKGROUND

BRS are designed to provide early support of the vessel wall postangioplasty, deliver antiproliferative agents to prevent excessive hyperplastic healing responses and finally "disappear" when no longer required. Emerging data have provided evidence of their use in specific clinical scenarios.

METHODS

A comprehensive literature search was performed by 2 independent reviewers utilizing MEDLINE, EMBASE, and Cochrane Library databases. The only 2 CE marked BRS: everolimus-eluting Bioresorbable vascular scaffold ABSORB BVS and the myolimus-eluting DESolve Bioresorbable Coronary Scaffold (BCS) System were included.

RESULTS

The studies were categorized into: ST elevation myocardial infarction (STEMI), stable CAD, and "all-comers" group. Thirty-one studies were included; 8 in STEMI patients (all ABSORB), 15 in stable CAD patients. In the STEMI group (n = 606), acute procedural success ranged from 96% to 100%, cardiac mortality 0-9.1%, recurrent MI and stent thrombosis rates were 0-4.3%. In the stable CAD group, the 13 ABSORB studies (n = 3259) demonstrated cardiac mortality rate of 0-0.6%, recurrent MI rate 0-4.5%, and stent thrombosis rate 0-4.3%

CONCLUSIONS

Current clinical data suggest the BRS, especially the ABSORB BVS, may represent a reasonable alternative to DES in uncomplicated coronary anatomy.

Percutaneous coronary intervention with everolimus-eluting bioresorbable vascular scaffolds in routine clinical practice: early and midterm outcomes from the European multicentre GHOST-EU registry

AIMS

Clinical data on the early and midterm outcomes of bioresorbable vascular scaffolds (BVS) in routine clinical practice are limited. To fill this gap, we report on the early and midterm clinical outcomes of PCI with everolimus-eluting BVS from the large multicentre GHOST-EU registry.

METHODS AND RESULTS

Between November 2011 and January 2014, 1,189 patients underwent percutaneous coronary intervention with one or more BVS (Absorb BVS; Abbott Vascular, Santa Clara, CA, USA) at 10 European centres. The primary outcome of interest was target lesion failure (TLF), defined as the combination of cardiac death, target vessel myocardial infarction, or clinically driven target lesion revascularisation (TLR). A total of 1,731 Absorb BVS were implanted at a mean of 12.3±3.4 atm. Technical success was achieved in 99.7% of cases. TLF was recorded in 67 of 1,189 patients at a median of 109 (interquartile range 8-227) days after implantation. The cumulative incidence of TLF was 2.2% at 30 days and 4.4% at six months. The annualised rate of TLF was 10.1%. At six months, the rate of cardiac death was 1.0%, target vessel myocardial infarction was 2.0%, TLR was 2.5%, and target vessel revascularisation was 4.0%. Diabetes mellitus was the only independent predictor of TLF (hazard ratio 2.41, 95% confidence interval: 1.28-4.53; p=0.006). The cumulative incidence of definite/probable scaffold thrombosis was 1.5% at 30 days and 2.1% at six months, with 16 of 23 cases occurring within 30 days.

CONCLUSIONS

"Real-world" outcomes of BVS showed acceptable rates of TLF at six months, although the rates of early and midterm scaffold thrombosis, mostly clustered within 30 days, were not negligible.

One-Year Clinical and Computed Tomography Angiographic Outcomes After Bioresorbable Vascular Scaffold Implantation During Primary Percutaneous Coronary Intervention for ST-Segment-Elevation Myocardial Infarction: The PRAGUE-19 Study

BACKGROUND

Bioresorbable vascular scaffolds (BVS) represent promising new technology, but data on their long-term outcomes in ST-segment-elevation myocardial infarction (STEMI) setting are missing. The aim was to analyze 1-year clinical and computed tomographic angiographic outcomes after BVS implantation in STEMI.

METHODS AND RESULTS

PRAGUE-19 is a prospective multicenter single-arm study enrolling consecutive STEMI patients undergoing primary percutaneous coronary intervention (pPCI) with intention-to-implant BVS. A total of 343 STEMI patients were screened during 15 months enrollment period, and 70 patients (mean age 58.6±10.3 and 74% males) fulfilled entry criteria and BVS was successfully implanted in 96% of them. All patients were invited for clinical and computed tomographic angiographic control 1 year after BVS implantation. Restenosis was defined as ≥75% area stenosis within the scaffolded segment. Three events were potentially related to BVS: 1 in-stent restenosis (treated 7 months after pPCI with drug-eluting balloon), 1 stent thrombosis (treated 2 weeks after pPCI by balloon dilatation-this patient stopped all medications after pPCI), and 1 sudden death at home 9 months after pPCI. Four other patients had events definitely unrelated to BVS. Overall, 1-year mortality was 2.9%. Computed tomographic angiography after 1 year was performed in 59 patients. All BVS were widely patent, and binary restenosis rate was 2% (the only restenosis mentioned above). Mean in-scaffold minimal luminal area was 7.8±2.6 mm(2), area stenosis was 20.1±16.3%, minimal luminal diameter was 3.0±0.6 mm, and diameter stenosis was 12.8±11.1%.

CONCLUSIONS

BVS implantation in STEMI is feasible and safe and offers excellent 1-year clinical and angiographic outcomes.

[What is the place of bioresorbable vascular scaffolds in setting of ST-segment elevation myocardial infarction?]

A 50-year-old woman was admitted for an inferior ST-segment elevation myocardial infarction; immediate coronary angiogram revealed a subocclusive stenosis of the right coronary artery. After optimal antithrombotic treatment, the type of stent could be discussed. The latest generation of drug-eluting stents showed excellent efficacy and safety in the long-term but has limitations such as potential chronic inflammation of the arterial wall and no recovery of vasoactive function. Bioresorbable vascular scaffolds, with complete resorption within several months, may reduce these limitations. Implantation of bioresorbable scaffold in the context of myocardial infarction may be interesting. However, very few studies are currently available in this setting. Preliminary results and perspectives are presented in this review.

Bioresorbable vascular scaffold implantation in acute coronary syndromes: clinical evidence, tips and tricks

Percutaneous coronary intervention (PCI) with a drug-eluting stent (DES) is routine treatment for patients with acute coronary syndromes (ACS). However, permanent metallic caging of the vessel has several shortcomings, such as side branch jailing and impossibility of late lumen enlargement. Moreover, DES PCI is affected by vasomotion impairment. In ACS a high thrombus burden and vasospasm lead to a higher risk of acute and late acquired stent malapposition than in stable patients. This increases the risk of acute, late and very late stent thrombosis. In this challenging clinical setting, the implantation of bioresorbable vascular scaffolds (BVS) could represent an appealing therapeutic option. Temporary vessel scaffolding has proved to have several advantages over metallic stent delivery, such as framework reabsorption, late lumen enlargement, side branch patency, and recovery of physiological reactivity to vasoactive stimuli. In the thrombotic environment of ACS, BVS implantation has the benefit of capping the thrombus and the vulnerable plaque. Bioresorbable vascular scaffolds also seems to reduce the incidence of angina during follow-up. Acute coronary syndromes patients may therefore benefit more from temporary polymeric caging than from permanent stent platform implantation. The aim of this review is to update the available knowledge concerning the use of BVS in ACS patients, by analyzing the potential pitfalls in this challenging clinical setting and presenting tricks to overcome these limitations.

The Safety and Efficacy of Absorb Bioresorbable Vascular Scaffold: A Systematic Review

Bioresorbable stents are novel devices designed to overcome the long-term limitations of permanent stent implantation. The Absorb bioresorbable vascular scaffold (BVS; Abbott Vascular, Santa Clara, CA) was the first bioresorbable stent with Conformité Européenne mark approval in coronary vessels and has been the subject of multiple clinical studies. Despite its potential advantages, the safety and efficacy of BVS remain unclear. To address this, we conducted a systematic review to examine the safety and efficacy of BVS. The MEDLINE, Embase, Current Index to Nursing & Allied Health Literature (CINAHL), Cochrane, and Science Citation Index Expanded (SCIE) databases were searched for studies examining BVS safety and efficacy. Our search was restricted to studies published in English or French. Outcomes of interest include cardiac death, myocardial infarction, target-lesion revascularization, restenosis, and composite endpoints. Eleven studies met our inclusion criteria (n = 2990), which included 1 randomized controlled trial and 10 cohort studies (2 controlled). These studies varied in size (11-1189) and follow-up duration (1-60 months). The incidence of major adverse cardiac events ranged from 2.6% to 15.5%, with no statistically significant difference between BVS and control in studies that included a comparison group. Although available data are limited, current evidence is promising and suggests that the use of BVS is not associated with a significant increase in major cardiac events in the short term. Numerous randomized controlled trials are currently in progress that will further improve our understanding of the safety and efficacy of this device.

Clinical Comparison With Short-Term Follow-Up of Bioresorbable Vascular Scaffold Versus Everolimus-Eluting Stent in Primary Percutaneous Coronary Interventions

Objective of this study was to assess the clinical performance of bioresorbable vascular scaffold (BVS) compared to everolimus-eluting stent (EES) in subjects with ST-segment elevation myocardial infarction (STEMI). We included all consecutive patients with STEMI who underwent percutaneous coronary intervention (PCI) with BVS implantation in centers participating to the Italian ABSORB Prospective Registry (BVS-RAI) and PCI with EES in the same centers during the same period. The 2 groups were compared. The primary end point was patient-oriented composite end point (POCE) including cardiac death, myocardial infarction, and target lesion revascularization (TLR) at the longest available follow-up. BVS or EES thrombosis at follow-up was also evaluated. Of the 563 patients with STEMI included, 122 received BVS and 441 EES. Procedural success was obtained in 549 (97.5%) cases without significant differences between the 2 groups (BVS 99.3% vs EES 97.0%, p = 0.2). At a median of 220-day (interquartile range 178 to 369) follow-up, no significant differences were observed in terms of POCE (BVS 4.9% vs EES 7.0%, p = 0.4); death (BVS 0.8%, EES 2.0%, p = 0.4), MI (BVS 4.1%, EES 2.0%, p = 0.2), TLR (BVS 4.1%, EES 4.5%, p = 0.8), device thrombosis (BVS 2.5%, EES 1.4%, p = 0.4). All TLR cases were successfully managed with re-PCI in both groups. A propensity matching of the study populations showed no significant differences regarding POCE at the longest available follow-up (odds ratio 0.53, 0.1 to 4.3). In conclusion, in this direct prospective comparison, BVS was associated with similar clinical results compared to EES in the STEMI setting. Larger and adequately powered randomized trials are needed to fully assess the potential clinical benefit of BVS versus the current standard of care in patients with STEMI.

Absorb bioresorbable vascular scaffold: What have we learned after 5 years of clinical experience?

Bioresorbable scaffolds have the potential to introduce a paradigm shift in interventional cardiology, a true anatomical and functional "vascular restoration" instead of an artificial stiff tube encased by persistent metallic foreign body. Early clinical studies using the first commercially available drug-eluting bioresorbable vascular scaffold (BVS) reported very promising safety and efficacy outcomes, comparable to best-in-class second-generation drug-eluting metal stent. To date, more than 60,000 Absorb BVSs have been implanted with only the interim analysis of one randomized trial (ABSORB II RCT) available. Recent registries have challenged the initial claim that BVS is immune from Scaffold Thrombosis (ST). However, suboptimal device expansion and insufficient intracoronary imaging guidance can explain higher than expected ST, especially in complex lesions. The aim of this review article is to critically evaluate the results of the available Absorb BVS studies and discuss the lessons learned to optimize lesion selection and implantation technique of such devices.

Short-term outcomes in patients with acute coronary syndrome treated with direct bioresorbable scaffold deployment

BACKGROUND

Direct coronary stenting is a validated therapeutic option for coronary lesions. We studied the feasibility of direct deployment with a bioresorbable vascular scaffold (BVS) in acute coronary syndrome (ACS).

METHODS

Demographic, procedural, and survival data were obtained for patients who had direct scaffold deployment with BVS from 1 May 2013 to 1 April 2014.

RESULTS

We performed a retrospective review of nine patients which included eight patients having ST-elevation myocardial infarction. There were no cases of worsening coronary flow, scaffold thrombosis, target lesion revascularization or death up to 30 days post intervention.

CONCLUSION

Direct BVS deployment in ACS appears safe and feasible.

Fully bioresorbable drug-eluting coronary scaffolds: A review

Following the development of stents, then drug-eluting stents (DES), bioresorbable scaffolds are proposed as a third evolution in coronary angioplasty, aiming to reduce the incidence of restenosis and stent thrombosis and to restore vascular physiology. At least 16 such devices are currently under development, but published clinical data were available for only three of them in September 2014. The first device is Abbott's BVS(®), a poly-L-lactic acid (PLLA)-based everolimus-eluting device, which has been tested in a registry and two non-randomized trials. Clinical results seem close to what is expected from a modern DES, but possibly with more post-procedural side-effects. Two randomized trials versus DES are underway. This device is already marketed in many European countries. The second device is Elixir's DESolve(®), a PLLA-based novolimus-eluting device, which has been evaluated in two single-arm trials. Results are not widely different from those expected from a DES. The third device is Biotronik's DREAMS(®), a metallic magnesium-based paclitaxel-eluting device, which has been assessed in an encouraging single-arm trial; its second version is currently undergoing evaluation in a single-arm trial. The available results suggest that the technological and clinical development of bioresorbable scaffolds is not yet complete: their possible clinical benefits are still unclear compared with third-generation DES; the impact of arterial physiology restoration has to be assessed over the long term; and their cost-effectiveness has to be established. From the perspective of a health technology assessment, there is no compelling reason to hasten the clinical use of these devices before the results of ongoing randomized controlled trials become available.

Alternative stents in ST-segment elevation myocardial infarction: improving the efficacy of primary percutaneous coronary intervention

Despite the efficacy of primary percutaneous coronary intervention in achieving epicardial reperfusion in ST-segment elevation myocardial infarction, it is often limited by impaired microvascular perfusion attributable to distal embolization of plaque and thrombus, and stent malappostion due to vessel constriction and thrombus apposition, attenuating the full benefits of myocardial reperfusion and resulting in unfavorable clinical outcomes. In the long run implantation of permanent metallic implants have negative effect the biological behavior of the target vessel with a continuous low device failure over the years. Recently, however, efforts have been realized to tackle these shortcomings and optimize mechanical reperfusion by improvements to stent design, as substantiated by the self-expanding stent, the mesh-covered stent and the bioresorbable vascular scaffold. In this article, we provide an overview of the role of these novel, innovatively designed, alternative devices in patients undergoing primary percutaneous coronary intervention for ST-segment elevation myocardial infarction.

A 12-month angiographic and optical coherence tomography follow-up after bioresorbable vascular scaffold implantation in patients with ST-segment elevation myocardial infarction

OBJECTIVES

The aim of the study was to evaluate the healing process at 12 months after ABSORB™ bioresorbable vascular scaffold (BVS) implantation in patients with ST-segment elevation myocardial infarction (STEMI).

BACKGROUND

There is currently no data on long-term BVS performance in the acute thrombotic setting. The underlying altered plaque pathomorphology may impact the neointima healing pattern, potentially making it different to that observed in stable coronary artery disease (CAD).

METHODS

We have performed an angiographic and optical coherence tomography (OCT) 12-month follow-up of 19 STEMI patients who were treated with a BVS implantation (23 scaffolds). An independent core laboratory performed a paired analysis of the corresponding frames at baseline and follow-up.

RESULTS

At 12 months, the OCT follow-up showed a decrease in the mean lumen area (8.29 ± 1.53 mm(2) vs. 6.82 ± 1.57 mm(2) , P < 0.001), but no significant change in the mean scaffold area (8.49 ± 1.53 mm(2) vs. 8.90 ± 1.51 mm(2) ). Significant decreases in malapposed strut ratio (4.9 ± 8.65% vs. 0.4 ± 1.55%, P < 0.001) and malapposition area (0.29 ± 0.60 mm(2) 0.08 ± 0.32 mm(2) , P = 0.002) were observed. A nonhomogenous proliferation of neointima was revealed with a symmetry index of 0.15 (0.08-0.27), a mean neointima thickness of 203 μm (183-249) and mean neointima area of 2.07 ± 0.51 mm(2) . The quantitative coronary angiography showed late lumen loss of 0.08 ± 0.23 mm and no significant change in the minimal lumen diameter (P = 0.11). There were no major adverse cardiovascular events (MACE), except for one nontarget vessel revascularization.

CONCLUSIONS

The OCT revealed a favorable healing pattern after BVS implantation in a STEMI population.

Neointimal coverage and late apposition of everolimus-eluting bioresorbable scaffolds implanted in the acute phase of myocardial infarction: OCT data from the PRAGUE-19 study

Incomplete stent apposition and uncovered struts are associated with a higher risk of stent thrombosis. No data exist on the process of neointimal coverage and late apposition status of the bioresorbable vascular scaffold (BVS) when implanted in the highly thrombogenic setting of ST-segment elevation acute myocardial infarction (STEMI). The aim of this study was to assess the serial changes in strut apposition and early neointimal coverage of the BVS using optical coherence tomography (OCT) in selected patients enrolled in the PRAGUE-19 study. Intracoronary OCT was performed in 50 patients at the end of primary percutaneous coronary intervention for acute STEMI. Repeated OCT of the implanted BVS was performed in 10 patients. Scaffold area, scaffold mean diameter and incomplete strut apposition (ISA) were compared between baseline and control OCT. Furthermore, strut neointimal coverage was assessed during the control OCT. Mean scaffold area and diameter did not change between the baseline and control OCT (8.59 vs. 9.06 mm(2); p = 0.129 and 3.31 vs. 3.37 mm; p = 0.202, respectively). Differences were observed in ISA between the baseline and control OCT (0.63 vs. 1.47 %; p < 0.05). We observed 83.1 % covered struts in eight patients in whom the control OCT was performed 4-6 weeks after BVS implantation, and 100 % covered struts in two patients 6 months after BVS implantation. Persistent strut apposition and early neointimal coverage were observed after biodegradable vascular scaffold implantation in patients with acute ST-segment elevation myocardial infarction.

Current status of clinically available bioresorbable scaffolds in percutaneous coronary interventions

Drug-eluting stents (DES) are widely used as first choice devices in percutaneous coronary interventions. However, certain concerns are associated with the use of DES, i.e. delayed arterial healing with a subsequent risk of neo-atherosclerosis, late stent thrombosis and hypersensitivity reactions to the DES polymer. Bioresorbable vascular scaffolds are the next step in percutaneous coronary interventions introducing the concept of supporting the natural healing process following initial intervention without leaving any foreign body materials resulting in late adverse events. The first-generation devices have shown encouraging results in multiple studies of selected patients up to the point of full bioresorption, supporting the introduction in regular patient care. During its introduction in daily clinical practice outside the previously selected patient groups, a careful approach should be followed in which outcome is continuously monitored.

ST-segment elevation myocardial infarction – ideal scenario for bioresorbable vascular scaffold implantation?

Bioresorbable vascular scaffolds (BVS) represent a breakthrough technology for percutaneous coronary intervention (PCI). In this context, because of the unique properties of bioresorbable devices, ST-segment elevation myocardial infarction (STEMI) may represent the ideal scenario for BVS implantation. Consistently, 57% of physicians declare they currently use BVS in this group of patients. However, continuous and growing evidence on the good performance of these devices has been actually shown only in small studies with short- and mid-term follow-up. For these reasons, we need data from sufficiently large observational studies, with long-term follow-up, to confirm that BVS can deliver the same results as 2nd-generation drug-eluting stents when using an appropriate implantation technique. In this review, we discuss the potential advantages of BVS implantation in STEMI patients, together with the most recent evidence from clinical studies, highlighting safety and procedural concerns.

OCT assessment of the long-term vascular healing response 5 years after everolimus-eluting bioresorbable vascular scaffold

BACKGROUND

Although recent observations suggest a favorable initial healing process of the everolimus-eluting bioresorbable vascular scaffold (BVS), little is known regarding long-term healing response.

OBJECTIVES

This study assessed the in vivo vascular healing response using optical coherence tomography (OCT) 5 years after elective first-in-man BVS implantation.

METHODS

Of the 14 living patients enrolled in the Thoraxcenter Rotterdam cohort of the ABSORB A study, 8 patients underwent invasive follow-up, including OCT, 5 years after implantation. Advanced OCT image analysis included luminal morphometry, assessment of the adluminal signal-rich layer separating the lumen from other plaque components, visual and quantitative tissue characterization, and assessment of side-branch ostia "jailed" at baseline.

RESULTS

In all patients, BVS struts were integrated in the vessel and were not discernible. Both minimum and mean luminal area increased from 2 to 5 years, whereas lumen eccentricity decreased over time. In most patients, plaques were covered by a signal-rich, low-attenuating layer. Minimum cap thickness over necrotic core was 155 ± 90 μm. One patient showed plaque progression and discontinuity of this layer. Side-branch ostia were preserved with tissue bridge thinning that had developed in the place of side-branch struts, creating a neo-carina.

CONCLUSIONS

At long-term BVS follow-up, we observed a favorable tissue response, with late luminal enlargement, side-branch patency, and development of a signal-rich, low-attenuating tissue layer that covered thrombogenic plaque components. The small size of the study and the observation of a different tissue response in 1 patient warrant judicious interpretation of our results and confirmation in larger studies.

"Leaving nothing behind": is the bioresorbable vascular scaffold a new hope for patients with coronary artery disease?

Despite significant advances in design and technology of drug eluting stents (DES) and improved long-term outcome of patients treated with percutaneous coronary intervention, the implantation of metallic stents is associated with some limitations. Multiple stents, covering long coronary segments substantially affect vasomotion, changing the vessel into a rigid tube. Bioresorbable vascular scaffolds (BVS) promise complete bioresorption after 2 to 3 years, vessel lumen enlargement, reduction of the plaque to media ratio, and restoration of vasomotion. Thus BVS seems to be a new, promising, and perhaps even a breakthrough invasive treatment for patients with coronary artery disease. The results of randomised trials and registries confirm the efficacy and safety of the BVS, provided the compliance with the technical aspects of implantation. A key role plays also the selection of patients who could potentially benefit most from the implantation of the BVS. The idea of "leaving nothing behind" after percutaneous coronary interventions is a very exiting concept in modern interventional cardiology. If current technology meets the challenge, major limitations will be overcome, and scaffolds prove to be at least as safe and effective as current DES, than in a long run we will be facing a real breakthrough not only in cardiology, but generally in medicine.

[Procedural aspects in primary PCI: arterial access, stent selection, thrombectomy and treatment of non-culprit lesions]

Acute myocardial infarction was one of the most common causes of death in Germany in 2011. According to the guidelines of the European Society for Cardiology, systemic fibrinolysis and primary percutaneous coronary intervention (PCI) are the methods of choice for acute treatment. Primary PCI should be given priority due to its superiority. The transradial access should be preferred due to the lower bleeding complication rate. In the selection of stents the new generation of drug-eluting stents (DES) are superior to the first generation of bare metal stents (BMS). It has now been demonstrated that the incident rates of DES (e.g. mortality, target vessel revascularization, early and late stent thrombosis and myocardial infarction) are significantly lower. For bioresorbable scaffolds (BRS) long-term results for the use in treatment of ST-elevation myocardial infarction (STEMI) are not yet available but initial results are very promising. However, the selection of a stent needs to be done on an individual basis in order to do justice to all aspects. Data with respect to thrombectomy in acute treatment are heterogeneous. Currently, a thorough consideration of all aspects is necessary because thrombus aspiration can also be associated with an increased rate of incidents. In a state of hemodynamic stability only so-called culprit lesions should currently be treated with a stent. Elective interventions on further stenoses should be carried out after consideration of individual factors and if necessary evaluation of the hemodynamic relevance.

Current and future perspectives on drug-eluting bioresorbable coronary scaffolds

Despite improvements in stent platform, polymer and drug elution, the permanent metallic stents have significant limitations as they distort vessel physiology, predispose to late thrombosis and may preclude surgical revascularization. Bioresorbable scaffold (BRS) technology has evolved over the last few years to overcome these drawbacks. Actually, different BRS are either available or under clinical and preclinical investigation. However, the use of BRS has largely been restricted to patients recruited into clinical trials with a relatively small number of 'real world' patients treated with these devices. Here, we highlight the potentialities of these devices, describe the evidence from the recent clinical trials and discuss the potential advantages, as well as challenges, that this novel technology may face in routine clinical practice.

Ubiquitin-free routes into the proteasome

The majority of proteasome substrates identified to date are marked for degradation by polyubiquitinylation. Exceptions to this principle, however, are well documented and can help us understand the process proteasomes use to recognize their substrates. Examples include ornithine decarboxylase, p21/Cip1, TCRalpha, IkappaBalpha, c-Jun, calmodulin and thymidylate synthase. Degradation of these proteins can be completely ubiquitin-independent or coexist with ubiquitin-dependent pathways. Uncoupling degradation from ubiquitin modification may reflect the evolutionary conservation of mechanisms optimized for highly specialized regulatory functions.