Current and Future Drug-eluting Coronary Stent Technology

Alfonso Ielasi; Azeem Latib; Antonio Colombo

Expert Rev Cardiovasc Ther. 2011;9(4):485-503.

Abstract and Introduction

Abstract

Despite the impressive benefits obtained following the introduction of the drug-eluting stent, safety concerns have been raised over their long-term safety with particular regard to stent thrombosis. Various mechanisms such as delayed endothelialization, local hypersensitivity and endothelial dysfunction owing to the durable polymer coating and/or the drug itself have been suggested as possible causes of this phenomenon. Therefore, to address these concerns, a newer-generation of drug-eluting stents has been developed and they are currently undergoing preclinical and clinical evaluation in order to increase both the safety and biocompatibility by optimizing the three major components of drug-eluting stents: the stent platform, the polymer and the drug. This article critically reviews the key clinical trials and the current status of these new coronary devices as well as preventing future perspectives for their continued development.

Introduction

The advent of drug-eluting stents (DESs) has considerably changed and revolutionized our ability to percutaneously approach more complex subsets of lesions in patients. In fact, DESs have resulted in lower rates of repeat revascularization as compared with bare-metal stents (BMSs)^[1,2] at immediate and long-term follow-up.^[3] Even though first-generation DESs (Cypher®, sirolimus-eluting stent, Cordis, a Johnson and Johnson Company, NJ, USA and Taxus®, Boston Scientific, MA, USA) have performed well in terms of efficacy, enthusiasm for this technology has been dampened in the last few years. In fact, some studies have raised concerns in relation to their long-term safety and the risk of late stent thrombosis (ST) that is often associated with fatal consequences. Various mechanisms such as delayed endothelialization, local hypersensitivity and endothelial dysfunction owing to the durable polymer coating and/or the drug itself have been suggested as possible causes of this phenomenon and the need to prolong dual antiplatelet therapy (DAT).^[4,5]

Therefore, the next generations of DESs were developed to increase both safety and biocompatibility by optimizing the three major components of DES: the stent platform, the polymer and the drug. Stents with thinner struts are endothelialized quicker, causing less vessel wall injury. Nondurable polymers applied thinly cause less inflammation and may result in better long-term safety. Furthermore, the use of biodegradable polymers, which fully degrade and leave a BMS, should result in less inflammation. Modern drugs with different release kinetics are also the object of study. In addition to the improvement in DES components, new approaches such as polymer-free drug delivery, a prohealing approach and fully biodegradable stents are also emerging.

Considering this background, we have reviewed the most important recent studies regarding new DES technology that are either under clinical evaluation or have recently been approved for use in everyday interventional practice.

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Cite this: Current and Future Drug-eluting Coronary Stent Technology - Medscape - Apr 01, 2011.

Table 1. Main characteristics of first- and second-generation drug-eluting stents with a durable polymer.
Stent	Drug	Concentration (mg/cm²)	Drug-release kinetic	Polymer	Polymer thickness (µm)	Platform	Strut thickness (µm)
Cypher®	Sirolimus	100	80%; 28 days	Polyethelyne co-vinyl acetate and poly-N-butyl methacrylate	12.6	Stainless steel	140
Taxus Express®	Paclitaxel	100	<10%; 28 days	Poly(styrene-b-isobutylene-b-styrene)	16	Stainless steel	132
Taxus Libertè®	Paclitaxel	100	<10%; 28 days	Poly(styrene-b-isobutylene-b-styrene)	16	Stainless steel	97
Taxus Element™	Paclitaxel	100	<10%; 90 days	Poly(styrene-b-isobutylene-b-styrene)	15	Platinum–chromium	81
Xience V™	Everolimus	100	80%; 28 days	Polyvinylidene fluorideco-hexafluoropropylene and poly-N-butyl methacrylate	7.6	Cobalt–chromium	81
XIENCE PRIME™	Everolimus	100	Cobalt–chromium
Promus™	Everolimus	100	80%; 28 days	Polyvinylidene fluorideco-hexafluoropropylene and poly-N-butyl methacrylate	7.6	Cobalt–chromium	81
Promus Element™	Everolimus	100	87%; 90 days	Poly(styrene-b-isobutylene-b-styrene)	6	Platinum–chromium	81
Endeavor®	Zotarolimus	100	95%; 14 days	Phosphorylcolyne	4.1	Cobalt–chromium	91
Endeavor® Resolute	Zotarolimus	100	85%; 60 days	Biolinx	4.1	Cobalt–chromium	81

Data taken from [51,108].

Table 2. Second-generation zotarolimus-eluting stent with biostable polymer: data from randomized trials and registries.
Trials	Patients (n)	Primary end point	Follow-up (months)	In-stent lumen loss (mm)	Binary restenosis (%)	Death (%)	MI (%)	TLR (%)	Definite/probable ST (%)	Ref.
*Randomized trials*
ENDEAVOR® I	100	In-stent LL In-segment LL Neo-intimal hyperplasia	12 60	0.61	5.4	0.0 4.1	1.0 1.0	2.0 3.1	1.0 1.0	[83,84]
ENDEAVOR II	ZES = 598 vs BMS = 599	TVF (cardiac death, MI or TVR)	9 60	0.61 vs 1.03*	9.4 vs 33.5*	1.2 vs 0.5 6.2 vs 7.6	2.7 vs 3.9 3.8 vs 4.8	4.6 vs 11.8^‡ 7.5 vs 16.3^‡	0.5 vs 1.2 0.9 vs 1.7	[85,86]
ENDEAVOR III	ZES = 323 vs SES = 113	In-segment LL	960	0.60 vs 0.15*	9.2 vs 2.1**	0.6 vs 0.0 5.2 vs 13**	0.6 vs 3.5 1.0 vs 4.6	6.3 vs 3.5^‡ 8.1 vs 6.5^‡	0.0 vs 0.0 0.7 vs 0.9	[87,88]
ENDEAVOR IV	ZES = 773 vs PES = 775	TVF (cardiac death, MI or TVR)	12 36	0.67 vs 0.42*	13.3 vs 6.7	1.1 vs 1.1 4.0 vs 4.5	1.6 vs 2.7 2.2 vs 4.9**	4.5 vs 3.2^‡ 6.5 vs 6.0^‡	0.9 vs 0.1 1.1 vs 1.6	[26,89]
ZEST	ZES = 880 vs SES = 880 vs PES = 880	MACE (death, MI, ischemia-driven TVR)	12			0.7 vs 0.8 vs 1.1	5.3 vs 6.3 vs 7.0	4.9 vs 1.4 vs 7.5*	0.7 vs 0.0 vs 0.8**	[90]
SORT OUT III	ZES = 1162 vs SES = 1170	MACE (cardiac death, MI or TVR)	9 18			2.0 vs 2.0 4.4 vs 2.7	1.4 vs 0.5 2.1 vs 0.9	4.0 vs 1.0^‡ 6.1 vs 1.7^‡	1.1 vs 0.2**^† vs 1.1 vs 0.5^†	[24]
*Registries*
E-registry	7832	MACE (cardiac death, MI or TLR)	12			2.4	1.6	4.5	1.1	[91]
E-Five registry	2116	MACE (cardiac death, MI or TLR)	12 24			1.7 2.9	1.2 1.5	4.5 5.1	0.6 0.7	[92]

*p < 0.001.
**p < 0.05.
^†Clinically driven.
^‡Definite only.
BMS: Bare-metal stent; ENDEAVOR I: Safety and Efficacy of the ABT-578-Coated Endeavor Stent – 12-Month Follow-up; ENDEAVOR II: A Randomized Comparison of the Endeavor ABT-578 Drug-Eluting Stent With a Bare Metal Stent for Coronary Revascularization; ENDEAVOR III: Randomized Controlled Trial of the Medtronic Endeavor Drug [ABT-578) Eluting Coronary Stent System Versus the Cypher Sirolimus-Eluting Coronary Stent System in De Novo Native Coronary Artery Lesions; ENDEAVOR IV: A Trial of a Coronary Stent System in Coronary Artery Lesions; LL: Late loss; MACE: Major adverse cardiovascular event; MI: Myocardial infarction; PES: Paclitaxel-eluting stent; SES: Sirolimus-eluting stent; SORT OUT III: Efficacy and Safety of Zotarolimus-Eluting and Sirolimus-Eluting Coronary Stents in Routine Clinical Care: a Randomized Controlled Superiority Trial; ST: Stent thrombosis; TLR: Target lesion revascularization; TVF: Target vessel failure; TVR: Target vessel failure; ZES: Zotarolimus-eluting stent.

Table 3. Second-generation everolimus-eluting stent with biostable polymer: data from major randomized trials.
Trials	Patients (n)	Follow-up (months)	Primary end points	In-stent LL (mm)	Binary restenosis (%)	Death (%)	MI (%)	TLR (%)	MACE (%)	Definite/probable ST (%)	Ref.
SPIRIT FIRST	EES = 27 vs BMS = 29	6 60	In-stent LL	0.10 vs 0.87*	0.0 vs 25.9**	0.0 vs 0.0 0.0 vs 7.4	3.8 vs 0.0 8.3 vs 0.0	3.8 vs 21.4 8.3 vs 28.0^†	7.7 vs 21.4^† 16.7 vs 28.0^†	0.0 vs 0.0 0.0 vs 0.0	[12,93]
SPIRIT II	EES = 223 vs PES = 77	6 48	In-stent LL	0.11 vs 0.36*	1.3 vs 3.5	0.0 vs 1.3 4.9 vs 9.9	0.9 vs 3.9 3.6 vs 7.5	2.7 vs 6.5 5.9 vs 12.7	2.7 vs 6.5 7.7 vs 16.4	0.5 vs 1.3 1.0 vs 3.0	[13,94]
SPIRIT III	EES = 669 vs PES = 333	12 36	In-segment LL	0.16 vs 0.30**	2.3 vs 5.7	1.2 vs 1.2 2.6 vs 4.1	2.8 vs 4.1 3.7 vs 6.3	3.4 vs 5.6 5.4 vs 8.9**^†	6.0 vs 10.3 9.1 vs 15.7	1.1 vs 0.6 1.3 vs 1.7	[14,95]
SPIRIT IV	EES = 2458 vs PES = 1229	12 24	TLF (cardiac death, TV-MI or ischemia-driven TLR)			1.0 vs 1.3 2.0 vs 2.7	1.9 vs 3.1 2.5 vs 3.9	2.5 vs 4.6**^† 4.5 vs 6.9	4.2 vs 6.9*^¶	0.3 vs 1.1** 0.42 vs 1.23*	[15,96]
SPIRIT V	EES = 218 vs PES = 106	12	In-stent LL in diabetic patients	0.19 vs 0.39*	3.1 vs 6.1	1.4 vs 2.9	3.3 vs 8.7**	11.2 vs 6.7	11.2 vs 12.5	0 vs 1.9	[97]
COMPARE	EES = 897 vs PES = 903	12 24	Composite of death, MI and TVR			2.0 vs 1.6	2.8 vs 5.3** 3.9 vs 7.6*	2.0 vs 5.3* 2.6 vs 5.9*	6.2 vs 9.1** 9.0 vs 13.7*	0.7 vs 2.6** 0.9 vs 3.9*	[16,98]
SORT OUT IV	EES = 1390 vs SES = 1384	9	Composite of cardiac death, MI, definite ST, clinically driven TVR			1.9 vs 1.4^§	1.1 vs 1.4	1.4 vs 1.7	4.9 vs 5.1**	0.9 vs 0.9	[99]
ISAR-TEST 4	EES = 652 vs SES = 652	12 24	Cardiac death, TV-MI or TLR	0.14 vs 0.17 0.29 vs 0.31	10.1 vs 13.4 12.7 vs 16.9**	4.5 vs 5.0 6.4 vs 6.7		8.1 vs 10.7 9.9 vs 13.5	13.6 vs 15.2 16.0 vs 18.8	1.4 vs 1.6 1.4 vs 1.9	[100]
RESOLUTE All Comers	EES = 1152 vs ZES = 1140	12	TLF (cardiac death, any MI or clinically driven TLR)	0.19 vs 0.27	6.5 vs 5.2	2.8 vs 1.6	4.2 vs 4.1	8.3 vs 8.2^¶	9.7 vs 8.7	0.7 vs 1.7	[101]

*p < 0.001.
**p < 0.05.
^†Clinically driven.
^‡p noninferiority < 0.01.
^§Cardiac death.
^¶Target lesion failure (a composite of cardiac death), any myocardial infarction; target lesion revascularization.
BMS: Bare-metal stent; COMPARE: A Trial of Everolimus-eluting Stents and Paclitaxel-eluting Stents for Coronary Revascularization in Daily Practice; EES: Everolimus-eluting stent; LL: Late loss; MACE: Major adverse cardiovascular event; MI: Myocardial infarction; ISAR-TEST 4: Intracoronary Stenting and Angiographic Results: test Efficacy of three Limus-Eluting Stents: Prospective, Randomized Trial of three limus Agent-Eluting Stents with Different Polymer Coatings; MI: Myocardial infarction; PES: Paclitaxel-eluting stent; SES: Sirolimus-eluting stent; SORT OUT IV: Randomized Clinical Comparison of the Xience V and the Cypher Coronary Stents in Non-Selected Patients with Coronary Heart Disease; SPIRIT: A Clinical Evaluation of an Investigational Device. The Abbott XIENCE V® Everolimus-Eluting Coronary Stent System in the Treatment of Patients with De Novo Native Coronary Artery Lesions; ST: Stent thrombosis; TLF: Target lesion failure; TLR: Target lesion revascularization; TV: Target vessel; ZES: Zotarolimus-eluting stent.

Table 4. Drug-eluting stents with a biodegradable polymer under investigation or already available outside US market.
Drug-eluting stent (manufacturer)	Drug (dose)	Platform	Polymer	Polymer complete biodegradation (months)	Drug-release kinetic	Study	Patients (n)	Primary end point	Angio follow-up (months)	In-stent LL (mm)	Binary restenosis (%)	Current status	Ref.
BioMatrix (Biosensors)	Biolimus A9 (15.6 µg/mm)	Stainless steel	Abluminal PLA	6–9	45%; 30 days	LEADERS	857 BES vs 850 SES	Cardiac death, MI or clinically driven TVR	9	0.13 vs 0.19	20.9 vs 23.3^†	CE	[49]
NOBORI (Terumo)	Biolimus A9 (15.6 µg/mm)	Stainless steel	Abluminal PLA	6–9	45%; 30 days	NOBORI 1	153 BES vs 90 PES	In-stent LL	9	0.11 vs 0.32**	0.7 vs 6.2**	CE	[53]
NEVO™ (Cordis)	Sirolimus (166 µg/17 mm)	Cobalt–Chromium	Reservoirs of PLGA	3	80%; 30 days	NEVO RES I	202 SES vs 192 PES	In-stent LL	6	0.13 vs 0.36*	1.1 vs 8.0**	Ongoing evaluation	[55]
Excel (JW Medical Systems)	Sirolimus (195–376 µg)	Stainless steel	PLA	6–9		Registry	2077	Cardiac death, MI or TLR	6–12	0.21	3.8	Ongoing evaluation	[57]
SYNERGY (Boston Scientific)	Everolimus (LD: 56 µg/20 mm; SD: 113 µg/20 mm)	Platinum–chromium	PLGA rollcoat abluminal	3	50%; 60 days	RCT	291 SD vs LD vs EES Element		6			Ongoing evaluation	[102]
Combo (OrbusNeich)	EPC and sirolimus (5 µg/mm)	Stainless steel	Abluminal			NA	NA					Ongoing evaluation	[201]
Elixir (Elixir Medical)	Miolimus (3 µg/mm)	Cobalt–chromium	Abluminal PLA	6–9	90%; 90 days	FIM	15		6	0.15	0	Ongoing evaluation	[36]
JACTAX (Boston Scientific)	Paclitaxel (9.2 µg/16 mm)	Stainless steel	JAC polymer abluminal	4	100%; 60 days	FIM	103		9	0.33	5.2	Ongoing evaluation	[103]
Supralimus (Sahajanand Medical)	Sirolimus (125 µg/19 mm)	Stainless steel	PLLA–PLGA–PLC–PVP	7	50%; 9–11 days	FIM	100		6	0.09	0.0	Ongoing evaluation	[104]
Infinnium (Sahajanand Medical)	Paclitaxel (122 µg/19 mm)	Stainless steel	PLLA–PLGA–PLC–PVP	7	50%; 9–11 days	RCT	111 PES vs 57 BMS		9	0.54 vs 0.90*	8.3 vs 25.5*	CE	[105]
MAHOROBA™ (Kaneka)	Tacrolimus (0.94 µg/mm²)	Cobalt–chromium	PLGA			FIM	47		4	0.99	26.7	Ongoing evaluation	[106]

*p < 0.001.
**p < 0.05.
^†p noninferiority < 0.01.
BES: Biolimus-eluting stent; CE: Conformitè Européene; EES: Everolimus-eluting stent; FIM: First-in-man; JAC: Juxtaposed abluminal coating; LD: Low dose; LL: Late loss; MI: Myocardial infarction; NA: Not available; PES: Paclitaxel-eluting stent; PLA: Poly-lactic acid; PLC: 75/25 poly L-lactide-co-caprolactone; PLGA: 50/50 poly-D-lactide-co-glycolide; PLLA: Poly-L-lactic acid; PVP: Polyvinyl pyrrolidone; RCT: Randomized controlled trial; SD: Standard dose; SES: Sirolimus-eluting stent; TLR: Target lesion revascularization; TVR: Target vessel failure.

Table 5. Polymer-free drug-eluting stents under investigation or already available outside the US market.
DES (manufacturer)	Drug (dose)	Platform	Drug-release kinetic	Surface modification	Study	Patients (n)	Primary end point	Angio follow-up (months)	In-stent LL (mm)	Binary restenosis (%)	Current status	Ref.
BioFreedom (Biosensors)	Biolimus A9 (SD 15.6 µg/mm; LD 7.8 µg/mm)	Stainless steel	90%; 50 h	Microporous surface	FIM	25 SD vs 25 LD vs 25 PES	In-stent LL	4	0.08 vs 0.37* 0.12 vs 0.37*		Ongoing evaluation	[73]
VESTAsync (MIV Therapeutics)	Sirolimus (55 µg)	Stainless steel	100%; 90 days	Nanoporous hydroxyapatite	FIM	15	In-stent LL In-stent % VO	9	0.36	0.0	Ongoing evaluation	[74]
YUKON (Translumina)	Sirolimus (11.7–21.9 µg)	Stainless steel	67%; 7 days	Microporous surface	RCT	225 SES vs 225 PES	In-stent LL	9	0.48 vs 0.48	12.6 vs 11.6	CE	[107]

*p < 0.001.
CE: Conformitè Européene; FIM: First-in-man; LD: Low dose; LL: Late loss; PES: Paclitaxel-eluting stent; RCT: Randomized controlled trial; SD: Standard dose; VO: Volume obstruction.
Data modified from [51].

Table 6. Main characteristics of fully bioabsorbable drug-eluting stents currently under investigation.
Drug-eluting stent (manufacturer)	Strut composition	Drug	Radiopacity	Radial support duration (months)	Absorption time (months)
BVS (Abbott Vascular)	Poly-(L-lactide)	Everolimus	Platinum markers	3	24
Combo (Orbus Neich)	3 × polymers	EPC + sirolimus	Tantalum markers	6
ReZolve™ (REVA Medical)	Tyrosine-derived polycarbonate	Sirolimus	Covalently bound iodine	3–6	36
IDEAL (Bioabsorbable Therapeutic Inc.)	Polymer + salicylate	Sirolimus + salicylate
AMS-3 (Biotronik)	Magnesium alloy				>4
Undisclosed (Elixir Medical Corporation)		Novolimus
Undisclosed (Endovasc-TissueGen Research Sponsor)		PGE-1

AMS-3: Absorbable metallic stent; BVS: Bioresorbable vascular scaffold; EPC: Endothelial progenitor cell; PGE-1: Prostaglandin E-1.
Data modified from [51].

Table 7. Main characteristics of the drug-eluting stents currently available or under investigation.
Stent name	Stent platform	Polymer coating	Drug	Manufacturer	Strut thickness (µm)
Amazonia Pax®	Cobalt–chromium	None	Paclitaxel	Mynvasis	73
Axxess™	Stainless steel	PLA (BA)	Biolimus A9	Devax Inc.	112
Axxion	Stainless steel	None	Paclitaxel	Biosensor Int. Ltd	119
BioMatrix	Stainless steel	PLA (BA)	Biolimus A9	Biosensor Int. Ltd	123
BioMatrix Flex™	Stainless steel	PLA (BA)	Biolimus A9	Biosensor Int. Ltd	118
BVS	PLLA	PLLA–PDLLA (BA)	Everolimus	Abbott Vascular	150
CardioMind sparrow	Nitinol	PLA–PLGA (BA)	Sirolimus	Cardioind inc.	67
Champion	Stainless steel	PLA (BA)	Everolimus	Boston Scientific	NA
Corio™	Cobalt–chromium	BD	Pimecrolimus	Cordis Corporation	89
CoStar™	Cobalt–chromium	PLGA (BA)	Paclitaxel	Cordis Corporation	89
Cura	Stainless steel	PLGA–PLA (BA)	Sirolimus	Orbus Neich	100
Cypher®	Stainless steel	PEVA + PBMA (BD)	Sirolimus	Cordis Corporation	140
Cypher Select™	Stainless steel	PEVA + PBMA (BD)	Sirolimus	Cordis Corporation	100
Dreams	Magnesium alloy	BA	Pimecrolimus	Biotronik	NA
Elixir	Cobalt–chromium	Methacrylate (BD)	Myolimus	Elixir Medical	81
Endeavor®	Cobalt–chromium	Phosphorylcolyne (BD)	Zotarolimus	Medtronic Inc.	91
Endeavor Resolute®	Cobalt–chromium	Biolinx (BD)	Zotarolimus	Medtronic Inc.	81
Excel	Stainless steel	PLA (BA)	Sirolimus	JW Medical Systems Ltd	150
Excella	Cobalt–chromium	Methacrylate (BD)	Novolimus	Elixir Medical	81
Genous™	Stainless steel	BD	Anti-CD34	Orbus Neich	NA
Infinnium	Stainless steel	PLLA + PLGA + PVP (BA)	Paclitaxel	Sahajanand Medical Tech.	84
Jactax	Stainless steel	PLA	Paclitaxel	Boston Scientific	97
Janus flex	Stainless steel	Carbofilm (BD)	Tacrolimus	Sorin Group	110
Nevo	Cobalt–chromium	PLGA (BA)	Sirolimus	Cordis Corporation	99
Nobori™	Stainless steel	PLA (BA)	Biolimus A9	Terumo Medical Corp.	123
Nyle pax	Cobalt–chromium	None	Paclitaxel	Mynvasis	73
Promus™	Cobalt–chromium	Fluoropolymer (BD)	Everolimus	Boston Scientific	81
Promus Element™	Platinum–chromium	SIBS (BD)	Everolimus	Boston Scientific	81
REVA	NA	Polycarbonate (BA)	Paclitaxel	REVA Medical Inc.	200
Stellium	Cobalt–chromium	PLGA (BA)	Paclitaxel	DISA Vascular Ltd	NA
SUPRALIMUS™	Stainless steel	PLLA + PLGA + PVP (BA)	Sirolimus	Sahajanand Medical Tech.	80
Symbiot™	Cobalt–chromium	PLGA (BD)	Pimecrolimus + paclitaxel	Cordis Corporation	89
SYNERGY™	Platinum–chromium	PLGA	Everolimus double doses	Boston Scientific	81
Synchronnium™	Stainless steel	BA	Sirolimus heparin	Sahajanand Medical Tech.	60
Taxus Express™	Stainless steel	SIBS (BD)	Paclitaxel	Boston scientific	132
Taxus Libertè®	Stainless steel	SIBS (BD)	Paclitaxel	Boston scientific	97
Taxus petal™	Platinum–chromium	SIBS (BS)	Paclitaxel	Boston Scientific	NA
Taxus element™	Platinum–chromium	SIBS (BS)	Paclitaxel	Boston Scientific	81
VESTAsync™	Stainless steel	None	Sirolimus	Miv Therapeutic Inc.	65
Xience V™	Cobalt–chromium	Fluoropolymer (BD)	Everolimus	Abbott Vascular	81
Yukon®	Stainless steel	None	Sirolimus	Transluminal GMBH	74
ZoMaxx	Stainless steel/tantalum	Phosphorylcoline (BD)	Zotarolimus	Abbott Vascular	74

BA: Bioabsorbable; BD: Biodurable; NA: Not available; PBMA: Poly-N-butyl methacrylate; PDLLA: poly-DL>-lactic acid; PEVA: Polyethylene co-vinyl-acetate; PLA: Poly-lactic acid; PLGA: Poly(lactic co-glycolic acid); PLLA: Poly-L-lactic acid; PVP: Polyvinylpyrrolidone; SIBS: Poly styrene-b-isobutylene-b-styrene.

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