The porcine coronary model of in-stent restenosis: Current status in the era of drug-eluting stents

A novel method to detect carotid artery in-stent restenosis

BACKGROUND AND PURPOSE

Carotid artery stenosis is a major risk factor for ischemic stroke. Despite carotid artery stenting, in-stent restenosis (ISR) remains challenging. Pigs serve as an ideal ISR model. This study aims to establish a novel porcine model of carotid ISR using open-loop and closed-loop stents and to assess ISR with optical coherence tomography (OCT) and histopathology, comparing incidence and vascular response between stent types.

METHODS

Twelve adult male Bama miniature pigs underwent carotid stenting with either open-loop or closed-loop stents. The animals received antiplatelet therapy pre- and postimplantation. Postimplantation evaluations at 90 days included carotid digital subtraction angiography (DSA), OCT, histopathological examination, and electron microscopy.

RESULTS

Both stent types showed ISR as detected by OCT and DSA. OCT revealed comparable neointimal proliferation within stent struts for both types, with no significant differences in stent, lumen, and neointimal dimensions. Histopathological analysis and electron microscopy provided insights into tissue responses and healing processes following stent implantation. No significant difference in ISR incidence was found between the stent types based on a χ2 test (p = .110). OCT and hematoxylin-eosin staining exhibit the highest consistency in evaluating neointimal area.

CONCLUSIONS

The novel porcine ISR model demonstrated similar ISR outcomes for open-loop and closed-loop stents. OCT proved to be a highly consistent and valuable tool for evaluating stent and arterial conditions, comparable to histopathological findings. However, due to a small sample size, the validity of these preliminary findings requires further investigation to be confirmed.

Plaque burden is associated with minimal intimal coverage following drug-eluting stent implantation in an adult familial hypercholesterolemia swine model

Safety and efficacy of coronary drug-eluting stents (DES) are often preclinically tested using healthy or minimally diseased swine. These generally show significant fibrotic neointima at follow-up, while in patients, incomplete healing is often observed. The aim of this study was to investigate neointima responses to DES in swine with significant coronary atherosclerosis. Adult familial hypercholesterolemic swine (n = 6) received a high fat diet to develop atherosclerosis. Serial OCT was performed before, directly after, and 28 days after DES implantation (n = 14 stents). Lumen, stent and plaque area, uncovered struts, neointima thickness and neointima type were analyzed for each frame and averaged per stent. Histology was performed to show differences in coronary atherosclerosis. A range of plaque size and severity was found, from healthy segments to lipid-rich plaques. Accordingly, neointima responses ranged from uncovered struts, to minimal neointima, to fibrotic neointima. Lower plaque burden resulted in a fibrotic neointima at follow-up, reminiscent of minimally diseased swine coronary models. In contrast, higher plaque burden resulted in minimal neointima and more uncovered struts at follow-up, similarly to patients' responses. The presence of lipid-rich plaques resulted in more uncovered struts, which underscores the importance of advanced disease when performing safety and efficacy testing of DES.

Ultrasound-guided femoral approach for coronary angiography and interventions in the porcine model

Coronary angiography and percutaneous coronary intervention (PCI) procedural details in swine are similar to those performed to humans, since their heart and coronary anatomy closely resembles. However, only a few detailed descriptions of the procedure are available, containing notable differences. We present a feasible and reproducible protocol for percutaneous coronary interventions in porcine experimental models, utilizing ultrasound-guided femoral approach. Nine female pigs were studied to explore the feasibility of superficial femoral arterial (SFA) access for coronary angiography and provisional PCI, as well as the most suitable guiding coronary catheters and angiographic projections for the above interventions. Experiments were performed under general anesthesia, using ultrasound-guided puncture of the SFA to gain arterial access. The Amplatzer AR1^® catheter, and the Right Coronary Bypass^® catheter were used for the selective engagement of the right and the left coronary artery, respectively. Successful arterial access and subsequent cardiac catheterization were performed in all pigs. Only one animal required a second puncture for femoral artery access. None of the 9 animals presented any significant tachycardia or hypotensive episode. One animal developed an access site-related complication following the first catheterization procedure. During follow-up, 100% success of SFA catheterization was achieved using the same ultrasound-guided technique. The ultrasound-guided superficial femoral artery access for coronary angiography and provisional interventions in porcine models is a quick and safe alternative to the carotid artery approach. The RCB and AR1 catheters may be the best choice for the quick and easy selective coronary engagement of the right and left ostia, respectively.

Long-Term Efficacy of Biodegradable Metal-Polymer Composite Stents After the First and the Second Implantations into Porcine Coronary Arteries

A biodegradable coronary stent is expected to eliminate the adverse events of an otherwise eternally implanting material after vessel remodeling. Both biocorrodible metals and biodegradable polymers have been tried as the matrix of the new-generation stent. Herein, we utilized a metal-polymer composite material to combine the advantages of the high mechanical strength of metals and the adjustable degradation rate of polymers to prepare the biodegradable stent. After coating polylactide (PLA) on the surface of iron, the degradation of iron was accelerated significantly owing to the decrease of local pH resulting from the hydrolysis of PLA, etc. We implanted the metal-polymer composite stent (MPS) into the porcine artery and examined its degradation in vivo, with the corresponding metal-based stent (MBS) as a control. Microcomputed tomography (micro-CT), coronary angiography (CA), and optical coherence tomography (OCT) were performed to observe the stents and vessels during the animal experiments. The MPS exhibited faster degradation than MBS, and the inflammatory response of MPS was acceptable 12 months after implantation. Additionally, we implanted another MPS after 1-year implantation of the first MPS to investigate the result of the MPS in the second implantation. The feasibility of the biodegradable MPS in second implantation in mammals was also confirmed.

Guidelines for animal exercise and training protocols for cardiovascular studies

Whole body exercise tolerance is the consummate example of integrative physiological function among the metabolic, neuromuscular, cardiovascular, and respiratory systems. Depending on the animal selected, the energetic demands and flux through the oxygen transport system can increase two orders of magnitude from rest to maximal exercise. Thus, animal models in health and disease present the scientist with flexible, powerful, and, in some instances, purpose-built tools to explore the mechanistic bases for physiological function and help unveil the causes for pathological or age-related exercise intolerance. Elegant experimental designs and analyses of kinetic parameters and steady-state responses permit acute and chronic exercise paradigms to identify therapeutic targets for drug development in disease and also present the opportunity to test the efficacy of pharmacological and behavioral countermeasures during aging, for example. However, for this promise to be fully realized, the correct or optimal animal model must be selected in conjunction with reproducible tests of physiological function (e.g., exercise capacity and maximal oxygen uptake) that can be compared equitably across laboratories, clinics, and other proving grounds. Rigorously controlled animal exercise and training studies constitute the foundation of translational research. This review presents the most commonly selected animal models with guidelines for their use and obtaining reproducible results and, crucially, translates state-of-the-art techniques and procedures developed on humans to those animal models.

Interleukin-35 promotes early endothelialization after stent implantation by regulating macrophage activation

Background: Early strut coverage after sirolimus-eluting stent (SES) implantation is associated with the activation of inflammation, but the underlying mechanisms are not completely understood. The present study aimed to identify the relationship between the anti-inflammatory cytokine interleukin (IL) 35 (IL-35) and early strut coverage in vivo and in vitro.

Methods: We utilized a retrospective study design to measure IL-35 levels in 68 stents from 68 patients with coronary artery disease and recorded serial optical coherence tomography (OCT) images (0 and 3 months) to assess stent endothelialization. The mechanism underlying the regulatory effects of IL-35 on macrophages and human umbilical vein endothelial cells (HUVECs) was also investigated. SESs were surgically implanted into the right common carotid arteries of 200 male New Zealand White rabbits receiving intravenous injections of IL-35 or a placebo.

Results: At the 3-month OCT evaluation, complete endothelium coverage was correlated with IL-35 levels. IL-35 induced the activation of an anti-inflammatory M2-like macrophage phenotype by targeting the signal transducer and activators of transcription (STAT)1/4 signalling pathway, and IL-35-treated macrophages induced endothelial proliferation and alleviated endothelial dysfunction. IL-35-treated New Zealand White rabbits with implanted SESs showed lower percentages of cross-sections with an uncovered strut, elevated mean neointimal hyperplasia (NIH) thickness, and inhibited inflammatory responses.

Conclusions: We investigated the effect of IL-35 expression on early stent endothelialization in vivo and in vitro and identified a crucial role for IL-35 in inducing the activation of an anti-inflammatory M2-like macrophage phenotype. The present study highlights a new therapeutic strategy for early stent endothelialization.

Swine model of in-stent stenosis in the iliac artery evaluating the serial time course

The aim of this study was to propose a new animal model evaluating the serial time course of in-stent stenosis by repeated carotid artery catheterization in the same animal. 16 bare-metal stents were implanted in the normal external and internal iliac artery of 8 miniature pigs. Repeated measurements were performed in the same animal every 2 weeks for 12 weeks through carotid artery catheterization. The time course and peak neointimal proliferation were evaluated by intravascular ultrasound. Health of all animals was assessed by clinical and hematological examinations. As a result, 7 times of carotid artery catheterization was performed per pig, but all animals remained healthy without both any complications and hematological inflammatory abnormalities. The time course of neointimal proliferation of each stent was observed from the stage of hyperplasia to partial regression. The peak neointimal proliferation varied from 6 to 12 weeks despite implantation of identical stents using the same deployment method. In conclusion, repeated carotid artery catheterization to the same animal is feasible without animal health deterioration. This model should be useful to evaluate the time course of neointimal proliferation after stent deployment in preclinical study.

Effects of Methotrexate in a Rabbit Model of In-Stent Neoatherosclerosis: An Optical Coherence Tomography Study

This study used optical coherence tomography (OCT) to investigate the effects of systemic methotrexate, in combination with a drug-eluting stent, on in-stent neoatherosclerosis in a rabbit model. Sirolimus-eluting stents were surgically implanted in the right common carotid arteries of 200 male New Zealand White rabbits; the animals received a high-fat diet, beginning one week before stent implantation. Each animal was randomly assigned to 1 of 4 groups, receiving intravenous injections of either methotrexate (0.4 mg/kg) or placebo weekly for 4 or 12 weeks. Stented arterial segments were harvested after stenting for 4 or 12 weeks, and processed for OCT and histological analysis. Prior to harvesting the arterial segments, blood was collected for the determinations of cytokine levels. Compared with the control animals, the methotrexate-treated animals showed lower rates of lipid-rich intima and per-strut low-signal intensity layers, smaller neointimal areas, and reduced neointimal thickness; larger fibrous cap thicknesses and smaller lumen areas were also seen in the animals receiving methotrexate. The levels of serum interleukin, adhesion molecules, and nuclear factor-κB p65 decreased and IL-10 level increased in the methotrexate-treated animals. Targeting the pro-inflammatory pathways may be an effective way to prevent restenosis without the long-term risk of late thrombosis.

Large animal models of cardiovascular disease

The human cardiovascular system is a complex arrangement of specialized structures with distinct functions. The molecular landscape, including the genome, transcriptome and proteome, is pivotal to the biological complexity of both normal and abnormal mammalian processes. Despite our advancing knowledge and understanding of cardiovascular disease (CVD) through the principal use of rodent models, this continues to be an increasing issue in today's world. For instance, as the ageing population increases, so does the incidence of heart valve dysfunction. This may be because of changes in molecular composition and structure of the extracellular matrix, or from the pathological process of vascular calcification in which bone-formation related factors cause ectopic mineralization. However, significant differences between mice and men exist in terms of cardiovascular anatomy, physiology and pathology. In contrast, large animal models can show considerably greater similarity to humans. Furthermore, precise and efficient genome editing techniques enable the generation of tailored models for translational research. These novel systems provide a huge potential for large animal models to investigate the regulatory factors and molecular pathways that contribute to CVD in vivo. In turn, this will help bridge the gap between basic science and clinical applications by facilitating the refinement of therapies for cardiovascular disease.

Potential Molecular Mechanism of Retrograde Aortic Arch Stenosis in the Hybrid Approach to Hypoplastic Left Heart Syndrome

BACKGROUND

The hybrid palliation for hypoplastic left heart syndrome has emerged as an alternative approach to the Norwood procedure. The development of patent ductus arteriosus (PDA) in-stent stenosis can cause retrograde aortic arch stenosis (RAAS), leading to significant morbidity. This study aimed to identify potential mechanisms of PDA in-stent stenosis contributing to RAAS.

METHODS

Tissues from stented PDAs were collected from 17 patients undergoing comprehensive stage II repair between 2009 and 2014. Patients requiring RAAS intervention based on cardiology-surgery consensus were defined as RAAS(+) (n = 10), whereas patients without any RAAS intervention were defined as RAAS(-) (n = 7). Tissues were examined by quantitative polymerase chain reaction analysis for vascular smooth muscle cell (VSMC) differentiation and proliferation markers.

RESULTS

Patient characteristics were hypoplastic left heart syndrome with aortic atresia in 6 and with aortic stenosis in 3; unbalanced atrioventricular canal in 3; double-inlet left ventricle/transposition of the great arteries in 3; and double-outlet right ventricle in 2. VSMC differentiation markers (β-actin, SM22, and calponin) and signaling pathways for VSMC modulation (transforming growth factor-β1, Notch, and platelet derived growth factor-BB) were significantly higher in the RAAS(+) than in RAAS(-) patients. The proliferation marker Ki67 was increased in RAAS(+) patients. Cell cycle markers were comparable in both groups.

CONCLUSIONS

Increased VSMC differentiation and proliferation markers suggest a mechanism for inward neointima formation of the PDA in RAAS. The apparent lack of change in cell cycle markers is contrary to coronary artery in-stent stenosis, suggesting further targets should be examined. Combined primary in vitro PDA cell culture and proteomics can be strong tools to elucidate targets to reduce PDA in-stent stenosis for RAAS in the future.

A new polymer-free drug-eluting stent with nanocarriers eluting sirolimus from stent-plus-balloon compared with bare-metal stent and with biolimus A9 eluting stent in porcine coronary arteries

BACKGROUND

Permanent polymers in first generation drug-eluting stent (DES) have been imputed to be a possible cause of persistent inflammation, remodeling, malapposition and late stent thrombosis. We aim to describe the in vivo experimental result of a new polymer-free DES eluting sirolimus from stent-plus-balloon (Focus np stent, Envision Scientific) compared with a bare-metal stent (BMS) (Amazonia CroCo, Minvasys) and with a biolimus A9 eluting stent (Biomatrix, Biosensors).

METHODS

In 10 juvenile pigs, 23 coronary stents were implanted in the coronary arteries (8 Amazonia CroCo, 8 Focus np, and 7 Biomatrix). At 28-day follow-up, optical coherence tomography (OCT) and histology were used to evaluate neointimal hyperplasia and healing response.

RESULTS

According to OCT analysis, Focus np stents had a greater lumen area and less neointimal hyperplasia response than BMS and Biomatrix had. Histomorphometry results showed less neointimal hyperplasia in Focus np than in BMS. Histology showed a higher fibrin deposition in Biomatrix stent compared to Focus np and BMS.

CONCLUSIONS

The new polymer-free DES with sirolimus eluted from stent-plus-balloon demonstrated safety and reduced neointimal proliferation compared with the BMS and Biomatrix stents at 28-day follow-up in this porcine coronary model. This new polymer-free DES is promising and warrants further clinical studies.

Development of a rabbit model for a preclinical comparison of coronary stent types in-vivo

Along with the development of innovative stent designs, preclinical trials in animal models are essential. Many animal models have been used and appear to yield comparable results to clinical trials despite substantial criticisms about their validity. Among the animal models, porcine coronary artery models have been the standard models for the preclinical evaluation of endovascular devices. However, rapid growth rate, high body weight potential, and the propensity to develop granulomatous inflammatory reactions are major limitations of the porcine coronary artery model. Compared with porcine coronary artery models, the comparative rabbit iliac artery model has the advantages of being small and easy to handle and relatively inexpensive. Furthermore, the rabbit model has been known to reliably reflect human restenosis histopathologically and have major advantages such as pairwise comparison, which makes each animal serve as its own control subject, therefore, maximizing its statistical power for comparative testing. However, despite the widespread use of this model, a systematic description of the procedure and harvest protocols has never been published. This article describes the surgical procedure, stent implantation procedure, method for tissue harvesting, and how measurements are performed. Although the results of animal models may not perfectly extrapolate to humans, the comparative rabbit iliac artery model may be a useful tool for assessing and comparing the efficacy of new coronary stents with conventional stent systems. This thorough description of the techniques required for vascular access, stent implantation, tissue preparation, and measurement, should aid investigators wishing to begin using the comparative rabbit iliac artery model.

Thinking inside the box: keeping tissue-engineered constructs in vitro for use as preclinical models

Tissue engineers have made great strides toward the creation of living tissue replacements for a wide range of tissue types and applications, with eventual patient implantation as the primary goal. However, an alternate use of tissue-engineered constructs exists: as in vitro preclinical models for purposes such as drug screening and device testing. Tissue-engineered preclinical models have numerous potential advantages over existing models, including cultivation in three-dimensional geometries, decreased cost, increased reproducibility, precise control over cultivation conditions, and the incorporation of human cells. Over the past decade, a number of researchers have developed and used tissue-engineered constructs as preclinical models for testing pharmaceuticals, gene therapies, stents, and other technologies, with examples including blood vessels, skeletal muscle, bone, cartilage, skin, cardiac muscle, liver, cornea, reproductive tissues, adipose, small intestine, neural tissue, and kidney. The focus of this article is to review accomplishments toward the creation and use of tissue-engineered preclinical models of each of these different tissue types.

Coronary angiography and percutaneous coronary intervention in the porcine model: a practical guide to the procedure

Assessment of safety and efficacy within the porcine coronary artery model remains a standard requirement for new therapies delivered to the coronary arteries before proceeding to clinical testing. Human coronary procedures carry a very low mortality rate; however, procedural mortality for porcine experiments is often high, despite these animals being young and free of atherosclerosis. Some of these deaths are due to poor technique, and therefore avoidable. However, despite the wide use of this model, a systematic description of the procedure has never been published. This article will detail how porcine angiography and stent implantation is performed in our institution and will discuss the relevant differences between humans and pigs with regard to anaesthesia, pharmacotherapy, vascular access, catheter selection and angiographic views. Important variations to the technique that have been reported are also covered.

Ribozyme-mediated gene knock down strategy to dissect the consequences of PDGF stimulation in vascular smooth muscle cells

Background

Vascular Smooth Muscle Cells (VSMCs), due to their plasticity and ability to shift from a physiological contractile-quiescent phenotype to a pathological proliferating-activated status, play a central role in the onset and progression of atherosclerosis and cardiovascular diseases. PDGF-BB, among a series of cytokines and growth factors, has been identified as the critical factor in this phenotypic switch. In order to obtain new insights on the molecular effects triggered by PDGF-BB, a hammerhead ribozyme targeting the membrane receptor PDGFR-β was applied to inhibit PDGF pathway in porcine VSMCs.

Findings

Ribozymes, loaded on a cationic polymer-based vehicle, were delivered into cultured VSMCs. A significant impairment of the activation mechanisms triggered by PDGF-BB was demonstrated since cell migration decreased after treatments. In order to functionally validate the effects of PDGFR-β partial knock down we focused on the phosphorylation status of two proteins, protein disulfide isomerase-A3 (PDI-A3) and heat shock protein-60 (HSP-60), previously identified as indicative of VSMC phenotypic switch after PDGF-BB stimulation. Interestingly, while PDI-A3 phosphorylation was counteracted by the ribozyme administration indicating that PDI-A3 is a factor downstream the receptor signalling cascade, the HSP-60 phosphorylation status was greatly increased by the ribozyme administration.

Conclusion

These contradictory observations suggested that PDGF-BB might trigger different parallel pathways that could be modulated by alternative isoforms of the receptors for the growth factor. In conclusion the knock down strategy here described enables to discriminate between two tightly intermingled pathways. Moreover it opens new attractive perspectives in functional investigations where combined gene knock down and proteomic technologies would allow the identification of key factors and pathways involved in VSMC-linked pathological disorders.

Late Positive Remodeling and Late Lumen Gain Contribute to Vascular Restoration by a Non-Drug Eluting Bioresorbable Scaffold

Background—

The interplay between mechanical dilatation, resorption, and arterial response following implantation of bioresorbable scaffolds is still poorly understood.

Methods and Results—

Long-term geometric changes in porcine coronary arteries in relation to gradual degradation of bioresorbable scaffolds were assessed in comparison with bare metal stents (BMS). Intravascular ultrasound (IVUS)-derived lumen, outer stent/scaffold, and reference vessel areas were evaluated in 94 polymer scaffolds and 46 BMS at 5 days and 3, 6, 12, 18, 24, and 55 months, in addition to polymer scaffold radial crush strength and molecular weight (M W ) at 3, 6, and 12 months. BMS outer stent area and lumen area remained constant through 55 months ( P =0.05, but within 1 standard deviation of 100%, and P =0.58, respectively), while significant increases were exhibited by polymer-scaffolded vessels with the maximum late lumen gain at 24 months, paralleled by the outer scaffold area increase, and then remaining at that increased level at 55 months ( P <0.01). By 12 months polymer scaffolds experienced significant reductions in radial strength and M W , while the animals underwent the largest weight gain. At 3 months and beyond, the patency ratio (lumen area/reference vessel area) of BMS remained constant (0.71 to 0.85, P =0.49). In contrast, that of polymer scaffolds increased and approached 1 ( P =0.13).

Conclusions—

Bioresorbable polymer scaffolds allow restoration of the treated segment's ability to remodel outward to achieve level lumen transition between reference vessel and scaffold-treated regions, a process mediated by animal growth and scaffold degradation. This also introduces a challenge to standard analyses of IVUS outcomes relying on constant stent diameters over time.

Histological analysis of cobalt-chromium stents with and without Camouflage® polymer coating: experimental porcine carotid artery model

This study evaluated the arterial response to cobalt-chromium stents with and without polymer coating (Camouflage®, Hemoteq AG, Wuerselen, Germany) implanted in pigs. Cobalt-chromium balloon-expandable stents (4 × 16 mm) were implanted in the common carotid arteries of nine pigs. Histological analysis of endothelialization, inflammation and injury was performed one month later. All stents were successfully deployed, and all but one animal survived the 30 study days. All arteries were patent. Endothelialization was nearly complete in most sections of all carotid stents in both groups. There were mild inflammatory infiltrate and mild-to-moderate injury, which were associated with the stent shafts and not significantly different between groups. Our findings suggest that, in porcine carotid arteries, the histological response to balloon-expandable cobalt-chromium stents coated with polymer (Camouflage®, Hemoteq AG) is similar to the response to non-coated cobalt-chromium stents.

Histopathologic evaluation of patent ductus arteriosus stents after hybrid stage I palliation

The aim of this study was to determine the histopathology of patent ductus arteriosus (PDA) in-stent stenosis after hybrid stage I palliation. The hybrid approach to palliation of hypoplastic left heart syndrome can be complicated by the development of in-stent stenosis of the PDA. This may obstruct retrograde aortic arch flow, decrease systemic circulation, and lead to interstage interventional procedures. Stented PDA samples removed from eight patients undergoing comprehensive stage II repair were examined by way of radiography and histochemistry (hematoxylin and eosin, Movat pentachrome, α-smooth muscle actin, and proliferating cell nuclear antigen). A retrospective chart review of the patients was also performed. PDA stents were in place in the PDA for a mean period of 169 ± 28 days in patients who had a mean age of 176 ± 30 days at the time of stent removal. Stent deployment caused chronic inflammation, caused fibrin deposition, and induced vascular smooth muscle-cell (VSMC) proliferation in the area immediately surrounding the stent struts. The neointimal region was composed largely of smooth muscle cells that appeared to be fully differentiated by the lack of PCNA staining. Neointimal thickening occurs in the PDA after stent placement for hybrid palliation of HLHS and is the result of inflammation, extracellular matrix deposition, and smooth muscle-cell proliferation in the peristrut region. This finding suggests that proliferating VSMCs in the peristrut region may provide the impetus for inward neointimal formation and therefore the manifestation of in-stent stenosis.

Coronary bare metal stent implantation in homozygous LDL receptor deficient swine induces a neointimal formation pattern similar to humans

INTRODUCTION

To date, most of all new developments in stent technologies are tested in normal animals. Although invaluable in the evaluation of device safety, the juvenile domestic swine (DS) do not follow the biological healing response occurring in humans following coronary stent implantation. By using a novel swine breed afflicted with familial hypercholesterolemia (FHS), we aimed to analyse the vascular response occurring following bare metal stent (BMS) implantation by comparing in vivo endovascular imaging and histological data.

METHODS

A total of 26 swine were included in this study (12 FHS and 14 DS). Sixty eight BMS (FHS=28 versus DS=40) were implanted using a 10% overstretch ratio. Imaging evaluation (IVUS and OCT) was conducted in all animals at 30 (n=14) or 90 (n=12) days following stent implantation. After imaging, the stented coronary segments were harvested for histological evaluation.

RESULTS

At 30 days, the degree of neointimal formation analysed by OCT (%AS=DS 21.9 ± 10% versus FHS 25.4 ± 12%; p=0.18) and histology (DS 24.6 ± 10% versus FHS 23.58 ± 10%; p=0.8) was similar between both animal groups. At 90 days, the degree of neointimal formation in the DS group decreased in all analysed variables (-40% in IVUS neointimal volume, -57% in OCT %AS, and -30% in %AS by histology) compared to the progression of neointimal formation observed in the FHS group (+29% in IVUS neointimal volume, +27% in OCT %AS and +43% in %AS by histology).

CONCLUSION

The pattern of neointimal formation following BMS implantation in the FHS follows a progressive course that does not occur in the DS. Therefore, by providing a progressive neointimal biological response to BMS implantation, the FHS could serve as an ideal efficacy model for the validation of drug eluting stent technologies.

A link between stent radial forces and vascular wall remodeling: the discovery of an optimal stent radial force for minimal vessel restenosis

Coronary and peripheral artery disease (PAD) continue to be primary causes of morbidity and mortality in western nations; percutaneous transluminal angioplasty (PTA) with stenting has become a popular treatment. Unfortunately, restenosis is a significant problem following intravascular stent placement. This study considers the contribution of stent forces in vascular stenosis and remodeling to develop an equation for identifying the optimal stent force. z-Type stents of three radial forces [low (3.4 N), high (16.4 N), and ultrahigh (19.4 N)] were deployed into the iliac arteries of a juvenile porcine model. Vessel diameters were measured before, after deployment, and again at 30 days. At 30 days animals were killed and the vessels fixed in situ. After implantation, there was a significant increase in total thickness and neointimal hyperplasia with increasing stent force. The model for vessel radius and experimental data was in agreement. The model shows that maximum late-term radius is achieved with a stent deployment stress of 480 kPa, which occurs at the end of the stress-strain curve nonlinear domain and beginning of the high-strain collagen domain. The results and calculations suggest that an optimal stent force exists that is subject to the geometry, structure, and mechanics of the target vessel. To achieve maximum late-term dilatation, stents should not produce stress in the vessel wall greater than the end of the transitional domain of the vessel's stress-strain curve. This finding is extremely important for vascular stent development and will be expanded to preliminary vessel wall injury and atherosclerotic models.

Preclinical Models of Restenosis and Their Application in the Evaluation of Drug-Eluting Stent Systems

Coronary arterial disease (CAD) is the leading cause of death in the United States, the European Union, and Canada. Percutaneous coronary intervention (PCI) has revolutionized the treatment of CAD, and it is the advent of drug-eluting stent (DES) systems that has effectively allayed much of the challenge of restenosis that has plagued the success of PCI through its 30-year history. However, DES systems have not been a panacea: There yet remain the challenges associated with interventions involving bare metallic stents as well as newly arisen concerns related to the application of DES systems. To effectively address these novel and ongoing issues, animal models are relied on both to project the safety and efficacy of endovascular devices and to provide insight into the pathophysiology underlying the vascular response to injury and mechanisms of restenosis. In this review, preclinical models of restenosis are presented, and their application and limitation in the evaluation of device-based interventional technologies for the treatment of CAD are discussed.

A novel mouse model of in-stent restenosis

BACKGROUND AND AIMS

In-stent restenosis (ISR) is the major complication that occurs after percutaneous coronary interventions to facilitate coronary revascularization. Herein we described a simple and cost-effective model, which reproduces important features of ISR in the mouse.

METHODS AND RESULTS

Microvascular bare metal stents were successfully implanted in the abdominal aorta of atherosclerotic ApoE-null mice. Patency of implanted stents was interrogated using ultrasound biomicroscopy. Aortas were harvested at different time points after implantation and processed for histopathological analysis. Thrombus formation was histologically detected after 1 day. Leukocyte adherence and infiltration were evident after 7 days and decreased thereafter. Neointimal formation, neointimal thickness and luminal stenosis simultaneously increased up to 28 days after stent implantation. Using multichannel fluorescence molecular tomography (FMT) for spatiotemporal resolution of MMP activities, we observed that MMP activity in the stented aorta of Apo-E null mice was 2-fold higher than that of wild-type mice. Finally, we compared neointimal formation in response to stenting in two genetically different mouse strains. In-stent neointimas in FVB/NJ mice were 2-fold thicker than in C57BL/6J mice (p=0.002).

CONCLUSION

We have developed a model that can take advantage of the multiple genetic resources available for the mouse to study the mechanisms of in-stent restenosis.

Reduction of neointimal hyperplasia in porcine coronary arteries by 2-deoxy-D-glucose

BACKGROUND

The drug eluting stents have been shown to play a substantial role in preventing in-stent restenosis. This study was initiated to determine the efficacy of 2-deoxy-D-glucose (2-DG) in an in-stent restenosis model for reducing neointimal hyperplasia after coronary stent placement.

METHODS

In a porcine overstretch model, three kinds of stents were investigated (n = 12 per group): bare metal stents (BMS), rapamycin-eluted stents (RES), and BMS after intracoronary short-term application of 2-DG (DGS). After 42 days histomorphometric and histopathological analyses were performed.

RESULTS

Neointimal thickness (BMS: 0.38 +/- 0.08, RES: 0.24 +/- 0.11, DGS: 0.15 +/- 0.01), area stenosis (BMS: 47.39 +/- 2.76, RES: 32.2 +/- 2.08, DGS: 29.30 +/- 2.98) did not differ after 42 days between the RES and DGS but were significantly lower as compared to BMS only. Lumen area (BMS: 3.15 +/- 1.53, RES: 4.37 +/- 1.72, DGS: 4.77 +/- 2.14) was significantly higher in the DGS group in comparison to the BMS group. The calculated injury and inflammation scores were similar and re-endothelialization was confirmed in all groups.

CONCLUSIONS

This study could demonstrate that in porcine stent model neointimal hyperplasia and re-endothelialization after application of 2-DG are comparable to those seen in RES. Thus, 2-DG might be a promising clinical application for coronary stent coating.

Complex porcine model of atherosclerosis: induction of early coronary lesions after long-term hyperlipidemia without sustained hyperglycemia

BACKGROUND

The incidence of coronary artery disease (CAD) is still increasing in industrialized countries and it is even higher in diabetic patients. For experimental studies investigating the pathophysiology of CAD, the use of an animal model comparable with the pathological situation in patients is crucial.

OBJECTIVE

To develop a model of advanced coronary atherosclerosis with induction of hyperlipidemia and hyperglycemia in domestic pigs.

METHODS

Six pigs were fed a standard pig chow (controls), two were fed a 2% cholesterol and 17% coconut fat diet (Chol group), and two pigs received a 4% cholesterol and 17% coconut fat diet combined with streptozotocin (STZ) injections to induce diabetes (High Chol+STZ group). Serum lipid and plasma glucose values were analyzed, and histochemical staining for morphometric analysis and immunohistochemistry were performed.

RESULTS

Pigs on the hyperlipidemic diet had elevated mean (+/- SD) serum lipid levels (total cholesterol 5.05+/-1.45 mmol/L [Chol] and 5.03+/-2.41 mmol/L [High Chol+STZ] versus 2.09+/-0.23 mmol/L [controls]). Histopathological evaluation revealed an initial stage of coronary atherosclerosis. None of the STZ-treated pigs showed a sustained elevation of plasma glucose (mean glucose before STZ injection was 5.11+/-0.94 mmol/L and thereafter was 6.03+/-2.39 mmol/L) or a decline in pancreatic beta cells.

CONCLUSIONS

The current data suggest that the domestic porcine model is not suitable to create severe CAD using an atherogenic diet in combination with STZ injections for experimental interventional vascular research. This may be due to different STZ sensitivities among species. However, hyperlipidemia induced early pathological lesions in coronary arteries resembling initial stages of atherosclerosis without severe luminal narrowing.

Rapamycin-eluting stents in the arterial duct: experimental observations in the pig model

BACKGROUND

Maintaining arterial duct patency by stent implantation may be advantageous in congenital heart disease management algorithms. Rapamycin, an immunosuppressant drug that demonstrates antiproliferative properties and inhibits smooth muscle cell migration, may deter the intimal hyperplasia that occurs during spontaneous closure and after-stent implantation of the arterial duct.

METHODS AND RESULTS

Twenty-eight Yorkshire piglets (7 to 11 days old; weight, 2.2 to 4.9 kg) underwent stent implantation of the arterial duct (rapamycin-eluting (n=14) or bare metal (n=14) stents, 3.5-mm diameter) and were euthanized at 2, 4, and 6 weeks. Dissected arterial ducts were analyzed for lumen diameter, smooth muscle cell, and extracellular matrix components. Isolated arterial duct-derived smooth muscle cells were cultured in the presence or absence of rapamycin. Cellular proliferation rates were assessed by Ki-67 detection and [(3)H]-thymidine incorporation. No significant neointimal proliferation was present in either stent type at 2 weeks. At 4 weeks, the median luminal diameters of the bare metal stents were 87% (P=0.009), 54% (P=0.004), and 77% (P=0.004) that of the drug-eluting stents at the middle and aortic and pulmonary artery ends, respectively. At 6 weeks, the median luminal diameters of the bare metal stents were 0% (P=0.18), 5% (P=0.25), and 61% (P=0.13) that of the drug-eluting stents at the same respective levels. Complete histological occlusion was found in at least 1 level of the lumen in 9 pigs: 1 (17%) in the BMS group at 4 weeks, 5 (83%) in the BMS group at 6 weeks, and 3 (50%) in the DES group at 6 weeks. In vitro studies demonstrated 50%-lower proliferation rates in rapamycin-treated cultures of duct-derived smooth muscle cell cultures (P<0.001).

CONCLUSIONS

Rapamycin has antiproliferative actions on the arterial duct. Drug-eluting stents may be a more efficient tool than current palliative options for maintaining patency in critically duct-dependent states, but there may be a finite time-related benefit.

Quinaprilat-eluting stents do not attenuate intimal thickening following stenting in porcine coronary arteries

OBJECTIVE

Vascular injury increases angiotensin-converting enzyme (ACE) activity in the vessel wall, and experimental evidence suggests that high-dose oral ACE inhibition reduces intimal hyperplasia following balloon angioplasty. Local drug delivery can achieve high local concentrations which may be especially efficacious in inhibiting tissue growth following stent implantation. The aim of this study was to evaluate the angiographic and histomorphologic effects of quinaprilat-eluting stents in normal porcine coronary arteries.

METHODS

Ten pigs received phosphorylcholine-coated stents in each of the three major coronary arteries: one loaded with 780 microg quinaprilat, one with the solvent and one non-loaded control. Quantitative angiography was performed before and after stenting and at 4 weeks follow-up. At this time point the stented arteries were also analyzed using histology and morphometry.

RESULTS

Repeated measures ANOVA yielded significantly smaller angiographic lumen in both quinaprilat and solvent groups: 2.62+/-0.31 and 2.65+/-0.31 mm, respectively versus control: 2.70+/-0.32 mm at follow-up, p<0.05. Histology confirmed this finding with an increment in intimal area (2.5+/-0.86 mm(2)) and thickness (0.57+/-0.29 mm) in the quinaprilat group; versus solvent (1.98+/-0.57 mm(2) 0.4+/-0.26 mm) and controls (1.92+/-0.50mm(2) and 0.41+/-0.18 mm).

CONCLUSION

Quinaprilat-eluting stents do not attenuate neointimal thickening following implantation in normal porcine coronary arteries, but rather show a tendency towards the opposite.

Effect of a novel drug-eluted balloon coated with genistein before stent implantation in porcine coronary arteries

BACKGROUND

The major drawback of stent implantation in native human coronary vessels is the occurrence of restenosis. Drug-eluting stents significantly reduce restenosis after percutaneous coronary intervention (PCI), but may be associated with persistent local inflammation involved in the restenosis mechanisms. In this setting coating coronary devices with anti-inflammatory agents represents an intriguing alternative to stent-based local drug delivery. The aim of the present study was to test in a porcine model the safety and efficacy of a novel Genistein-eluting balloon preceding coronary stenting.

DESIGN

Female piglets underwent PCI in a randomized fashion with either a Genistein-eluting or a standard balloon angioplasty, followed in all vessels by bare-metal stent implantation. Pigs were sacrificed at different time points to appraise safety (i.e. endothelialization) and efficacy (i.e. anti-inflammatory and anti-proliferative effects): 1, 4, and 6-8 weeks following PCI.

RESULTS

Overall analysis was conducted on 14 piglets. Twenty-five bare-metal stents were implanted preceded by angioplasty with a conventional balloon in 13 vessels and by the Genistein-eluted balloon in 12. No untoward effects were reported in either group. Healing and endothelialization appeared universal within 4 weeks. The Genistein-eluted balloon group disclosed a significant reduction, at four weeks from implantation, of the peri-stent inflammatory cells count (mononucleocytes 39 +/- 32 Vs. 96 +/- 29 per square millimetre, P = 0.019). This effect did not clearly translate into a trend towards a reduced neointimal hyperplasia at 6-8 weeks (0.13 +/- 0.11 Vs. 0.14 +/- 0.09, P = 0.835).

CONCLUSION

This study provides the first in vivo demonstration of the anti-inflammatory effects of a Genistein-eluting balloon in PCI, warranting further research including the combination of a Genistein-eluting balloon with standard drug-eluting stent.

Drug release kinetics from stent device-based delivery systems

In an effort to overcome the limitations of balloon-expandible intravascular metal stent-induced neointimal formation, drug-coated stent devices have been developed. The stent platform allows the local delivery of drugs to an injury site, thereby reducing the amount of drug exposure to the systemic circulation and other organs. The drug carrier matrix allows the release of the drug in a diffusion-controlled manner over an extended time period after the stent implant. The drugs are chosen such that the complex cascade of events that occurs after stent implantation that leads to smooth muscle cell proliferation and migration towards the intima are inhibited. The success of an antirestenotic drug therapy from a drug-coated stent is dependent, at least partially, on the extent of drug elution from the stent, the duration and rate of release, and accumulation of drug in the arterial wall in such a way that it covers the initiation and progression of vessel wall remodeling. The local vascular drug concentrations achieved are directly correlated with the biological effects and local vascular toxicity, and there is therefore a challenge in finding an optimum dose of drug to be delivered to tissues (ie, one that has the desired therapeutic effect without local adverse effects). There is increased focus on optimization of various factors that affect drug release from the stent system, including the physicochemical properties of the drugs, carrier vehicle formulation, and profile of elution kinetics. This review highlights the various factors involved in drug release kinetics, local vascular toxicity, carrier vehicle matrix, tissue deposition, and distribution through the arterial wall from stent-based drug delivery systems.

Contribution of recipient-derived cells in allograft neointima formation and the response to stent implantation

Allograft coronary disease is the dominant cause of increased risk of death after cardiac transplantation. While the percutaneous insertion of stents is the most efficacious revascularization strategy for allograft coronary disease there is a high incidence of stent renarrowing. We developed a novel rabbit model of sex-mismatched allograft vascular disease as well as the response to stent implantation. In situ hybridization for the Y-chromosome was employed to detect male cells in the neointima of stented allograft, and the population of recipient derived neointimal cells was measured by quantitative polymerase chain reaction and characterized by immunohistochemistry. To demonstrate the participation of circulatory derived cells in stent neointima formation we infused ex vivo labeled peripheral blood mononuclear cells into native rabbit carotid arteries immediately after stenting. Fourteen days after stenting the neointima area was 58% greater in the stented vs. non-stented allograft segments (p = 0.02). Male cells were detected in the neointima of stented female-to-male allografts. Recipient-derived cells constituted 72.1±5.7% and 81.5±4.2% of neointimal cell population in the non-stented and stented segments, respectively and the corresponding proliferation rates were only 2.7±0.5% and 2.3±0.2%. Some of the recipient-derived neointimal cells were of endothelial lineage. The ex vivo tagged cells constituted 9.0±0.4% of the cells per high power field in the stent neointima 14 days after stenting. These experiments provide important quantitative data regarding the degree to which host-derived blood-borne cells contribute to neointima formation in allograft vasculopathy and the early response to stent implantation.

Biocompatibility of tetramethylpyrazine-eluting stents in normal porcine coronary arteries

OBJECTIVE

Drug-eluting stents have been used to markedly decrease in-stent restenosis in 6 months, but they are noticed due to the late thrombogenicity. The purpose of the present study was to evaluate the biocompatibility of Tetramethylpyrazine-eluting stents by investigating the intimal response and thrombogenicity in normal porcine coronary arteries by quantitative coronary angiography (QCA), intravascular ultrasound (IVUS) and histomorphometry.

METHODS

Bare metal stents (BMS) were uniformly spray-coated with Tetramethylpyrazine (TMP 200 microg) and prepared for TMP-eluting stents (TES). Fourteen coronary arteries in 14 pigs underwent stent implantation. Seven TES were implanted in 7 pigs and 7 BMS in other 7 pigs. The stents were deployed with a stent-to-artery ratio of 1.1-1.2/1.0 in order to induce vascular wall injury. QCA and IVUS were performed before and immediately after the implantations and at 28 days (end time point). The analysis on blood cell count, biochemical parameters, status of behavior of pigs were evaluated before the implantation and at the time of 1 and 28 days. Stented-coronary arteries, stented-coronary arteries related ventricular wall, lung, liver and kidney were harvested after euthanasia of animals at the endpoint. Histopathology and histomorphometry had been done to assess the local toxicity of TES to these organs.

RESULTS

All the stents were successfully implanted, however, 4 pigs died of cardiac tamponade or anesthesia. No bone marrow depression and hemolysis was seen. No damage to the function and metabolism of liver and kidney was discovered. No thrombosis was found in control and test groups. Few inflammatory cells were found in the stented-coronary artery walls at each endpoint in both groups. No damage to stented-coronary arteries related ventricular wall, lung, liver and kidney was detected due to TES implantation. Compared with the control group, the neointimal area was significantly reduced in the TES group (60.2+/-23.5% vs 10.0+/-2.1%, P=0.01) by IVUS analysis, but the lumen area in the TES group was increased (4.34+/-0. 93 mm(2) vs 1.29+/-1.02 mm(2), P=0.011), the neointimal area was reduced markedly (1.51+/-0.45 mm(2) vs 4.60+/-1.39 mm(2), P=0.004).

CONCLUSIONS

The biocompatibility of TES in porcine model at 28 days seems to be good and acceptable. Biocompatibility can be evaluated by IVUS and histopathology in a porcine restenosis model.

Coronary artery overexpansion increases neointimal hyperplasia after stent placement in a porcine model

OBJECTIVE

To determine if an aggressive approach to coronary revascularisation with oversized balloons is counterproductive, we studied the effect of increasing balloon-to-artery (B:A) ratio on neointimal hyperplasia following primary stent placement using a non-atherosclerotic porcine coronary overstretch model.

METHODS

60 vessels in 33 Yorkshire swine were randomly assigned to one of five B:A ratios between 1.0:1 and 1.4:1. Intravascular ultrasound (IVUS) imaging was performed before bare-metal stent placement to accurately determine vessel size, after stent placement, and at 28 days.

RESULTS

The mean prestent vessel diameter was 3.05 (0.31) (SD) mm. In-stent neointimal volume, in-stent volume stenosis and cross-sectional area stenosis at the stent minimum lumen diameter increased significantly with increasing achieved B:A ratio (multilevel regression test for slope, p<0.001, p = 0.002 and p<0.001, respectively) and were independent of vessel size. Even minor vessel overstretch at an achieved B:A ratio of 1.1:1 resulted in significant neointimal hyperplasia. Larger B:A ratios were also associated with more neointima beyond the stent edges (p = 0.008). For vessels from the same animal, neointimal response at a given B:A ratio was dependent upon the animal treated.

CONCLUSIONS

In a porcine model of IVUS-guided coronary primary stent placement, vessel overexpansion is counterproductive. Neointimal hyperplasia at 28 days is strongly associated with increasing B:A ratio. In addition, vessels do not respond independently of each other when multiple stents are placed within the same animal using a range of B:A ratios.

Effects of Oral Prednisone After Stenting in a Rabbit Model of Established Atherosclerosis

OBJECTIVES

The aim of this study was to compare the effects of systemic prednisone in combination with a bare-metal stent (BMS) or a paclitaxel-eluting stent (Taxus, Boston Scientific Corp., Natick, Massachusetts) on neointimal inhibition and vessel healing in an atherosclerotic rabbit model.

BACKGROUND

Inflammation plays a critical role in neointimal formation after coronary artery stenting. The efficacy of immunosuppressive doses of oral prednisone to inhibit in-stent neointimal proliferation was compared with BMS and with a commercially available paclitaxel-eluting stent (Taxus) in a rabbit model of established atherosclerosis.

METHODS

Bilateral iliac artery injury in atherosclerotic New Zealand White rabbits fed an atherogenic diet was followed by stent implantation. Animals randomly received Taxus stents, BMS (Express, Boston Scientific Corp.) and placebo, or BMS and oral prednisone (2.1 mg/kg/day for the first 7 days, followed by 1.4 mg/kg/day for 14 days and 0.7 mg/kg/day for 21 days). Stented arterial segments were harvested at 42 days and processed for light microscopy, immunohistochemistry, and organoid culture.

RESULTS

Compared with control subjects, prednisone-treated animals showed a 30% reduction in percent stenosis (p = 0.009), a 35% decrease in neointimal area (p < 0.003), and a 66% decrement in neointimal thickness (p < 0.001). Taxus stents also reduced all 3 parameters significantly (-34%, -39%, and -83%, respectively), but showed significantly more inflammatory cells and fibrin deposition and less endothelialization compared with the other 2 groups. Plaque burden was equal among groups, as shown by the identical stent and vessel area, and no remodeling was observed.

CONCLUSIONS

Systemic prednisone treatment and Taxus stents reduce neointimal formation compared with BMS. The extent of neointimal reduction is similar between prednisone- and Taxus stent-treated animals; however, Taxus stents resulted in a significantly greater delay in healing. Targeting of inflammatory pathways after percutaneous coronary intervention may be an efficacious way to prevent restenosis without the long-term risk of late thrombosis.

Success, safety, and efficacy of implantation of diamond-like carbon-coated stents

The aim of this study was to evaluate safety and clinically defined efficacy of the implantation of a new stent coated with diamond-like carbon (DLC stent), in a group of patients who underwent percutaneous transluminal coronary revascularization procedures in two hemodynamic centers. This study was an observational prospective nonrandomized study that included 196 patients with a total of 236 significant de novo atheromatous coronary lesions, in which 245 DLC stents were implanted. The primary end point of this study was a composite of major cardiovascular events (death or acute myocardial infarction with or without Q) and need for target lesion revascularization (TLR) or target vessel revascularization (TVR) procedure during the first 48 hours and at 6 months after the DLC stent implantation. All patients had a myocardial perfusion imaging study with Tl(201) at 6 months after DLC stent implantation. Only patients with a myocardial perfusion imaging study indicative of myocardial ischemia were then submitted for a new coronary angiogram. No major cardiovascular event or emergency TVR occurred during hospitalization. At 6-month follow-up no major cardiovascular event occurred either, whereas the rate for TLR was 5.6% and that for TVR was 7.65%. This preliminary study provides enough clinical evidence that implantation of intracoronary bare metal stents coated with diamond-like carbon is associated with high success rates, safety, and efficacy, both in the hospital and at the 6-month follow-up after the interventional procedure.

Inhibition of Neointima Formation by a Novel Drug-Eluting Stent System That Allows for Dose-Adjustable, Multiple, and On-Site Stent Coating

OBJECTIVE

The risk of in-stent restenosis can be considerably reduced by stents eluting cytostatic compounds. We created a novel drug-eluting stent system that includes several new features in the rapidly evolving field of stent-based drug delivery.

METHODS AND RESULTS

The aim of the present study was the preclinical evaluation of a stent-coating system permitting individual, on-site coating of stents with a unique microporous surface allowing for individualizable, dose-adjustable, and multiple coatings with identical or various compounds, designated ISAR (individualizable drug-eluting stent system to abrogate restenosis). Stents were coated with 0.75% rapamycin solution, and high-performance liquid chromatography (HPLC)-based determination of drug release profile indicated drug release for >21 days. Rapamycin-eluting microporous (REMP) stents implanted in porcine coronary arteries were safe. To determine the efficacy of REMP stents, this novel drug-eluting stent platform was compared with the standard sirolimus-eluting stent. At 30 days, in-stent neointima formation in porcine coronary arteries was similar in both groups, yielding a significant decrease of neointimal area and injury-dependent neointimal thickness compared with bare-metal stents.

CONCLUSIONS

The ISAR drug-eluting stent platform as a novel concept for stent coating allows for a safe, effective, on-site stent coating process, thus justifying further clinical evaluation to decrease in-stent restenosis in humans.