March 15, 2018 (Vol. 38, No. 6)
Lynne Lederman Ph.D.
Cardiologists May Now Feel Differently about Quick Patch Ups
Pipes, valves, and pumps. If they were parts of an ordinary plumbing system, they could be repaired or replaced quickly and easily, should the need arise. Why, even duct tape might do in a pinch. Or silicon putty.
But nothing like strips of tape or beads of putty could be used to mend the living plumbing that is the cardiovascular system. Or could they? As it happens, the cardiovascular system might benefit from what would strike a master plumber as slapdash work: the application of patches or the injection of gels.
Such repair work is anything but a DIY undertaking. You don’t learn about it by watching YouTube. You need to attend events like the Tissue Engineering and Regenerative Medicine conferences. The most recent of these took place in Charlotte, NC, and was sponsored by the Tissue Engineering and Regenerative Medicine International Society (TERMIS).
At this event, experts in the field of cardiovascular repair explained how cell-based and cell-free technologies—patches and gels—may be used to repair or promote the healing of injured or diseased cardiovascular tissues.
Engineered Patches for Infarct Repair
Nenad Bursac, Ph.D., professor of biomedical engineering, Duke University, discussed the use of heart patches of cardiomyocytes derived from pluripotent stem cells (PSCs) for infarct repair.1 Dr. Bursac’s team collaborates with the group of Jianyi “Jay” Zhang, M.D., Ph.D., professor and chair, Department of Biomedical Engineering, University of Alabama School of Engineering. The road to clinical use will pass through large-animal experimentation and the development of techniques for generating heart patches under good manufacturing practices (GMP). Developers traveling this road will also encounter obstacles.
“Ideally, we want these patches to electrically couple to hearts,” said Dr. Bursac. “So far, they have positive effects on heart function, but mostly through paracrine effects. We would like them to directly electrically integrate with the host heart.”
In porcine studies, the patches have been administered via sternotomy, which is quite invasive. Another obstacle is the thinness of current patches. Growing thicker patches would mean more muscle that could contract more strongly, particularly if the electrical coupling problem could be solved.
Human research is still in its initial stages. In one study, embryonic stem cell–derived cardiac progenitors were embedded in a fibrin scaffold and surgically delivered into the infarcted area of the hearts of six patients in combination with coronary artery bypass surgery. Although the cardiac patches were not associated with adverse events such as arrhythmias or tumors, alloimmunization occurred in half of patients.2,3 Symptomatic improvements were reported3, but the use of concomitant bypass surgery precludes any conclusions about efficacy.
Alloimmunization and immunosuppression could be avoided by the use of autologous cells. However, generating patches from a patient’s own cells would take about six months, noted Dr. Bursac.
“We can envision that most of the PSC therapy will be made from immune-matched cells,” he continued. “That means we will have banks of PSCs that are immune-matched as for bone marrow.” He added that his group is using genome editing to generate universal donor cells by knocking out major histocompatibility genes to make the cells “invisible” to the host immune system.
Dr. Zhang’s talk was on the major roadblocks to overcome in making the engineered heart muscle a cardiac therapy option. He agreed with Dr. Bursac that the major obstacles are optimization of graft size and electrical and mechanical integration into the recipient heart.
A recent study conducted by Dr. Zhang and colleagues showed that human cardiac muscle patches of a clinically relevant size (4 cm × 2 cm × 1.25 mm) can be generated by suspending cardiomyocytes, smooth-muscle cells, and endothelial cells that have been differentiated from human induced pluripotent stem cells (hiPSCs) in a fibrin matrix and cultured on a rocking platform to induce myocyte maturation.4 Dr. Zhang said that this engineered heart tissue beats synchronously with significant force generation per myocyte. Results this encouraging, he noted, had not been previously reported.
In a porcine model of myocardial infarction (MI), the human cardiac muscle patch significantly improved left ventricle function, reduced infarct size, and decreased apoptosis in the scar border area around the infarcted tissue. Engraftment of patch cells at four weeks was 10.9%, which is relatively high for this model. No arrhythmias were observed during the short follow-up time; immunosuppressive therapy was required.
In a murine model of MI, Dr. Zhang and colleagues have shown that overexpression of the cell cycle activator CCND2 (cyclin D2) in hiPSCs increases proliferation, enhances remuscularization of injured myocardium, and improves left ventricular function.5
However, as in the porcine model, the duration of the in vivo portion of the study was only four weeks, so the long-term outcome of the transplanted hiPSCs is not known. Although telomerase activity in cultured hiPSCs declined over 24 weeks, the group believes future studies in large-animal models are warranted.
Cell-Free Bioengineering Approach
Other approaches to cardiac tissue repair avoid some of the obstacles inherent in the use of cell patches by adopting cell-free methods. Karen L. Christman, Ph.D., professor, Department of Bioengineering, University of California, San Diego, gave her keynote talk on designing materials for minimally invasive treatment of MI and heart failure, which is the focus of her research.
Rather than implanting cell patches or injecting cells, Dr. Christman’s group uses a biomaterials scaffold approach, investigating the use of injectable hydrogels derived from decellularized cardiac extracellular matrix (ECM) for treating subacute and chronic MI. “Everything we do,” she said, “is injectable and could be delivered minimally invasively without surgery using the biomaterial to stimulate tissue repair.”
Because this biomaterials scaffold approach relies on injectable materials rather than surgery, it allows therapy to be delivered under conscious sedation versus general anesthesia. Dr. Christman noted that it is a lot more economical to use just the biomaterial alone than it is to incorporate cells or other biologic therapies, which can increase the cost by an order or two of magnitude.
Dr. Christman is a scientific co-founder of Ventrix,6 which recently completed a Phase I trial7 of VentriGel™, an off-the-shelf, ECM biomaterial scaffold designed to facilitate the repair of cardiac tissue. Trial objectives included assessment of safety and efficacy in patients with a history of MI. The open-label study was conducted at five sites in the United States, and enrolled 18 individuals aged 30 to 75 years who had experienced a MI between 60 days and 3 years prior to enrollment and had evidence of left ventricular dysfunction.
VentriGel was injected via a femoral artery catheter. Six months of follow-up to assess adverse events and efficacy variables for halting the progression of heart failure, (including heart stroke volume, ejection fraction, and scar mass) is ongoing. Patients were post-reperfusion, meaning they have had angioplasty and a stent, but not bypass therapy, in contrast to the patients who received cardiomyocyte patches.
“I have a lot of optimism for biomaterial-based therapies,” commented Dr. Christman. “Having an acellular biomaterial is potentially less challenging from a regulatory standpoint and also less costly. I am hopeful that there will be more of these biomaterial-based therapeutics reaching the clinic. The cell-based therapies have more barriers to the clinic in applications.”
Dr. Christman’s lab has looked at the immunogenicity of VentriGel, which is a porcine-derived product. Like other porcine-derived products, such as heart valves and decellularized ECM patches used for surgical repair with a long history of good safety profiles, there does not seem to be a problem with immunogenicity. “As long as you remove the cells, the remaining scaffold promotes a pro-remodeling response as opposed to a rejection immune response,” Dr. Christman asserted. She recognized that larger trials will be needed to confirm efficacy.
Dr. Christman’s group is also investigating injectable nanoparticles that can be delivered intravenously to treat acute MI, but this approach will require a lot more developmental research to get to the clinic.
When asked about how long these therapies would take to reach the clinic for wide-spread use, Katja Schenke-Layland, Ph.D., professor for medical technologies and regenerative medicine, University Tübingen, said, “Every 5 years we have this conversation, and it may continue for another 5 or 10 years.”
She noted that cellular-based therapies face several obstacles. These include deriving appropriate cells from appropriate sources so that they will function in a clinical setting. So far, cardiomyocytes are derivable only from iPSC or embryonic stem cells, not adult stem cells. Although there is some improvement of heart function, there is no regeneration of heart muscle. Dr. Schenke-Layland believes this is due to limitations of the cells being used, not the skills of the engineering.
Dr. Schenke-Layland is not sure that it is possible to have a fully functional, cultured, engineered patch to treat infarction, given the difficulty of getting the in vitro derived cells to integrate into injured heart muscle. She thinks Dr. Christman’s approach, using a cell-free product, is more promising.
It may be possible to create an engineered, cell-free patch that could attract cells already in the body to regenerate damaged tissue, as has been shown for other tissues, such as skeletal muscle,8 although Dr. Schenke-Layland does not expect this to work for heart muscle. Where it could work is for heart valves, where a cell-free implant is repopulated after implantation.
One challenge is loss of flexibility and function of valve leaflets if they are encapsulated by cells that recognize them as foreign. Products that avoid this reaction are being developed, such as polymer-based, cell-free cardiac and pulmonary valves from Xeltis.9
As cell-free products are developed, in vitro grown human myocyte patches and related cellular products will remain valuable for preclinical testing and drug development, if not for therapy. This could one day replace animal testing, maybe even allowing the use of patient-specific cells for personalized drug development.
1. Shadrin IY, Allen BW, Qian Y, et al. Cardiopatch platform enables maturation and scale-up of human pluripotent stem cell-derived engineered heart tissues. Nat. Commun. 2017; 8: 1825. DOI: 10.1038/s41467-017-01946-x.
2. Menasche P, Vanneaux V, Hagege A, et al. Human embryonic stem cell-derived cardiac progenitors for severe heart failure treatment: First clinical case report. Eur. Heart J. 2015; 36: 2011–2017. DOI: 10.1093/eurheartj/ehv189.
3. Menasche P, Vanneuax V, Hagege A, et al. Transplantation of human embryonic stem cell-derived cardiovascular progenitors for severe ischemic left ventricular dysfunction. J. Am. Coll. Cardiol. 2018; 71. DOI: 10.1016/j.jacc.2017.11.047.
4. Gao L, Gregorich ZR, Zhu W, et al. Large cardiac-muscle patches engineered from human induced-pluripotent stem-cell-derived cardiac cells improve recovery from myocardial infarction in swine. Circulation 2017.
5. Zhu W, Zhao M, Mattapally S, et al. CCND2 overexpression enhances the regenerative potency of human induced pluripotent stem cell-derived cardiomyocytes: Remuscularization of injured ventricle. Circ. Res. 2018; 122: 88–96. DOI: 10.1161/ CIRCRESAHA.117.311504.
7. A Phase I, Open-Label Study of the Effects of Percutaneous Administration of an Extracellular Matrix Hydrogel, VentriGel, Following Myocardial Infarction. ClinicalTrials.gov Identifier: NCT02305602.
8. Zhang J, Hu ZQ, Turner NJ, et al. Perfusion-decellularized skeletal muscle as a three-dimensional scaffold with a vascular network template. Biomaterials 2016; 89: 114–26.