Bioengineered tissues are showing promise in the treatment of valvular heart disease, according to the results of several early clinical and preclinical studies demonstrating the benefits of using heterologous substrates as scaffolding capable of promoting in vivo cell infiltration and remodeling.
Ultimately, cardiac tissue engineering seeks to develop the ideal material for replacement and repair of a variety of heart components – a material able to integrate, function, and, when necessary, grow and develop in a manner undifferentiated from normal cellular structures.
One of the most important areas of focus for such efforts is the development of heart valves and valve patches suitable for use in replacement or repair. The need is obvious: “The fact that literature continues to be published debating the best type of valve prosthesis is proof that, to date, the ideal valve substitute has not been found,” according to Cristian Rosu, Ph.D. and Dr. Edward G. Soltesz of the Cleveland Clinic (Semin Thorac Cardiovasc Surg. 2015 June 30 [doi: 10.1053/j.semtcvs.2015.06.007]).
Dr. Roşu and Dr. Soltesz go on to say that this lack of an ideal prosthetic heart valve leaves surgeons and their patients with a difficult choice at the time of valve surgery. The focus of their concern relates to how current bioprostheses are being used in younger and younger patients, and how these devices have a finite lifespan – requiring eventual reoperation and replacement, perhaps multiple times. In addition, calcification is a prominent and well-known risk of bioprosthetic valves, especially in children (Circulation. 2014 Jul 1;130[1]:51-60).
As for mechanical valves, although durable, they require a lifetime of anticoagulation therapy to prevent thrombosis. And the lack of growth potential found in both mechanical and current bioprosthetic devices is obviously a bane to pediatric valve therapy.
The latest efforts in tissue engineering therefore seek to develop valves and valve components that may be more permanent solutions by better mimicking natural valves.
But better mimicking of a natural valve is not an easy task, when such valves exist in “a near-perfect correlation of structure and function, enabling the valve to avoid excess stress on the cusps while simultaneously withstanding the wear and tear of 40-million repetitive deformations per year, equivalent to some 3 billion over a 75-year lifetime.” (“Principles of Tissue Engineering,” 4th ed. [London: Academic Press 2014, p. 813]).
The “holy grail” of tissue engineering, therefore, is to provide a completely in vitro–developed, autologous, fully cellularized, functional scaffolding for implantation that can live up to these requirements. In order to do this, extensive research into the search for the best cell sources and growth matrices and methods is underway, as illustrated by several recent reviews (Front Cell Dev Biol. 30 June 2015 [doi.org/10.3389/fcell.2015.00039] and Mater Sci Eng C. 2015 March 1;48:556-65).
Candidate cell types include a variety of embryonic stem cells, as well as adult cell types that have proven amenable to rejuvenation and redifferentiation (Adv Drug Deliv Rev. 2014;69-70:254-69).
In one example of the quest for completely in vitro human-tissue designed valves, Dr. Jean Dubé of Laval University, Quebec, and his colleagues reported research on a human tissue–engineered trileaflet heart valve assembled in vitro using human fibroblasts. These cells self-assembled into living tissue sheets when cultured in the presence of sodium ascorbate. These sheets could be layered together to create a thick construct, with the ultimate goal of replacing the use of bovine pericardium tissue implants with ones made of autologous cells from the patient (Acta Biomater. 2014 Aug;10[8]:3563-70).
Currently, however, many of the preclinical studies of fully tissue engineered heart valves have shown retraction of the heart valve leaflets as a major mechanism of functional failure. This retraction is caused by both passive and active cell stress and passive matrix stress, according to a review by Inge A.E.W. van Loosdregt, Ph.D., and her colleagues at the Eindhoven (the Netherlands) University of Technology (J Biomech. 2014 Jun 27;47[9]:2064-9).
While all of these developmental issues regarding the use of fully tissue-engineered valves are being worked out, early clinical applications are already being found for a new generation of valve prostheses that take an intermediate approach, one that uses an implanted scaffolding material that allows autologous cell infiltration and replacement in vivo.
Two of the most prominent examples of these scaffoldings currently in investigation and in early clinical use for heart valve repair or replacement are the bovine pericardium-derived CardioCel (Admedus, Brisbane, Australia) and the porcine intestinal submucosa-derived CorMatrix ECM(CorMatrix Cardiovascular, Roswell, Ga.). CorMatrix ECM was approved by the Food and Drug Administration in 2005 for pericardial repair and reconstruction, and in 2007 for cardiac tissue repair. The FDA approved CardioCel in 2014 for use in the United States in pericardial closure and for the repair of cardiac and vascular defects in both adults and children.