Cardiovascular disease is the leading cause of death in Western countries. The damage that occurs during a heart attack results from the death and loss of the cells called cardiomyocytes that make up the majority of the myocardium. Medical therapy, balloon angioplasty, stent deployment, coronary artery bypass surgery, and heart transplantation are the current options available to the patient. However, with the exception of total heart transplantation, none of the therapies are able to replace the dead tissue with viable tissue. Cell transplantation has been suggested as a method to regenerate scarred myocardium.
The concept underlying cell therapy is that cells removed from a different part of the body can be transplanted into diseased tissue to restore decreased or lost function. Cardiac muscle is a good example for this approach since the remaining cardiomyocytes that survive following injury have an extremely limited ability to regenerate. Unlike cardiac muscle, skeletal muscle is capable of robust self-repair. The inability of cardiac muscle to self-repair is thought to be a consequence of the heart being deficient in a particular cell type called a myoblast. Myoblasts are small cells located between the basal lamina and sarcolemma in the skeletal muscle fiber. They respond to injury by extensive proliferation and can differentiate into nascent muscle, thereby providing a source for growth and repair. Myoblasts display a remarkable tolerance to reduced perfusion, making them a suitable choice for repair of ischemic heart disease. In particular, autologous skeletal myoblasts (ASM) are preferred since these are isolated from the same person into whom they will be later transplanted. This avoids many issues related to ethics and transplant rejection.
Autologous skeletal myoblast transplantation is not an approved therapy by the US Food and Drug Administration (FDA). Since it is considered an investigational new drug (IND), institutions participating in this type of human research must apply to the FDA for approval prior to screening for patients. In general, the acceptance criteria for entry into these studies require that patients have a documented previous myocardial infarction and an impaired left ventricular ejection fraction.
Autologous Skeletal Myoblast Isolation and Growth
The basic outline for ASM isolation and growth is as follows: a surgeon in a surgical suite makes an incision into the quadriceps muscle and aseptically removes a biopsy between 0.5g and 5g. The biopsy is placed in a transport solution and immediately taken to a tissue culture facility. Once inside a pre-irradiated laminar flow hood, the biopsy is removed and cut into small chunks approximately 1mm3 with sterile scissors.
These chunks are placed into an enzymatic solution of digestive enzymes used to degrade extracellular matrix and connective tissue releasing individual cells. After the digestive process is complete, the enzymes are inactivated and the ASM are extensively washed to remove red blood cells and debris. At this point, the cells are resuspended in a rich medium containing growth factors and necessary nutrients and seeded onto cell culture dishes.The ASM are transferred to a 37┬║C incubator and grown in high humidity until they reach approximately 70% to 80% confluency.ASM cultures must be carefully monitored and split before reaching confluency since they have a tendency to convert to fibroblasts that are not thought to be of therapeutic benefit. After two to three weeks, when a sufficient number of ASM have been grown, they are harvested, loaded into a syringe, and transported to the surgical suite where the patient awaiting cell transplantation has been prepared.
Preliminary Human Results
Encouraging results from initial Phase I human studies have confirmed the safety, survival, differentiation, and functional improvement of myoblast transplantation.1-4
In these experiments, direct epicardial injection was used to transfer myoblasts into patients with severe left ventricular dysfunction in addition to bypass surgery. Initially, low numbers of myoblasts were injected into patients so that potential toxicity from the injected myoblasts could be determined. As the studies continued, higher doses were safely administered without signs of toxicity.All of the patients survived the process of myoblast injection.
The methods to assess the safety and effectiveness of treatment include Holter monitoring, echocardiography, magnetic resonance imaging (MRI), and positron emission tomography (PET). Although the majority of patients did not experience serious adverse events, Holter monitoring revealed that a small subset of patients experienced clinically non-significant ventricular arrhythmias. Neither the myoblast transplantation process nor the presence of the myoblasts could be directly linked to the arrhythmias. Left ventricular ejection fraction and left ventricular wall motion is improved by echocardiographic analysis. PET scanning showed increased metabolic activity in the regions that received myoblast injections suggesting that myoblasts had grafted and survived for up to two years.
These encouraging results must be tempered with the knowledge that for ethical reasons the operations were done in the absence of a control group and with concomitant coronary artery bypass grafting (CABG). CABG is known to benefit the heart after MI by affecting all of the biological parameters measured in the previously mentioned experiments so with these results it is not possible to conclusively state that myoblast transplantation is responsible for the observed positive effects. One recent study has successfully demonstrated catheter-based delivery of myoblasts to ischemic myocardium as a stand-alone therapy.5
Autologous Skeletal Myoblast Delivery Systems
Although surgical delivery has been performed safely, only a small number of patients are candidates. In response, investigators have focused on using endovascular catheter-based technologies for cell transplantation. Several companies have manufactured injection catheters including the MyoCath™ (Bioheart, Inc.), TransAccess┬« MicroLume™ Delivery System (Transvascular), the Stiletto™ (Boston-Scientific Scimed, Inc.), Myostar™ (Biosense-Webster). These catheters have several common features including a 7-9F-diameter sheath, a coaxial cable to control the catheter tip orientation, an injection needle, and a leur lock adapter for syringe attachment, and are positioned in the left ventricle using fluoroscopic guidance.
Endoventricular delivery is more challenging than surgical injection. Safe, accurate, and reproducible endoventricular delivery of molecules or cells would ideally incorporate knowledge of the internal architecture of the ventricle, the position and orientation of the catheter tip in space, catheter maneuverability, and the ability to penetrate and inject into the thin, scarred myocardium. Although targeting an injection catheter using only fluoroscopic guidance has been demonstrated, the procedure is limited by low spatial resolution, the inability to accurately navigate to a predefined site, prolonged X-ray exposure, and risks local toxicity if multiple injections occurred at the same site.
To circumvent these limitations, Biosense-Webster developed a novel non-fluoroscopic three-dimensional (3-D) catheter-based in vivo navigational system for the left ventricle. The Biosense system consists of a locator pad having three coils that emit a low-energy magnetic field, a stationary reference catheter with a magnetic field sensor, separate mapping, and injection catheters with a magnetic field sensor, and a computer processor.
When the catheter is in contact with endocardium, two electrodes on the distal tip permit measurement of a voltage potential across a short segment of endocardium.The system uses a triangulation algorithm to determine the co-ordinates of the catheter tip in 3-D space. Data taken during these measurements are processed to generate a 3-D, color-coded voltage map reconstruction of the left ventricle (LV) that clearly demarcates the area of myocardial infarction (see Figure 1).When the LV map is complete, the mapping catheter is exchanged for a modified mapping catheter that can perform an endocardial injection through a needle on the distal tip. This hollow, steerable catheter has a 27-gauge nitinol needle on the distal tip that can be advanced or retracted by the operator and a three-way luer lock adapter on the proximal end so that tuberculin syringes can be fitted to load sample into the lumen of the needle (see Figure 2). Using the computer-generated image as a roadmap, endocardial injections can be performed in realtime and their position marked throughout the procedure.
Safety and Other Considerations
Endoventricular injection catheters have an exemplary safety record.There are few reports in the literature of myocardial rupture leading cardiac tamponade from overly aggressive catheter manipulation or from injection. Instances of non-sustained ventricular tachycardia occur, but are primarily related to the presence of the catheter itself, and not to the injection process. Premature ventricular contractions (PVC) and other short arrhythmias (<10 seconds) are relatively common, being associated with physical irritation of the endocardium during mapping or injection. However, in general, they resolve quickly without the need for cardiac defibrillation.
Endoventricular injection carries the risk of pericardial effusion or cell leakage back into the circulatory system. Post-mortem visualization of marker gene expression in uninjected tissues suggests that complete retention is unlikely to occur during the injection process. The consequence of leakage may be as simple as cell loss or more detrimental such as distal embolization. Quantitative studies regarding the bioretention and biodistribution of injected substances will need to be performed to better understand these issues.
Autologous cells derived from the person into whom they will be re-administered are the preferred choice for transplantation because immunosupression of the treated individual should not be required to prevent graft rejection. However, autologous cell transplantation will require that an individualized batch of cells be cultured for each transplant recipient.
This requires expensive, sophisticated good manufacturing practice (GMP) cell culture facilities staffed with highly trained individuals. Commercial development of cell therapy is likely to prefer an allogenic approach because large numbers of cells can be grown and stored for 'off-the-shelf' use, but it is not clear how they will be safely administered to patients without risking cell transplant rejection. In such cases, immunosupression is likely to be necessary.
Conclusion
Numerous studies have demonstrated the safety and efficacy of cell transplantation in animal models, and initial studies have demonstrated the safety of injection catheters and cell transplantation in the human heart. Catheter injection of cells is a relatively new innovation with only a handful of published studies, but the ability to access the heart without having to perform a median sternotomy will continue to be explored by more physicians and researchers as a technique to deliver therapeutic biologics. The capability to grow a pure and robust population of stem cells, coupled with technological advances in catheter design that facilitate the injection of substances into the myocardium percutaneously, is likely to have a profound impact on modern cardiac medicine.
The Arizona Heart Institute is expected to receive FDA approval for catheter-based autologous skeletal myoblast therapy. For more information regarding this type of therapy for heart disease please visit our website at www.azheart.com. Requests for participation in our research program should be directed to (602)-707-3535 or e-mailed to myoblast@azheart.com.