Therapeutic Technology

For decades, researchers and clinicians have advanced gene- and cell-based therapies to the cusp of fundamentally altering the way we treat human disease. The ability to transplant multipotent or pluripotent cells or to constitutively express a therapeutic gene in the target diseased tissue provides an attractive alternative to continuous pharmacological intervention. Cellular therapies have been approved by the U.S. Food and Drug Administration (FDA) to treat cosmetic flaws (Laviv®, Fibrocell Technologies Inc.), blood disorders (Ducord, Duke University), cartilage deficiencies (Carticel, Genzyme Biosurgery), dental pathologies (Gintuit, Organogenesis Incorporated), and cancer (Provenge®, Dendreon Corporation). There is widespread research in numerous diseases that utilize viral-vector-mediated therapies. A great many preclinical trials have shown efficacy with viral-mediated gene delivery in animal models of disease. Gene-based therapeutics have been shown to be generally safe for human application, however there were serious complications on the path to acceptability. Specifically, a patient died from a therapeutically induced catastrophic immune response in an early clinical trial. The death of this patient caused a significant delay in the development of gene-based technology as the safety of viral vectors was called into question. Some cellular therapeutic products are already approved for public use but gene therapy products are not yet approved for market release. A particular clinical trial (NCT00643890) using viral-mediated gene therapy in Parkinson’s disease did show efficacy above the placebo range but the study was terminated because it did not elevate the current standard of care. Modern generations of viral vectors have been demonstrated to be safe in multiple trials but their efficacy in clinical trials rarely surpass either the placebo-effect range or the current standard of care.

Viral-Mediated Gene Therapy

Gene therapy uses viral or synthetic vectors to deliver exogenous genetic material to living cells and tissues to elicit therapeutic benefit. Preclinical studies commonly use modified viral vectors derived from naturally occurring viruses that infect human cells. Modified viruses include retroviruses, such as lenti and HIV; Herpes simplex virus 1 presents as cold sores; adenovirus presents as respiratory sickness; or adeno-associated viruses, which have no known pathogenicity in humans. Researchers use the naturally evolved mechanisms of infectious viruses to transduce diseased cells and express a potentially therapeutic gene. For example, the adeno-associated virus serotype 9 (AAV9) is particularly useful for peripheral administration (i.e., intravenous) because it can cross the blood–brain barrier to express exogenous gene products within the central nervous system (CNS). While some particular genetic characteristics of these viral vectors make them attractive for therapeutic engineering, other pathogenic viral genes must be removed to make them safe, and therefore useful for therapeutic development.

Stem Cell Therapy

Stem cells are extremely promising therapeutics that have the potential to support or replace cells vulnerable to dysfunction and/or death in a wide range of human diseases. The term “stem cell” was first used by German scientist Ernst Haeckel to describe the fertilized egg that will give rise to a complicated organism. Stem cells demonstrate these three general characteristics: (1) they can divide and multiply for prolonged periods; (2) they are unspecialized cells; and (3) they can differentiate into specific cell types depending on stimuli ( http://stemcells.nih.gov.easyaccess1.lib.cuhk.edu.hk/info/basics/pages/basics2.aspx ). There are different types of stem cells depending on how they were isolated and from what tissue they were taken. Stem cells after transplantation have been shown to align with and become tissue-specific cells given specific physiological or experimental stimuli. Other stem cell strategies involve transplanting cells that will express growth factors or neurotrophic factors to support viability or function in vulnerable cell populations. Although to date there have been many stem cell-based clinical trials launched to treat neurological diseases, the field remains in the early stages of development and application. Many preclinical studies have shown great efficacy in a large number of wide-ranging disease models, but successful implementation into human therapies have been incremental.

The advancement of stem cell therapy has been relatively slow because early stem cell lines were embryo-derived, a stem cell source that has remained a national ethical dilemma from its inception. Many nations have government-imposed regulations on the development and use of stem cells in biomedical research. The controversial issues surrounding stem cell therapeutics span ethical, moral, and political debates.

The two main types of stem cells are human embryonic derived stem cells (hESCs) and somatic (adult) stem cells (ASCs) that are found in and isolated from specific body tissues. A particular advantage of hESCs is their ability to differentiate to become any cell found within the human body, which is termed “pluripotency.” Pluripotent cells are amenable to a broad range of therapeutic applications given their ability to divide for long periods of time in addition to their vast differentiation potential. Conversely, isolated ASCs do not readily produce large quantities of stem cells and may only be able to differentiate into the limited cell types found in the tissue of origin. Within the bone marrow, a common tissue where stem cells are isolated from, there are two types of stem cells: hematopoietic (HSCs; give rise to all blood cells) and mesenchymal stem cells (MSCs; give rise to bone, cartilage, and fat cells). In the 1990s it was discovered that neural stem cells (NSCs) inhabited the adult brain and could give rise to the three major brain cell types: astrocytes, oligodendrocytes, and neurons. This discovery was of particular interest to those researching neurological disease in that it could allow for isolation of neural precursors to develop novel strategies for advanced treatments.

The deleterious effect of the host’s immune system on stem cell survival after transplantation is of particular concern. Patients that receive allogeneic stem cells (usually hESCs) are likely to need continuous administration of immunosuppressive drugs, which can leave the patient vulnerable to infection. Stem cell transplantations using “autologous” stem cells are believed to reduce the risk of host immune-response mediated rejection as the patient is receiving their own cells and not cells from a foreign donor. While a patient is less likely to have a robust immune response after autologous cell transplantation, when compared to donor cell transplantation, the risk remains that newly transplanted autologous cells within diseased tissue will not function as they did within the tissue of origin. There is also potential that the transplanted cells isolated from a diseased patient could be in a diseased state as well. Since many millions of cells are needed for cell-mediated therapy, ESCs are more amenable to growing large numbers of therapeutic cells for the treatment of disease. Therefore, more research is warranted on the specific conditions that are required to achieve successful transplantation and subsequent engraftment.

Another type of stem cell, termed the induced pluripotent stem cell (iPSC) has recently been elevated as the new improved stem cell therapy that has characteristics of both hESCs and MSCs. iPSCs are pluripotent because they have been reprogrammed to replicate gene expression observed in ESCs. Human iPSCs express embryonic stem cell markers and are capable of generating cells characteristic of all three germ layers. This technology enables the transplantation of reprogrammed autologous cells that maintain somatic genetic homogeneity with the patient (as with autologous MSCs) but with the potential to differentiate into any cell found in the body (as with hESCs). While these cells have been met with great excitement in the therapeutic arena, a concerning issue with advancing the clinical use of iPSCs is their propensity to form tumors. As with any new medical technology, a patient’s safety must be paramount.

In recent years the FDA has approved many trials that aim to use stem cell therapy to treat many devastating diseases. These trials include evaluation of stem cell therapies in diseases such as neurodegenerative disorders, cardiovascular disorders, genetic disorders, cancer, musculoskeletal disorders, and degenerative eye conditions. Moreover, HSCs have been included in blood transfusions for almost half a century, and transplantations of isolated and expanded HSCs have been conducted ( http://stemcells.nih.gov.easyaccess1.lib.cuhk.edu.hk/info/scireport/pages/chapter5.aspx ).

Review of Clinical Trial Process

Clinical trials are used to determine whether or not novel therapies that exhibit therapeutic benefit in preclinical studies can be “translated” into effective treatments for human disease. In the United States, the FDA has regulated the clinical trial process since the 1970s. Prior to the FDA approving the commencement of human clinical trials, they first require that these clinical trials must be designed to comply with established good clinical practices (GCPs) to ensure the safety of human trial subjects. There are now international guidelines established in concert with the FDA to standardize the GCPs used to conduct clinical trials throughout the world. Therapeutics should only advance to early-stage clinical trials if there is sufficient preclinical data supporting the probable safe usage in human subjects. Therapies validated by the clinical trial process can be chemical, biological, thermal electrical, or behavioral. Depending on the nature of the therapy to be tested, there is a department within the FDA responsible for approval and oversight of the trial. Studies in gene and cellular therapies are specifically regulated by the FDA’s Center for Biologics Evaluation and Research (CBER). In addition to establishing safe effective parameters for the development of therapeutic biologics, the CBER also educates the public on the safe and appropriate usage of biological products.

The clinical trial process ( http://www.fda.gov/ForPatients/Approvals/Drugs/ucm405622.htm ) utilizes a series of trials to scientifically establish parameters for use of an investigational therapeutic pertaining to safety, dosage, efficacy, and side effects. In phase I trials, a team of investigators will test a new therapeutic in a relatively limited group of people to establish safety data. In addition to safety, the dosing range and potential efficacy may be monitored prior to launching the phase II trial. Phase I trials are usually conducted in healthy subjects to ensure that the therapeutic itself causes no adverse effects in the absence of a disease state. However, in cell- and gene-based therapeutics, phase I trials usually evaluate safety as a primary measure, and efficacy as a secondary measure in diseased patients. This diversion from enrolling healthy participants in phase I clinical trials is due to the potential for long-term or permanent adverse effects associated with gene- or cell-based therapies when compared to transient small-molecule drug therapies. Phase I trials are usually open-label, which means that the patient is aware of the potential therapeutic outcome. This means that there is a likely placebo effect that must be taken into consideration when looking to move therapeutics into the next trial phase.

The phase II trial remains committed to monitoring safety of treatment in a larger cohort while determining whether the therapeutic represents an effective treatment for the relevant disease. If possible, these larger cohorts will be conducted under “closed-label” and “blinded” conditions, meaning that that trial subjects are not told what therapeutic they are given or even whether or not they have been given the therapeutic or the inert placebo. This gives the trial more validity if there is evidence of efficacy in the treated group versus the control because neither group knows what they had received. Phase I and II clinical trials involving terminally ill patients or complex invasive procedures may not include a true “control or placebo” group. While the ideal trial involves blinding the researchers and the patients to the treatments it is unlikely that there will be true control groups in late-stage neurodegenerative disease studies, unless they advance to phase III, due to the invasive nature of intraparenchymal or intrathecal CNS delivery. In cases such as these, efficacy is usually established by a comparison with established disease progression rates.

If the therapeutic shows efficacy in a phase II trial then it will be moved into the phase III trial to be administered in multiple-center trials to confirm efficacy, generate more safety and dosage data, and compare it with currently available treatments. Phase III trials have large control and therapeutic cohorts (usually minimum of 100 participants) where randomized patients and investigational clinicians are blinded to who gets what. Finally, if the novel therapeutic drug or biologic advances successfully through the first three clinical trial phases, and it exceeds the current standard of care, it will be brought to market for distribution and clinical use. During this open-market period, ongoing phase IV studies continue to evaluate the therapeutic efficacy in diverse populations and determine long-term effects of use. These studies are of particular use to determine future applications for new therapeutics.