Gene Therapy for Rheumatoid Arthritis

Introduction.
Gene therapy offers great possibilities for the treatment rheumatoid arthritis (RA). RA was the first orthopedic condition to be targeted by gene therapy. Initially, RA, a non-lethal disease that is not regulated by a single gene, may not have been an obvious target for gene therapy. However, while traditional surgical and pharmaceutical methods of treating RA have met with limited therapeutic success and have failed to produce a cure, the past several years have seen extensive progress toward development of gene therapy for arthritis. Numerous vectors and therapeutic genes have been investigated in animal models of arthritis, and the potential of gene therapy to treat or manage RA has been demonstrated in several clinical studies. Gene therapy offers the possibility of overcoming many of the limitations of current biologic therapies by providing long-term, high-level localized expression of therapeutic genes, potentially in as little as a single dose.

Gene therapy emerged as a novel strategy to treat arthritis in the early 1990s. Fundamentally, arthritis gene therapy involves the transfer of complementary DNA (cDNA) encoding antiarthritis gene products, which may be difficult to administer by more conventional methods. Gene therapy offers the promise of long-term expression of antiarthritis gene products, as well as targeted delivery to and expression in affected tissues, limiting potential systemic side effects.

Background on the Disease.
Arthritis is the leading cause of disability among adults in the United States, affecting approximately 21% (more than 46 million) adults. Rheumatoid arthritis (RA) affects approximately 1.3 million adults in the United States, and more than 60 million people worldwide [1]. RA is characterized by inflammation of the synovial lining and destruction of extraarticular bone and cartilage [2]. It is believed that arthritogenic peptides, either from foreign or self proteins, are presented to T cells preferentially by RA-associated MHC molecules on antigen-presenting cells [3]. Activated T cells produce a variety of proinflammatory cytokines, including TNF?, IL-1 and IL-6. The inflammatory response induced by these cytokines is directly responsible for the overt RA symptoms, including joint pain, swelling, effusion and stiffness.

Existing Therapies.
Despite the prevalence and rising economic burden of RA, effective therapies for this disease remain limited, and there is no cure. Current therapies consist of early, aggressive and continuous treatment with non-steroidal anti-inflammatory drugs (NSAIDs) and disease-modifying antirheumatic drugs (DMARDs). Among the DMARDs, methotrexate has long been the drug of choice for RA, but newer biologic targeted therapies have emerged in recent years. These targeted therapies include Orencia®, a fusion protein composed of the extracellular domain of cytotoxic T lymphocyte antigen-4 fused to an immunoglobulin (CTLA4-Ig); Kineret®, an interleukin-1 receptor antagonist (IL-1Ra); Remicade®, a chimeric anti–TNF? antibody; Humira®, a fully human anti–TNF? antibody; and Enbrel®, a soluble TNF? receptor [4]. As a result of these therapies, the outlook for RA patients is better than at any time in our history. However, it is important to remember that these drugs provide treatment, not cures. In addition, the chronic nature of the disease necessitates lifelong treatment for most patients. Therefore, the long-term efficacy, safety and expense of these drugs remain concerns, particularly due to the high systemic doses and repeated intravenous or subcutaneous administrations necessary to achieve therapeutic results.

Overview of Preclinical Gene Therapy Studies.
Given the success of biologic targeted therapies, various strategies have been employed to utilize these immunomodulatory agents in RA gene therapy. Multiple preclinical RA gene therapy studies have demonstrated that gene therapy vectors, including retrovirus and adenovirus vectors, expressing IL-1Ra produce high levels of the transgene in target tissues and inhibit inflammation and cartilage loss in animal models. Similarly, numerous studies have demonstrated the effectiveness of blocking TNF? activity via gene therapy, including the suppression of inflammatory cell infiltration, pannus formation, cartilage and bone destruction, and expression of joint proinflammatory cytokines in animal models [4].

Furthermore, AAV expressing a TNFR:Fc fusion gene under the control of an inflammation-inducible NF-?B promoter delayed disease onset and decreased the incidence and severity of joint damage in mouse and rat arthritis models [4]. This type of gene therapy, utilizing a disease-inducible promoter, is of particular interest in autoimmune diseases like RA, which are characterized by flare-ups followed by periods of disease regression. By utilizing the inflammation-inducible NF-?B promoter, high levels of transgene expression are obtained only during disease flares, preventing unnecessary exposure of the patient to immunosuppressive agents during periods of disease regression.

In addition to proinflammatory cytokines, an important role for NF-?B signaling has been established in RA. NF-?B controls the expression of proinflammatory mediators of RA, including TNF?, thus may serve as a master regulator of the disease. In terms of gene therapy, both adenovirus and AAV have been utilized to deliver NF-?B inhibitors to synovium, successfully preventing expression of proinflammatory cytokines [5, 6].

A variety of growth factors have also been studied as therapeutics for RA. Bone morphogenetic proteins (BMPs), particularly BMP-2 and BMP-7, have been shown to induce chondrogenesis and osteogenesis when delivered by adenovirus or AAV vectors [4]. Additional growth factors that have been studied in relation to RA gene therapy include transforming growth factor (TGF)-?1 and insulin-like growth factor (IGF)-1. Adenovirus or AAV vectors expressing TGF-?1 have been used to transduce mesenchymal stem cells (MSCs) and drive ex vivo differentiation of MSCs into chondrocytes, facilitating cartilage repair in animal models [7]. Similarly, ex vivo chondrocytes transduced with an adenovirus expressing IGF-1 enhanced matrix synthesis and cartilage repair in horses [8]. These studies suggest important roles for growth factors in RA gene therapy, and indicate that a variety of growth factors may be able to join the list of effective RA therapies.

Overview of Clinical Gene Therapy Studies.
Early arthritis gene transfer trials used retrovirus vectors in ex vivo protocols to deliver IL-1Ra to the metacarpophalangeal joints of RA patients [9]. These safety and feasibility studies illustrated that gene transfer to RA joints could be safe and effective, but the low numbers of patients enrolled prevented any conclusions with regard to efficacy. A similar Phase I trial has been initiated by TissueGene, Inc utilizing retroviral vectors to transduce human chondrocytes, which are then mixed with normal human chondrocytes and injected into the knee of patients with degenerative joint disease. Currently, 16 patients have been treated in the United States and South Korea, with approximately 50% of the treated patients demonstrating symptomatic improvement [4, 10].

Another ongoing RA gene therapy trial is sponsored by Targeted Genetics, Inc and is evaluating the safety and efficacy of a single-stranded rAAV2 virus expressing the complete coding sequence of a TNFR:Fc fusion protein, which is identical to Enbrel®. This trial received much publicity in 2007 because of the death of an enrolled patient. Subsequent investigation determined that the death of the subject was not likely due to the virus vector [4]. Despite the death of this subject, the Targeted Genetics trial has shown promising results. The AAV vector appears safe, in that there is no evidence of circulating TNFR:Fc or extraarticular over-expression of TNFR-Fc, which would have indicated vector dissemination and amplification in extraarticular tissues. Clinical response was assessed using patient reported outcomes, revealing moderate improvement in target joint pain and swelling [11].

Conclusions.
Over the past several years, significant advancement has been made in the field of arthritis gene therapy. Numerous vectors and therapeutic genes have been tested and shown to have varying degrees of efficacy. However, while more than 1000 gene therapy clinical trials have been conducted or are ongoing, at least 32 of which have entered Phase III, only a limited number of these trials have been in the field of arthritis. Those trials that have attempted to combat arthritis have shown significant promise, while also serving as reminders that more work is needed. Future clinical trials are already being designed that will incorporate lessons learned from earlier trials, helping the field continue to move forward. With the recent Chinese approval of Gendicin [12], the world’s first commercially available gene therapy, for head and neck cancers, the field of gene therapy seems poised to finally realize its long-standing potential. The hope remains that gene therapy will soon join the arsenal against RA and other orthopedic diseases.

References.
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10.    Lee K.H. (2008). Preclincal and early clinical analysis of allogeneic chondrocytes transfected retrovirally with TGF-beta1 gene for degenerative arthritis patients. 5th International Meeting of Gene Therapy of Arthritis and Related Disorders.
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