Gene Therapy Clinical Trials Regulatory Affairs
Regulatory and ethical issues of gene transfer are usually a secondary preoccupation of researchers. Conducting gene therapy clinical trials with genetically modified organisms as the vectors presents unique safety and infection control issues. The area is governed by a range of legislation and guidelines, some unique to this field, as well as those pertinent to any area of clinical work. Gene therapy legislation aims at protecting the human subject, the general public and the environment. This site is an attempt to give an overview of the different regulations and guidelines in Europe (nearly all countries including European member states), United States of America and the rest of the world[23].
The major regulatory institutes are as follows:
CLINICAL TRIALS
Worldwide, over 700 clinical trials have been conducted or are underway, with enrollment of over 6000 patients. A substantial portion of these clinical trials (over 70%) are cancer related and often carried out on terminal patients. The most commonly used vectors are retroviral vectors based on MLV, which were the first viral vectors to be used in a gene therapy trial [20]. The targets of that first clinical trial were the T-lymphocytes of two children suffering from severe combined immunodeficiency (SCID) [20]. Unfortunately, a clear judgment of the success of this trial was not possible because the patients received supportive conventional therapy. This trial, however, did set the stage for other gene therapy trials.
Gene transfer into multipotent hematopoietic stem cells has received much attention owing to its relevance for a broad variety of human diseases, ranging from hematological disorders to cancer. In addition, it allows the use of ex vivo transduction protocols, thereby minimizing the exposure of the patient with viral particles. The use of retroviral vectors in this setting is hampered by the low frequency of gene delivery, as transduction by retrovirus vectors occurs only in cells that are replicating at the time of infection, and therefore, the transduction of slowly or nondividing stem cells and progenitors is inefficient. A number of growth factor combinations have been used to prestimulate hematopoietic stem/progenitor cells and have been shown to increase transduction efficacy. The drawback of this approach is that exposure of progenitors to growth factors over several days markedly impaires their ability for long-term engraftment. However, a brief (<24 h) exposure to specific cytokines and stromal support allows engraftment with a high number (10%–20%) of retrovirally modified cells.[21]
Given the positive effect of bone marrow stroma on retroviral gene delivery to stem cells, attempts were made to mimic the bone marrow milieu by the addition of purified extracellular matrix molecules. Certain fibronectin fragments (e.g., fibronectin CH296) proved to significantly increase the gene transfer efficiency as a result of colocalization of retroviral particles and target cells. This procedure resulted in a relatively high level (median 14%) of gene transfer in human CD34-positive cells as assayed by the number of transduced progenitor colonies. Although a median engraftment level of 12% transduced cells was observed in human bone marrow 1 month after transplantation, the number of transduced cells fell to 5% over the next 11 months. The combination of fibronectin and cytokine cocultivation is a promising approach to retroviral transduction of stem cells, which has resulted in an improved gene transfer to baboon marrow stem cells, and was used for the correction of the SCID phenotype in two patients.[22]
Among the clinical trials presently conducted in the United States that involve AAV, one has received much attention. In this Phase I trial, patients suffering from hemophilia B were injected into the skeletal muscle with an AAV vector carrying the cDNA for factor IX. Preliminary data on three patients that received the starting dose of the dose-escalation study—i.e., 1011 I.U./kg—suggest that the transduced muscle cells expressed factor IX protein for at least 2–3 months. In addition, an 80% reduction of factor IX infusion was observed in one patient and was interpreted as evidence for a modest clinical response. However, the reduction in factor usage is a quite surprising finding, given the fact that the factor IX levels were not affected by the AAV-factor IX injection in the same patient. The fact that so far neither vector-related toxicity (at least at the initial dose) nor evidence for germline transmission of vector sequenceswas found in these patients is an important finding, but it must be confirmed at higher, therapeutic vector doses. Another important question involves the immune response of the patients against the vector. Studies addressing this issue show a preexisting humoral immunity against AAV in virtually all patients. In addition, administration of the AAV vector elicited a 10- to 1000-fold increase in the neutralizing antibody titer. This boost of the humoral immunity against the vector is of major concern, especially if the AAV vectors must be re-administered.[11][43]
