Questions about Systemic Adenovirus Delivery
Authors: Barbara Nasto
Journal: Molecular Therapy (2002)

As of September 2001, 600 gene therapy trials had taken place. Used in 27% of those trials, adenovirus is clearly a favored choice among gene therapy delivery vehicles. It is also rapidly becoming the most contentious. “Adenovirus has great characteristics,” says Estuardo Aguilar-Cordova, Deputy Director of the Harvard Gene Therapy Initiative (Boston). However, Aguilar-Cordova is among a growing number of researchers who believe that systemic delivery of adenovirus may be inappropriate. Data derived from the ornithine transcarbamylase (OTC) deficiency trial at the University of Pennsylvania, presented by Jim Wilson at Cabo II (a recent meeting focused on the immunobiology of gene therapy [1Mol. Ther. 2002; 5: 486-491Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar]), have called into question the safety of the systemic delivery of adenoviral gene therapy. As Aguilar-Cordova points out, “the timing [of the immune reaction] is what is of concern.” According to the data presented by Wilson, the onset of Jesse Gelsinger's immune response was so rapid it was indicative of an innate immune reaction, perhaps a result of preexisting memory cells afloat in his circulation touching off an immune reaction so virulent that it shut down all of his organs. The prospect that the adenoviral coat possesses the potential to prompt a rapid and lethal immune reaction is not the only immune obstacle encountered by the use of this vector to systemically deliver a sustained therapeutic effect. The first generation of adenoviral vectors applied to promote the production of factor IX in hemophilia patients was rendered impotent because the adenoviral vector proteins they produced triggered cellular and humoral immune responses that destroyed the transgenic cells and neutralized the human factor IX produced by the cells [2Dai Y. Cellular and humoral responses to adenoviral vectors containing factor IX gene: tolerization of factor IX and vector antigen allow for long-term expression.Proc. Natl. Acad. Sci. USA. 1995; 92: 1401-1405Crossref PubMed Scopus (582) Google Scholar]. As a result, researchers refined the vector by removing the adenoviral-protein producing genes (that is, E1, E3 and/or E4). Even though the alterations render the vectors less immunogenic, removal of the expression proteins may not be enough to circumvent toxicity concerns (the vector associated with Gelsinger's death was an E1/E4-deleted vector). Most humans already possess immunity to adenoviruses due to a long history of human and virus interaction. The protein coat of the virus alerts the system and prompts clinical reactions that may range from the commonly recorded benign-transient fever to what Wilson and others feel led to Gelsinger's death. Savio Woo, Director and Professor of the Institute for Gene Therapy Mount Sinai School of Medicine (NY, NY), points out, “when you have an ‘n of 1′ it is difficult to know who will and won't respond.” Engineering the vector to avoid the immune system's first line of response “is like trying to circumvent 30 million years of evolution,” says Malcolm Brenner, ASGT President and Director at the Center for Cell and Gene Therapy at Baylor College of Medicine (Houston, TX). Aguilar-Cordova expressed the reservation that “with today's data it is difficult to envision the easy application of systemic delivery.” Brenner goes as far as to say adenoviral vectors are “inappropriate for systemic replacement of proteins,” citing immunogenicity along with other factors like the short duration of protein expression. Brenner, Woo, Aguilar-Cordova, and others would at least like to see systemic delivery of any adenovirus put on hold until more is known about its interaction with human immune system. This view is not, however, shared by all gene therapy researchers. GenStar Therapeutics is a biotechnology company in San Diego, CA, seeking to license the delivery of an adenoviral vector (MAX-AD-Factor VIII) into the systemic circulation for the treatment of hemophilia A, a hereditary bleeding disorder characterized by a deficiency in the factor VIII blood clotting protein. The company is in the midst of a phase I trial using an adenovirus genetically modified to replace all of the viral genes (so-called “gutless” adenovirus) with therapeutic genes that are responsible for the production of factor VIII. So far they have treated one patient, and low-level delivery of the factor (1% of normal) has persisted for eight months. Robert Sobol, CEO of GenStar, is not concerned about lethal toxicities because he believes that “the severity [of Gelsinger's reaction] was contributed to by his underlying liver disease.” Adenoviral vectors do pose less of a toxicity risk in cancer trials when short-term, high-level expression is the goal. In most cancer trials so far, engineered virus is administered directly to the tumor to try to elicit an immune response against the tumor tissue. Thus, as Aguilar-Cordova notes, “adenovirus is good if you want an immunoflammatory response.” Although intratumoral delivery seems for now to be a safer approach, issues of systemic delivery still come into play even when dealing with some cancer indications. Leonard Post, Senior Vice President of Research and Development at Onyx Pharmaceuticals, sees systemic delivery in the future of Onyx-015, an adenovirus that replicates selectively in cancer cells. According to Post, “the company will eventually be interested in systemic delivery of the agent to manage metastatic disease.” For the most part, Onyx-015 trials have used intratumoral delivery of the virus so far. However, a phase I/II clinical trial that employs intrahepatic artery infusion in patients with liver metastases of colorectal cancer is now underway. GenStar and Onyx are not alone in their perception of toxicity as a gradation issue. David Curiel, Director of the Division of Human Gene Therapy and Gene Therapy Center at the University of Alabama at Birmingham, notes that, “current toxicity data embodies the notion of a dose-relative phenomena.” This perspective is also held by Kristen Hege of Cell Genesys (Foster City, CA). Cell Genesys’ engineered adenoviruses are delivered either by direct injection into tumors or by intravenous administration. Hege says that “we are pondering the issue [of toxicity] like everyone else. You see a dose safety curve.” She adds that “There is a steep dose-toxicity curve between approximately 10e12 and 10e13 viral particles where toxicities go from mild to potentially life threatening. Since preexisting neutralizing antibody titer might impact this and since current viral titer assays can vary by as much as 1 log dose, escalation in this range is challenging. There is a big effort underway to standardize assays for measurement of viral titer to eliminate this issue.” Curiel sees enhanced vector activity and reduced dose as key to the safer systemic use of adenoviral vectors. His laboratory is working to improve the targeting and internalization of adenoviral vectors. Another recently published strategy, such as surrounding the adenovirus with polyethylene glycol or encapsulating it in liposomes so that it could go undetected in the bloodstream, may overcome any acute response, but is still vulnerable to acquired immune reactions to the vector and/or transgene [3Yotnda P. Bilamellar cationic liposomes protect adenovectors from preexisting humoral immune responses.Mol. Ther. 2002; 5: 233-241Abstract Full Text Full Text PDF PubMed Scopus (105) Google Scholar]. Of course, there are other viral and nonviral delivery vehicles that could be used for systemic long-term induction of deficient proteins, but these have their own well-documented limitations. In the US, where 80% of all gene therapy trials take place, the policies implemented by the American government will have a lasting and extensive impact on the future of gene therapy. Given the current uncertainty about systemic adenovirus use, it is perhaps not surprising that the government regulators are unwilling to make a decision either way at this point. When asked if the Food and Drug Administration (FDA) would prohibit systemic delivery of adenoviral vectors in gene therapy protocols, Phil Noguchi, Chief of the FDA's Division of Cellular and Gene Therapies, replied, “We cannot make a blanket statement. There are so many different diseases and populations.” Noguchi did say that “the science will drive the path,” adding that “some approaches are not going to be tolerated.” Whether there is ultimately a decision to refrain from proceeding with systemic treatment until more is known about the immunogenicity of adenoviruses or to continue the current trials and “learn as you go,” representatives along the continuum concur that there is a need for more immunological data. “You need a database of patients who don't have liver disease,” says Curiel. Little is known about the immune effects of systemic adenoviral treatment of healthy humans. Consequently the Cabo II Working Group suggested that the incompletely analyzed clinical and preclinical data be gathered to help address immunological issues. Who or what will gather it? A centralized effort directed by the NIH had made the workshop committee's list of possibilities. Theodore Friedman, a professor at UCSD and Chairman of the NIH-RAC, endorses this idea. “I couldn't agree more we should look at the available immunogenic and pharmacological data,” says Friedmann, but adds that “those types of studies tend not to be easy to get money for.” When asked about the feasibility of the FDA augmenting such an initiative, Noguchi said that “it goes in the ‘something to be pondered’ category,” but added, “there are limits on FDA resources.” In the meantime, many gene therapy researchers, both supporters and detractors of systemic adenovirus use, are hoping that the death of Jesse Gelsinger was an aberrant incident, and that a better grasp on the mysterious relationship between adenovirus and the human immune system will soon be gained.