Working Toward an Adenoviral Vector Testing Standard
Authors: Beth Hutchins, Nancy Sajjadi, Sally Seaver, Alasdair Shepherd, Steven R. Bauer, Stephanie Simek, Keith Carson, Estuardo Aguilar-Cordova
Journal: Molecular Therapy (2000)

As the development of adenoviral vectors for gene delivery applications broadens and moves forward, the possibility of approved therapeutic products draws nearer. However, regulatory approval of new classes of drugs–including gene therapy–requires a good safety profile of the product, composed of product-specific data as well as collective data from products that share common elements in the platform technology. In the case of adenovirus, the analysis of safety and efficacy across the product class currently relies on data generated by nonstandardized methods and comparability between quantities such as viral particle, which are relatively imprecise (see below). Thus, confidence in safety assessments is lacking. In these cases, especially when efficacy has not been convincingly demonstrated, regulators and clinicians often take a conservative approach and assume that the relative risk is high when making decisions about dose escalation and other risk/benefit decisions. In other circumstances, development of reference testing reagents has been a successful approach to standardization of measurements among laboratories. This approach has been recommended by many parties with a stake in adenovirus vector delivery. To begin to address the question of standards for adenoviral testing, the Williamsburg BioProcessing Foundation hosted a 1-day meeting on October 5 (Washington, DC). The point was to bring together representatives from industry, regulatory agencies, contract testing laboratories, academia, and standard setting organizations to discuss testing of adenoviral gene therapy vectors and whether and how to develop a standard. More than 115 representatives from all sectors attended the meeting. The overall goals were to determine if there was consensus that a standard was useful, to identify technical and practical issues related to the development of a standard(s) for adenoviral vector testing, and to determine how to proceed with the development of a standard. The following outlines the problems facing the development of a standard and the recommendations of the conferees for moving ahead. There are two primary safety issues raised by the use of adenoviral vectors. The first is the toxicity of the vector particles themselves, and the second is the risk associated with replication-competent adenovirus (RCA). Although vector dosing is based on particle number, different methods are being used by different laboratories and investigators to define particle dose. The most common method, recommended in the 1998 FDA Guidance on Human Somatic Cell Therapy and Gene Therapy, is to measure the absorbance of the preparation at OD 260 nm after lysing the viral particles and convert that value to particle number based on a published extinction coefficient. Unfortunately, values derived from this type of measurement cannot be directly compared because the composition of a given vector preparation (particle concentration, purity, formulation) can influence the absorbance of the sample. Values derived from analytical methods that may measure a subcomponent of the vector preparation (e.g., DNA dye binding, intact particles by anion-exchange HPLC, reverse-phase HPLC analysis of viral protein components) make comparisons between different preparations even more difficult. This inability to precisely compare viral particle counts obtained by different manufacturers makes the analysis of dose-related clinical safety effects–and thus toxicity–difficult at best. Another concern in establishing particle number relates to the infectious titer of preparations, which can also have an impact on both safety and efficacy. Currently, adenoviral vector lots with particle-to-infectious titer ratios of 100:1 or less can be used in clinical trials per the 1998 Guidance. This ratio is used to monitor production lot consistency and stability. However, the methods for determining vector infectious titer are not standardized across sponsors. There are several other issues with infectious titer determinations that complicate their use. Because infectious titer is a biological assay, it is a measure that carries considerable imprecision, often greater than 30% interday precision. By comparison, physical methods to determine particle number frequently have interday precision values of less than 5%. Additionally the methodology used in determining infectious titer rarely takes into account the slow diffusion of adenoviral vector particles and how that can affect the number of particles that actually come into contact with the cells during the assay. Lack of consideration of this property of adenovirus leads to infectious titers that typically overestimate the number of particles that are able to interact with the cells to create an infection event. Thus, current infectious titer assays can significantly underestimate the actual number of infectious particles present in the sample. Until infectious titer assays become more standardized, comparing the safety or efficacy of doses that have the same particle number but vary in their infectious titer measurement presents a significant challenge for regulators in the field of adenovirus vector-based gene transfer. Testing methodology for RCA is also variable and results are difficult to compare among manufacturers. Variables include the volume of a production lot that should be tested, what analytical methods to use to quantity RCA, and how to report RCA testing results. As a result, there are limited data by which to determine the risk associated with the presence of specific amounts of RCA as an impurity in different clinical settings. Although it is understandable and acceptable to have differences in the specific analytical tools used by drug developers to assess their products, the lack of an international reference standard for both dose and RCA hampers comparison analyses. It is with these problems in mind that the Williamsburg Bio Processing Foundation meeting began. The goal of the meeting's morning agenda was to highlight the perspectives of those involved in, and impacted by, standards development. Representatives from industry, academia, FDA, U.S. Pharmacopeia (USP), the UK's National Institute for Biological Standards and Control (NIBSC), American Type Culture Collection (ATCC), and the Biotechnology Division of the National Institute of Standards and Technology (NIST) described their perspectives, interests, and possible roles in the development of an adenoviral testing standard. Representatives from 10 different companies then made short technical presentations that highlighted specific issues in determination of particle concentration, measuring infectivity, and detecting and quantifying RCA. Participants then separated into small groups to discuss in further detail and identify the issues related to developing standards for adenoviral vector testing. Appointed discussion group leaders presented the key points identified by each group when the entire conference reconvened for the afternoon session. The conference ended with consensus reached on several points after further discussions regarding RCA testing and limits, potency assays, and use of the term m.o.i. Consensus was reached on the following points: 1Development of a well-characterized adenoviral standard is a good idea;2Development of a standard must proceed rapidly to help regain public trust;3FDA should take responsibility for leading the process using a working group approach to accomplish the goal;4The Working Group will be responsible for identifying the process to evaluate the standard and to select appropriate group(s) to manufacture, characterize, and distribute the standard;5Decisions will be made available for public comment via Web sites, journals, and meetings;6A wild-type adenovirus should be the primary standard. Development of a second standard that is replication defective and tied to the first standard will be acceptable for physical characterization assays;7The standard must be characterized using detailed procedures, which will be made available to all participants;8There will not be any standardization of specific methods at this time;9The standard vial label will list both particle concentration and infectious units. An orthogonal approach was recommended to establish particle concentration including analyses by reverse-phase HPLC, quantitative real-time PCR, Pico Green DNA dye binding assay, absorbance at 260 nm, and anion ion-exchange HPLC.10A specific method for using absorbance at 260 nm to determine the particle concentration from the new standard will be made available, i.e., a new extinction coefficient will be determined for adenovirus based on the standard and a specific particle lysis method. Representatives from FDA/CBER made several points of clarification. First, in reference to the 1998 Guidance recommendation that "patient doses should contain no more than 1 pfu of RCA or equivalent," FDA representatives pointed out that this recommendation was made based on what were typical dose levels in the mid-1990s, i.e., 109 particles. Because dose levels commonly rise above this level today, the levels of RCA that are acceptable on a per dose basis require a risk-benefit analysis. The 1998 Guidance actually addresses this point, stating that "if sponsors wish to propose a different specification, data should be provided demonstrating that the level of RCA present represents an acceptable risk for the intended patient population, route of administration, and dose." However, until commonly agreed-upon units of quantifying RCA are available, the agency cannot complete this assessment. The FDA considers RCA to be a safety concern as well as an indicator of the quality and consistency of adenovirus vector manufacturing. Until there are enough data to establish the true risk of this agent, the agency has a current recommendation of <1 RCA in 109 infectious units of adenoviral vector product. When asked whether the agency will standardize the amount to be tested for RCA following a similar model as that developed for replication-competent retrovirus (RCR) testing, i.e., a statistical confidence-based method, FDA representatives said that the agency is discussing this internally, but that no specifics had yet been decided. A second point made by the FDA was that a measure of infectious titer does not constitute a potency assay. Potency assays must quantify vector function. This typically means quantification of the biological function of the specific transgene. In the case of adenoviruses that do not express an exogenous transgene, it should still be possible to develop a potency assay based on the intended clinical effect. This does not necessarily imply that the mechanism of action of the vector is completely understood: The purpose of a potency assay is to enable one to distinguish lots that are too potent or not potent enough in terms of inducing the intended clinical effect. When asked how the FDA views the requirement for potency assays when the genes are species-specific, agency representatives commented that they would encourage discussions with sponsors early in clinical development so that they could help provide guidance in the development of an appropriate method. One intense discussion point of the meeting was the continued use of "multiplicity of infection" or m.o.i., a term frequently used in describing assay conditions. It was agreed by participants in the meeting that m.o.i. may not be a scientifically valid way to standardize infection efficiency across different procedures. However, m.o.i. can be a shorthand way to standardize assay input amounts across samples within one procedure (standard assay volumes and infection time) and to scale samples within one procedure (e.g., infecting cells in a flask versus a roller bottle). An example was given that interference and toxicity can be seen in RCA bioassays at "high m.o.i.'s." It was pointed out by several meeting participants that if toxicity is truly associated with the number of infection events, then infection efficiency is a function of virus concentration and infection time and not related to m.o.i. (virus-to-cell ratio). A consensus was reached by participants that use of the term m.o.i. should only be in conjunction with other information, specifically virus concentration in particles per milliliter and infection time. The probability of an infection event can then be calculated based on a diffusion model. The Working Group recommended by the meeting participants currently consists of more than 25 people from industry (both United States and Europe), the FDA, testing companies, NIBSC, ATCC, academia, and the Williamsburg BioProcessing Foundation. Additional researchers and others who wish to participate in the Working Group described above should contact Keith Carson at the Williamsburg BioProcessing Foundation. The Foundation can be reached by e-mail at [email protected] or by phone at 757-423-8823.