By Dr. Ray Nims
There is a new general chapter being prepared for inclusion in the United States Pharmacopeia (USP). It will be entitled “Design, Evaluation, and Characterization of Viral Clearance Procedures” and will be numbered 1050.1 to associate it with the current General Chapter <1050>.
A little history is called for to make this association more clear. Chapter <1050> first appeared in supplement 10 of USP23-NF-18 in 1999. It was, and still is, a verbatim copy of the International Conference on Harmonisation (ICH) document Q5A R1. In fact, it has an identical title: “Viral Safety Evaluation of Biotechnology Products Derived from Cell Lines of Human or Animal Origin”.
In many respects, U5P Chapter <1050> (and ICH Q5A R1) is similar in content and philosophy to the 1993 US FDA Points to Consider (PTC) document entitled "Characterization of Cell Lines used to Produce Biologicals". Like the 1993 PTC, USP Chapter <1050> expresses an overall safety paradigm composed of three orthogonal approaches. These approaches may be summed up as consisting of testing of raw materials and cell banks, testing of unprocessed bulk harvest, and evaluation of viral clearance steps used during purification. Also like the 1993 PTC, Chapter <1050> describes potential sources of viral contamination, including raw materials and the cell substrate. The scopes of the documents are similar, applying to products derived from cell lines. Transmissible spongiform encephalopathy agents and live or inactivated intact viral vaccines and gene therapy vectors are out of scope and are covered by other guidance documents.
Due to the potential for introducing a viral contaminant through use of an infected cell substrate (e.g., SV40 in polio vaccine, or more recently PCV-1 in rotavirus vaccine), USP Chapter <1050> and the 1993 PTC address the concept of cell banking and the extensive viral testing that is required for cell banks at the Master Bank, Working Bank, and Limit of Cell Growth levels. The second approach of the safety paradigm is the requirement for lot-to-lot testing of the unprocessed bulk harvest. Testing is done at this level as opposed to purified product since purification is designed to eliminate viruses and therefore might preclude the manufacturer from discovering a viral infection. It is the intent of the regulatory agencies that potential opportunities for introduction of a viral contaminant be investigated and remediated, so testing is done prior to purification. The viral testing that is required for unprocessed bulk harvest is a subset of the tests done on the cell banks.
Both USP Chapter <1050> and the 1993 PTC discuss the limitations of viral testing as a sole means to assure viral safety, and this provides a segue for discussion of the need for viral clearance steps and the validation of the ability of such steps to clear viruses from the product. In fact, USP Chapter <1050> does a good job of providing the fundamentals of viral clearance evaluation, however there was a feeling expressed by some in the industry (and especially Mike Rubino, a member of the original ad hoc advisory panel for this chapter) that more detailed guidance on experimental design was needed. This, by the way, is true in general about Chapter <1050> (and IQH Q5A Rl upon which USP <1050> was based) that it is strong on philosophy but weak on specific methodological detail.
The original USP ad hoc advisory panel for Chapter <1050> revision was assembled with the purpose of updating the chapter to include this missing experimental detail. The panel went through each section of the existing chapter and added the detail that was felt to be lacking. This was done with the overall mandate to avoid introducing any new language that might conflict with the original language (and ICH Q5A Rl). The revised Chapter <1050> was published in the Pharmacopeial Forum in 2010. Responses obtained indicated that there was reluctance to modify in any way the language of this chapter without also modifying ICH Q5A. The situation as of February 2011 was described in an earlier posting.
As a result, the USP decided to keep Chapter <1050> intact and identical to ICH Q5A Rl. A new ad hoc advisory panel was assembled with the purpose of producing a new chapter in the general information series that would be a companion to the existing Chapter <1050>. The new chapter received the number 1050.1 and provided the vehicle for adding the desired methodological detail.
The proposed new General Chapter <1050.1>, entitled "Design, Evaluation, and Characterization of Viral Clearance Procedures" consists of some background information on process evaluation and process characterization, then launches into experimental design for evaluating both inactivation and removal steps. The latter section includes some specific experimental design flow charts addressing virus removal by filtration and chromatography and virus inactivation. Along with the flow charts describing the design of the studies are description of the methods used to assess potential cytotoxicity or interference caused by the process materials themselves.
The subheadings of the background information are shown in the text box below.
The information presented under these subheadings reflects the panel's understanding of current regulatory expectations. Regulatory input obtained during the public comment period will assure that the panel's perceptions were correct. Discrepancies will be corrected as required.
Finally, a goal of the new chapter was to provide an updated list of the types of viruses that have been and may be used in viral clearance evaluations. For studies enabling clinical trials, it is common for evaluations to use a parvovirus such as MMV and a retrovirus such as X-MuLV as models. This provides a small non-enveloped virus to challenge filtration steps, as well as an enveloped virus to assess inactivation steps designed for lipid enveloped viruses. For BLA enabling studies, a few additional viruses may be selected from this updated list, keeping in mind that the viruses should represent a diversity of characteristics (envelope status, genome type, size, etc.).
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