How much Clearance?
The question of how much viral clearance is enough is the million dollar question, according to Luke A. Pallansch, Ph.D., director, pharmaceutical research services at Cell Trends (www.celltrends.com).
According to Dr. Rosenblatt, The answer is readily available if you know the parameters, noting that it depends upon the number of virus particles per unit volume detected in the feed stock, the anticipated dose, process productivity, and cell line yield. Consequently, Dr. Pallansch believes, fixed log reduction values probably would not be appropriate for all manufacturer biopharmaceuticals. Instead, he advocates understanding the critical parameters of individual unit operations within a manufacturing process in order to assure the robustness of viral clearance associated with those unit operations.
The fairest answer, says James Gilbert, Ph.D., senior director, MDS Pharma Services Biopharmaceuticals (www. mdsinc.com), is that there should be a demonstration of sufficient viral clearance by the manufacturing process to provide a significant margin of safety in potential viral load based upon the viral content of the starting materialgenerally below the limits of assay detection. Of course, what is a significant margin of safety?
FDA and ICH guidances indicate the margin of safety has been three to five log reduction value higher than the estimated viral particles in one dose.
For Mabs and recombinant proteins produced using well-defined cell lines, such as CHO and NSO, this roughly translates to about 1215 log10 clearance for endogenous retroviruses and about 6 log10 clearance of adventitious viruses, Krishnan elaborates. Typically, x-MuLV is used as a specific model virus for endogenous retroviruses, and MMV is used as a relevant adventitious virus. That said, Dr. Hughes notes that, because of high endogenous retroviral particles or high dose levels, some products have a target of 1622 log10 clearance.
Tissue samples, however, are a little different, and are undergoing some changes, Dr. Hughes says. Historically, viral clearance has been achieved by chemical inactivation, followed by titration to determine how much virus remained. Now regulatory bodies question whether the tissue absorbed and protected the virus, so you must measure the inactivation by taking off the supernatant, disrupting the tissue sample, and taking a second sample to measure any protective effect from the tissue.
Dr. Gilbert suggests that the desired extent of viral clearance be determined by case-by-case evaluation of the data, testing results, nature of the product, nature of the starting material, and a variety of other considerations. The important thing, in my mind, is for drug developers to get guidance from the appropriate regulatory agencies on the expectation of the agency.
That guidance, however, is often open to interpretation. As Martin says, None of the regulatory agenciesin the U.S. or internationallyhave documented how much viral clearance is needed. It is done on a case-by-case method.
The Committee for Proprietary Medicinal Products of the EMEA, for example, says a given viral clearance step with 4 log reduction is robust. The industry widely interprets that standard as requiring a 4 log reduction but, that same document later states that if log reduction is less than 1 log is it not significant. Therefore, viral clearance doesnt have to be 4 logs, but is better if it is, Martin concludes.
Regulatory bodies dont want manufacturers to rely upon such a number analysis, however, Dr. Wisher says. Companies sometimes lack a good sense of their goals for target removal based upon dosage size and, therefore, may not have planned enough clearance steps. Most have a five or six log safety factor, so with a viral load of 1012 particles per liter, and a safety factor of 5 logs, the target viral clearance will be 1017. To achieve this, you have to add three or four steps together, Dr. Hughes says.
Regulators want manufacturers to perform a virus risk assessment and to discuss the viral clearance obtained in the light of this assessment, Dr. Wisher says. In that vein, The European Pharmacopoeia has published a draft monograph that requires all manufacturers of biological products to prepare a virological risk assessment.