Gamma Irradiation of Animal Serum: Validation of Efficacy for Pathogen Reduction and Assessment of Impacts on Serum Performance

T he treatment of animal serum by gamma irradiation, for the purpose of mitigating the risk of introducing a pathogen (virus, mollicute, or other microbe) into a cell culture, is a process that has been executed (and perhaps understood) primarily by irradiation contractors utilized by serum manufacturers. The selection of appropriate exposure conditions and irradiation doses is driven by a number of critical factors including: (1) the validation and control of the irradiation process itself; (2) the efficacy of the applied irradiation dose range for inactivating pathogens of interest; (3) determination and control of critical process attributes; (4) the potential impacts of these irradiation dose levels on the serum being irradiated; and finally, (5) the potential impact of irradiated serum on the medicinal product and the associated manufacturing process where serum is ultimately used. In order to increase awareness of these topics throughout the cell culture community, we have addressed these critical factors in the current review. Introduction This article is part of a series of papers that are being authored under the sponsorship of the International Serum Industry Association (ISIA) with the purpose of establishing best practices for processes employed in the gamma irradiation of animal serum. In the present article, we describe best practices for validating the efficacy of viral inactivation during gamma irradiation of serum. In general, these practices can be applied for validating pathogen reduction for other microbes. A survey of gamma irradiation efficacy for the inactivation of pathogens, especially viruses and mollicutes (mycoplasmas and acholeplasmas), in frozen serum will be presented. Finally, as will be explained further, a useful window of irradiation dosage must be established, not only in terms of fluency required for pathogen inactivation, but also keeping in mind that the desired performance characteristics of the material being irradiated must remain intact. The latter must be empirically evaluated both by serum manufacturers and end-users of the irradiated serum. Best practices for these evaluations have also been described. As will become apparent, there is a necessary trade-off between optimization of pathogen reduction by gamma irradiation and preservation of performance of the irradiated serum as a medium additive for cell culture applications. 1. Validation of the Efficacy of Gamma Irradiation for Viral Inactivation The main objectives of a validation study involving gamma irradiation of animal serum (or other animalderived liquid materials) are to: (1) establish the kinetics of virus inactivation in such matrices at various increasing irradiation doses; and (2) select a minimum irradiation dose at which adequate virus inactivation is achieved. An irradiation dose is expressed as fluency, in units of kiloGrays (kGy) or in units of megarads (MR), where 1 MR = 10 kGy. The term “fluency” incorporates both dose-rate and time, making this a very useful term for the comparison of efficacy in different exposure scenarios. Efficacy information obtained through validation is necessary for establishing the commercial product irradiation dose range. Serum performance at increasing irradiation doses must be taken into account in order to determine the highest irradiation dose at which physicochemical, biochemical, and biological performance properties of serum product are deemed acceptable. IMAGE: A negatively-stained transmission electron micrograph of rotavirus particles. Rotavirus is a member of the Reoviridae family. (Credit: CDC/Dr. Erskine Palmer, http://phil.cdc.gov/phil/)