Use of a Quantitative Product-Enhanced Reverse Transcriptase Assay to Monitor Retrovirus Levels in mAb Cell-Culture and Downstream Processing

Murine hybridoma cells used in the production of monoclonal antibodies (mAbʹs) produce endogenous type C retrovirus particles. Regulatory agencies require a demonstration that mAbʹs intended for human use are free of retrovirus with an adequate margin of safety. This is usually achieved by validation studies, performed at small scale, to demonstrate that the manufacturing process is capable of removing or inactivating several different model viruses, including a murine retrovirus. In this report, we assess the utility of the TaqMan fluorogenic 5ʹ-nuclease Product-Enhanced Reverse Transcriptase (TM-PERT) assay for measuring reverse transcriptase (RT) activity in cell-culture samples and RT removal by models of processing steps. The levels of RT activity contained in laboratory-scale cell-culture harvests (10^8—10^13 pU/ mL) were substantially above the detection limit of the TM-PERT assay (~10^6 pU/ mL). The nature of the RT activity from cell culture was complex, but the bulk ofRT activity in clarified mAb harvests appears to be contained in large molecular weight viral particles. In laboratory-scale chromatographic runs, sufficient RT activity was present in mAb-containing eluates to accurately calculate its log(10) reduction value (LRV), typically between 2 and 4 log(10) per step. Monoclonal antibody purified using a model purification scheme consisting of three serial columns contained some residual RT activity near the limit of detection. The data indicate that the TM-PERT assay, because it is quantitative and highly sensitive and can be used to analyze a large number of samples in a short period, is ideally suited to investigate and optimize retrovirus clearance in purification processes.