Calibration of pertussis toxin BRP batch 1 in a standardised CHO cell-based clustering assay

The European Pharmacopoeia (Ph. Eur.) pertussis toxin (PT) Biological Reference Preparation (BRP) is used as a working standard for safety testing of acellular pertussis vaccines as prescribed in the Ph. Eur. monographs 1356 "Pertussis vaccine (acellular, component, adsorbed)" and 1595 "Pertussis vaccine (acellular, co-purified, adsorbed)". The BRP was calibrated in 2006 in the murine histamine sensitisation test (HIST) against the World Health Organization (WHO) 1^st International Standard (IS) for PT. In recent years, there have been increasing efforts to replace the in vivo test with in vitro methods. The Chinese hamster ovary (CHO) cell clustering assay has been used for many years by manufacturers to monitor residual PT activity in detoxified non-adjuvanted bulks. More recently a standardised protocol has been developed for this assay and a PT reference preparation was needed. Due to low stocks, the WHO 1^st International Standard for Pertussis Toxin (JNIH-5) needed to be replaced and therefore a joint study between the European Directorate for the Quality of Medicines & HealthCare (EDQM) and WHO was initiated to calibrate the PT BRP for the CHO clustering assay and to replace the IS. The collaborative study involved 14 laboratories from Europe, North America and Asia. The outcome of the study confirmed that the BRP is suitable for use as a reference preparation in the CHO clustering assay. The material was assigned a potency of 1360 IU per vial for the CHO clustering assay.