Calibration of the Ph. Eur. Biological Reference Preparation (BRP) for tetanus vaccine (adsorbed) batch 3

A joint collaborative study was organised by the European Directorate for the Quality of Medicines & HealthCare (EDQM) and the World Health Organization (WHO)/National Institute for Biological Standards and Control (NIBSC) to establish replacement batches for the European Pharmacopoeia (Ph. Eur.) Tetanus Vaccine (adsorbed) Biological Reference Preparation (BRP) batch 2 and for the WHO 3rd International Standard (IS) for Tetanus toxoid (adsorbed). Two freeze-dried stabilised tetanus vaccine (adsorbed) candidate preparations (Preparation A, 08/218 and Preparation B, 08/102) were calibrated against the current 3rd IS/BRP batch 2 (Preparation C) using challenge methods in guinea pigs and mice as described in the Ph. Eur. general chapter 2.7.8. Assay of tetanus vaccine (adsorbed). They were also assayed by serology methods. The WHO 2nd IS for Tetanus toxoid adsorbed (TEXA-2) was additionally included in the sample panel as Preparation D. Thirty-four laboratories (regulatory organisations and manufacturers) from 22 countries participated in the collaborative study. The majority of participants performed 2 independent challenge tests. Nine laboratories performed challenge assays in guinea pigs and 30 laboratories performed challenge assays in mice. Eight laboratories performed serology in guinea pigs and 1 laboratory performed serology in mice. For Preparation A, the geometric mean (GM) potency estimate (with 95 % confidence interval (CI)) in guinea pigs for all laboratories that provided valid results (n = 6) was 488.5 (354.2-673.6) IU/ampoule. For valid mouse assays (n = 25) the GM potency (with 95 % CI) was 259.8 (223.5-302.0) IU/ampoule. The inter-laboratory geometric coefficient of variation (GCV) was 36 % for guinea pig assays and 45 % for mouse assays. This compared favourably with the calibration of the 3rd IS/BRP batch 2 where the inter-laboratory GCV was 36 % and 42 % in guinea pigs and mice, respectively. For Preparation B, the GM potency estimate (with 95 % CI) in guinea pigs for all laboratories that provided valid results (n = 6) was 107.9 (64.1-181.7) IU/ampoule. For valid mouse assays (n = 24) the GM potency (with 95 % CI) was 147.9 (126.3-173.1) IU/ampoule. The inter-laboratory GCV was 64.3 % for guinea pig assays and 45.2 % for mouse assays. From the collaborative study, Preparation A appeared more suitable to be the replacement Ph. Eur. BRP as it is similar to the Tetanus vaccine (adsorbed) BRP batch 2, except for nature of the stabiliser. Preparation A was confirmed to have higher potency, readily detectable tetanus toxoid, and confirmed satisfactory stability and performance in challenge assays. Preparation A was adopted in January 2011 by the Ph. Eur. Commission as the Tetanus vaccine (adsorbed) BRP batch 3, with assigned potencies of 490 IU/ampoule in the guinea pig challenge assay and of 260 IU/ampoule in the mouse challenge assay. The same Preparation A was adopted in October 2010 as the WHO 4th IS for Tetanus toxoid (adsorbed), with the assigned activity of 490 IU/ampoule from guinea pig challenge assays. A follow-up study (reporting study) was organised by the EDQM to assess the impact of the potency assigned to the BRP batch 3 for mouse challenge assays on the outcome of batch release testing in Europe. Eight laboratories including official medicines control laboratories (OMCLs) and manufacturers reported the results of their routine testing, using the BRP batch 3 in addition to their regular reference preparation. For each tested product, participants calculated the potency relative to their routine reference and relative to the BRP batch 3. No common sample panel was distributed to participants. In total, data on 40 batches of different marketed tetanus vaccines were reported. Overall, a good concordance was observed between the potencies calculated relative to the BRP batch 2 and relative to the BRP batch 3. On average, the potency estimates were 10 % lower when expressed relative to the BRP batch 3. Cases of discrepant decisions for batch release were very limited and affected mainly batches with specifications close to the pharmacopoeial requirements. The reasons for differences in estimated potencies are discussed. The study showed that the use of the BRP batch 3 with an assigned potency of 260 IU/ampoule does not result in substantial change in the potency of different marketed products. This confirmed that the mouse challenge potency value assigned to the BRP batch 3 is suitable.