Transferability of dermal temperature histamine sensitization test for estimation of pertussis toxin activity in vaccines.

All current acellular pertussis vaccines (ACVs) contain detoxified pertussis toxin (PT) as a major component. An essential part of the safety evaluation of these vaccines, required by regulatory authorities, is to monitor their active PT content and to check for reversion to toxicity of the detoxified PT. Although various in vitro tests are under investigation, the only practicable means for detecting active PT at present is the histamine sensitization test. The methods given in the European Pharmacopoeia and in the US Pharmacopoeia are based on recording a binary response to histamine challenge (using a lethal end point). A more sensitive method based on measurement of rectal temperature is given in the Japanese Minimum Requirements for Biological Products. More recently, a refinement of this method based on dermal temperature measurement has been developed for ACVs in combination with diphtheria and tetanus vaccines (DTaP). We show that this method also can be used for more complex combination vaccines and is readily transferable. Furthermore use of dermal temperature provides a more precise quantitative estimate of toxin activity than the binary response, leading to an increase in information from a specified number of animals, or allowing a reduction in the number of animals required. We suggest that, pending the development of an alternative in vitro replacement method, the temperature based method may serve as an intermediate solution to the estimation of PT activity giving a precise estimate with reduction in animal numbers.