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.

Transferability study of CHO cell clustering assays for monitoring of pertussis toxin activity in acellular pertussis vaccines

Current regulations for acellular pertussis (aP) vaccines require that they are tested for the presence of residual or reversion-derived pertussis toxin (PTx) activity using the mouse histamine sensitisation test (HIST). Although a CHO cell clustering assay can be used by manufacturers to verify if sufficient inactivation of the substance has occurred in-process, this assay cannot be used at present for the final product due to the presence of aluminium adjuvants which interfere with mammalian cell cultures. Recently, 2 modified CHO cell clustering assays which accommodate for the adjuvant effects have been proposed as alternatives to the HIST. These modified assays eliminate the adjuvant-induced cytotoxicity either through dilution of the vaccine (called the Direct Method) or by introducing a porous barrier between the adjuvant and the cells (the Indirect Method). Transferability and suitability of these methods for testing of products present on the European market were investigated during a collaborative study organised by the European Directorate for the Quality of Medicines & HealthCare (EDQM). Thirteen laboratories participated in this study which included 4 aP-containing vaccines spiked by addition of PTx. This study also assessed the transferability of a standardised CHO cell clustering assay protocol for use with non-adjuvanted PTx preparations. Results showed that the majority of laboratories were able to detect the PTx spike in all 4 vaccines at concentrations of 4 IU/mL or lower using the Indirect Method. This sensitivity is in the range of the theoretical sensitivity of the HIST. The Direct Method however did not show the expected results and would need additional development work.

Acellular pertussis vaccines in Japan: past, present and future

An antivaccine movement developed in Japan as a consequence of increasing numbers of adverse reactions to whole-cell pertussis vaccines in the mid-1970s. After two infants died within 24 h of the vaccination from 1974 to 1975, the Japanese government temporarily suspended vaccinations. Subsequently, the public and the government witnessed the re-emergence of whooping cough, with 41 deaths in 1979. This series of unfortunate events revealed to the public that the vaccine had, in fact, been beneficial. Furthermore, researchers and the Japanese government proceeded to develop safer pertussis vaccines. Japan now has the most experience worldwide with acellular pertussis vaccines, being the first country to have approved their use. This review describes the major events associated with the Japanese vaccination program. The Japanese experience should be valuable to other countries that are considering the development and use of such vaccines.

Collaborative study for the standardisation of the histamine sensitizing test in mice and the CHO cell-based assay for the residual toxicity testing of acellular pertussis vaccines

The European Pharmacopoeia (Ph. Eur.) and the World Health Organisation (WHO) require the performance of extensive quality and safety control testing before the release on the market of vaccine products for human use. Safety testing with regard to residual pertussis toxin (PT) in acellular pertussis combination vaccines is performed through assessment of fatal sensitisation of mice to histamine challenge by the vaccine product under test. Currently, use of different in-house procedures and no requirement for the inclusion of a standard reference in each assay render comparisons of results obtained for identical vaccine batches between different control laboratories very difficult. At the initiative of the European Directorate for the Quality of Medicines and HealthCare (EDQM), an international collaborative study was organised for the standardization of the Histamine Sensitizing Test (HIST) in mice and the Chinese Hamster Ovary (CHO)-cell-based assay (performed at the bulk product level) for the residual toxicity testing of acellular pertussis vaccines or acellular pertussis-based combination vaccines. The study was run under the aegis of the Biological Standardisation Programme, jointly supported by the Council of Europe and the European Commission under the project code BSP076. Ten (10) laboratories participated in the study and were requested to perform 3 independent Histamine Sensitizing Tests in mice and to report results of the lethal end-point measurement as prescribed by the Ph. Eur. monographs. Some of them also reported data from an in-house validated CHO-cell-based assay. In addition, some of the laboratories reported concomitantly data obtained by measurement of the drop in temperature induced after the histamine challenge, a method currently under investigation to be added as an alternative end-point for the HIST in the Ph. Eur. monographs for acellular pertussis-based combination vaccines in order to alleviate animal suffering (in application of the 3Rs principle). Based on the results of the collaborative study, a potency of 7500 IU/vial (International Units per vial) was assigned to the current Ph. Eur. Biological Reference Preparation (BRP) for PT. The results of the study also show that 1) intra- and inter-laboratory variations can be improved by the use of a validated standard operating procedure; 2) inclusion in each assay of a standard reference sample, calibrated in IU, can increase comparability of results among laboratories and thus help to reduce repeat testing; 3) a correlation between mortality data and temperature data was observed although, due to the limited number of data sets and the lack of a common method for the temperature end-point, further investigation of this point is required; 4) the CHO-cell-based assay did not yield comparable results and further standardisation of the assay procedure may be investigated in a follow-up project.

An alternative temperature-based histamine-sensitisation test for absence of residual pertussis toxin in acellular pertussis vaccines

As an extension of the BSP076 study (see the article 'Collaborative Study for the Standardisation of the Histamine Sensitizing Test in Mice and the CHO Cell-based Assay for the Residual Toxicity Testing of Acellular Pertussis Vaccines (BSP076)', page 51 of this issue of Pharmeuropa Bio & Scientific Notes), it was decided to publish the following experimental method for the temperature-based histamine-sensitisation test, validated at the SSI (Statens Serum Institut, Denmark), as a working basis for the growth of the method in individual laboratories.

Collaborative study on a Guinea pig serological method for the assay of acellular pertussis vaccines

An international collaborative study (coded BSP083) was performed under the aegis of the Biological Standardisation Programme supported by the Council of Europe and the European Commission, with the aim of replacing the in vivo challenge assays for potency determination of combined acellular pertussis (aP) vaccines by a refined procedure also allowing reduction of animal use. This study investigates whether the immunogenicity of aP vaccine components could be assayed in a guinea pig (gp) serology model, using the same vaccine immunising doses as for D and T components potency testing, instead of using separate animals as is currently done. The BSP83 project is a follow up of 3 former collaborative studies (coded BSP019, BSP034 and BSP035) on serological methods for the potency testing of tetanus (T) and diphtheria (D) vaccines for human use. The use of gp instead of mice serology has the advantage of providing a larger volume of good quality antiserum for the assay of several vaccine components in the same sample, hence providing the opportunity for animal sparing. The results of Phase I of the study demonstrated that gp serology may be a useful method for the immunogenicity assay of acellular pertussis vaccines. This was confirmed in Phase II of the study, using 7 different combined aP vaccines in an international collaborative study involving 17 laboratories from both public and private sectors. Clear dose-response relationships were observed for different vaccines by ELISA, for antibodies against aP antigens, i.e. pertussis toxin (PT), filamentous haemagglutinin (FHA), fimbrial agglutinogens-2/3 (Fim 2/3) and pertactin (PRN). Intra- and inter-laboratory variations of aP ELISA results were found to be within an acceptable range. For some combined vaccines, however, the range of vaccine dilutions for immunisation confirmed to be optimal for D and T potency testing may not provide optimal dose-response for all aP components. Method adjustments may thus be required and suitability should therefore be demonstrated for each vaccine combination and product prior to the application of this assay. The results of this study support the use of the gp serological method for the determination of the immunogenicity of aP vaccines. The application of the method for batch release testing of combined D, T and aP vaccines could significantly contribute to the implementation of the 3R principles through reduction of animal use and improved animal welfare, whilst reducing costs. As an outcome of this study, the Group of Experts No. 15 on Sera and Vaccines of the European Pharmacopoeia (Ph. Eur.) decided in February 2009 to include the gp serological assay as an example in the Ph. Eur. General chapter 2.7.16. on acellular pertussis vaccine assay.

Improved efficacy of a licensed acellular pertussis vaccine, reformulated in an adjuvant emulsion of liposomes in oil, in a murine model

The immunogenicities and efficacies of a licensed diphtheria, tetanus, acellular pertussis, and inactivated poliovirus vaccine and the same vaccine formulated in a liposome/oil emulsion adjuvant were compared in a mouse model of pertussis respiratory infection. A single dose of the liposome/oil emulsion-adjuvanted vaccine produced significantly higher antibody levels than one dose of the licensed vaccine and protected mice from Bordetella pertussis infection with an efficacy equivalent to that of three doses of the licensed vaccine.

Evaluation of respiratory model employing conventional NIH mice to access the immunity induced by cellular and acellular pertussis vaccines

The increasing number of pertussis cases reported on the last twenty years and the existence of new acellular vaccines reinforce the need of research for experimental models to assure the quality of available pertussis vaccines. In this study, allotments of whole-cell and acellular pertussis vaccines were tested through the Intranasal Challenge Model (INM) using conventional NIH mice. The results have been compared to those achieved by the "Gold standard" Intracerebral Challenge Model (ICM). In contrast to ICM, INM results did not show intralaboratorial variations. Statistical analysis by Anova and Ancova tests revealed that the INM presented reproducibility and allowed identification and separation of different products, including three-component and four-component accellular pertussis vaccines. INM revealed differences between pertussis vaccines. INM provides lower distress to the mice allowing the reduction of mice number including the possibility of using conventional mice (less expensive) under non-aseptic environment. Thus, INM may be used as an alternative method of verifying the consistence of allotment production, including acellular pertussis vaccines.

[Laboratory assessment problems of the efficacy of acellular pertussis vaccines]

Although composition of acellular pertussis vaccines is better defined than whole-cell vaccines, differences in the formulation, content, and detoxification of pertussis vaccine antigens led to a unique character of each of differently produced acellular vaccine. Currently used methods for laboratory evaluation of whole-cell pertussis vaccine efficacy were found not suitable for acellular vaccines. There is a strong need to perform analysis and evaluation of the safety and efficacy profiles of acellular pertussis vaccines combined with other vaccine antigens (e.g. Hib) both before and after conjugation. Mechanisms of interactions seen after conjugation inducing weaker immunogenicity or efficacy are still poorly recognized.

[Prevention of pertussis and high expectations concerning vaccines]

The basic vaccine used in the prevention of pertussis is the combined vaccine including a whole cell pertussis component and tetanus and diphtheria toxoids. Although this type of vaccine has been used more than 50 years in USA and more than 40 years in Poland it is still effective what can be evidenced by the decreased number of pertussis cases since the vaccine has been implemented. There are however some evidences that the whole cell vaccine may lead to the acute encephalopathy, fever seizures, hypotonic-hyporeactive episodes, inconsolable crying or anaphylactic reactions. But still is a lack of convincing evidences that the vaccine may be a cause of persistent brain damage. It was also shown that the longer is the period after the last dose of the vaccine the lower effectiveness was observed. Improving the safety of the pertussis vaccine was the reason of introducing the acellular vaccines in the eightieth. All these products contain pertussis toxoid and some of them contain also filamentous hemagglutinin, pertactin and fimbrial agglutinogens. Some published studies have shown that the effectiveness of these vaccines is similar to the whole cells vaccines and that the incidence of some adverse events especially seizures, hypotonic-hyporeactive episodes and inconsolable crying is lower.

Detection of residual pertussis toxin in vaccines using a modified ribosylation assay

Pertussis toxin (PTx) in its detoxified form is an important component of both whole cell and acellular pertussis vaccines (ACVs). For safety reasons, it is imperative to ensure that the quantity of residual PTx in vaccines does not exceed permissible levels. The majority of the toxic effects of PTx have been attributed to the consequences of PTx-catalyzed ribosylation of the alpha-subunits of signal-transducing guanine-nucleotide-binding proteins. In this report PTx ribosylation activity was determined by an improved enzymatic-high performance liquid chromatography coupled assay using a fluorescein labeled Galpha(i3)C20 peptide. The effect of aluminum salts and other vaccine components on the assay system were also studied. The enzymatic assay system was shown to be a convenient, sensitive method and correlate well with the toxicity observed in vivo by the histamine sensitization assay. This method forms the basis of a new assay which could replace the unsatisfactory animal test currently used in pertussis vaccines control.

[Vaccine protection of Swedish children. Time for revision of replacing doses of diphtheria, tetanus, polio and pertussis]

The Swedish vaccination programme against diphtheria, tetanus, pertussis, polio and Haemophilus influenzae type b is characterized by few, widely spaced doses. Swedish schoolchildren have low diphtheria antibody levels compared to children in other countries. The introduction of acellular pertussis vaccine in 1996 has markedly cut the pertussis incidence in all ages except school-age children. There are now reasons to consider replacing the present fourth dose of inactivated polio vaccine given at 5 years of age and the diphtheria-tetanus toxoids given at 10 years of age with a combined diphtheria-tetanus-polio-acellular pertussis vaccine dose to be given at 5-6 years of age. A fifth dose of a DTPaPolio vaccine should be considered at 15-18 years of age.

A quantitative analysis for the ADP-ribosylation activity of pertussis toxin: an enzymatic-HPLC coupled assay applicable to formulated whole cell and acellular pertussis vaccine products

The majority of the biological effects of pertussis toxin (PT) are the result of a toxin-catalyzed transfer of an adenosine diphosphate-ribose (ADP-ribose) moiety from NAD(+)to the alpha-subunits of a subset of signal-transducing guanine-nucleotide-binding proteins (G-proteins). This generally leads to an uncoupling of the modified G-protein from the corresponding receptor and the loss of effector regulation. This assay is based on the PT S1 subunit enzymatic transfer of ADP-ribose from NAD to the cysteine moiety of a fluorescent tagged synthetic peptide homologous to the 20 amino acid residue carboxyl-terminal sequence of the alpha-subunit of the G(i3)protein. The tagged peptide and the ADP-ribosylated product were characterized by HPLC/MS and MS/MS for structure confirmation. Quantitation of this characterized ADP-ribosylated fluorescently tagged peptide was by HPLC fluorescence using Standard Addition methodology. The assay was linear over a five hr incubation period at 20 degrees C at PT concentrations between 0.0625 and 4.0 microg/ml and the sensitivity of the assay could be increased several fold by increasing the incubation time to 24 h. Purified S1 subunit of PT exhibited 68.1+/-10.1% of the activity of the intact toxin on a molar basis, whereas the pertussis toxin B oligomer, the genetically engineered toxoid, (PT-9K/129G), and several of the other components of the Bordetella pertussis organism possessed little (<0.6%) or no detectable ribosylation activity. Commonly used pertussis vaccine reference materials, US PV Lot #11, BRP PV 66/303, and BRP PV 88/522, were assayed by this method against Bordetella pertussis Toxin Standard 90/518 and demonstrated to contain, respectively, 0.323+/-0.007, 0.682+/-0.045, and 0.757+/-0.006 microg PT/ml (Mean+/-SEM) or in terms of microg/vial: 3.63, 4.09 and 4.54, respectively. A survey of several multivalent pertussis vaccine products formulated with both whole cell as well as acellular components indicated that products possessed a wide range of ribosylation activities. The pertussis toxin S1 subunit catalyzed ADP- ribosylation of the FAC-Galpha(i3)C20 peptide substrate and its subsequent quantitation by HPLC was demonstrated to be a sensitive and quantitative method for measuring intrinsic pertussis toxin activity. This methodology not only has the potential to be an alternative physicochemical method to replace existing bioassay methodology, but has the added advantage of being a universal method applicable to the assay of pertussis toxin in both whole cell and acellular vaccines as well as bulk and final formulated vaccine products. Acceptance of this method by regulatory agencies and industry as a credible alternative to existing methods would, however, require validation in an international collaborative study against the widely accepted bioassay methods.

International collaborative study: evaluation of proposed International Reference Reagent of pertussis antiserum (mouse) 97/642

A freeze dried preparation of mouse serum in vials coded 97/642 containing antibodies to five pertussis antigens [pertussis toxin (PT), filamentous hemagglutinin (FHA), pertactin (PRN), fimbriae type 2 and 3 (Fim 2 and 3)] has been assessed for its suitability as an international reference reagent in an international collaborative study by thirteen laboratories in nine countries. This serum has been compared with U.S. Standard Pertussis Antiserum (mouse) Lot No. 1 (US Lot 1), which has been in use since 1995, for antibodies for each antigen. Calibration of the proposed International Reference Reagent of Pertussis Antiserum (pIRR) in terms of US Lot 1 gives results which are broadly consistent between laboratories for antibodies to each antigen, although the between-laboratory differences are larger than those seen for comparison of identical sera. Calibration of two positive control sera in terms of the pIRR gave similar between laboratory variability of estimates to that obtained when the same sera were calibrated in terms of US Lot 1. Overall continuity of estimates is maintained if units are assigned to the pIRR based on its calibration in terms of US Lot 1 in this study. Data presently available indicate that the pIRR is sufficiently stable to serve as a reference reagent. It was therefore recommended, with the agreement of all participants, that the preparation in vials coded 97/642 be established as the First International Reference Reagent for Pertussis Antiserum, mouse, with assigned unitages 16 units of anti-PT per vial, 143 units of anti-FHA per vial and 30 units of anti-PRN per vial based on its calibration in terms of US Lot 1. These unitages are also consistent with calibration of 97/642 in terms of the Japanese preparations JNIH-11 for anti-FHA and of JNIH-12 for anti-PT. Purified antigens for Fim 2 and Fim 3 are not readily available and an arbitrary value of 32 units per vial is suggested for anti-Fim 2 and 3 mixture. These recommendations were agreed by the Expert Committee on Biological Standardization of the World Health Organization.

Key points for the development of mouse immunogenicity test as potency assay for acellular pertussis vaccines

According to WHO and the European Pharmacopoeia, the current potency test for acellular pertussis vaccines is a mouse immunogenicity assay assessing consistency of production from batch to batch. The assay compares the batch under control with a reference vaccine of documented clinical efficacy. This study describes and illustrates critical aspects of the assay, based on our experience on a tricomponent vaccine: validation of immunoassay to quantify mouse antibody response, choice of vaccine immunising doses in the three-doses model, treatment of non-responder mice for calculations, establishment of assay validity criteria.