Phase I clinical trials of aroA aroD and aroA aroD htrA attenuated S. typhi vaccines; effect of formulation on safety and immunogenicity
Introduction
Live attenuated typhoid vaccines have shown the promise to confer long lasting immunity to S. typhi infections. The currently licensed vaccine strain (Vivotif™ Berna), though completely safe, requires three oral doses for maximum efficacy. Ty21a was generated through nitrosoguanidine mutagenesis and the basis for attenuation is not completely understood [1]. Second generation vaccines are being developed to reduce the immunization schedule from three to one dose and also to create defined genetic lesions which account for the attenuated phenotype [2], [3], [4].
Second generation attenuated typhoid vaccines should exhibit the safety profile demonstrated by Ty21a while improving upon the immunogenicity of the vaccine. New S. typhi candidate vaccines strive to be at least as efficacious in one dose as that achieved by three doses of Ty21a in sachet formulation, which is the most immunogenic formulation of that vaccine to date [5]. The effective dose of vaccine should be at least 10 fold lower than the symptomatic dose to allow for fill variation upon manufacture while ensuring safety.
Initial phase I clinical studies on aroC aroD, aroC aroD htrA and phoPQ attenuated S. typhi strains have used freshly grown bacterial cultures [2], [4], [6], [7]. While nonbacteremic and immunogenic doses of these strains were determined, vaccination with fresh bacterial cultures is not commercially feasible. The formulation of these strains for large scale vaccination will effect their safety and immunogenicity. Although Ty21a has been safe both as a fresh culture and after lyophilization and formulation into capsules and sachets, the same may not hold true for less attenuated strains.
We present here a series of phase I trials examining the safety and immunogenicity of aroA aroD attenuated S. typhi grown and formulated under a variety of conditions. In addition, data were obtained using an aroA aroD htrA strain formulated in lyophilized sachets.
Section snippets
Media
Routine culturing of S. typhi strains was performed on SOB [8] plus 0.01 g/L p-aminobenzoic acid (PABA) and 0.01 g/L 2,3-dihydroxybenzoic acid (DHB) or M9CAA media which is M9 media [9] supplemented with 5 g/L casamino acids, 2 g/L glucose, 0. 2 g/L MgSO4·7H2O, 0.5 g/L thiamine, 0.3 g/L Na citrate 2-hydrate, 0.01 g/L nicotinic acid. An aromatic amino acid (Aro) and precursor supplement was added to M9 for growth of aro-strains: 0.03 g/L each of phenylalanine, tyrosine and tryptophan, 0.02 g/L
Trial 1
In total, 34 subjects were vaccinated in three separate vaccine dosage groups with reconstituted lyophilized aroA aroD S. typhi, PBCC211. Thirteen adults received a single dose of 3.4×104 cfu, 10 adults received a single dose of 9×105 cfu and 11 adults received a single dose of 5×107 cfu. Safety evaluations of the adults in these three groups revealed no clinically significant systemic events or fever (Table 2). In addition, no positive blood cultures for S. typhi were noted. Bacterial stool
Discussion
Evaluation of aroA aroD S. typhi strain PBCC211 prepared and administered to a total of 92 volunteers in three different growth and formulation conditions revealed a range of safety profiles. Many symptoms were either not temporally related to vaccine administration or judged to be unrelated to vaccine administration. Eight of the 92 subjects developed S. typhi positive blood cultures after receiving the vaccine. Among these individuals, there was no correlation between the presence of positive
Acknowledgements
We wish to thank Dr. B.A.D. Stocker for strains and professional advice and Dr. Anders Kärnell for his helpful comments and suggestions for the manuscript.
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Cited by (0)
- 1
Present address: Innherred Hospital, 7600 Levanger, Norway.
- 2
Present address: Endorex Corporation, Lake Bluff, IL, USA.
- 3
Present address: Pasteur Mérieux Connaught, Marcy-L'Étoile, France.