Extended safety studies of the attenuated live tuberculosis vaccine SO2 based on phoP mutant

Safety is one of the main concerns for attenuated live vaccine candidates. Here we extend the stability and attenuation studies of the promising tuberculosis vaccine candidate based on Mycobacterium tuberculosis phoP mutant strain, SO2. Stability of the phoP mutation was tested after sub-culturing SO2 strain for 6 months in laboratory media and also after 3 months of infection in SCID mice. Results showed no reversion of the phoP mutation either in vitro or in vivo. In addition, SO2 was fully sensitive to four major first-line antituberculous drugs against tuberculosis. Safety and toxicity studies were performed in guinea pigs. Animals were infected with a quantity of SO2 equivalent to 50 vaccination doses (2.5 × 10⁶ CFUs) and weight was monitored for 6 months. All animals survived and no histological lesions were found, showing full attenuation of SO2. Studies in a post-exposure model of guinea pigs and mice, previously infected with M. tuberculosis, were performed and no toxicity effects were found after inoculation of SO2. All these results together confirm that SO2 has a secure safety profile that encourages its use in clinical trials.