Re-evaluating the antibacterial properties of DMARD and pro-drug sulphasalazine against autoimmune bacterial triggers after eighty years

Sulphasalazine (SSZ) has been used to treat a range of inflammatory conditions since the 1940s. It functions as a pro-drug that, upon azoreduction by selected gastrointestinal bacteria (including the bacterial triggers of some inflammatory diseases), releases an antioxidant protective molecule, 5-aminosalicylate (5-AS), and the antibacterial molecule sulfapyridine (SP). SSZ, 5-AS and SP were evaluated for growth inhibitory activity against some bacterial triggers of rheumatoid arthritis ( Proteus spp.), ankylosing spondylitis ( Klebsiella pnumoniae ), multiple sclerosis ( Acinetobacter baylyi and Pseudomonas aeruginosa ) and rheumatic fever ( Streptococcus pyogenes ). These bacteria have previously been reported to have azoreductase activity and therefore they may locally convert the SSZ pro-drug into 5-AS and SP. The potency of all compounds, as well as a combination of 5-AS and SP, were evaluated under aerobic conditions by MIC, ƩFIC and isobologram analysis. Noteworthy antibacterial activity was calculated for SSZ, with MIC values as low as 625 µg/mL against P. mirabilis and K. pneumoniae . The azoreduction product SP had substantially more potent antibacterial activity (MICs 78–625 µg/mL). It was particularly potent against the Proteus spp. triggers of rheumatoid arthritis. Whilst 5-AS also inhibited bacterial growth, it was substantially less potent than SP. However, 5-AS potentiated the activity of SP when tested in combination. Indeed, synergy was noted for the combination against P. vulgaris , whilst additive effects were recorded against P. mirabilis and K. pneumoniae . Taken together, these results highlight the pro-drug properties of SZ against the bacterial triggers of selected inflammatory diseases. Future studies into the pharmacological properties of SSZ, as well as the 5-AS and SP combination are warranted. In particular, these compounds should be evaluated against additional strains of these bacteria (including antibiotic-resistant strains), as well as against bacterial triggers of further inflammatory diseases.