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.