Influence of B-ring modifications on proton affinity, transmembrane anion transport and anti-cancer properties of synthetic prodigiosenes

Prodigiosin is the parent compound of the tripyrrolic natural products known as the prodigiosenes. Some of these natural products and their synthetic analogs show anti-cancer, immunosuppressive and antimicrobial actions, amongst other biological activities. One mechanism put forth to explain their biological activity is that since prodigiosenes are typically protonated at physiological pH they can alter intracellular pH via HCl co-transport (or Cl-/OH- exchange) across cell membranes. In this study we synthesized a series of prodigiosene analogs with different -O-aryl substituents attached to the B-ring of the tripyrrolic skeleton. NMR studies showed that these analogs can exist as a mixture of two stable alpha and beta conformers in acidic solution, and that both conformers can bind anions in solution. We found that the electronic nature of the O-aryl substituent on the B-ring influences the rate at which these prodigiosenes catalyze transmembrane anion transport, i.e. the prodigiosenes with the higher pK(a) had greater Cl-/NO3- exchange rates. Four of the synthetic prodigiosenes were tested for their in vitro anti-cancer activities in the NCI60 human tumour panel. Despite their promising in vitro anti-cancer activity (GI(50) values ranging from 18 to 74 nM), there was no evidence that this activity is influenced by the extent of protonation of these synthetic prodigiosenes.