Synthesis and biological properties of synthetic methylene blue analogues

Abstract

Methylene blue (MB) has diverse medical applications. Among these, MB may act as an antimalarial agent and has demonstrated potential for the treatment of neurodegenerative disorders such as Alzheimer’s disease. MB also possesses promising antidepressant and anxiolytic activity. While MB acts at numerous pharmacological targets, MB is a noteworthy inhibitor of monoamine oxidase A (MAO-A), exhibiting an in vitro IC50 value of 0.07 μM. MB is a much less potent MAO-B inhibitor with an IC50 value of 4.37 μM. While MB possesses an excellent safety profile, in combination with serotonergic agents such as serotonin reuptake inhibitors, MB can induce serotonin toxicity, which is a direct result of its ability to potently inhibit MAO-A. It may therefore be advantageous to "design-out" the MAO-A inhibition properties of MB in order to improve its safety profile. During this study, two new derivatives of MB were synthesized and their in vitro interactions with recombinant human MAO-A and MAO-B characterised. The MB analogues, compounds 1 and 2, were synthesized by reacting phenothiazine with iodine to yield phenothiazin-5-ium tetraiodide hydrate. This intermediate was subsequently treated with an appropriate amine to yield the monosubstituted MB analogues. These new compounds were characterised by nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). The two MB analogues were evaluated as potential inhibitors of human MAO-A and MAO-B. The results showed that compounds 1 and 2 are significantly weaker as MAO-A inhibitors compared to MB, but display more potent inhibition of the MAO-B isoform compared to MB. The analogues were also found to be reversible MAO inhibitors, although 2 may exhibit some tight-binding to MAO-B. Interestingly, the Lineweaver-Burk plots constructed for the inhibition of the MAOs by 1 and 2 are not typical of competitive inhibition. For the inhibition of MAO-A by 1, the Lineweaver-Burk plots are indicative of non-competitive inhibition. A modelling study was carried out to propose potential binding orientations of the MB analogues in MAO-A and MAO-B. For this study it may be concluded that, compounds (such as 1) that potently inhibit both MAO-A and MAO-B would be applicable for the treatment of Parkinson’s disease where depression is a comorbidity. Since MAO-A is inhibited reversibly, the possibility of tyramine-induced hypertension is reduced. Serotonin toxicity, however, remain a concern of MB analogues that inhibit MAO-A