In vitro Effect of Harmine Alkaloid and Its N-Methyl Derivatives Against Toxoplasma gondii

Abstract

Toxoplasmosis is one of the most prevalent and neglected zoonotic global diseases caused by <i>Toxoplasma gondii</i>. The current pharmacological treatments show clinical limitations, and therefore, the search for new drugs is an urgent need in order to eradicate this infection. Due to their intrinsic biological activities, β-carboline (βC) alkaloids might represent a good alternative that deserves further investigations. In this context, the <i>in vitro</i> anti-<i>T. gondii</i> activity of three βCs, harmine (<b>1</b>), 2-methyl-harminium (<b>2</b>), and 9-methyl-harmine (<b>3</b>), was evaluated herein. Briefly, the three alkaloids exerted direct effects on the parasite invasion and/or replication capability. Replication rates of intracellular treated tachyzoites were also affected in a dose-dependent manner, at noncytotoxic concentrations for host cells. Additionally, cell cycle analysis revealed that both methyl-derivatives <b>2</b> and <b>3</b> induce parasite arrest in S/M phases. Compound <b>3</b> showed the highest irreversible parasite growth inhibition, with a half maximal inhibitory concentration (IC50) value of 1.8 ± 0.2 μM and a selectivity index (SI) of 17.2 at 4 days post infection. Due to high replication rates, tachyzoites are frequently subjected to DNA double-strand breaks (DSBs). This highly toxic lesion triggers a series of DNA damage response reactions, starting with a kinase cascade that phosphorylates a large number of substrates, including the histone H2A.X to lead the early DSB marker γH2A.X. Western blot studies showed that basal expression of γH2A.X was reduced in the presence of <b>3</b>. Interestingly, the typical increase in γH2A.X levels produced by camptothecin (CPT), a drug that generates DSB, was not observed when CPT was co-administered with <b>3</b>. These findings suggest that <b>3</b> might disrupt <i>Toxoplasma</i> DNA damage response.