The synthesis, characterization, photophysical and biological properties of 13 new conjugate coumarin-diruthenium(II)•arene complexes against Toxoplasma gondii are presented. For all conjugate organometallic unit/coumarins, an almost complete loss of fluorescence efficacy was observed. However, the nature of the fluorophore, the type of bonding, the presence and length of a linker between the coumarin dye and the ruthenium(II) moiety, and the number of dye units influenced their biological properties. The in vitro activity against a trans-genic T. gondii strain grown in human foreskin fibroblasts (HFF) leads to IC 50 values for T. gondii β-gal from 105 to 735 nM. Of note is that nine compounds displayed lower IC 50 than the standard drug pyrimethamine. One compound applied at its IC 50 did not affect B-cell proliferation but had an impact on Tcell proliferation in murine splenocyte cultures. Transmission electron microscopy of T. gondii β-gal-infected HFF showed that treatment predominantly affected the parasites' mitochondrion.

Aiming toward compounds with improved anti-Toxoplasma activity by exploiting the parasite auxotrophies, a library of nucleobase-tethered trithiolato-bridged dinuclear ruthenium(II)-arene conjugates was synthesized and evaluated. Structural features such as the type of nucleobase and linking unit were progressively modified. For comparison, diruthenium hybrids with other type of molecules were also synthesized and assessed. A total of 37 compounds (diruthenium conjugates and intermediates) were evaluated in a primary screening for in vitro activity against transgenic Toxoplasma gondii tachyzoites constitutively expressing β-galactosidase (T. gondii β-gal) at 0.1 and 1 µM. In parallel, the cytotoxicity in non-infected host cells (human foreskin fibroblasts, HFF) was determined by alamarBlue assay. Twenty compounds strongly impairing parasite proliferation with little effect on HFF viability were subjected to T. gondii β-gal half maximal inhibitory concentration determination (IC50) and their toxicity for HFF was assessed at 2.5 µM. Two promising compounds were identified: 14, ester conjugate with 9-(2-oxyethyl)adenine, and 36, a click conjugate bearing a 2-(4-(hydroxymethyl)-1H-1,2,3-triazol-1-yl)methyl substituent, with IC50 values of 0.059 and 0.111 µM respectively, significantly lower compared to pyrimethamine standard (IC50 = 0.326 µM). Both 14 and 36 exhibited low toxicity against HFF when applied at 2.5 µM and are candidates for potential treatment options in a suitable in vivo model.

Aiming to generate novel compounds with improved antitoxoplama activity by exploiting the parasite auxotrophies, a library of nucleic base-tethered trithiolato-bridged dinuclear ruthenium(II)-arene conjugates - was synthesized and evaluated. Various structural features as the nucleobase (adenine, uracil, cytosine, thymine, and xanthine) and the linker between the two units were progressively modified. To substantiate the concept, hybrid molecules comprising the diruthenium moiety and other type of pendant molecules were also synthesized and assessed. The use of the CuAAC (copper catalyzed azide-alkyne cycloaddition) approach for the obtainment of conjugates bearing the trithiolato diruthenium scaffold was validated. 37 Compounds (diruthenium conjugates and corresponding intermediates) were assessed in a primary screening for in vitro activity against transgenic Toxoplasma gondii tachyzoites constitutively expressing β-galactosidase (T. gondii β-gal) at 0.1 and 1 μM. In parallel the cytotoxicity in non-infected host cells (human foreskin fibroblasts, HFF) was determined by alamarBlue assay. 20 Compounds strongly impairing parasite proliferation with little effect on HFF viability were subjected to T. gondii β-gal dose-response studies (IC50, half maximal inhibitory concentration determination) and their toxicity for HFF was assessed at 2.5 μM. Two compounds showing promise for further development were identified: 14, ester conjugate with 9-(2-oxyethyl)adenine, and 36, a click conjugate bearing a 2-(4-(hydroxymethyl)-1H-1,2,3-triazol-1-yl)methyl substituent, with IC50 values of 0.059 and 0.111 μM respectively, significantly lower compared to the standard drug pyrimethamine (IC50 = 0.326 μM). Both 14 and 36 exhibited low toxicity against HFF when applied at 2.5 μM and are candidates as potential treatment options in a suitable in vivo model.