New Nucleic Base-Tethered Trithiolato-Bridged Dinuclear Ruthenium(II)-Arene Compounds: Synthesis and Antiparasitic Activity

Abstract

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.

Synthesis and Antiparasitic Activity of New Trithiolato-Bridged Dinuclear Ruthenium(II)-arene-carbohydrate Conjugates

“…Intermediates 2 – 4 can be modified using ester and amide coupling reactions as previously described [ 89 , 92 , 93 , 94 ]. The alkyne ester 6 was obtained in moderate yield (47%) by reacting 2 with 5-hexynoic acid using EDCI ( N -(3-dimethylaminopropyl)- N′ -ethylcarbodiimide hydrochloride) as coupling agent, in basic conditions (DMAP, 4-(dimethylamino)-pyridine) ( Scheme 2 , top).…”

“…The alkyne ester 6 was obtained in moderate yield (47%) by reacting 2 with 5-hexynoic acid using EDCI ( N -(3-dimethylaminopropyl)- N′ -ethylcarbodiimide hydrochloride) as coupling agent, in basic conditions (DMAP, 4-(dimethylamino)-pyridine) ( Scheme 2 , top). 5-Hexynoic acid was also reacted with the amino diruthenium derivative 3 using EDCI and HOBt (1-hydroxybenzotriazole) as coupling agents, in basic conditions (DIPEA, N , N -diisopropylethylamine), to afford the amido alkyne compound 7 as reported [ 93 ] ( Scheme 2 , top). Similar reaction conditions were used for the synthesis of amide 8 from carboxylic acid diruthenium derivative 4 and propargylic amine as formerly described ( Scheme 2 , bottom) [ 92 , 93 ].…”

“…5-Hexynoic acid was also reacted with the amino diruthenium derivative 3 using EDCI and HOBt (1-hydroxybenzotriazole) as coupling agents, in basic conditions (DIPEA, N , N -diisopropylethylamine), to afford the amido alkyne compound 7 as reported [ 93 ] ( Scheme 2 , top). Similar reaction conditions were used for the synthesis of amide 8 from carboxylic acid diruthenium derivative 4 and propargylic amine as formerly described ( Scheme 2 , bottom) [ 92 , 93 ].…”

“…The synthesis of trithiolato diruthenium complexes is generally straightforward and efficient [ 87 , 88 , 89 ], this scaffold being robust to chemical modification and easily adaptable to the conjugate strategy as demonstrated by the various series of hybrids with peptides [ 90 ], drugs [ 91 , 92 ], fluorophores [ 89 , 93 ] or metabolites [ 93 ]. Ester and amide couplings [ 89 , 94 ], but also CuAAC (Cu(I)-catalyzed azide-alkyne cycloaddition) click reactions [ 92 , 93 ] proved to be useful tools for the functionalization of the diruthenium trithiolato unit at the level of the bridge thiols. CuAAC offer the advantage of mild reaction conditions, compatible with various ligands [ 44 , 46 , 71 , 72 , 73 , 95 , 96 , 97 ] but also with organometallics [ 81 , 98 , 99 , 100 , 101 , 102 ], and enables the construction of libraries of compounds [ 103 , 104 , 105 ].…”

“…CuAAC offer the advantage of mild reaction conditions, compatible with various ligands [ 44 , 46 , 71 , 72 , 73 , 95 , 96 , 97 ] but also with organometallics [ 81 , 98 , 99 , 100 , 101 , 102 ], and enables the construction of libraries of compounds [ 103 , 104 , 105 ]. Additionally, trithiolato diruthenium(II)-arene compounds suitably substituted with alkyne or azide groups were already used in CuAAC reactions for obtaining conjugates with molecules of interest e.g., various nucleic bases or drugs [ 92 , 93 ].…”