Inhibitory effects of the phytohormone inhibitors fluridone and inabenfide against Babesia gibsoni in vitro

Anti-babesial activity of a series of 6,7-dimethoxyquinazoline-2,4-diamines (DMQDAs)

Diminazene aceturate (DA), imidocarb dipropionate (ID), atovaquone (ATO), azithromycin (AZI), clindamycin, and quinine have been used to treat animal and human babesiosis for many years, despite their negative effects and rising indications of resistance. Thus, finding anti-babesial compounds that can either treat the infection or lower the dose of drugs given has been a primary objective. Quinazolines are one of the most important nitrogen heterocycles, with a wide range of pharmacological activities including analgesic, anti-inflammatory, sedative-hypnotic, anti-histaminic, anti-cancer, and anti-protozoan properties. The present study investigated the anti-babesial activities of twenty 6,7-dimethoxyquinazoline-2,4-diamines on Babesia spp. One candidate, 6,7-dimethoxy-N4-ethylisopropyl-N2-ethyl(pyridin-4-yl)quinazoline-2,4-diamine (SHG02), showed potent inhibition on Babesia gibsoni in vitro, as well as on B. microti and B. rodhaini in mice. Our findings indicate that the candidate compound SHG02 is promising for further development of anti-babesial drugs and provides a new structure to be explored for developing anti-Babesia therapeutics.

In vitro screening of novel anti-Babesia gibsoni drugs from natural products

Babesia gibsoni is a tick-transmitted intraerythrocytic apicomplexan parasite that causes babesiosis in dogs. Due to the strong side effects and lack of efficacy of current drugs, novel drugs against B. gibsoni are urgently needed. Natural products as a source for new drugs is a good choice for screening drugs against B. gibsoni. The current study focuses on identifying novel potential drugs from natural products against B. gibsoniin vitro. Parasite inhibition was verified using a SYBR green I-based fluorescence assay. A total of 502 natural product compounds were screened for anti-B. gibsoni activity in vitro. Twenty-four compounds showed high growth inhibition (>80%) on B. gibsoni and 5 plant-derived compounds were selected for further study. The half-maximal inhibitory concentration (IC₅₀) values of lycorine (LY), vincristine sulfate (VS), emetine·2HCl (EME), harringtonine (HT) and cephaeline·HBr (CEP) were 784.4 ± 3.3, 643.0 ± 2.8, 253.1 ± 1.4, 23.4 ± 1.2, and 108.1 ± 4.3 nM, respectively. The Madin-Darby canine kidney (MDCK) cell line was used to assess cytotoxicity of hit compounds. All compounds showed minimal toxicity to the MDCK cells. The effects of hit compounds combined with diminazene aceturate (DA) on B. gibsoni were further evaluated in vitro. VS, EME, HT or CEP combined with DA showed synergistic effects against B. gibsoni, whereas LY combined with DA showed an antagonistic effect against B. gibsoni. The results obtained in this study indicate that LY, VS, EME, HT and CEP are promising compounds for B. gibsoni treatment.

Closing the empty anti-Babesia gibsoni drug pipeline in vitro using fluorescence-based high throughput screening assay

In the current study, we evaluated the usefulness of a SYBR Green I (SG I) fluorescence assay for evaluation of the inhibitory effects of antibabesial drugs against the in vitro growth of Babesia gibsoni. Linearity and high-throughput screening (HTS) assays exhibited the validity of the SG I fluorescence assay for B. gibsoni parasite when performed at low hematocrits (HCTs) (2.5% and 5%) without daily changing of the medium. Interestingly, 5% HCT showed the highest value of the signal/noise ratio. Of note, there were no significant differences (P > .05) in the IC₅₀s of the commonly used antibabesial drugs (diminazene aceturate and/or imidocarb dipropionate) that calculated by either the SG I fluorescence assay with and without daily medium changing or by the fluorescence and microscopy methods at 2.5% and 5% HCTs. Such results confirmed that both HCTs are valid for mass drug screening against the in vitro growth of B. gibsoni. While the results of the HTS assay add merit to the assay when performed at 5% HCT especially when incubating the plates for 2 h in a dark after adding lysis buffer with SG I stain. Next, nine different drugs were screened to confirm the assay's usefulness. MMV396693, pyronaridine tetraphosphate and nerolidol drugs exhibited the highest effectiveness against the in vitro growth of B. gibsoni, next to diminazene aceturate. In summary, SG I fluorescence assay with 5% HCT without daily changing of the medium for B. gibsoni offers a novel approach for the large-scale screening of huge chemical libraries in in vitro cultures.