Evaluation of the biological activity of venom extracts from the toads Rhinella marina and Rhaebo guttatus on peripheral blood monocytes from healthy human subjects

Skin secretions from toads of the Bufonidae family contain immunomodulatory and antioxidant substances and have pharmaceutical potential for treating cancer, degenerative diseases, and cardiovascular conditions. Previous studies have demonstrated that the methanolic extracts of Rhinella marina and Rhaebo guttatus have an immunomodulatory effect on murine macrophages, stimulating the production of pro-inflammatory cytokines and hydrogen peroxide. However, little is known about its effect on human monocytes. Therefore, the aim of this study was to evaluate the in vitro effect of these extracts on the function of human peripheral blood monocytes by assessing cytotoxicity and the production of nitric oxide and hydrogen peroxide. The amphibians were collected in Mato Grosso, Brazil. In the laboratory, their paratoid glands were compressed to extract the venom. Voucher specimens were deposited in the Biological Collection of the Southern Amazon (Sinop/MT). To obtain monocytes, peripheral blood samples were collected from 16 healthy individuals (aged 30 – 45 years) who had not used any class of medication for the previous 15 days. The monocytes were cultured (5% CO2 at 37°C) with different extract concentrations for 18 h (for cytotoxicity assessment) or 24 h (for activity assessment). For R. marina venom, the most toxic concentration was 1000 µL mL-1, whereas non-toxic concentrations were 62.5 µL mL-1, 15.63 µL mL-1, and 7.8 µL mL-1. For R. guttatus, concentrations ranging from 1000 µL mL-1 to 31.25 µL mL-1 were the most toxic, and the non-toxic concentrations were 15.63 µL mL-1 and 7.8 µL mL-1. The cells did not produce nitric oxide or hydrogen peroxide in response to stimulation with the extracts. We conclude that extracts from the skin secretions of R. marina and R. guttatus exhibit a concentration-dependent cytotoxic effect on human monocytes but do not modulate the cells’ in vitro ability to produce nitric oxide and hydrogen peroxide.