In vitro inhibition of snake venom toxins by varespladib, marimastat, nafamostat and dimercaprol

Snakebite envenoming causes more than 130,000 deaths and more than 400,000 disabilities per year and has been classified as a priority Neglected Tropical Disease by the World Health Organization (WHO). While antivenom therapy remains the mainstay of snakebite treatment, small molecule therapeutics (SMTs) have been proposed as potential adjuncts to antivenom, particularly as oral treatment in the prehospital setting. Several SMTs have demonstrated efficacy in preclinical models of snakebite envenoming, with varespladib, a secreted phospholipase A₂ (sPLA₂) inhibitor, being granted orphan drug status for its potential to treat snakebite. The present study investigated the potential of four SMTs (e.g., varespladib, marimastat, nafamostat and dimercaprol) to neutralise toxic components present in the venom of southern African snake species. In vitro experimentation found that varespladib potently inhibited snake venom phospholipase A₂ (svPLA₂) activity in Bitis arietans (IC₅₀ = 0.221 μM) and B. gabonica (IC₅₀ = 0.276 μM). Marimastat exhibited potent inhibition of snake venom metalloproteinase (svMP) in several snake species with IC₅₀ values ranging from 0.0042 to 3.06 μM, while dimercaprol, a metal chelator, was a lower potency svMP inhibitor with IC₅₀ values ranging from 5.01 to 79.8 μM. Nafamostat proved to be an inhibitor of snake venom serine protease (svSP) in B. arietans (IC₅₀ = 3.72 μM), B. gabonica (IC₅₀ = 3.80 μM) and Causus rhombeatus (IC₅₀ = 0.261 μM). These data demonstrate that SMTs are effective inhibitors of the relevant enzymes in several snake species and support the proposal that SMTs could be developed for therapeutic intervention in snakebite envenoming.