**2. Biochemical Structure and Production**

The cardiac glycosides include bufadienolides and cardenolides, which substantially differ from each other in both biochemical structure and the cellular [1,2] mechanism of action. Bufadienolides are steroid compounds with a δ-lactone ring at carbon C17, primarily synthesized from cholesterol as a precursor via the mevalonate-independent pathway [28]. They have a double unsaturated six-membered lactone ring while cardenolides have an unsaturated five-membered lactone ring [29]. However, at present, the bufadienolide biosynthesis mechanism is still unknown [26].

Bufadienolides are found in both animals and plants [30,31] and those of mammalian origin are produced by the placenta [32] and adrenal cortex [33] under the control of the bile acid CYP27A1 enzyme, although other production sites cannot be excluded [34]. Bufadienolides are eliminated unchanged via renal excretion [13]. The bufadienolides are so named because they are extracted from the venom of a common toad, *bufo marinus* or *rhinella marina*, both members of the Bufonidae family [35]. Skin [36] and parotoid gland secretions [37] of *bufo marinus* are considered the main natural sources of MBG. This toad species is native to South and Central America and has remained almost unchanged since the late Miocene period. Later, the species was introduced to Australia and the Oceanian islands and is currently one of the worst invasive species in many countries. Both sexes of the *Bufo* species possess huge parotid glands, stretching through the retro-orbital level and releasing a venom composed of different types of molecules, such as alkaloids, peptides, biogenicamines, steroids (bufogenins and bufotoxins) and proteins [38–40], which have antimicrobial activity and a defense action against potential predators [41–43]. The chemical and pharmacological characteristics of the secretions from the parotid gland and skin of the family Bufonidae have been studied for some time [44], and as early as 1972, 50 compounds were recognized in 39 species collected from different locations around the world [45].
