*3.3. Bioactivity of Choline Esters*

Unlike the brominated indole derivatives, the choline esters naturally occurring in *D. orbita* obtained high bioactivity scores in the Molinspiration online chemoinformatics prediction software (Table 2b). In particular, all three choline esters were predicted to inhibit enzymes and modulate ion channels. Murexine, with an imidazole moiety, obtained the highest bioactivity scores and was also the only choline ester predicted to bind to GPCR (Table 2b). The prediction for ion channel modulation is consistent with the known biological activities of these choline esters. Murexine in particular has been thoroughly investigated for toxicity, paralysis of the skeletal musculature, neuromuscular blocking activity and nicotinic action [64,73,82,83] and similar pharmacological properties have been reported for senecioylcholine [64]. Both compounds are almost devoid of muscarinic effects on acetylcholine receptors [64,73]. Murexine was shown to stimulate ganglion, in addition to having depolarizing neuromuscular blocking actions in cat, dog and rat [82].

The intravenous LD50 for murexine in mice has been established at 6.5 mg/kg [64] to 8.7 mg/kg [73] and death is caused by anoxia secondary to peripheral respiratory arrest [64,73]. Tigloylcholine was estimated to be more toxic, with an i.v. LD50 of 0.92 mg/kg in mice [65]. When administered subcutaneously the LD50 of murexine in mice was approximately 50 mg/kg and oral delivery was ineffective in doses up to 1 g/kg [64]. Preliminary human clinical trials were conducted with murexine as muscle relaxant on 160 patients. The mean paralysing dose in adult patients was approximately 1 mg/kg i.v. with the paralysis lasting for 3–6 min after a single dose and longer lasting muscular relaxation could be obtained by slow i.v. infusion of a 1/1000 solution of murexine in physiological saline [64,73]. However, murexine caused several side-effects, which were mainly attributable to the nicotinic actions of the drug.
