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The more polar metabolites from the Venezuelan plant Verbesina caracasana, i.e., N³-prenylagmatine, (3,4-dimethoxycinnamoyl)-N¹-agmatine, agmatine, and galegine (prenylguanidine), previously reported (Delle Monache, G.; et al. BioMed. Chem. Lett. 1999, 9, 3249−3254), have been synthesized following a biosynthetic strategy. The pharmacologic profiles of various synthetic analogues of (3,4-dimethoxycinnamoyl)-N¹-agmatine (G5) were also analyzed, to shed some light on the structure−activity relationship of these compounds. Derivatives with the (E)-configuration and/or with a p-methoxybenzoyl moiety were found to be responsible for higher hypotensive effects, which were associated with a slight and, in some cases, not dose-related increase of cardiac inotropism, with variable and not significant chronotopic responses, and, only at higher doses, with effects of respiratory depression. Either an increase (to six) or a decrease (to two) of the number of methylene groups in the alkyl chain of (E)-G5 did not change blood pressure responses, while slightly increasing the positive inotropic ones. At pharmacological doses, all the studied compounds showed hypotensive and slight positive inotropic effects without relevant chronotropic and respiratory actions.

Caracasandiamide, a second hypotensive agent isolated from Verbesina caracasana, is the cyclobutane dimer (truxinic type) of the previously reported 1-[(3,4-dimethoxycinnamoyl)amino]-4-[(3-methyl-2-butenyl)guanidino]butane (caracasanamide) (Delle Monache, G.; et al. BioMed. Chem. Lett. 1992, 25, 415−418). The structure was confirmed by synthesis starting from β-truxinic acid obtained by photoaddition of 3,4-dimethoxycinnamic acid. The dimer was coupled with 2 mol of prenylagmatine to give caracasandiamide in satisfactory yield. By contrast, the direct photodimerization of caracasanamide was unsuccessful. Caracasandiamide, assayed by the iv route in anesthetized rats at doses ranging from 50 to 3200 μg/kg of body weight, was found to have no appreciable effect on heart rate. At lower doses, the drug stimulates breathing and increases cardiac inotropism, stroke volume, and cardiac output, thus augmenting blood pressure and aortic flow. At higher doses, caracasandiamide depresses breathing likely through central neurogenic mechanisms (not involved in the cardiovascular effects), continues to stimulate cardiac inotropism, and induces, by reducing peripheral vascular resistance, arterial hypotension with reduction of both aortic flow and stroke volume. These cardiovascular effects appear to involve complex interactions at the level of the peripheral β1-, β2-, and α2-adrenoreceptor-dependent as well as M2- and M4-cholinergic receptor-dependent transductional pathways both in cardiovascular myocells and at the level of the postganglionic sympathetic endings (with reserpine- and guanethidine-like mechanisms). The cardiovascular effects of caracasandiamide, different from those of caracasanamide, do not depend on significant actions on the central nervous system and on baroreflex pathways. In a similar manner and more effective than caracasanamide, caracasandiamide may be considered a hypotensive and antihypertensive drug. It is devoid of some of the negative side effects, e.g., reflex tachycardia and decreased cardiac inotropism, which are shown by the majority of the most common antihypertensive and vasodilator drugs.