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The antiprotozoal activities of three naturally occurring isomeric indoloquinoline alkaloids, i.e., cryptolepine (1), neocryptolepine (2), and isocryptolepine (3), and two dimeric indoloquinoline alkaloids, cryptoquindoline (6) and biscryptolepine (7), originally obtained from the plant Cryptolepis sanguinolenta, were compared with those of a new synthetic indoloquinoline isomer, isoneocryptolepine (4), and a quaternary derivative, N-methyl-isocryptolepinium iodide (5). The latter compounds showed a high antiplasmodial activity against the chloroquine-resistant Plasmodium falciparum strain K1 (IC50 of 0.23 ± 0.04 and 0.017 ± 0.004 μM, respectively), while the cytotoxicity (L6 cells) was 4.32 ± 0.04 and 12.7 ± 2.0 μM, respectively. Isoneocryptolepine (4) was found to act as an inhibitor of β-hematin formation and as a DNA-intercalating agent.

Morinda morindoides (Baker) Milne-Redhead (syn. Gaertnera morindoides Bak.) is one of the most popular medicinal plants in the Democratic Republic of Congo. In relation to its traditional use against rheumatic pains, fractionation of both the EtOAc- and the n-BuOH-soluble fraction of the 80% MeOH extract of the leaves, guided by the anticomplementary activity on the classical activation pathway, yielded eight novel iridoids (1−8), all containing a spirolactone functionality. Their structure was elucidated using spectroscopic methods. Gaertneroside 1, acetylgaertneroside 2, and gaertneric acid 5 were found to inhibit the activation of the classical pathway of the complement system, with IC50 values between 58 and 69 μM. In addition to the biologically active flavonoids reported before from the same plant, these complement-inhibiting iridoids may contribute at least in part to the traditional use against rheumatic pains.

The structure−activity relationship of flavonoids as inhibitors of xanthine oxidase and as scavengers of the superoxide radical, produced by the action of the enzyme xanthine oxidase, was investigated. The hydroxyl groups at C-5 and C-7 and the double bond between C-2 and C-3 were essential for a high inhibitory activity on xanthine oxidase. Flavones showed slightly higher inhibitory activity than flavonols. All flavonoid derivatives except isorhamnetin (30) were less active than the original compounds. For a high superoxide scavenging activity on the other hand, a hydroxyl group at C-3‘ in ring B and at C-3 were essential. According to their effect on xanthine oxidase and as superoxide scavengers, the flavonoids could be classified into six groups:  superoxide scavengers without inhibitory activity on xanthine oxidase (category A), xanthine oxidase inhibitors without any additional superoxide scavenging activity (category B), xanthine oxidase inhibitors with an additional superoxide scavenging activity (category C), xanthine oxidase inhibitors with an additional pro-oxidant effect on the production of superoxide (category D), flavonoids with a marginal effect on xanthine oxidase but with a pro-oxidant effect on the production of superoxide (category E), and finally, flavonoids with no effect on xanthine oxidase or superoxide (category F).

Three different extracts and four alkaloids from the root bark of Cryptolepis sanguinolenta have been assessed in vitro against Plasmodium falciparum D-6 (chloroquine-sensitive strain), K-1, and W-2 (chloroquine-resistant strains). Cryptolepine (1) and its hydrochloride (2), 11-hydroxycryptolepine (3), and neocryptolepine (5) showed a strong antiplasmodial activity against P. falciparum chloroquine-resistant strains. Quindoline (4) was less active. The highest activity was obtained with compound 1. In vivo tests on infected mice showed that crytpolepine (1), when tested as its hydrochloride (2), exhibited a significant chemosuppressive effect against Plasmodium berghei yoelii and Plasmodium berghei berghei, while 1 had the same effect against P. berghei yoelii only. Compounds 3 and 4 did not show activity in this in vivo test system.