Traditional medicines have contributed greatly over the centuries to the discovery and development of new therapeutic agents and indoloquinoline alkaloids may represent a new class of drug leads. Cryptolepine (5-methyl-5H-indolo[ 3,2-b]quinoline), neocryptolepine (5-methyl-5H-indolo[2,3-b]quinoline), isocryptolepine (5-methyl-5H-indolo[3,2- c]quinoline, extracted from the African medicinal plant Cryptolepis sanguinolenta, and isoneocryptolepine (5-methyl-5H-indolo[ 2,3-c]quinoline), which has never been found in nature, are isomeric tetracyclic compounds of particular interest due to their broad spectrum of biological activities including antiparasitic, antifungal, antibacterial, cytotoxic, antiinflammatory and antihyperglycaemic. As a result, in the last 30 years hundreds of indoloquinoline analogues were synthesized and their biological activities evaluated. In this paper, we present an overview of the potential of indoloquinolines as scaffolds in drug discovery by reviewing the in vitro and in vivo biological activities of natural and synthetic analogues, as well as the proposed mechanisms of action and structure-activity relationships.

The roots of Cryptolepis sanguinolenta have been investigated for their chemical composition since 1931 but so far no studies on the leaves have been reported although they are used in traditional medicine in Guinea-Bissau. Two new alkaloids identified as cryptolepinoic acid (1) and methyl cryptolepinoate (2) and the known alkaloids cryptolepine (4), hydroxycryptolepine (5/5 a) and quindoline (6), were isolated from the ethanolic and chloroformic leaf extracts. Aqueous and ethanolic extracts of the leaves and roots and seven alkaloids isolated from those extracts were tested in vitro against Plasmodium falciparum K1 (multidrug-resistant strain) and T996 (chloroquine-sensitive clone). All the extracts were shown to give 90 % inhibition of P. falciparum K1 growth at concentrations < 23 μg/ml. Cryptolepine (4) was the most active alkaloid tested with IC₅₀ values (0.23 μM to K1; 0.059 μM to T996) comparable with chloroquine (0.26 μM to K1; 0.019 μM to T996). The indolobenzazepine alkaloid cryptoheptine (7) was the second most active with IC₅₀ values of 0.8 μM (K1) and 1.2 μM (T996). Cryptolepinoic acid (1) showed no significant activity while its ethyl ester derivative 3 was active against P. falciparum K1 (IC₅₀ = 3.7 μM). All the indoloquinoline alkaloids showed cross-resistance with chloroquine but not the indolobenzazepine alkaloid 7. It was noticed that alkaloids with weakly basic characteristics were active whereas other structurally related alkaloids with different acid-base profiles were inactive. These observations are in agreement with the antimalarial mechanism of action for quinolines.

KRAS is one of the most frequently mutated oncogenes in human cancer, yet remaining undruggable. To explore a new therapeutic strategy, a library of 5-methyl-indolo[3,2-c]quinoline derivatives (IQc) with a range of alkyldiamine side chains was designed to target DNA and RNA G-quadruplexes (G4) in the promoter and 5′-UTR mRNA of the KRAS gene. Biophysical experiments showed that di-substituted IQc compounds are potent and selective KRAS G4 stabilizers. They preferentially inhibit the proliferation of KRAS mutant cancer cell lines (0.22 < IC₅₀ < 4.80 μM), down-regulate KRAS promoter activity in a luciferase reporter assay and reduce both KRAS mRNA and p21^KRAS steady-state levels in mutant KRAS colon cancer cell lines. Additionally, IQcs induce cancer cell death by apoptosis, explained in part by their capacity to repress KRAS expression. Overall, the results suggest that targeting mutant KRAS at the gene level with G4 binding small molecules is a promising anticancer strategy.