Synthesis and in vitro antimalarial activity of some indolo[3,2-c]quinolines

Role of basic aminoalkyl chains in the lead optimization of Indoloquinoline alkaloids

Indoloquinoline (IQ) is an important class of naturally occurring antimalarial alkaloids, mainly represented by cryptolepine, isocryptolepine, and neocryptolepine. The IQ structural framework consists of four isomeric ring systems differing via the linkage of indole with quinoline as [3,2-b], [3,2-c], [2,3-c], and [2,3-b]. Structurally, IQs are planar and thus they bind strongly to the DNA which largely contributes to their biological properties. The structural rigidity and associated nonspecific cellular toxicity is a key shortcoming of the IQ structural framework for preclinical development. Thus, the lead optimization efforts were aimed at improving the therapeutic window and ADME properties of IQs. The structural modifications mainly involved attaching the basic aminoalkyl chains that positively modulates the vital physicochemical and topological parameters, thereby improves biological activity. Our analysis has found that the aminoalkylation consistently improved the selectivity index and provided acceptable in-vivo antimalarial/anticancer activity. Herein, we critically review the role of aminoalkylation in deciphering the antimalarial and cytotoxic activity of IQs.

Synthesis and Evaluation of the Tetracyclic Ring-System of Isocryptolepine and Regioiso-Mers for Antimalarial, Antiproliferative and Antimicrobial Activities

A series of novel quinoline-based tetracyclic ring-systems were synthesized and evaluated in vitro for their antiplasmodial, antiproliferative and antimicrobial activities. The novel hydroiodide salts 10 and 21 showed the most promising antiplasmodial inhibition, with compound 10 displaying higher selectivity than the employed standards. The antiproliferative assay revealed novel pyridophenanthridine 4b to be significantly more active against human prostate cancer (IC₅₀ = 24 nM) than Puromycin (IC₅₀ = 270 nM) and Doxorubicin (IC₅₀ = 830 nM), which are used for clinical treatment. Pyridocarbazoles 9 was also moderately effective against all the employed cancer cell lines and moreover showed excellent biofilm inhibition (9a: MBIC = 100 µM; 9b: MBIC = 100 µM).

Synthesis, β-haematin inhibition, and in vitro antimalarial testing of isocryptolepine analogues: SAR study of indolo[3,2-c]quinolines with various substituents at C2, C6, and N11

A series of indolo[3,2-c]quinolines were synthesized by modifying the side chains of the ω-aminoalkylamines at the C6 position and introducing substituents at the C2 position, such as F, Cl, Br, Me, MeO and NO2, and a methyl group at the N11 position for an SAR study. The in vitro antiplasmodial activities of the derivative agents against two different strains (CQS: NF54 and CQR: K1) and the cytotoxic activity against normal L6 cells were evaluated. The test results showed that compounds 6k and 6l containing the branched methyl groups of 3-aminopropylamino at C6 with a Cl atom at C2 exhibited a very low cytotoxicity with IC50 values above 4000 nM, high antimalarial activities with IC50 values of about 11 nM for CQS (NF54), IC50 values of about 17 nM for CQR (K1), and RI resistance indices of 1.6. Furthermore, the compounds were tested for β-haematic inhibition, and QSAR revealed an interesting linear correlation between the biological activity of CQS (NF54) and three contributing factors, namely solubility, hydrophilic surface area, and β-haematin inhibition for this series. In vivo testing of 6l showed a reduction in parasitaemia on day 4 with an activity of 38%.

Structure—activity relationships of some indolo[3,2-c]quinolines with antimalarial activity

The synthesis, physicochemical characterization and in vitro antimalarial activity of a series of indolo[3,2-c]quinolines (9a-f) are described. There is only a poor correlation between the activity and hydrophobicity. In contrast, 33% of the observed variation in antimalarial activity can be attributed to the size of the side chain attached to position 9 of the indoloquinoline ring. An increase in the size of this dibasic side chain generally results in a reduction in activity, suggesting that it is accommodated in a site/cavity of limited size on the receptor. More significantly, the charge on the distal nitrogen (N3) on the side chain, located 10-11 A from the quinoline N, could account for 75% of the observed variation. Since a large charge on N3 is associated with improved antimalarial activity, it is suggested that N3 is protonated and functions as a H bond donor in the drug-receptor interaction.

An exploration of the structure-activity relationships of 4-aminoquinolines: novel antimalarials with activity in-vivo

The structure-activity relationships of bisquinolines, a potentially important group of novel antimalarial drugs, were studied. The high-temperature (180-250 degrees C) synthesis of 4-aminoquinolines, including bisquinolines, by nucleophilic displacement was both fast and efficient Several bisquinolines including (+/-)-trans-N1,N2-bis(7-trifluoroquinolin-4-yl)cyclohexane-1, 2-diamine and 1R,2R-(-)-, 1S,2S-(+)-, (+/-)-trans- and cis-N1, N2-bis(7-chloroquinolin-4-yl)cyclohexane-1,2-diamine exhibited potent activity against Plasmodium berghei in mice; (+/-)-trans-N1,N2-bis(7-chloroquinolin-4-yl)cyclohexane-1, 2-diamine was orally active. Our results indicate that these compounds conform to a putative receptor for quinoline antimalarials. In addition, a 7-haloquinoline linked by a heterocyclic bridge, at the 4-position, to another heterocycle (such as an acridine at the 9-position) maximally occupies the active site of our postulated target.