Synthesis, analysis and biological evaluation of novel indoloquinoline cryptolepine analogues as potential antitumor agents
Cryptolepine is a tetracyclic, aromatic natural product, isolated from West African shrubs of the Cryptolepis species, consisting of fused indole and quinoline rings. Due to its polyaromatic character it is able to intercalate into DNA showing a unique selectivity for non-alternating GC base pairs. Cryptolepine, and especially its halogenated analogues, have been investigated for their anti-malarial and anti-cancer activity, the latter being the area of interest of this thesis. A number of novel halogenated indoloquinoline cryptolepine analogues were synthesised in order to determine their abilities to interact with DNA, and to inhibit topoisomerase II activity. In vitro cytotoxicity testing was carried out using human tumor cell lines MCF-7 (breast cancer), 5637 (bladder cancer), DLD-1 (colon cancer) and A-549 (lung cancer) via the MTT assay. The halogenated cryptolepine analogues were allowed to bind to calf thymus DNA and the melting points of these complexes measured. An increase in melting temperature between 4.5 and 12°C was seen when compared to naked DNA. Inhibition of topoisomerase II was seen at concentrations between 0.5 and 0.020 μM compared to 5 μM for cryptolepine. All compounds showed cytotoxic activity in the range 0.07 to 73.87 μM against the tumor cell lines after 96 hr exposure. 11-lodocryptolepine was found to be the most potent on all of the cell lines tested, with IC₅₀ values down to sub micro molar levels. This analogue was also one of the most potent at topoisomerase II inhibition (0.5 μM) and had a ΔTm of 12.0°C. However, despite 11-chlorocryptolepine being the most potent at inhibiting topoisomerase II (0.02 μM), it was found to be the least potent in vitro and had one of the lowest ΔTm values (5.0°C). Cellular incorporation of the analogues was investigated by using laser confocal microscopy which showed that 11-iodo and 11-bromocryptolepines had nuclear localisation, whereas, the 11-chlorocryptolepine analogues remain in the cell membrane.