Preclinical methods for the identification and characterization of molecules for development into new cancer drugs were investigated. Based on repurposing, i.e. the exploration of currently prescribed drugs for new indications, and as a result of a new high throughput screening (HTS) approach, the benzimidazoles benomyl and carbendazim, the alkaloid cryptolepine and the acridine acriflavine were found interesting to characterize using these methods.

In mice the benzimidazoles inhibited ³H-thymidine incorporation in tissues with high cell renewal, with benomyl being more active than carbendazim.  They were rapidly absorbed with highest amounts seen in the liver, kidneys and gastro-intestinal lumen as evidenced from distribution of ¹⁴C-labeled drugs. In human tumour cell lines, the benzimidazoles showed a similar activity pattern but benomyl was more potent. This was true also in tumour cells from patients but carbendazim was relatively more active against solid tumours. Analyses of drug activity cross-resistance patterns and of drug activity – gene expression correlations in a cell line panel suggested multiple mechanisms of action for the benzimidazoles.

Cryptolepine was widely distributed to tissues in vivo in the mice. It was more potent than the benzimidazoles in tumour cells, with highest activity in haematological malignancies but some patient samples of breast, colon and non small-cell lung cancer were sensitive. Cross-resistance analysis indicated cryptolepine to be a topoisomerase II inhibitor whereas drug activity – gene expression correlations suggested additional mechanisms of action.

HTS on 2 000 molecules in colon cancer cell lines and normal cells identified acriflavine as a hit molecule, subsequently shown to have unprecedented activity against colorectal cancer tumour cells in patient tumour samples. Connectivity map analysis, based on drug induced gene expression perturbation patterns in a tumour cell line, indicated acriflavine to be a topoisomerase inhibitor, subsequently confirmed in a plasmid relaxation assay. In conclusion, repurposing of drugs and HTS using stringent activity criteria followed by preclinical characterization might contribute to more efficient development of new cancer drugs.