Illudin S

Illudins are novel low molecular weight natural products cytotoxic to human tumor cells in vitro. Illudin-derived analogs are effective against experimental human cancers nonresponsive to conventional anticancer agents. It is not known why some illudin analogs are more efficacious in vitro and in vivo than other analogs. Therefore, the in vitro cytotoxicity of the parent compound illudin S towards tumor cells was characterized using radiolabeled drug. Two cell lines sensitive at nanomolar concentrations using only a 15-min exposure period displayed a saturable, energy-dependent accumulation of illudins with relatively low K(m) and high Vmax values. A nonsensitive cell line, requiring millimolar concentrations to achieve in vitro toxicity, showed minimal illudin uptake with higher K(m) and lower Vmax values. No release of radioactivity could be demonstrated from tumor cells, indicating that there was no efflux of illudin S (or metabolites) from these cells. The number of intracellular illudin S molecules required to kill 50% of cells of different tumor cell lines varied from 78000 to 1114000 molecules per cell and was correlated with the 2-h IC50 value determined using a colony-forming assay. Illudin S was cytotoxic to a variety of multidrug-resistant tumor cell lines regardless of whether resistance was mediated by gp170/mdrl, gp180/MRP, GSHTR-pi, topoisomerase I, topoisomerase II, increased DNA repair capacity, or alterations in intracellular thiol content. Information obtained in this study could be used to design clinical phase I trials and to develop analogs with improved therapeutic indexes.

Illudin S is a natural sesquiterpene drug with strong anti-tumour activity. Inside cells, unstable active metabolites of illudin cause the formation of as yet poorly characterised DNA lesions. In order to identify factors involved in their repair, we have performed a detailed genetic survey of repair-defective mutants for responses to the drug. We show that 90% of illudin's lethal effects in human fibroblasts can be prevented by an active nucleotide excision repair (NER) system. Core NER enzymes XPA, XPF, XPG, and TFIIH are essential for recovery. However, the presence of global NER initiators XPC, HR23A/HR23B and XPE is not required, whereas survival, repair and recovery from transcription inhibition critically depend on CSA, CSB and UVS, the factors specific for transcription-coupled NER. Base excision repair and non-homologous end-joining of DNA breaks do not play a major role in the processing of illudin lesions. However, active RAD18 is required for optimal cell survival, indicating that the lesions also block replication forks, eliciting post-replication-repair-like responses. However, the translesion-polymerase DNA pol eta is not involved. We conclude that illudin-induced lesions are exceptional in that they appear to be ignored by all of the known global repair systems, and can only be repaired when trapped in stalled replication or transcription complexes. We show that the semisynthetic illudin derivative hydroxymethylacylfulvene (HMAF, Irofulven), currently under clinical trial for anti-tumour therapy, acts via the same mechanism.

The effect of the antitumor antibiotic illudin S on bacterial macromolecular synthesis was investigated. Illudin S was found to be inhibitory to in vivo deoxyribonucleic acid (DNA) synthesis from thymidine. Ribonucleic acid (RNA) synthesis was inhibited only at a concentration of illudin S 10 times that which inhibited DNA synthesis. The rate of protein synthesis remained the same except for a brief initial inhibition. When thymidine triphosphate was used for in vitro DNA synthesis, inhibition by illudin S did not occur, as tested with partially purified DNA polymerase II from Escherichia coli pol A(1) (-), with E. coli DNA-dependent RNA polymerase, with E. coli pol A(1) (-) spheroplasts, and with frozen and thawed Bacillus subtilis cells. A protein fraction isolated from B. subtilis capable of forming thymidine mono-, di-, and triphosphates from thymidine was not inhibited by illudin S. Furthermore, (14)C-illudin S taken up by B. subtilis cells was reisolated unchanged, making an intracellular activation of illudin S unlikely. Therefore, an attractive hypothesis is that illudin S inhibits DNA synthesis from thymidine which does not proceed through deoxyribonucleoside triphosphates, the generally accepted substrates for DNA synthesis.