and Andrew D. Westwell

The development of the antitumour benzothiazole prodrug, Phortress, as a clinical candidate.

This review traces the development of a series of potent and selective antitumour benzothiazoles from the discovery of the initial lead compound, 2-(4-amino-3-methylphenyl)benzothiazole (DF 203) in 1995 to the identification of a clinical candidate, Phortress, scheduled to enter Phase 1 trials in Q1 2004 under the auspices of Cancer Research U.K. Advances in our understanding of the mechanism of action of this unique series of agents are described and can be summarised as follows: selective uptake into sensitive cells followed by Arylhydrocarbon Receptor (AhR) binding and translocation into the nucleus, induction of the cytochrome P450 isoform (CYP) 1A1, conversion of the drug into an electrophilic reactive intermediate and formation of extensive DNA adducts resulting in cell death. Our understanding of this mechanistic scenario has played a crucial role in the drug development process, most notably in the synthesis of fluorinated DF 203 analogues to thwart deactivating oxidative metabolism (5F 203) and water-soluble prodrug design for parenteral administration. Aspects of mechanism of action studies, in vitro and in vivo screening, synthetic chemistry and pharmacokinetics are reviewed here.

Cytotoxic and antiangiogenic activity of AW464 (NSC 706704), a novel thioredoxin inhibitor: an in vitro study

AW464 (NSC 706704) is a novel benzothiazole substituted quinol compound active against colon, renal and certain breast cancer cell lines. NCI COMPARE analysis indicates possible interaction with thioredoxin/thioredoxin reductase, which is upregulated under hypoxia. Through activity on HIF1α, VEGF levels are regulated and angiogenesis controlled. A thioredoxin inhibitor could therefore exhibit enhanced hypoxic toxicity and indirect antiangiogenic effects. In vitro experiments were performed on colorectal and breast cancer cell lines under both normoxic and hypoxic conditions and results compared against those obtained with normal cell lines, fibroblasts and keratinocytes. Antiangiogenic effects were studied using both large and microvessel cells. Indirect antiangiogenic effects (production of angiogenic growth factors) were studied via ELISA. We show that AW464 exerts antiproliferative effects on tumour cell lines as well as endothelial cells with an IC50 of ∼0.5 μM. Fibroblasts are however resistant. Proliferating, rather than quiescent, endothelial cells are sensitive to the drug indicating potential antiangiogenic rather than antivascular action. Endothelial differentiation is also inhibited in vitro. Hypoxia (1% O2 for 48 h) sensitises colorectal cells to lower drug concentrations, and in HT29s greater inhibition of VEGF is observed under such conditions. In contrast, bFGF levels are unaffected, suggesting possible involvement of HIF1α. Thus, AW464 is a promising chemotherapeutic drug that may have enhanced potency under hypoxic conditions and also additional antiangiogenic activity.

The aryl hydrocarbon receptor in anticancer drug discovery: friend or foe?

Binding of ligands such as polycyclic aromatic hydrocarbons to the Aryl hydrocarbon Receptor (AhR) and the sequence of events leading to induction of xenobiotic-metabolising enzymes such as the cytochrome P450 isoform 1A1 and subsequent generation of DNA adducts is historically associated with the process of chemical carcinogenesis. Cancer chemopreventative agents, on the other hand, often exert their biological effect at least in part through antagonism of AhR-induced carcinogenesis. A third scenario associated with AhR binding could occur if the induction of xenobiotic enzymes and subsequent DNA damage causes apoptosis. If this occurs selectively in tumour cells whilst sparing normal tissue, the AhR ligand would have a therapeutic cytotoxic effect. In this review we survey for the first time the major classes of reported AhR ligands and discuss the biological consequences of AhR binding in each case. The use of AhR ligands as cancer chemotherapeutic agents, as illustrated by the case of the 2-(4-aminophenyl)benzothiazole prodrug Phortress, is discussed as a therapeutic strategy.

Update to: The Aryl Hydrocarbon Receptor in Anticancer Drug Discovery: Friend or Foe?

Binding of ligands such as polycyclic aromatic hydrocarbons to the Aryl hydrocarbon Receptor (AhR) and the sequence of events leading to induction of xenobiotic-metabolising enzymes such as the cytochrome P450 isoform 1A1 and subsequent generation of DNA adducts is historically associated with the process of chemical carcinogenesis. Cancer chemopreventative agents, on the other hand, often exert their biological effect at least in part through antagonism of AhR-induced carcinogenesis. A third scenario associated with AhR binding could occur if the induction of xenobiotic enzymes and subsequent DNA damage causes apoptosis. If this occurs selectively in tumour cells whilst sparing normal tissue, the AhR ligand would have a therapeutic cytotoxic effect. In this review we survey for the first time the major classes of reported AhR ligands and discuss the biological consequences of AhR binding in each case. The use of AhR ligands as cancer chemotherapeutic agents, as illustrated by the case of the 2-(4-aminophenyl)benzothiazole prodrug Phortress, is discussed as a therapeutic strategy.