Chairman's summary of session D

Antitumor properties of phenothiazines

The purpose of this review is to describe and evaluate the antineoplastic properties of phenothiazines. The present research studies suggest that many phenothiazines do not show significant antitumor and antineoplastic activity and so they cannot be used as potential drugs to treat cancer. However some of their derivatives help indirectly in decreasing cytotoxic effects caused by radiation and other chemical carcinogens. Additionally, some phenothiazine derivatives provide protection against cancers caused by metabolic activation of carcinogens such as dimethylbenzanthracene. The selective accumulation of phenothiazine derivatives in certain tissues such as brain and melanoma tumors may provide an effective treatment of such tumors. Current studies suggest that highly potent phenothiazine derivatives can be made by nitro substitution on the aromatic ring of phenothiazines.

The nitroreductase enzyme in Walker cells that activates 5-(aziridin-1-yl)-2,4-dinitrobenzamide (CB 1954) to 5-(aziridin-1-yl)-4-hydroxylamino-2-nitrobenzamide is a form of NAD(P)H dehydrogenase (quinone) (EC 1.6.99.2)

A nitroreductase enzyme has been isolated from Walker 256 rat carcinoma cells which can convert 5-(aziridin-1-yl)-2,4-dinitrobenzamide (CB 1954) to a cytotoxic DNA interstrand crosslinking agent by reduction of its 4-nitro group to the corresponding hydroxylamino species (Roberts JJ et al., Biochem Biophys Res Commun 140: 1073-1078, 1986; Knox RJ et al., Biochem Pharmacol 37: 4661-4669, 1988). The enzyme has now been identified as a form of NAD(P)H dehydrogenase (quinone) (DT diaphorase, menadione reductase (NMOR), phylloquinone reductase, quinone reductase, EC 1.6.99.2) by comparison of partial protein sequences, coenzymes, substrate and inhibitor specificities, and spectroscopic data. 2-Phenyl-5(4)-aminoimidazole-4(5)-carboxamide and 5(4)-aminoimidazole-4(5)-carboxamide were shown to be inhibitors of the isolated Walker cell enzyme. This observation could explain the reported antagonistic action of the aminoimidazole carboxamides to the antitumour effects of CB 1954.