The differential cytotoxicity of RSU 1069: Cell survival studies indicating interaction with DNA as a possible mode of action

Abstract

Summary The hypoxic cell radiosensitizer RSU 1069 (1-(2-nitro-1-imidazolyl)-3-(1-aziridinyl)-2-propanol) shows, on a concentration basis, a 100-fold greater toxicity towards hypoxic relative to aerobic cells. This toxicity is substantially greater than that of misonidazole, a compound of similar electron affinity. Reductive processes are important for hypoxic toxicity; this is demonstrated by the fact that misonidazole, in excess, can protect against the hypoxic but not aerobic toxicity of RSU 1069. The importance of the interaction of RSU 1069 with DNA, suggested initially by molecular studies, is supported by the fact that cells containing 5-bromodeoxyuridine (5-BUdR) incorporated into their DNA show greater sensitivity towards the lethal effects of RSU 1069 both in air and nitrogen, compared to cells not treated with 5-BUdR. Experiments with RSU 1069 and 3-aminobenzamide (3-AB) show the latter compound to potentiate aerobic toxicity, consistent with monofunctional alkylation by RSU 1069. In contrast, 3-AB has no effect on the hypoxic cytotoxicity of RSU 1069, which would be predicted if RSU 1069 is functioning as a bifunctional agent under these conditions. It is our contention that in air, RSU 1069 functions as a typical monofunctional alkylating agent, presumably due to the presence of the aziridine group whereas, in hypoxia, reduction of the nitro group provides an additional alkylating species, converting the compound into a bifunctional agent.The compound RSU 1069 (NSC 347503), 1-(2-nitro-l-imidazolyl)-3-(1-aziridinyl)-2-propanol, has a similar one-electron reduction potential to that of misonidazole (Adams, 1984a). However it is both much more efficient as a radiosensitizer and considerably more cytotoxic than misonidazole both in vitro and in vivo (Adams et al., 1984a,b). Structurally RSU 1069 differs from misonidazole in that an aziridine replaces the methoxy group in the NI side-chain of the 2-nitroimidazole. Aziridines are monofunctional alkylating agents which can react with cellular macromolecules such as DNA (see e.g. Ross, 1962) and it has been recently shown that RSU 1069 and a product(s) of its reduction can bind to calf thymus DNA and cause single strand breaks in plasmid DNA (Silver et al., 1985).The aim of the present work was to characterize the cytotoxic effect of RSU 1069 at the cellular level. Results will be discussed with regard to those obtained in molecular studies (Silver et al., 1985;Silver & O'Neill, 1986)

Potentiation of the anti-tumour effect of melphalan by the vasoactive agent, hydralazine

“…Reduced blood flow in tumours can cause lowering of the oxygen status of the tumour, thereby causing radiation resistance (Kruuv et al, 1967). This so-called 'stealing' effect has recently been exploited by Chaplin and Acker (1987) in order to increase the anti-tumour effect of the bio-reductive agent, RSU 1069 (Adams et al, 1984) a compound which is activated under hypoxic conditions to give a species 100 x more toxic than the parent compound (Stratford et al, 1986).…”

“…Reduced blood flow in tumours can cause lowering of the oxygen status of the tumour, thereby causing radiation resistance (Kruuv et al, 1967). This so-called 'stealing' effect has recently been exploited by Chaplin and Acker (1987) in order to increase the anti-tumour effect of the bio-reductive agent, RSU 1069 (Adams et al, 1984) a compound which is activated under hypoxic conditions to give a species 100 x more toxic than the parent compound (Stratford et al, 1986).Reduction of blood flow in tumours may also be potentially useful for enhancing the effects of some anti-cancer drugs. The rationale for this is that, administration of the vaso-active drug at the time at which the chemotherapeutic agent has reached its maximum tumour concentration, will inhibit loss of active drug from the tumour.…”

Distribution of pimonidazole and RSU 1069 in tumour and normal tissues

“…It is thought to act as a monofunctional alkylating agent (aziridine group) under normoxic conditions and under hypoxic conditions as a bifunctional alkylating group through the aziridine group and the reduced nitro group (Stratford et al, 1986). As a hypoxic-cell cytotoxin it is 100 times more toxic to anoxic than to normoxic cells in vitro (Ahmed et al, 1986;Stratford et al, 1986). In a limited clinical trial it has been found to cause the side-effects of nausea and vomiting (Horwich et al, 1986).…”

“…RSU 1069 (1-(2-nitro-1-imidazolyl)-3-(1-aziridinyl)-2-propanol) is a MISO analogue in which an aziridine group replaces the methoxy group in the Nl sidechain (Adams et al, 1984). It is thought to act as a monofunctional alkylating agent (aziridine group) under normoxic conditions and under hypoxic conditions as a bifunctional alkylating group through the aziridine group and the reduced nitro group (Stratford et al, 1986). As a hypoxic-cell cytotoxin it is 100 times more toxic to anoxic than to normoxic cells in vitro (Ahmed et al, 1986;Stratford et al, 1986).…”

Potentiation of RSU-1069 tumour cytotoxicity by 5-hydroxytryptamine (5-HT)

“…This compound has been shown to be selectively toxic to hypoxic cells both in vitro (Stratford et al, 1986a) and in vivo (Chaplin et al, 1986).…”