Synthesis and evaluation of .alpha.-[[(2-haloethyl)amino]methyl]-2-nitro-1H-imidazole-1-ethanols as prodrugs of .alpha.-[(1-aziridinyl)methyl]-2-nitro-1H-imidazole-1-ethanol (RSU-1069) and its analogs which are radiosensitizers and bioreductively activated cytotoxins

Abstract

alpha-[(1-Aziridinyl)methyl]-2-nitro-1H-imidazole-1-ethanols, of general formula ImCH2CH(OH)CH2NCR1R2CR3R4, where Im = 2-nitroimidazole and R1, R2, R3, R4 = H, Me, are radiosensitizers and selective bioreductively activated cytotoxins toward hypoxic tumor cells in vitro and in vivo. Treatment of the aziridines with hydrogen halide in acetone or aqueous acetone gave the corresponding 2-haloethylamines of general formula ImCH2CH(OH)CH2(+)-NH2CR1R2CR3R4X X-, where R1, R2, R3, R4 = H, Me, and X = F, Cl, Br, I. These 2-haloethylamines were evaluated as prodrugs of the parent aziridines. The rates of ring closure in aqueous solution at pH approximately 6 were found to increase with increasing methyl substitution and to depend on the nature of the leaving group (I approximately Br greater than Cl much greater than F). A competing reaction of ImCH2CH(OH)CH2+NH2CH2CH2X X- (X = Cl, Br) with aqueous HCO3- ions gives 3-[2-hyroxy-3-(2-nitro-1H-imidazol-1-yl)propyl]-2-oxazolidinone. The activities of these prodrugs as radiosensitizers or as bioreductively activated cytotoxins were consistent with the proportion converted to the parent aziridine during the course of the experiment. alpha-[[(2-Bromoethyl)amino]methyl]-2-nitro-1H-imidazole-1- ethanol (RB 6145, 10), the prodrug of alpha-[(1-aziridinyl)methyl]-2-nitro-1H-imidazole-1-ethanol (RSU-1069, 3), is identified as the most useful compound in terms of biological activity and rate of ring closure under physiological conditions.

Oxygen-sensitive enzyme-prodrug gene therapy for the eradication of radiation-resistant solid tumours

“…GFP/R9 and GFP5 cells grown as xenografts were allowed to reach exponential growth (Figure 3, tumour doubling time of approximately 4 days) before combined treatment with radiotherapy and RB6145 -a chemical precursor of RSU1069 that exhibits reduced systemic toxicity in vivo. 22 This allowed us to evaluate the ability of our therapeutic approach to potentiate the radioresponse of an established tumour with a clinically relevant hypoxic fraction. 24 Consistent with our in vitro data, hypoxia-regulated P450R expression proved a successful approach to enhance bioreductive hypoxic cell cytotoxicity and hence improve the efficacy of radiotherapy.…”

“…For these studies, the prodrug RB6145 (1-[3-(2-bromoethylamino)-2-hydroxypropyl]-2-nitroimidazole) was used, which when hydrolysed in vivo releases RSU1069. 22 As highlighted above, RSU1069 shows preferential toxicity towards hypoxic cells. Radiotherapy was chosen to control the oxic cell population within the tumours.…”

“…22 NADPH:cytochrome P450 reductase activity determination NADPH:cytochrome P450 reductase activity was determined spectrophotometrically as the NADPH dependent reduction of cytochrome c. The reaction comprised 400 l of cytochrome c (final concentration 50 M), 100 l of 10 mM potassium cyanide (final concentration 1 mM), 10-300 g protein lysate (10-100 l volume) and 100 mM phosphate buffer pH 7.6, to a total volume of 0.98 ml. The reaction was equilibrated to 37°C and was initiated by addition of 20 l of 10 mM NADPH to the test cuvette (final concentration 200 M) and the rate of reduction of cytochrome c was monitored at 550 nm for 3 min against a blank without NADPH.…”

Enhancement of bioreductive drug toxicity in murine tumours by inhibition of the activity of nitric oxide synthase

“…RB6145 is completely converted to its active product RSU1069 within minutes of administration in vivo (Jenkins et al, 1990), and the plasma half-life of RSU1069 is about 20 min in C3H mice (Walton and Workman, 1988;Binger and Workman, 1990). It is thought that RSU1069 would reach its maximum tumour concentration within 15 min of administration, hence any interference with tumour blood supply thereafter would not only create hypoxia but also prevent efflux of the drug from the tumour, thereby enhancing tumour toxicity (Bremner et al, 1990).…”

“…More recently, a prodrug of RSU1069 has been prepared and licensed to the pharmaceutical industry for clinical development. This drug, RB6145 (Jenkins et al, 1990), has also been shown to have considerable anti-tumour activity when combined with treatments that increase the level of tumour hypoxia (Bremner 1993;Stratford et al, 1994). Thus, in view of the findings on induction of hypoxia in murine tumours after the inhibition of NOS activity, experiments were carried out in which RB6145 was combined with nitro-L-arginine (Wood et al, 1994a).…”