35446-36-7

Isothiocyanates (ITCs) found in cruciferous vegetables have demonstrated cancer preventive activity in animals, and increased dietary intake of ITCs has been shown to be associated with a reduced cancer risk in humans. ITCs exert their cancer chemopreventive action by multiple mechanisms, for example, by modulating the activities of phase I and phase II drug metabolism enzymes, by inhibiting the cell cycle and histone deacetylase, and by causing apoptotic cell death. In cells, protein adducts account for most of total cellular ITC uptake at 4 h after treatment. The time course of this protein binding correlates well with the inhibition of proliferation and the induction of apoptosis. Animal studies have shown that glutathione conjugates are the major products of ITCs. The major urinary excretion products of ITCs in human are N-acetyl cysteine conjugates. Urinary metabolites might provide the exposure history of the last 24 h, if the urine of the full next day is collected. However, this is not feasible in large epidemiological studies. Furthermore, the mercapturic acids of ITC are not stable. Therefore, stable biomarkers are needed that reflect a larger time span of the ITC exposure history. We developed a method to determine stable (not cysteine adducts) reaction products of ITCs with albumin and hemoglobin in humans and mice. We reacted albumin with the ITCs: benzyl isothiocyanate (BITC), phenylethyl isothiocyanate (PEITC), sulforaphane (SFN), and allyl isothiocyanate (AITC). After enzymatic digestion, we found one major product with lysine using LC-MS/MS. The identity of the adducts was confirmed by comparing the analyses with synthetic standards: N6-[(benzylamino) carbonothioyl]lysine (BITC-Lys), N6-{[(2-phenylethyl)amino] carbonothioyl}lysine (PEITC-Lys), N6-({[3-(methylsulfinyl)propyl] amino}carbonothioyl)lysine (SFN-Lys), and N6-[(allylamino] carbonothioyl]lysine (AITC-Lys). The adduct levels were quantified by isotope dilution mass spectrometry using the corresponding new ITC-[13C 615N2]lysines as internal standards. The applicability of the method was tested for biological samples obtained from different experiments. In humans consuming garden cress, watercress, and broccoli and/or in mice exposed chronically to N-acetyl-S-{[(2-phenylethyl) amino]carbonothioyl}-l-cysteine, albumin and hemoglobin adducts were found. BITC-Lys, PEITC-Lys, and SFN-Lys released after enzymatic digestion of the proteins were quantified with LC-MS/MS. This new method will enable quantification of ITC adducts in blood proteins from large prospective studies about diet and cancer. Protein adducts are involved in the chemopreventive effects of ITCs. Therefore, blood protein adducts are a potential surrogate marker for the effects of ITCs at the cellular level. This new technique will improve the assessment of ITC exposure and the power of studies on the relationship between ITC intake and cancer.

Mercapturic acid pathway metabolites of phenylethyl isothiocyanate inhibited the growth of human leukaemia 60 (HL60) cells in vitro. The adduct with L-cysteine, S-(N-phenylethylthiocarbamoyl)cysteine, was the most potent with strong antileukaemic activity: the median growth inhibitory concentration (GC50) value was 336 ± 1 nM (N = 18) compared with GC50 values of the precursor formed from dietary glucosinolates, phenylethyl isothiocyanate, 1.49 ± 0.01 μM (N = 8), and the initial mercapturic acid pathway metabolite S-(N-phenylethylthiocarbamoyl)glutathione 5.46 ± 0.36 μM (N = 18). S-(N-Benzylthiocarbamoyl)cysteine and S-(N-phenylpropylthiocarbamoyl)cysteine also had antiproliferative activity but S-(N-phenylethylthiocarbamoyl)cysteine was the most potent compound studied. The latter induced DNA fragmentation in HL60 cells but DNA laddering characteristic of apoptosis was not observed. It had low toxicity to corresponding differentiated cells, neutrophils, in culture, and therefore the cytotoxicity had selectivity for leukaemia cells. The antiproliferative activity of S-(N-phenylethylthiocarbamoyl)cysteine was lost during preincubation with culture medium, attributed to S-thiocarbamoyl transfer to serum proteins, which may decrease its effectiveness in vivo. The antiproliferative activity of S-(N-phenylalkylthiocarbamoyl)cysteine derivatives, by inhibiting tumour growth in pre-clinical development, may contribute to the association of decreased cancer incidence with dietary glucosinolate consumption.

The develop analogues of phenylalkyl isothiocyanate with less toxicity and better biological activity, two water-soluble phenylalkyl isothiocyanate-cysteine conjugates, S--L-cysteine (1) and S--L