The available literature, which has been considered in this review, suggests that urinary DNA adducts can provide valuable information about human exposure to alkylating carcinogens. In many cases the received dose of carcinogen may be unknown, particularly if a major part of it is derived from endogenous processes [as may well be the case for some N-nitroso compounds (117)]. As a result of efficient DNA repair or chemical depurination of the major adducts of many alkylating carcinogens, alkylated nucleic acid bases are excreted in urine. These urinary adducts either are largely derived from modifications in the target organ (for example, liver in the case of AFB1) or represent an integrated measure of whole-body dose for carcinogens which show organotropism but which form adducts in many, if not all, tissues (for example, ethylene oxide; 118). The extensive studies which have been carried out on AFB1 exposure in both experimental animals and humans provide a valuable paradigm for the exploitation of urinary DNA adducts in the rapidly developing field of molecular epidemiology. Early experimental studies demonstrated a clear relationship between the excretion of AFB1-Gua and adduct formation in the liver. A convincing demonstration of the link between urinary adduct excretion and biological outcome (both preneoplastic liver foci and hepatocellular carcinoma) in experimental animals was made using inhibitors of carcinogenesis based on the dithiolethiones. Not only was excretion of AFB1-Gua a good predictor of carcinogenic outcome following AFB1 treatment but its reduction in inhibition experiments predicted the consequent reduction in tumors. This overall approach was borne out in a case-control study in China in which elevated levels of urinary AFB1-Gua (as well as some other AFB1 metabolites) were associated with risk of developing hepatocellular carcinoma (116). A logical extension of these studies is to study the excretion of AFB1-Gua in subjects taking part in intervention studies aimed at either reducing exposure to AFB1 or evaluating the effectiveness of chemopreventive agents. This illustrates the use of urinary adducts as short-term indicators of carcinogenic risk. The situation for many carcinogens is not so clear-cut as that for AFB1, but the field of excreted DNA adducts is still relatively unexplored and gives every indication of developing rapidly. The suspected human leukemogen, ethylene oxide (EO), has been extensively studied and the dosimetric and biomonitoring approach based on its protein adducts by Ehrenberg and co-workers (8) is widely used (119). However, experimental studies (118) indicated that EO formed DNA adducts and that these adducts were repaired, to more or less the same extent, in all tissues. Recent results by Skopek et al. (120) on the mutational spectra induced by EO in exon 3 of the hprt locus in rats indicate that apurinic sites induced by depurination of 7-HOEtGua and 3-HOEtAde may be important mutagenic lesions and suggested that excreted hydroxyethylated bases may be a direct measure of this phenomenon. The use of alkylating chemotherapeutic drugs offers a unique opportunity to study human exposure to welldefined doses of known or suspected carcinogens. As survival of cancer patients increases, the risk of developing iatrogenic second cancers also increases, and it would be of great benefit to reduce this risk while maintaining or even improving therapeutic efficacy. In view of the ease of obtaining blood samples from cancer patients there have been a number of studies on leucocyte DNA adducts arising from alkylating agent use (most notably with cis-platin; 7). Nonetheless, the studies which have been carried out using urinary adducts suggest that this technique can provide interesting results by noninvasive means. For example, the use of MNU in chemotherapy demonstrated that urinary 3-MeAde was a good indicator of internal dose (see above) and showed that there was wide interindividual variation in adduct formation for the same dose, which may have consequences not only for clinical response but also for the risk of second cancer in patients who survive long enough after treatment. Urine collection from healthy, nonhospitalized subjects is easy, and it is relatively safe to handle compared to other biological fluids. The apparent lack of cultural barriers to its collection makes it amenable, in practical terms, to ready incorporation into molecular epidemiological studies. The resistance to the use of urinary markers is most often related to their intrinsic lack of long-term exposure information compared to, for example, blood protein adducts or stable DNA adducts. However, with an increasing tendency toward the conduct of prospective epidemiological studies with the aim of detecting previously unknown risk factors, this apparent shortcoming of urinary DNA adducts is less relevant. In addition, the analytical potential of currently available techniques for the identification and quantification of urinary DNA adducts provides a powerful tool for the molecular epidemiologist to evaluate human exposure to carcinogenic alkylating agent exposures. © 1992, American Chemical Society. All rights reserved.
