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Recent analytical developments in the determination of adducts of DNA and protein with alkylating carcinogens are described which have considerably extended the number of carcinogens that can be examined. While sensitivity of detection equal to or better than one modified DNA base per 10(8) normal bases is now achievable for many specific alkylating carcinogens, further developments in the analytical methods are still needed for the identification and quantification of adducts derived from unknown and/or mixed exposures to carcinogens.

Tandem mass spectrometric techniques have been developed for analysing alkylated purines in human urine. These techniques are designed with the objectives firstly of identifying unknown adducts derived from mixed exposures to alkylating agents, and secondly of quantifying these adducts with minimal sample purification.

Using very specific antibodies in sensitive radioimmunoassays for O6-methyldeoxyguanosine (O6-medGuo), we have been able to detect the presence of this modification in human DNA. Since O6-medGuo is not likely to be a normal component of DNA, its presence must be due to exposure to environmental alkylating agents. We have studied two groups of samples, one of which appears to have received exposure to an alkylating agent (so that most members of the group show a detectable level of O6-medGuo). Although some individuals in the other group showed detectable O6-medGuo (sometimes at levels exceeding those observed in the first group), the majority showed undetectable levels. This may indicate that they had much less exposure to environmental alkylating agents than the first group and that some individuals may have received additional exposures due to other factors such as life style or drugs that they may have been given.

Measurement of reaction products of alkylating agents with proteins has been used as a monitor of in-vivo exposure to over 30 such compounds. Doses in animals exposed to directly-acting alkylating agents (e.g., methyl methanesulfonate, ethylene oxide) are directly related to the production of alkylated amino acids in haemoglobin. The erythrocyte dose of alkylating agent, calculated from the extent of haemoglobin alkylation, is in some cases related to liver and extra-hepatic DNA doses; thus, detection of alkylation of haemoglobin may be taken as an indication of a reaction at the carcinogenic target site.

Several recent and major advances in the technology of mass spectrometry (MS) have greatly promoted use of this technique for the study of the interaction of alkylating agents with biomolecules. MS, in combination with gas chromatography (GC), may now be used to quantify adducts of carcinogens with proteins at levels down to 20 pmol/g protein. Soft ionization techniques have proved invaluable in determining the structure of carcinogen adducts with both DNA and proteins, and the newly developed tandem MS promises to be of considerable use in the characterization of complex carcinogen adduct mixtures.

Transplacental carcinogenesis by N-ethyl-N-nitrosourea (ENU) was studied in patas (Erythrocebus patas) and rhesus (Macaca mulatta) monkeys. Repeated intravenous injections throughout pregnancy caused gestational choriocarcinoma in female patas monkeys and a variety of non-trophoblastic neoplasms in their offspring. Latent periods for transplacentally induced tumours varied from less than one month to more than ten years. One case of congenital neoplasm was observed. Certain kinds of neoplasms were observed only in transplacentally exposed offspring, and not in monkeys given the same carcinogen during juvenile or adult life. These included a variety of embryonal tumours, especially nephroblastoma, and tumours of the brain, mostly gliomas. Schwannomas of the peripheral nervous system were not observed in patas monkeys given ENU. One embryonal tumour yielded DNA that transformed NIH 3T3 cells in a transfection assay. Similar protocols performed in rhesus monkeys also yielded a variety of tumours in the offspring, including brain tumours; but in this species the most common embryonal tumour was pulmonary blastoma, and no choriocarcinoma was seen in adult females given ENU during pregnancy. Administration of N-nitrosodiethylamine to patas monkeys during pregnancy at first appeared to have had no effect on the offspring, but administration of phenobarbital beginning at four years of age resulted in the rapid appearance of multiple hepatocellular tumours. With regard to potential human risk from prenatal exposure to carcinogens, three conclusions deserve special emphasis: (1) the extreme susceptibility of the fetal primate central nervous system to certain chemical carcinogens, which confirms and reinforces what was previously known from studies in rodents; (2) the prolonged latency of transplacentally initiated epithelial tumours and the importance of subsequent postnatal exposure to promoting agents; and (3) the concurrent risk of transplacental chemical carcinogenesis in offspring and gestational choriocarcinoma in the mother, suggesting that gestational choriocarcinoma with its short latent period may serve as an epidemiologically exploitable marker for human populations in which transplacental carcinogenesis is likely.