Metabolic oxidation phenotypes as markers for susceptibility to lung cancer

Genetic basis for a lower prevalence of deficient CYP2D6 oxidative drug metabolism phenotypes in black Americans

Debrisoquin hydroxylase (CYP2D6) is a cytochrome P450 enzyme that catalyzes the metabolism of > 30 commonly prescribed medications. Deficiency in CYP2D6 activity, inherited as an autosomal recessive trait, was found to be significantly less common in American blacks (1.9%) than whites (7.7%). To determine the genetic basis for this difference, inactivating CYP2D6 mutations were assessed by allele-specific PCR amplification and RFLP analyses of genomic DNA from 126 unrelated whites and 127 unrelated blacks. Blacks had a twofold lower frequency (8.5 versus 23%, P = 0.001) of the CYP2D6(B) mutation (point mutation at intron 3/exon 4 splice site), while complete deletion of the CYP2D6 gene (5.5% blacks, 2.4% whites), and the CYP2D6(A) mutation (single nucleotide deletion in exon 5; 0.24% blacks, 1.4% whites) were not different between the two groups. The prevalence of heterozygous genotypes was significantly lower in blacks (25 versus 42% of extensive metabolizers, P = 0.009), consistent with the observed prevalence of the deficient trait in blacks and whites. We conclude that the same CYP2D6 mutations lead to a loss of functional expression in blacks and whites, but American blacks have a lower prevalence of the deficient trait due to a lower frequency of the CYP2D6(B) mutation. This could explain racial differences in drug effects and disease risk.

Recent developments in the epidemiology of lung cancer

Lung cancer is currently the leading cause of cancer death in the United States and also the most common tumor worldwide. Changes in the distribution of histologic types over the past two decades in the United States, as well as high rates of lung cancer in certain subpopulations, require explanation. While cigarette smoking and specific occupational exposures are firmly established as important risk factors for lung cancer, recent work provides evidence that other factors may play a role either as independent risk factors or as modifiers of the effect of smoking. This paper reviews the epidemiology of lung cancer, with an emphasis on developments in the past decade.

The etiology of lung cancer

The association between tobacco smoking and lung cancer has been noted for more than 50 years and continues to dominate the etiologic milieu of this malignant disease. Other agents, many discovered in the occupational setting, have also been substantiated as lung carcinogens. Inherent predisposition to the disease has long been suspected, and recent investigations suggest several potential mechanisms and a possible mode of inheritance. Considerable progress has been made in deciphiring the molecular defects present in lung cancer cells. These recent findings have been incorporated into two well-known models of lung carcinogenesis. As the details of the carcinogenic process are unraveled, one goal is to identify intermediate (preneoplastic) markers of exposure and inherent predisposition that will help assess the risk of lung cancer for individuals as well as for groups.

Metabolic polymorphisms

Polymorphisms have been detected in a variety of xenobiotic-metabolizing enzymes at both the phenotypic and genotypic level. In the case of four enzymes, the cytochrome P450 CYP2D6, glutathione S-transferase mu, N-acetyltransferase 2 and serum cholinesterase, the majority of mutations which give rise to a defective phenotype have now been identified. Another group of enzymes show definite polymorphism at the phenotypic level but the exact genetic mechanisms responsible are not yet clear. These enzymes include the cytochromes P450 CYP1A1, CYP1A2 and a CYP2C form which metabolizes mephenytoin, a flavin-linked monooxygenase (fish-odour syndrome), paraoxonase, UDP-glucuronosyltransferase (Gilbert's syndrome) and thiopurine S-methyltransferase. In the case of a further group of enzymes, there is some evidence for polymorphism at either the phenotypic or genotypic level but this has not been unambiguously demonstrated. Examples of this class include the cytochrome P450 enzymes CYP2A6, CYP2E1, CYP2C9 and CYP3A4, xanthine oxidase, an S-oxidase which metabolizes carbocysteine, epoxide hydrolase, two forms of sulphotransferase and several methyltransferases. The nature of all these polymorphisms and possible polymorphisms is discussed in detail, with particular reference to the effects of this variation on drug metabolism and susceptibility to chemically-induced diseases.

The CYP1A1 gene and cancer susceptibility

A close correlation between cigarette smoking associated lung cancer incidence and an Msp I restriction fragment length polymorphism (RFLP) of the human P-450 1A1 (CYP1A1) gene was found in a Japanese population in terms of genotype frequency and cigarette dose. A Val/Ile codon difference in the primary structure of the CYP1A1 protein (Val-, Ile-type) was in linkage disequilibrium with the Msp I RFLP. A synergistic increase in susceptibility to lung cancer was found when combining genotyping of CYP1A1 and the Mu-class of glutathione S-transferase (GST1). Interindividual variability in the genetic make-up of carcinogen metabolizing enzymes may thus be a key host factor to explain the differences in susceptibility to chemical carcinogenesis among individuals.

Cancer risk related to genetic polymorphisms in carcinogen metabolism and DNA repair

Chemical carcinogenesis involves metabolism in the body of the carcinogen to the ultimate carcinogen and its interaction with DNA. There is considerable interindividual variation in the metabolic ability to activate as well as detoxify the carcinogens and in the ability to repair the carcinogen-DNA adducts. In many cases such differences occur as genetic polymorphisms and form the basis for variation in susceptibility to carcinogens and thereby to cancer risk. The activation mechanism is particularly related to the cytochromes P-450 (CYPs), and four of these are known to activate carcinogens: CYP1A1, CYP1A2, CYP2E1, and CYP3A4. Increased cancer risk has been related to polymorphisms in the CYPs and other activating enzymes. The DNA repair mechanisms show considerable complexity, and deficient repair mechanisms in certain human disorders are clearly related to increased cancer risk. Yet, there is no unambiguous epidemiological evidence available for cancer risk among individuals in general. In vivo methods have to be refined and developed for use in epidemiological studies.

The Role of Pharmacogenetics in Cancer Chemotherapy and the Development of Malignancy

The role of pharmacogenetics in the area of cancer chemotherapy and the development of malignancy has not been well defined. Only four chemotherapeutic agents have been evaluated for toxicity or clinical response based on genetic differences in metabolism. These include 5-fluorouracil, 6-mercaptopurine, amonafide, and cyclophosphamide. Severe toxicity of 5-fluorouracil and amonafide due to individual differences in drug metabolism has been reported in the literature. Tumor response in leukemic children may be associated with genetic differences in metabolism of 6-mercaptorpurine. The development of malignancy may be secondary to an individual's ability to detoxify carcinogens found in the environment. For example, the incidence of bladder cancer appears higher in subjects who have occupational exposure to aromatic amines and the slow acetylator phenotype. Some evidence also exists that smokers who are very extensive metabolizers of debrisoquin may be more prone to developing lung cancer. Strong evidence for an association between other cancer types and pharmacogenetics requires more study.

Relationship between personality and debrisoquine hydroxylation capacity. Suggestion of an endogenous neuroactive substrate or product of the cytochrome P4502D6

We administered the Karolinska Scales of Personality to 225 healthy subjects in Spain selected from a group of 925 individuals previously phenotyped with regard to their capacity to hydroxylate debrisoquine. A significant relationship was found between the scores in as many as 4 of the 15 subscales (psychic anxiety, psychasthenia, inhibition of aggression and socialization) and the debrisoquine hydroxylation capacity. Poor metabolizers were more anxiety-prone and less successfully socialized than extensive metabolizers of debrisoquine. This and a previous study among subjects in Sweden suggest that there may be a relationship between personality and the activity of the enzyme hydroxylating debrisoquine (cytochrome P4502D6). This polymorphic enzyme may have an endogenous neuroactive substrate or product, such as a biogenic neurotransmitter amine.

The epidemiology of lung cancer

Lung cancer rates and mortality have risen in epidemic proportions in the United States and other industrialized nations during the 20th century. Case-control and cohort studies performed in the 1950s and 1960s firmly established cigarette smoking as the single greatest risk factor for lung cancer. In the United States, overall lung cancer mortality rates in men and women rose progressively from the 1950s. Fortunately, lung cancer incidence and mortality are now declining in middle-aged men. Smoking has significantly increased lung cancer rates among women and is on the rise in developing countries. Environmental agents found in the home and workplace, including radon and asbestos, have also been shown to increase lung cancer risk in both smokers and nonsmokers. Government regulations have helped curtail quantities of these and other atmospheric carcinogens. Efforts to reduce lung cancer risk must be continued and their scope expanded in order to have a global impact on the incidence and mortality of this fatal malignancy.

Genetically Determined Polymorphisms in Drug Metabolism

Many factors can influence the metabolism and disposition of drugs. Genetically determined differences in an individual's capacity to metabolize drugs are known causes of interindividual and interethnic variabilities in drug disposition and response. In general, a poor metabolizer for a specific metabolic pathway would likely develop adverse effects, and an extensive metabolizer for the same metabolic pathway might have less than optimal response. Although there are different types of polymorphism in drug metabolism, polymorphisms in debrisoquine-type oxidation, S-mephenytoin oxidation, and N-acetylation have been the most extensively studied. This article will present the basic concepts of pharmacogenetics, review the major types of metabolic polymorphisms, outline ways to determine phenotyping and genotyping differences in metabolizing enzyme activities, and discuss how these differences relate to drug metabolism, response, and toxicity. When evaluating drug response and adverse reactions in individual patients, an awareness of genetic differences in metabolic capacities would help contribute to optimization in drug therapy.

The human glutathione S-transferase supergene family, its polymorphism, and its effects on susceptibility to lung cancer

Cytosolic glutathione S-transferases (GSTs) are a supergene family of dimeric enzymes capable of detoxifying a number of carcinogenic electrophiles. Of the numerous components of tobacco smoke, the polycyclic aromatic hydrocarbons appear to be the principal compounds that yield substrates for these enzymes, GSTM1-1 being effective with those PAH derivatives so far studied; however, the gene locus for GSTM1 is polymorphic, containing two well-characterized expressing genes and a null allele. Use of cDNA for GSTM1-1 or appropriate fragments of genomic clones as probes in Southern blots indicated that the null allele is due to the absence of GSTM1. In preliminary experiments, described here, with lung tissue from smokers, levels of 32P-postlabeled nuclease P1-enhanced DNA adducts were inversely correlated with levels of antigen cross-reacting with antibody to GSTM1-1, suggesting that initiation depends on the expression of GSTM1-1. Since similar quantities of DNA adducts and GSTM1-1 activity have been shown to occur in bronchial and peripheral lung, however, the development of malignancy, which is usually in the bronchial region, presumably depends on additional factors that bring about promotion and progression, which are not necessarily affected by GSTM1 expression. Two epidemiological studies have been carried out in which a possible correlation between the absence of GSTM1 and lung cancer incidence is considered. In the first, involving a U.S. population sample, smokers with and without lung cancer were phenotyped, and a highly significant correlation between the absence of GSTM1-1 activity and adenocarcinoma of the lung was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Carcinogen metabolism in human lung tissues and the effect of tobacco smoking: results from a case--control multicenter study on lung cancer patients

Cigarette smoking is the strongest risk factor for lung cancer, but genetically determined variations in the activities of pulmonary enzyme that metabolize tobacco-derived carcinogens may affect individual risk. To investigate whether these enzymes (e.g., CYP1A-related) can serve as markers for carcinogen-DNA damage, lung tissue specimens were taken during surgery from middle-aged men with either lung cancer or non-neoplastic lung disease. Phase I [aryl hydrocarbon hydroxylase (AHH), ethoxycoumarin O-deethylase (ECOD)] and phase II (epoxide hydrolase, UDP-glucuronosyltransferase, glutathione S-transferase) enzyme activities, glutathione and malondialdehyde contents were determined in lung parenchyma and/or bronchial tissues; some samples were also analyzed for DNA adducts, using 32P-postlabeling. The data were then analyzed for the following: a) differences in metabolic profiles between bronchial and parenchymal lung tissue; b) the effect of recent exposure to tobacco smoke on enzyme inducibility and benzo[a]pyrene metabolism; c) differences in enzyme inducibility between lung cancer and non-lung cancer patients; d) the effect of smoking on metabolism of mutagens in vitro; e) pulmonary DNA adduct levels and AHH activity in lung parenchyma of smokers and ex-smokers; f) lipid peroxidation products in lung tissue from lung cancer and non-lung cancer patients, as related to smoking habits and degree of airway obstruction; and g) prognostic value of AHH pulmonary activity in lung cancer patients. The results demonstrate a pronounced effect of tobacco smoke on pulmonary metabolism of xenobiotics and prooxidant state and suggest the existence of a metabolic phenotype at higher risk for tobacco-associated lung cancer.

Mutant genes of cytochrome P-450IID6, glutathione S-transferase class Mu, and arylamine N-acetyltransferase in lung cancer patients

Epidemiological studies suggested a protective effect of certain phenotypes of polymorphic foreign-compound-metabolizing enzymes in some types of cancer. Poor metabolizers (PM) of debrisoquine 4-hydroxylase (cytochrome P-450IID6, CYP2D6) were found to be underrepresented among patients with lung cancer. Recent advances in molecular genetic characterization of CYP2D6, glutathione S-transferase (GST) class Mu, and arylamine N-acetyltransferase enabled genotypical determination of mutant alleles in lung cancer patients. Restriction fragment length polymorphism (RFLP) with a cDNA gene probe of CYP2D6 was analyzed in 79 lung cancer patients who were phenotyped with debrisoquine. Mutant alleles were detected by allele-specific polymerase chain reaction (PCR). In the same individuals, genotype of GST class Mu was analyzed by PCR and correlated with ex vivo activity of glutathione conjugation towards trans-stilbene oxide. RFLP patterns allowed discrimination between the slow and fast genotype of N-acetyltransferase as well as the heterozygotes. Three phenotypical PMs of debrisoquine (3.8%) were confirmed by PCR and RFLP. No PM could be unambiguously recognized only by RFLP patterns. The PMs were characterized by PCR and RFLP as carriers of the 29B/29B (n = 1), 29A/29B (n = 1), and 29A/44 (n = 1) mutant alleles. Higher debrisoquine hydroxylase activities were found in the homozygous EMs, who possess two active genes, as compared to heterozygous EMs, who have only one active gene. The patients with phenotypically impaired GST Mu activity were confirmed as such by PCR. A complete correspondence between phenotyping of N-acetyltransferase (with caffeine) and genotyping was found. The new genetic techniques proved to be powerful tools for molecular-epidemiological studies aimed at establishing host factors of cancer susceptibility.

Mutagenic activation of aflatoxin B1 by pulmonary, renal, and hepatic cytochrome P450s from rats

The genotoxic and mutagenic activation of aflatoxin B1 (AFB1) by hepatic, renal, and pulmonary microsomes and purified cytochrome P450s was investigated in Salmonella typhimurium TA1535/pSK1002 cells in which an umu response shows DNA damage. The activity of the hepatic microsomes was greatest. Pulmonary microsomes had moderate activity and renal microsomes had low activity. P450 2C11, 2B1, 3A2, 4A2, 4B1, K-2, and K-4 were assayed in a reconstituted system with dilauroylphosphatidylcholine (DLPC). P450 2C11 (a major hepatic cytochrome P450 in male rats) had high activity. P450 2B1 (a major form as well as P450 4B1 in pulmonary microsomes) and K-2 (a minor form in renal microsomes) had moderate activity. P450 4A2 (a major form in renal microsomes), P450 K-4 (a renal form), and P450 4B1 had low activity. P450 3A2 did not have high activity in these conditions but it had high activity toward AFB1 in a modified reconstituted system with a lipid mixture and sodium cholate instead of DLPC only. The activities of other forms were not enhanced by the modification of reconstituted system. Anti-P450 2C11 or 3A2 antibodies inhibited the bioactivation of AFB1 by hepatic microsomes to 50%. These results suggest that the greater ability of hepatic microsomes as compared with pulmonary and renal microsomes to metabolize AFB1 to mutagenic products is a function of the relative proportions of the highly active cytochrome P450s, P450 2C11 and 3A2, in the liver.

Characterization of the cytochrome P-450IID subfamily in bovine liver. Nucleotide sequences and microheterogeneity

To elucidate the molecular mechanisms underlying drug detoxification, the structures of the members of the microsomal cytochrome P-450IID subfamily were analyzed by isolating, mapping and sequencing cytochrome P-450IID (CYP2D) cDNA clones from bovine liver. The screening was performed under nonstringent conditions so that most of the P-450IID subfamily members could be obtained. 114 of the 147 positive clones were classified into four groups on the basis of their restriction-enzyme maps. The maps of the four groups were highly similar, however, the clones of one group contained an insertion of approximately 500 bp in the coding region. Analysis of partial nucleotide sequences of several representative clones from each group showed that the bovine P-450IID subfamily in liver consisted of several, not many, highly similar members, differing by less than 7% in their nucleotide sequences. The location of the insertion found in the minor group corresponded to intron 7 and the GT/AG rule was found at the exon/intron boundary, suggesting that intron 7 was retained in this group. The complete nucleotide sequences of two clones from the major group were examined to determine the structures of the P-450IID subfamily in bovine liver. A full-length cDNA clone (1615 bp) and a partial cDNA clone (1538 bp) contained open reading frames encoding 500 and 487 amino acid residues, respectively. The partial clone lacked the nucleotide sequence corresponding to the first 13 N-terminal amino acid residues. The deduced amino acid sequences of the two clones were 98% similar, and 80% and 68% similar to those from human CYP2D6 and rat CYP2D1, respectively. Comparisons of the amino acid sequences of the P-450IID subfamily members showed the highly conserved C-terminal region of their molecules and the high similarity between the members in one species, especially in cattle and man.

Recombinant DNA approaches for the development of metabolic systems used in in vitro toxicology

In the past few years there has been considerable progress in the development of mammalian cell systems for use in genetic toxicology by the stable transfer of genes/cDNAs coding for drug metabolizing enzymes directly into the target cell. Alternative approaches have also been developed in which mammalian cells are transiently transfected with cDNAs coding for drug-metabolizing enzymes and S9 preparations expressing a single metabolizing enzyme isolated and used for metabolic activation. Progress in these areas is reviewed here and the relative merits of the different approaches are discussed. Work to date has focused primarily on the cytochrome P450 family of enzymes, although other enzyme systems involved in xenobiotic metabolism have been used. The central theme of this review is the transfer of genetic information to improve the metabolic capability of cell systems used in genetic toxicology. However, a basic philosophy of the review is that genetic manipulation of cultured mammalian cells has the potential for developing systems to be used to better understand chemically induced toxicological effects.

A tobacco-specific N-nitrosamine or cigarette smoke condensate causes neoplastic transformation of xenotransplanted human bronchial epithelial cells

Using a xenotransplantation system in which immortalized nontumorigenic human bronchial epithelial cells (BEAS-2B cells) are grown in deepithelialized rat tracheas that are subcutaneously transplanted into athymic nude mice, we exposed BEAS-2B cells either to cigarette smoke condensate or to the tobacco-specific N-nitrosamine 4-(methylnitrosamine)-1-(3-pyridyl)-1- butanone. After 6 mo the carcinogen-exposed BEAS-2B cells were neoplastically transformed to invasive adenocarcinomas. Cell lines obtained from xenografts exposed in vivo to chemicals exhibited several features typical of malignant lung cancer cells, such as increased in vivo invasiveness that correlated well with enhanced type IV collagenolytic activity, resistance to serum-induced growth inhibition, and increased expression of transforming growth factor alpha and its cellular-membrane receptor. Invasiveness, similar to that seen after exposure to phorbol esters, was also detected after in vitro exposure of BEAS-2B cells to cigarette smoke condensate. Collectively, these data indicate that cigarette smoke condensate and N-nitrosamine 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone induce in vivo phenotypic changes in BEAS-2B cells similar to the progressive changes that occur during human lung carcinogenesis.

Debrisoquine metabolism in Chinese patients with Alzheimer's and Parkinson's diseases

We determined the oxidative phenotype and metabolic ratio of debrisoquine in 96 Chinese patients with Alzheimer's disease (n = 12), Parkinson's disease (n = 55), and using patients with stroke and cervical spondylosis as controls (n = 29). We did not find any difference in debrisoquine metabolic phenotype among Parkinson's disease, Alzheimer's disease, and control patients as judged by chi-square analysis. In addition, the metabolic ratio of all our patients was less than 12.6. The result suggested that Chinese patients with Parkinson's disease and Alzheimer's disease metabolize debrisoquine at a velocity not different from that of their Western counterparts even though the frequency distribution of debrisoquine metabolism phenotyping in these two populations is quite different.

High susceptibility to lung cancer analyzed in terms of combined genotypes of P450IA1 and Mu-class glutathione S-transferase genes

Lung cancer is closely associated with cigarette smoking. Aromatic hydrocarbons in smoke, including benzo[a]pyrene, first require metabolic activation by Phase I enzymes, cytochrome P450, to their ultimate forms, and these activated forms are then subjected to detoxification by Phase II enzymes, especially glutathione S-transferases. Thus, genetically determined susceptibility to lung cancer may depend on the metabolic balance between Phase I and Phase II enzymes. In this study, we identified individuals genetically at high risk of lung cancer in terms of polymorphisms of the P450IA1 gene and GST1 gene. The relative risk of individuals with a combination of the genotypes of both a homozygous rare allele of the P450IA1 gene and the nulled GST1 gene was remarkably high at 5.8 for lung cancer and 9.1 for squamous cell carcinoma compared with other combinations of genotypes.

Coefficients of relationship by isonymy among registrations for five common cancers in Scottish males

STUDY OBJECTIVE

The aim was to assess the relative importance of genetic factors in carcinoma of the stomach, colon, rectum, prostate, and bladder in Scottish males.

DESIGN

Cancer cases and controls were compared in terms of the coefficient of relationship by isonymy (Ri).

SETTING

Surname distributions for cancer cases were derived from the Scottish Cancer Register for the years 1959-85. Control distributions were derived from all births, marriages and deaths in Scotland for 1976.

SUBJECTS

Analysis was carried out on a total of 60,933 cancer registrations and 101,836 births, marriages, and deaths over the 12 local government regions of Scotland.

MAIN RESULTS

Comparisons of Ri within and between regions indicated that inherited susceptibility was of greatest importance in carcinoma of the prostate and colon, of intermediate importance in carcinoma of the rectum and stomach, and of minimal importance in carcinoma of the bladder. Familial aggregation of cancers was most pronounced in Highland, Tayside, and Borders Regions. For Highland, this appeared to be the result of region-specific familial influences, while Tayside and the Borders shared genetic factors contributing to cancer aetiology with neighbouring regions in south east Scotland.

CONCLUSIONS

Surname analysis is a simple but useful tool for studying population genetic structure and its relationship to disease incidence.

Cytochrome-P450-dependent hydroxylation in cluster headache

Two isozymes of the cytochrome-P450-dependent drug oxidizing system exhibit polymorphism. Five to 10% of a Caucasian population are deficient in debrisoquine-hydroxylase activity and about 3% in mephenytoin-hydroxylase activity (poor metabolizers). We tested the hypothesis of a possible over-representation of poor metabolizers among patients with cluster headache. The individual metabolic capacity was determined in 30 cluster headache patients after administration of a test dose of 10 mg of debrisoquine and 100 mg of mephenytoin. Two patients (6.7%) were poor metabolizers of debrisoquine and one (3.3%) a poor metabolizer of mephenytoin. This was no different from the rate of poor metabolizers, 7.1% and 3.3% respectively, in a reference panel of healthy Swedish volunteers.

Environmental tobacco smoke: current assessment and future directions

Scientific information on environmental tobacco smoke (ETS) is critically reviewed. Key areas addressed are: differences in chemical composition between mainstream smoke, sidestream smoke, and ETS; techniques for measurement of ETS; epidemiology; in vitro and in vivo toxicology; and chamber and field studies of perceptual or physiological effects. Questions concerning estimation of ETS exposure, suitability of various biomarkers, calculation of lifetime dose, control of confounding variables, use of meta-analysis, and the relationship between ETS concentrations and human responses all emphasize the need for additional research in order to assess potential effects of ETS on health or comfort.

The human hepatic cytochromes P450 involved in drug metabolism

The cytochromes P450 are a superfamily of hemoproteins that catalyze the metabolism of a large number of xenobiotics and endobiotics. The type and amount (i.e., the animal's phenotype) of the P450s expressed by the animal, primarily in the liver, thus determine the metabolic response of the animal to a chemical challenge. A majority of the characterized P450s involved in hepatic drug metabolism have been identified in experimental animals. However, recently at least 12 human drug-metabolizing P450s have been characterized at the molecular and/or enzyme level. The characterization of these P450s has made it possible to "phenotype" microsomal samples with respect to their relative levels of the various P450s and their metabolic capabilities. The purpose of this review is to compare and contrast the human P450s involved in drug metabolism with their related forms in the rat and other experimental species.

Metabolic activation of carcinogens

Most chemical carcinogens are not active in themselves but require bioactivation to electrophiles that bind covalently to DNA and often act by producing mutations. In recent years it has been realized that mutations can be important at many stages of carcinogenesis. A variety of different enzymes are involved in bioactivation reactions, which include oxidation, reduction, thiol conjugation, acetyl transfer, sulfur transfer, methyl transfer, glucuronosyl transfer, and epoxide hydrolysis. These processes often occur in concert with a single carcinogen. Humans vary considerably in activities of these enzymes and this variation may contribute to differences in risk.

P450 and human cancer

Most of the chemical carcinogens in our environment are activated mainly by a restricted number of cytochrome P450 species, P450 1A1, 1A2, 2E1, and 3A. This metabolic activation of procarcinogens is a crucial part of the initial host response to the environmental exposure, since most chemical carcinogens do not show any carcinogenicity by themselves. Inter-individual variability in the metabolic activity may thus be a key host factor to explain the differences in susceptibility to chemical carcinogenesis among individuals. Recent studies on P450s in cancer etiology have provided some valuable insights into this problem.

Mapping genes encoding drug-metabolizing enzymes in recombinant inbred mice

Probes for cytochrome P450IVA (P450IVA), alpha- and pi-class glutathione S-transferases (GST), and phenol-metabolizing UDP-glucuronyltransferase (UDPGT-K39) detected restriction fragment length variants (RFLVs) between C57BL/6J and DBA/2J mice. These variants were used to map the P450IVA genes (Cyp4 alpha) to chromosome 4, close to Mtv-13 and Pmv-19, midway between brown (b) and Gpd-1; GST alpha genes were mapped to chromosome 9, with a cross-hybridizing sequence mapping to another chromosome; the GST pi genes were mapped to the distal end of chromosome 1 near Pmv-21; one UDPGT-K39 variant to chromosome 1, between Acrg and Emv-17, and another showed linkage to Odc-10 on an unidentified chromosome. No RFLVs were detected with probes for P450IID, P450 reductase, androsterone-metabolizing UDPGT, GST mu, or microsomal GST.

N-acetylation and debrisoquine hydroxylation polymorphisms in patients with Gilbert's syndrome

1. N-acetylation and debrisoquine hydroxylation phenotypes were determined in 54 patients with Gilbert's syndrome and in 247 (sulphamethazine) and 76 (debrisoquine) non-related healthy volunteers. 2. Forty (74.1%) of the patients and 135 (54.7%) of the healthy volunteers were slow acetylators (chi 2 = 6.87). In the patients, the cumulative urinary excretion of sulphamethazine up to 6 h (Ae(0,6)) was significantly lower. No differences in the frequency of debrisoquine poor metabolizers were observed: Gilbert's syndrome 5/54 (9.3%), healthy volunteers 5/76 (6.6%). The metabolic ratios were similar in both groups as well as the urinary recoveries of debrisoquine and its 4-hydroxy metabolite. 3. Gilbert's syndrome seems to be related in some way to N-acetylation but not to the debrisoquine hydroxylation polymorphism.

Carcinogen metabolism and DNA adducts in human lung tissues as affected by tobacco smoking or metabolic phenotype: a case-control study on lung cancer patients

Individual variations in activity of pulmonary enzymes that metabolize tobacco-derived carcinogens may affect an individual's cancer risk from cigarette smoking. To investigate whether some of these enzymes (e.g., cytochrome P450IA-related) can serve as markers for carcinogen-induced DNA damage accumulating in the lungs of smokers, non-tumorous lung tissue specimens were taken during surgery from middle-aged men with either lung cancer (n = 54) or non-neoplastic lung disease (n = 20). Phase I (AHH, ECDE) and phase II (EH, UDPGT, GST) enzyme activities, glutathione and malondialdehyde contents were determined in lung parenchyma and/or bronchial tissues; some samples were analyzed for DNA adducts, using 32P-postlabeling. Data analysis of subsets or the whole group of patients yielded the following results. (1) Phase I and II drug-metabolizing enzyme (AHH, EH, UDPGT, GST) activities in histologically normal surgical specimens of lung parenchyma were correlated with the respective enzyme activities in bronchial tissues of the same subject. (2) In lung parenchyma, enzyme (AHH, ECDE, EH, UDPGT) activities were significantly and positively related to each other, implying a similar regulatory control of their expression. (3) Mean activities of pulmonary enzymes (AHH, ECDE) were significantly (2- and 7-fold, respectively) higher in lung cancer patients who had smoked within 30 days before surgery (except GST, which was depressed) than in cancer-free subjects with a similar smoking history. (4) In the cancer patients, the time required for AHH, EH and UDPGT activities to return to the level found in non-smoking subjects was several weeks. (5) Bronchial tree and peripheral lung parenchyma preparations exhibited a poor efficiency in activating promutagens to bacterial mutagens in Salmonella. However, they decreased the mutagenicity of several direct-acting mutagens, an effect which was more pronounced in tissue from recent smokers. GSH concentration and GST activity were positively correlated with mutagen inactivation in the same sample. (6) In recent smokers, AHH activity in lung parenchyma was positively correlated with the level of tobacco smoke-derived DNA adducts. (7) Pulmonary AHH and EH activity had prognostic value in tobacco-related lung cancer patients. (8) An enhanced level of pro-oxidant state in the lungs was associated with recent cigarette smoking. Malondialdehyde level in lung parenchyma was associated with the degree of small airway obstruction, suggesting a common free radical-mediated pathway for both lung cancer induction and small airway obstruction.(ABSTRACT TRUNCATED AT 250 WORDS)

Phenotypic debrisoquine 4-hydroxylase activity among extensive metabolizers is unrelated to genotype as determined by the Xba-I restriction fragment length polymorphism

1. The major pathway for 4-hydroxylation of debrisoquine in man is polymorphic and under genetic control. More than 90% of subjects (extensive metabolizers, EMs) have active debrisoquine 4-hydroxylase (cytochrome P450IID6) while in the remainder (poor metabolizers, PMs), cytochrome P450IID6 activity is greatly impaired. 2. Within the EM group, cytochrome P450IID6-mediated metabolism of a range of substrates varies widely. Some of this intra-phenotype non-uniformity may be explained by the presence of two subsets of subjects with different genotypes (heterozygotes and homozygotes). 3. Cytochrome P450IID6 substrates have not differentiated between these two genotypes. However, a restriction fragment length polymorphism (RFLP) which identifies mutant alleles of cytochrome P450IID6 locus has been described and can definitively assign genotype in some heterozygous EM subjects. 4. In this study, we used RFLP analysis and encainide as a model substrate to determine if non-uniformity in cytochrome P450IID6 activity among EMs is related to genotype. We tested the hypothesis that heterozygotes exhibit intermediate metabolic activity and that homozygous dominants exhibit the highest activity. We proposed encainide as a useful substrate for this purpose since cytochrome P450IID6 catalyzes not only its biotransformation to O-desmethyl encainide (ODE) but also the subsequent metabolism of ODE to 3-methoxy-O-desmethyl encainide (MODE). 5. A single 50 mg oral dose of encainide was administered to 139 normal volunteers and 14 PMs were identified. Urinary ratios among encainide, ODE and MODE in the remaining 125 EM subjects revealed a wide range of cytochrome P450IID6 activity. However, Southern blotting of genomic DNA digested with XbaI identified obligate heterozygotes in both extremes of all ratio distributions.(ABSTRACT TRUNCATED AT 250 WORDS)

Polymorphisms in oxidative drug metabolism: relationship to food preference

To determine whether a correlation exists between polymorphisms of oxidative drug metabolism and dietary preference, 29 poor metabolizers of dextromethorphan, 18 poor metabolizers of mephenytoin and 134 extensive metabolizers of both drugs were screened for their preferences for various food items. Poor metabolizers of dextromethorphan showed a diminished stated preference for cauliflower and coconut, and poor metabolizers of mephenytoin showed a diminished stated preference for spinach and cabbage.

A protocol for the safe administration of debrisoquine in biochemical epidemiologic research protocols for hospitalized patients

The genetically determined ability to metabolize the antihypertensive drug debrisoquine has been proposed as a genetic risk factor for primary carcinomas of the lung. To test this hypothesis, the metabolism of the drug was evaluated in a case control study. The subjects were characterized by their ability to metabolize debrisoquine after receiving a test dose of the drug followed by the collection of an 8-hour urine sample. They were classified by laboratory analysis into one of the following three groups: extensive, intermediate, and poor metabolizers. Poor metabolizers comprise 10% of the population and are unable to hydroxylate the drug. This group was expected to be at highest risk for deleterious effects from this medication. A protocol was created that included patient education and blood pressure monitoring to administer this medication safely to a group of patients with cancer who were already compromised. Although poor metabolizers showed a small decrease in systolic and diastolic blood pressure, no significant hypotensive episodes or clinical sequelae were observed in any of the groups. These data suggest that debrisoquine can be administered safely in a controlled clinical setting and will be useful for the characterization of lung cancer patients in biochemical epidemiology studies.

Oxidative polymorphism of debrisoquine is not related to human colo-rectal cancer

The oxidative polymorphism of debrisoquine (DBQ) has been determined in 89 patients with colo-rectal cancer and in 556 normal control subjects. Four patients and 34 controls, with a metabolic ratio greater than 12.6, were classified as poor metabolisers of DBQ (n.s.). No difference was found in the distribution of the frequencies of the MR of DBQ between patients and controls. It is concluded that polymorphic oxidation of DBQ is not related to the risk of developing colo-rectal cancer in human beings.

Lack of a relationship between the polymorphism of debrisoquine oxidation and lung cancer

1. Determination of debrisoquine oxidation phenotype was carried out in 119 healthy subjects, 135 patients with chronic bronchitis and 153 patients with lung cancer, all of Caucasian origin. 2. A non-Gaussian distribution of the log D/HD ratio was observed in the three groups. 3. Assuming an antimode of 1.12, the proportion of PMs was found to be 6.7% in healthy subjects, 8.9% in chronic bronchitics and 6.5% in patients with lung cancer. These differences were not significant. 4. The presence of a lung tumour itself had no influence on phenotype in a group of 14 patients who were phenotyped before and after surgery. 5. We conclude that a link between debrisoquine phenotype and lung cancer is unlikely.

Role of genetics and drug metabolism in human cancer risk

The research field concerning responses to drugs having a hereditary basis is called 'pharmacogenetics'. At least 5 dozen pharmacogenetic polymorphisms have been described in clinical medicine; many are responsible for marked differences in genetic predisposition toward toxicity or cancer. Three are detailed here: the acetylation, the debrisoquine, and the AH locus polymorphism. All 3 are very common among the United States' population: 1 in 2 is a 'slow acetylator', 1 in 12 is a 'poor metabolizer' for more than 2 dozen commonly prescribed drugs in the debrisoquine panel, and the CYP1A1 and CYP1A2 (cytochromes P(1)450 and P(3)450) genes are highly inducible by cigarette smoke in 1 of 10 patients. Differences in xenobiotic metabolism between individuals in the same family can be greater than 200-fold, suggesting that occupationally hazardous chemicals, as well as prescribed drugs having a narrow therapeutic window, might cause strikingly dissimilar effects between patients of differing genotypes. Our ultimate goal is 'preventive toxicology', i.e. the development of simple, inexpensive, unequivocal and sensitive assays to predict individual risk of toxicity or cancer. These tests could help the individual in choosing a safer life style or place of work and might aid the physician in deciding which drug to prescribe.

Future research directions in cancer ecogenetics

There are many productive directions for future research in cancer ecogenetics. Genetic variation in susceptibility to chemicals and other carcinogenic agents has been neglected in most epidemiologic and rodent investigations of cancer etiology. Genetic variation is important to characterization of risks for population subgroups. Genetic investigations also may enhance inquiries into the underlying mechanisms of carcinogenesis and of cancer prevention. Polymorphisms of cytochrome P450 mono-oxygenases, epoxide hydrolase, glutathione-S-transferases, and N-acetyltransferase offer important windows on biotransformation of pro-carcinogens. Assays in peripheral blood cells need to be related closely to variation in activity in target organs. Tumor suppressor genes, signal transduction pathways, and cell surface receptors are additional sites where genetic variation would be highly important to cancer risks.

Clinical consequences of polymorphic drug oxidation

Many characters are genetically regulated as polymorphisms. This means that discrete groups are seen within the distribution of a certain character. Drug metabolism is no exception and the polymorphism of acetylation is recognised since the 50's. Polymorphic drug oxidation was discovered in the 70's and has been extensively studied. There are two fully established polymorphisms in drug oxidation named as the debrisoquine/sparteine and the s-mephenytoin hydroxylation polymorphisms. The metabolism of a number of important drugs cosegregates with that of debrisoquine. Among these drugs are beta-blockers, antiarrhythmics, tricyclic antidepressants and neuroleptics. Apart from accumulation of parent drug and active metabolite, also reduced formation of active metabolite occur for some drugs in slow metabolisers. There are, however, few cases where the presence of polymorphic drug metabolism is of significant disadvantage. The polymorphisms will add to variability in drug clearance but the potential clinical importance should be evaluated for each drug. The cytochrome P-450 isozyme responsible for debrisoquine hydroxylation is of high affinity-low capacity character, which means that it can be saturated under certain circumstances. This will decrease the difference in drug metabolic rate between rapid and low metabolisers as will inhibitors of the debrisoquine isozyme like cimetidine, quinidine and propafenone. The debrisoquine isozyme is not readily inducible. In cases where a major metabolic route or the formation of an active metabolite are polymorphically controlled, knowledge about a patient's oxidator status might be of practical value for dose adjustments especially if there is a narrow therapeutic ratio or an established concentration-effect relationship. For some drugs it is difficult to differentiate between insufficient therapeutic effect and symptoms of overdosage. Tricyclic antidepressants and neuroleptics meet some of these criteria and patients who get recurrent treatment may benefit if the physician has knowledge about debrisoquine metabolic phenotype. Otherwise, the clinical consequences of polymorphisms in drug oxidation seem so far to be limited, considering that a number of disease conditions have not shown any clear association with oxidation status. The polymorphisms in drug metabolism should be considered as a part of natural variability which could in fact be larger with other drugs that do not show polymorphic elimination.

Is environmental carcinogenesis modulated by host polymorphism?

It is now clear that tobacco smoking, alcohol consumption, dietary factors and occupation can all interact with genetic host factors to place one individual at a greater or lesser risk of a particular cancer than another. Phenotypes which confer significantly elevated risks arise from the human CYP1A1. CYP2D6, GST1 and NAT (N-acetyltransferase) genes. The last is the only one remaining to be cloned. It is envisaged that several of these genes may interact in a given circumstance cooperatively to increase susceptibility. For example CYP1A1, CYP2D6 and GST1 genotypes may have additive or multiplicative risks of bronchogenic carcinoma in cigarette smokers. CYP2D6 and NAT genotypes may interact in bladder cancer. The advent of molecular genetics brings us closer to the day when, for example, factory workers in potentially hazardous environments might be screened using PCR methodology on skin scrapings or buccal swabs for their innate susceptibility to combined workplace and lifestyle cancer risks.

Non-small cell lung cancer. Part I: Biology, diagnosis, and staging

Squamous, large cell, and adenocarcinoma, collectively termed non-small cell lung cancer (NSCLC), are diagnosed in approximately 75% of patients with lung cancer in the United States. The treatment of these three tumor cell types is approached in virtually identical fashion because, in contrast to small cell carcinoma of the lung, NSCLC more frequently presents with localized disease at the time of diagnosis and is thus more often amenable to surgical resection but less frequently responds to chemotherapy and irradiation. Cigarette smoking is etiologically related to the development of NSCLC in the great majority of cases. Genetic mutations in dominant oncogenes such as K-ras, loss of genetic material on chromosomes 3p, 11p, and 17p, and deletions or mutations in tumor suppressor genes such as rb and p53 have been documented in NSCLC tumors and tumor cell lines. NSCLC is diagnosed because of symptoms related to the primary tumor or regional or distant metastases, as an incidental finding on chest radiograph, or rarely because of a paraneoplastic syndrome such as hypercalcemia or hypertrophic pulmonary osteoarthropathy. Screening smokers with periodic chest radiographs and sputum cytologic examination has not been shown to reduce mortality. The diagnosis of NSCLC is usually established by fiberoptic bronchoscopy or percutaneous fine-needle aspiration, by biopsy of a regional or distant metastatic site, or at the time of thoracotomy. Pathologically, NSCLC arises in a setting of bronchial mucosal metaplasia and dysplasia that progressively increase over time. Squamous carcinoma more often presents as a central endobronchial lesion, while large cell and adenocarcinoma have a tendency to arise in the lung periphery and invade the pleura. Once the diagnosis is made, the extent of tumor dissemination is determined. Since most NSCLC patients who survive 5 years or longer have undergone surgical resection of their cancers, the focus of the staging process is to determine whether the patient is a candidate for thoracotomy with curative intent. The dominant prognostic factors in NSCLC are extent of tumor dissemination, ambulatory or performance status, and degree of weight loss. Stages I and II NSCLC, which are confined within the pleural reflection, are managed by surgical resection whenever possible, with approximate 5-year survival of 45% and 25%, respectively. Patients with stage IIIa cancers, in which the primary tumor has extended through the pleura or metastasized to ipsilateral or subcarinal lymph nodes, can occasionally be surgically resected but are often managed with definitive thoracic irradiation and have 5-year survival of approximately 15%.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolism of debrisoquine and susceptibility to breast cancer

There may exist an association between the genetically determined oxidation status of the antihypertensive agent debrisoquine (DEB) and the propensity to develop tumours. The metabolism of DEB is extensive in 90% of healthy subjects (metabolic ratio = MR = 0-12.6; MR = % DEB excreted divided by % 4-hydroxy-DEB excreted) and poor in 10% (MR greater than 12.6). In patients with cancer of the lung, urinary bladder, and gastrointestinum, the percentage of high metabolizers is increased to greater than 98%. The poor metabolizer mode is almost devoid of cancer patients. It was investigated whether breast cancer patients show a similar association with respect to the oxidative status of DEB. 108 breast cancer patients and 123 women with benign gynecologic disorders received 1 tablet of 10 mg DEB orally in the evening. Urine was collected for the subsequent 8 hrs and analysed for its content of DEB and its main urinary metabolite 4-OH-DEB by means of HPLC. No decreased amount of poor metabolizers was seen in the cancer group.

Debrisoquine oxidation in Parkinson's disease

Variations in the activities of xenobiotic metabolizing liver enzymes may be involved in the pathophysiology of diseases, including Parkinson's disease. We therefore studied the activity of the debrisoquine metabolizing enzyme in 97 patients with newly diagnosed Parkinson's disease. The urine debrisoquine metabolic ratios (MR) of the patients were compared with a group of 176 healthy subjects. There were 4 poor metabolizers (4.1%) among the parkinsonians. This proportion did not differ from that found in the group of healthy subjects (51%). In contrast to earlier finding, the parkinsonian poor metabolizers (PM) had the onset of the disease later than the parkinsonian extensive metabolizers (EM). In the parkinsonian patients, it was observed that the excretion of debrisoquine and 4-OH-debrisoquine into urine correlated inversely with the actual age and age at disease onset. Our results indicate that in patients with Parkinson's disease, debrisoquine hydroxylation is comparable with healthy subjects.

Pharmacogenetics and drug metabolism: an Irish perspective

Genetic factors, particularly in relation to control of liver drug metabolism, are a major cause of variability in the response to drugs. In 145 Irish subjects 48% were fast acetylators of sulphadimidine in contrast to 80% in Chinese subjects. Eleven (7.6%) of our Irish population showed an improved ability to oxidise delrisoquine. The therapeutic implications of these findings are discussed.

Drug metabolism in carcinogenesis and cancer chemotherapy

Cytotoxic drugs have become invaluable for the clinical oncologist in the treatment of neoplastic disease. Frequently, these therapeutic agents are used in combination in order to combat the heterogeneity imposed by the variable tumor cell biochemistry of the neoplastic cell population. Hence, one could argue polypharmacy has become the rule rather than the exception in cancer chemotherapy. The use of such regimens obviously increases the potential for drug-drug interactions and also may potentiate the effects of interindividual variation in drug metabolism. Altered expression of drug metabolizing enzymes may also predispose certain individuals to cancer through enhanced metabolic activation and decreased detoxication of environmental, dietary and possibly endogenous procarcinogens. Many anticancer drugs can be considered as prodrugs which require metabolic activation to exert their selective cytotoxic effects. Recent molecular and biochemical advances have increased our understanding of the factors which govern the regulation of drug metabolizing enzymes and have improved our knowledge of the metabolism and action of anticancer agents. The aim of this review is not to exhaustively document all the work in the area of drug metabolism in relation to cancer, but to provide a comprehensive update of some of the recent advances in drug metabolism which have helped to rationalize the mechanism of action of some anticancer drugs and which may help to optimize future patient selection for certain novel chemotherapeutic regimens. This review also discusses some of the more recent breakthroughs in the area of carcinogenesis and highlights directions for future studies.