Expression of intestinal drug-metabolizing enzymes in patients with chronic inflammatory bowel disease

Small molecule drug absorption in inflammatory bowel disease and current implementation in physiologically- based pharmacokinetic models

Inflammatory bowel disease (IBD) is characterized by a chronic inflammation of the intestinal mucosa, with predominant localization in the colon in ulcerative colitis (UC) or affecting the entire length of the gastrointestinal tract in Crohn's disease (CD). Recent advances in the drug development space have been marked by a return to orally administered small molecules with novel mechanisms of action such as Janus kinase inhibitors. Additionally, the prevalence of certain chronic conditions is higher in IBD patients, many of which are treated with orally administered drugs. Given the pathophysiology and localization of IBD, altered drug absorption from the gastrointestinal tract can be expected. This review discusses several physiological differences between the small and large intestine with the potential to influence drug absorption including pathophysiology related alterations associated with IBD. The main physiological parameters which are identified include luminal fluid volume, luminal pH, transit time, bile salt concentration, microbiome, absorptive surface area, permeability and metabolizing enzymes and transporters. Literature regarding these factors in IBD patients is marked with high heterogeneity in reporting of disease severity and location leading to difficulties in interpreting data across different studies. While the influence of most of these factors has been directly assessed in healthy volunteers, this is rarely the case for IBD patients. Furthermore, studies which used PBPK modelling to describe the PK of an orally administered drug in an IBD population and were able to verify their findings using clinical data are critically examined. These models were able to incorporate the pathophysiological changes associated with IBD and partly succeeded in adequately predicting drug absorption in this population. Given the limited amount of PBPK studies performed on a limited number of drugs, the developed models are most likely not suitable to be used as a general PBPK model for the IBD population.

Role of Cytochromes P450 in Intestinal Barrier Function: Possible Involvement in the Pathogenesis of Leaky Gut Syndrome

The intestinal barrier constitutes the largest surface of the human body communicating with the external environment. Alterations affecting elements of intestinal wall may lead to increased intestinal permeability and resulting translocation of bacteria or its components to the bloodstream in the form of the "leaky gut syndrome" (LGS). One of the most common causes of LGS is the disruption of tight junctions (TJ) maintained by tight junction proteins (TJP). LGS and associated alterations in TJP are observed in numerous gastrointestinal (GI) diseases, including inflammatory bowel diseases (IBD) such as Crohn's disease (CD) and ulcerative colitis (UC). Current literature indicates the key role of LGS in many pathological processes, further emphasizing the need for effective pharmacological approaches to treat this syndrome. One of the potential pharmacological targets in LGS treatment are members of the cytochrome P450 (CYP450) superfamily. By affecting intestinal permeability, they may lead to LGS development. It was found that the expression of CYP8B1 synthesizing cholic acid and CYP26 degrading all-trans retinoic acid indirectly influence TJs. CYP2E1 responsible for the metabolism of a wide variety of chemicals, including ethanol, plays a crucial role in the impairment of the intestinal wall. Contrarily, the overexpression of CYP27B1 has a protective effect on the intestinal integrity. CYP1A1, CYP2A6, CYP2J2 and CYP3A were also suggested to influence the GI tract, through their capability to metabolize serotonin, nicotine, endocannabinoids and gemcitabine, respectively. This review summarizes the findings on the role of CYP450 isoforms in intestinal hyperpermeability and their potential involvement in the pathophysiology of LGS.

The Use of Tissue Concentrations of Biological and Small-Molecule Therapies in Clinical Studies of Inflammatory Bowel Diseases

The introduction of biological therapies has revolutionized inflammatory bowel disease (IBD) management. A critical consideration in developing these therapies is ensuring adequate drug concentrations at the site of action. While blood-based biomarkers have shown limited utility in optimizing treatment (except for TNF-alpha inhibitors and thiopurines), tissue drug concentrations may offer valuable insights. In antimicrobial therapies, tissue concentration monitoring is standard practice and could provide a new avenue for understanding the pharmacokinetics of biological and small-molecule therapies in IBD. Various methods exist for measuring tissue concentrations, including whole tissue sampling, MALDI-MSI, microdialysis, and fluorescent labeling. These techniques offer unique advantages, such as spatial drug-distribution mapping, continuous sampling, or cellular-level analysis. However, challenges remain, including sampling invasiveness, heterogeneity in tissue compartments, and a lack of standardized bioanalytical guidelines. Drug pharmacokinetics are influenced by multiple factors, including molecular properties, disease-induced changes in the gastrointestinal tract, and the timing of sample collection. For example, drug permeability, solubility, and interaction with transporters may vary between Crohn's disease and ulcerative colitis. Research into the tissue concentrations of drugs like anti-TNF agents, ustekinumab, vedolizumab, and tofacitinib has shown variable correlations with clinical outcomes, suggesting potential roles for tissue concentration monitoring in therapeutic drug management. Although routine clinical application is not yet established, exploring tissue drug concentrations may enhance understanding of IBD pharmacotherapy.

Cytochrome P450 1A1 is essential for the microbial metabolite, Urolithin A-mediated protection against colitis

Background

Cytochrome P450 Family 1 Subfamily A Member 1 (CYP1A1) pathway, which is regulated by aryl hydrocarbon receptor (AhR) plays an important role in chemical carcinogenesis and xenobiotic metabolism. Recently, we demonstrated that the microbial metabolite Urolithin A (UroA) mitigates colitis through its gut barrier protective and anti-inflammatory activities in an AhR-dependent manner. Here, we explored role of CYP1A1 in UroA-mediated gut barrier and immune functions in regulation of inflammatory bowel disease (IBD).

Methods

To determine the role of CYP1A1 in UroA-mediated protectives activities against colitis, we subjected C57BL/6 mice and Cyp1a1^-/- mice to dextran sodium sulphate (DSS)-induced acute colitis model. The phenotypes of the mice were characterized by determining loss of body weight, intestinal permeability, systemic and colonic inflammation. Further, we evaluated the impact of UroA on regulation of immune cell populations by flow cytometry and confocal imaging using both in vivo and ex vivo model systems.

Results

UroA treatment mitigated DSS-induced acute colitis in the wildtype mice. However, UroA-failed to protect Cyp1a1^-/- mice against colitis, as evident from non-recovery of body weight loss, shortened colon lengths and colon weight/length ratios. Further, UroA failed to reduce DSS-induced inflammation, intestinal permeability and upregulate tight junction proteins in Cyp1a1^-/- mice. Interestingly, UroA induced the expansion of T-reg cells in a CYP1A1-dependent manner both in vivo and ex vivo models.

Conclusion

Our results suggest that CYP1A1 expression is essential for UroA-mediated enhanced gut barrier functions and protective activities against colitis. We postulate that CYP1A1 plays critical and yet unknown functions beyond xenobiotic metabolism in the regulation of gut epithelial integrity and immune systems to maintain gut homeostasis in IBD pathogenesis.

Altered Bioavailability and Pharmacokinetics in Crohn's Disease: Capturing Systems Parameters for PBPK to Assist with Predicting the Fate of Orally Administered Drugs

Backgrond and Objective

Crohn’s disease (CD) is a chronic inflammatory bowel disease that affects a wide age range. Hence, CD patients receive a variety of drugs over their life beyond those used for CD itself. The changes to the integrity of the intestine and its drug metabolising enzymes and transporters (DMETs) can alter the oral bioavailability of drugs. However, there are other changes in systems parameters determining the fate of drugs in CD, and understanding these is essential for dose adjustment in patients with CD.

Methods

The current analysis gathered all the available clinical data on the kinetics of drugs in CD (by March 2021), focusing on orally administered small molecule drugs. A meta-analysis of the systems parameters affecting oral drug pharmacokinetics was conducted. The systems information gathered on intestine, liver and blood proteins and other physiological parameters was incorporated into a physiologically based pharmacokinetic (PBPK) platform to create a virtual population of CD patients, with a view for guiding dose adjustment in the absence of clinical data in CD.

Results

There were no uniform trends in the reported changes in reported oral bioavailability. The nature of the drug as well as the formulation affected the direction and magnitude of variation in kinetics in CD patients relative to healthy volunteers. Even for the same drug, the reported changes in exposure varied, possibly due to a lack of distinction between the activity states of CD. The highest alteration was seen with S-verapamil and midazolam, 8.7- and 5.3-fold greater exposure, respectively, in active CD patients relative to healthy volunteers. Only one report was available on liver DMETs in CD, and indicated reduced CYP3A4 activity. In a number of reports, mRNA expression of DMETs in the ileum and colon of CD patients was measured, focussing on P-glycoprotein (p-gp) transporter and CYP3A4 enzyme, and showed contradictory results. No data were available on protein expression in duodenum and jejunum despite their dominant role in oral drug absorption.

Conclusion

There are currently inadequate dedicated clinical or quantitative proteomic studies in CD to enable predictive PBPK models with high confidence and adequate verification. The PBPK models for CD with the available systems parameters were able to capture the major physiological influencers and the gaps to be filled by future research. Quantification of DMETs in the intestine and the liver in CD is warranted, alongside well-defined clinical drug disposition studies with a number of index drugs as biomarkers of changes in DMETs in these patients, to avoid large-scale dedicated studies for every drug to determine the effects of disease on the drug’s metabolism and disposition and the consequential safety and therapeutic concerns.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40262-022-01169-4.

Quantitative Assessment of the Impact of Crohn's Disease on Protein Abundance of Human Intestinal Drug-Metabolising Enzymes and Transporters

Crohn's disease affects the mucosal layer of the intestine, predominantly ileum and colon segments, with the potential to affect the expression of intestinal enzymes and transporters, and consequently, oral drug bioavailability. We carried out a quantitative proteomic analysis of inflamed and non-inflamed ileum and colon tissues from Crohn's disease patients and healthy donors. Homogenates from samples in each group were pooled and protein abundance determined by liquid chromatography-mass spectrometry (LC-MS). In inflamed Crohn's ileum, CYP3A4, CYP20A1, CYP51A1, ADH1B, ALPI, FOM1, SULT1A2, SULT1B1 and ABCB7 showed ≥10-fold reduction in abundance compared with healthy baseline. By contrast, only MGST1 showed ≥10 fold reduction in inflamed colon. Ileal UGT1A1, MGST1, MGST2, and MAOA levels increased by ≥2 fold in Crohn's patients, while only ALPI showed ≥2 fold increase in the colon. Counter-intuitively, non-inflamed ileum had a higher magnitude of fold change than inflamed tissue when compared with healthy tissue. Marked but non-uniform alterations were observed in the expression of various enzymes and transporters in ileum and colon compared with healthy samples. Modelling will allow improved understanding of the variable effects of Crohn's disease on bioavailability of orally administered drugs.

Pharmacokinetics of omeprazole in rats with dextran sulfate sodium-induced ulcerative colitis

Omeprazole is a commonly used drug in patients with ulcerative colitis (UC). This study investigated the pharmacokinetics of omeprazole in rats with UC induced by dextran sulfate sodium (DSS). The pharmacokinetics of intravenously administered omeprazole (20 mg/kg) was investigated in normal and UC rats using LC-MS/MS. The formation of 5-OH omeprazole, a main metabolite of omeprazole, in rat liver microsomes (RLMs) from normal and UC rats was compared. The protein levels of CYP1A2, CYP2D1, and CYP3A1 in the liver were measured by Western blot. Compared with normal rats, UC rats had increased plasma concentrations of omeprazole, resulting in an increased AUC_0-240 min and decreased CL. DSS treatment decreased the formation rate of 5-OH omeprazole in RLMs but did not change the affinity of the enzymes. The V_max and CL_int of RLMs from UC rats were 62% and 48% those of RLMs from normal rats, respectively. The hepatic CYP1A2 and CYP3A1 protein levels in UC rats were 42.6 and 45.2% lower than those in normal rats, respectively; however, the protein levels of CYP2D1 in the two groups were similar. The activity and expression of some hepatic CYP450 isoforms were decreased by UC, leading to changes in the pharmacokinetics of omeprazole.

Role of cytochrome P450 polymorphisms and functions in development of ulcerative colitis

Cytochromes P450s (CYPs) are terminal enzymes in CYP dependent monooxygenases, which constitute a superfamily of enzymes catalysing the metabolism of both endogenous and exogenous substances. One of their main tasks is to facilitate the excretion of these substances and eliminate their toxicities in most phase 1 reactions. Endogenous substrates of CYPs include steroids, bile acids, eicosanoids, cholesterol, vitamin D and neurotransmitters. About 80% of currently used drugs and environmental chemicals comprise exogenous substrates for CYPs. Genetic polymorphisms of CYPs may affect the enzyme functions and have been reported to be associated with various diseases and adverse drug reactions among different populations. In this review, we discuss the role of some critical CYP isoforms (CYP1A1, CYP2D6, CYP2J2, CYP2R1, CYP3A5, CYP3A7, CYP4F3, CYP24A1, CYP26B1 and CYP27B1) in the pathogenesis or aetiology of ulcerative colitis concerning gene polymorphisms. In addition, their significance in metabolism concerning ulcerative colitis in patients is also discussed showing a clear underestimation in genetic studies performed so far.

Impact of gastrointestinal disease states on oral drug absorption – implications for formulation design – a PEARRL review

Objectives

Drug product performance in patients with gastrointestinal (GI) diseases can be altered compared to healthy subjects due to pathophysiological changes. In this review, relevant differences in patients with inflammatory bowel diseases, coeliac disease, irritable bowel syndrome and short bowel syndrome are discussed and possible in vitro and in silico tools to predict drug product performance in this patient population are assessed.

Key findings

Drug product performance was altered in patients with GI diseases compared to healthy subjects, as assessed in a limited number of studies for some drugs. Underlying causes can be observed pathophysiological alterations such as the differences in GI transit time, the composition of the GI fluids and GI permeability. Additionally, alterations in the abundance of metabolising enzymes and transporter systems were observed. The effect of the GI diseases on each parameter is not always evident as it may depend on the location and the state of the disease. The impact of the pathophysiological change on drug bioavailability depends on the physicochemical characteristics of the drug, the pharmaceutical formulation and drug metabolism. In vitro and in silico methods to predict drug product performance in patients with GI diseases are currently limited but could be a useful tool to improve drug therapy.

Summary

Development of suitable in vitro dissolution and in silico models for patients with GI diseases can improve their drug therapy. The likeliness of the models to provide accurate predictions depends on the knowledge of pathophysiological alterations, and thus, further assessment of physiological differences is essential.

Effects of dextran sulfate sodium induced experimental colitis on cytochrome P450 activities in rat liver, kidney and intestine

Dextran sulfate sodium (DSS) induced experimental colitis presents a histologic resemblance to human ulcerative colitis (UC). Altered cytochrome P450s (CYPs) have been reported in this model and patients with UC. In this study, six CYPs activities were quantitatively determined in microsomes of liver (RLMs), kidney (RRMs) and intestine (RIMs) from rats with colitis at acute (5% DSS for 7 days, UCA) and remission (7-day DSS treatment followed by 7-day cessation, UCR) phases and compared with normal rats. Generally, CYPs activities varied with isoform, organ, and disease status. Hepatic CYP1A2, 2B1, 2C6/11, 2E1 and 3A1/2 activities were reduced by acute colitis and completely or partially restored after DSS was halted. Although DSS treatment decreased the V_max of renal CYP2C6/11 and increased that of CYP2D2, their CL_{int, in vitro} were comparable among normal, acute and remission stages. DSS treatment changed the kinetics of CYP3A1/2-mediated nifedipine metabolism in RRMs from biphasic to classical kinetics. Notably, CYP2D2 activity was elevated in liver and kidney in acute UC, while enhanced in liver and decreased in kidney in remission. In intestine, CYP3A1/2 activity was increased in UCA and further enhanced after DSS withdrawal. These findings highlight the necessity of quantifying enzyme activity for precision drug therapy.

Biotransformation of xenobiotics in the human colon and rectum and its association with colorectal cancer

In humans, the liver is generally considered to be the major organ contributing to drug metabolism, but studies during the last years have suggested an important role of the extra-hepatic drug metabolism. The gastrointestinal tract (GI-tract) is the major path of entry for a wide variety of compounds including food, and orally administered drugs, but also compounds - with neither nutrient nor other functional value - such as carcinogens. These compounds are metabolized by a large number of enzymes, including the cytochrome P450 (CYP), the glutathione S-transferase (GST) family, the uridine 5'-diphospho- glucuronosyltransferase (UDP-glucuronosyltransferase - UGT) superfamily, alcohol-metabolizing enzymes, sulfotransferases, etc. These enzymes can either inactivate carcinogens or, in some cases, generate reactive species with higher reactivity compared to the original compound. Most data in this field of research originate from animal or in vitro studies, wherein human studies are limited. Here, we review the human studies, in particular the studies on the phenotypic expression of these enzymes in the colon and rectum to get an impression of the actual enzyme levels in this primary organ of exposure. The aim of this review is to give a summary of currently available data on the relation between the CYP, the GST and the UGT biotransformation system and colorectal cancer obtained from clinical and epidemiological studies in humans.

Intestinal metabolism and transport of drugs in children: the effects of age and disease

The presence of cytochrome P450 enzymes in the small bowel results in the reduced bioavailability of several drugs. Recently, there has been much research on the interplay between these enzymes and transporter proteins such as P-glycoprotein. Intestinal drug metabolism not only has an effect on drug disposition but also may have a role in protecting the host from xenobiotics. Although there is some information on how both the enzymes and transporters develop in the small bowel with age, there is little information regarding the clinical effects of these changes. In addition to developmental changes, the influence of gastrointestinal disease and nutrition are additional covariates in the intestinal metabolism of drugs.

Imbalances in dietary consumption of fatty acids, vegetables, and fruits are associated with risk for Crohn's disease in children

BACKGROUND AND OBJECTIVES

The role of dietary factors in the etiology of Crohn's disease (CD) is inconsistent largely due to difficulties in acquiring valid information on consumption habits. We examined the impact of diet on new onset CD in children using a validated food-frequency questionnaire (FFQ).

METHODOLOGY

A case-control study was carried out. Children < or =20 yr, newly diagnosed with CD, were recruited from 3 pediatric gastroenterology clinics across Canada. Population or hospital controls were selected matched to cases for time of diagnosis (+/-6 months) and area of residence. Dietary consumption 1 yr prior to disease diagnosis was evaluated using a validated FFQ, administered within 1 month of diagnosis. Conditional logistic regression analysis adjusting for potential confounding variables (energy intake, age, gender, body mass index) was carried out.

RESULTS

A total of 130 CD patients and 202 controls were studied. Mean age at diagnosis (+/-SD) was 14.2 (2.7). There were more male patients (59%). Comparing the highest to the lowest levels of consumption, higher amounts of vegetables (OR 0.69, 95% CI 0.33-1.44, P= 0.03), fruits (OR 0.49, 95% CI 0.25-0.96, P= 0.02), fish (OR 0.46, 95% CI 0.20-1.06, P= 0.02), and dietary fiber (OR 0.12, 95% CI 0.04-0.37, P < 0.001) protected from CD. Consumption of long-chain omega-3 fatty acids (LCN-omega-3) was negatively associated with CD (OR 0.44, 95% CI 0.19-1.00, P < 0.001). A higher ratio of LCN-omega-3/omega-6 fatty acids was significantly associated with lower risks for CD (OR 0.32, 95% CI 0.14-0.71, P= 0.02).

CONCLUSIONS

Our findings indicate that an imbalance in consumption of fatty acids, vegetables, and fruits is associated with increased risks for CD among Canadian children.

Expression of cytochrome P450 and MDR1 in patients with proctitis

BACKGROUND

The aim of this study was to investigate the effect of inflammation on the gene expression of three cytochrome P450's (CYP) and P-glycoprotein (P-gp) in the rectal and colonic mucosa in patients with proctitis.

METHODS

Biopsies were obtained from inflamed and normal mucosa in association with routine sigmoidoscopy in patients with proctitis. The biopsies were snap-frozen in liquid nitrogen. Real time PCR (polymerase chain reaction) was used for quantitative analyses of mRNA specific for the CYP2E1, CYP3A4 and CYP3A5 gene and the MDR1 genes. Values were normalised based on gene expression of beta-actin to enable comparisons between samples.

RESULTS

The gene expression of CYP2E1 and CYP3A4 was lower in mucosa with severe inflammation vs normal mucosa (p<0.05). For CYP3A5 and P-gp there was no significant difference when comparing normal and inflammatory changed mucosa.

CONCLUSION

Our study suggests that at least for some of the CYP enzymes the expression decreases in response to the inflammatory process in the gastrointestinal tract.

Cytochromes P450 and MDR1 mRNA expression along the human gastrointestinal tract

AIM

The aim of this study was to quantify the mRNA expression of three cytochromes P450 (CYP) and P-glycoprotein (P-gp) in the human gastrointestinal (GI) tract.

METHOD

Biopsies were obtained from gastric, duodenal, colonic and rectal mucosa during routine gastro-colonoscopy in 27 patients. The biopsies were snap-frozen in liquid nitrogen. Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) was used for the quantitative analyses of mRNA expressed by the CYP2E1, CYP3A4 and CYP3A5 genes, and the MDR1 gene coding for P-gp protein. The mRNA expression of b-actin was used as an internal standard for comparisons between samples.

RESULTS

All CYP genes were expressed at all locations throughout the GI tract, although all showed substantial interindividual variation. CYP2E1 had the highest expression at all locations (P < 0.05 to P < 0.0001), except in the right colon. CYP3A4 and CYP3A5 had their highest mRNA expression in the duodenum (P < 0.001 and P < 0.000 001, respectively) and CYP2E1 in the stomach (P < 0.01). MDR1 mRNA concentrations increased along the GI tract with the highest expression being in the left colon (P < 0.000001).

CONCLUSION

Multiple sampling within the same individual enabled us to study the intraindividual variation in expression of CYP and MDR1 genes along the GI tract. We find that CYP2E1 mRNA expression is higher than that of the other CYPs. CYP3A expression is highest in the duodenum and that of MDR1 increases from stomach and duodenum to colon.

Etiopathogenesis of pediatric Crohn's disease. Biologic pathways based on interactions between genetic and environmental factors

The incidence and prevalence of Crohn's disease (CD) among children in developed countries is extremely high. Although, dietary factors have long been implicated, to date no single causative element has been identified. Risks or benefits for CD from dietary elements would depend on their appropriate metabolism within the gut by xenobiotic metabolising enzymes (XME). The metabolising capacity among individuals differs and is determined by the level and/or activity of the specific XME. As the latter is the function of the DNA variants that code them, we propose that certain individuals exposed to dietary elements are likely to be more or less susceptible for CD. We highlight this hypothesis by describing gene-environment interaction (GxE) mechanisms that could determine susceptibility for CD. We suggest that investigating these mechanisms will be paramount for the appropriate identification of susceptible populations, such that preventive and/or therapeutic interventions could be adequately targeted.

Host determinants of DNA alkylation and DNA repair activity in human colorectal tissue: O(6)-methylguanine levels are associated with GSTT1 genotype and O(6)-alkylguanine-DNA alkyltransferase activity with CYP2D6 genotype

There is increasing evidence that alkylating agent exposure may increase large bowel cancer risk and factors which either alter such exposure or its effects may modify risk. Hence, in a cross-sectional study of 78 patients with colorectal disease, we have examined whether (i) metabolic genotypes (GSTT1, GSTM1, CYP2D6, CYP2E1) are associated with O(6)-methyldeoxyguanosine (O(6)-MedG) levels, O(6)-alkylguanine-DNA alkyltransferase (ATase) activity or K-ras mutations, and (ii) there was an association between ATase activity and O(6)-MedG levels. Patients with colon tumours and who were homozygous GSTT1(*)2 genotype carriers were more likely than patients who expressed GSTT1 to have their DNA alkylated (83 versus 32%, P=0.03) and to have higher O(6)-MedG levels (0.178+/-0.374 versus 0.016+/-0.023 micromol O(6)-MedG/mol dG, P=0.04) in normal, but not tumour, DNA. No such association was observed between the GSTT1 genotype and the frequency of DNA alkylation or O(6)-MedG levels in patients with benign colon disease or rectal tumours. Patients with colon tumours or benign colon disease who were CYP2D6-poor metabolisers had higher ATase activity in normal tissue than patients who were CYP2D6 extensive metabolisers or CYP2D6 heterozygotes. Patients with the CYP2E1 Dra cd genotype were less likely to have a K-ras mutation: of 55 patients with the wild-type CYP2E1 genotype (dd), 23 had K-ras mutations, whereas none of the 7 individuals with cd genotype had a K-ras mutation (P=0.04). No other associations were observed between GSTT1, GSTM1, CYP2D6 and CYP2E1 Pst genotypes and adduct levels, ATase activity or mutational status. O(6)-MedG levels were not associated with ATase activity in either normal or tumour tissue. However, in 15 patients for whom both normal and tumour DNA contained detectable O(6)-MedG levels, there was a strong positive association between the normal DNA/tumour DNA adduct ratio and the normal tissue/tumour tissue ATase ratio (r(2)=0.66, P=0.001). These results indicate that host factors can affect levels both of the biologically effective dose arising from methylating agent exposure and of a susceptibility factor, the DNA repair phenotype.