Potential role of UGT pharmacogenetics in cancer treatment and prevention: focus on tamoxifen and aromatase inhibitors

Quantification of the aromatase inhibitor letrozole and its carbinol metabolite in mouse plasma by UHPLC-MS/MS

A liquid chromatography - electrospray ionization-mass spectrometry (LC-ESI-MS) method was developed for the quantification of letrozole, a third-generation aromatase inhibitor, and its main carbinol metabolite (CM) in support of murine pharmacokinetic studies. Using polarity switching, simultaneous ESI-MS measurement of letrozole and CM was achieved in positive and negative mode, respectively. The assay procedure involved a one-step protein precipitation and extraction of all analytes from mouse plasma requiring only 5 μL of sample. Separation was optimized on an Accucore aQ column with gradient elution at a flow rate of 0.4 mL/min in 5 min. Two calibration curves per day over four consecutive measurement days showed satisfactory linear responses (r2 > 0.99) over concentration ranges of 5-1000 ng/mL and 20-2000 ng/mL for letrozole and CM, respectively. No matrix effect was found, and the mean extraction recoveries were 103-108 % for letrozole and 99.8-107 % for CM. Precision and accuracy within a single run and over four consecutive measurement days were verified to be within acceptable limits. Application of the developed method to preclinical pharmacokinetic studies in mice receiving oral letrozole at a dose 1 or 10 mg/kg revealed that the systemic exposure to letrozole was dose-, formulation-, and strain-dependent. These findings may inform the future design of preclinical studies aimed at refining the pharmacological profile of this clinically important drug.

Anastrozole and Related Glucuronic Acid Conjugate are Electrophilic Species

Anastrozole (ANA), is an inhibitor of non-steroidal aromatase, widely employed for the treatment of breast cancer. However, ANA-associated liver injury cases have been documented in the application of the drug.The major purposes of the present study were to identify the structure of reactive metabolites derived from ANA and to study related metabolic pathways of ANA.We found ANA itself is an electrophilic species reactive to GSH. ANA can be metabolized to ANA-N⁺-glucuronide (1) catalyzed by UGT1A4. An ANA GSH conjugate (2) was detected in bile and livers of rats treated with ANA. UGT1A4 participated in the phase II metabolic pathway.This work allowed us to better understand the mechanisms of the hepatotoxicity of ANA and provided new avenue to define the possible role of metabolic activation in hepatotoxicity.

Effect of Genetic Variability in 20 Pharmacogenes on Concentrations of Tamoxifen and Its Metabolites

BACKGROUND

Tamoxifen, as a treatment of estrogen receptor positive (ER+) breast cancer, is a weak anti-estrogen that requires metabolic activation to form metabolites with higher anti-estrogenic activity. Endoxifen is the most-studied active tamoxifen metabolite, and endoxifen concentrations are highly associated with CYP2D6 activity. Associations of tamoxifen efficacy with measured or CYP2D6-predicted endoxifen concentrations have been inconclusive. Another active metabolite, 4-OHtam, and other, less active metabolites, Z-4'-endoxifen and Z-4'-OHtam, have also been reported to be associated with tamoxifen efficacy.

METHOD

Genotype for 20 pharmacogenes was determined by VeriDose^® Core Panel and VeriDose^®CYP2D6 CNV Panel, followed by translation to metabolic activity phenotype following standard activity scoring. Concentrations of tamoxifen and seven metabolites were measured by UPLC-MS/MS in serum samples collected from patients receiving 20 mg tamoxifen per day. Metabolic activity was tested for association with tamoxifen and its metabolites using linear regression with adjustment for upstream metabolites to identify genes associated with each step in the tamoxifen metabolism pathway.

RESULTS

A total of 187 patients with genetic and tamoxifen concentration data were included in the analysis. CYP2D6 was the primary gene associated with the tamoxifen metabolism pathway, especially the conversion of tamoxifen to endoxifen. CYP3A4 and CYP2C9 were also responsible for the metabolism of tamoxifen. CYP2C9 especially impacted the hydroxylation to 4-OHtam, and this involved the OATP1B1 (SLCO1B1) transporter.

CONCLUSION

Multiple genes are involved in tamoxifen metabolism and multi-gene panels could be useful to predict active metabolite concentrations and guide tamoxifen dosing.

Berberine induces apoptosis and arrests the cell cycle in multiple cancer cell lines

Introduction

To examine the anti-cancer effects of berberine on multiple cancer cell lines, and to clarify the underlying molecular mechanisms.

Material and methods

The IC50 values for the action of berberine on Tca8113 (oral squamous cell carcinoma), CNE2 (nasopharyngeal carcinoma cell), MCF-7 (breast cancer), Hela (cervical carcinoma), and HT29 (colon cancer) cells were determined by MTT cell viability assay. Early apoptosis and cell cycle arrest were examined by flow cytometry with annexin V and propidium iodide (PI) staining, respectively. For expression of BAX and BCL-2 genes and proteins were detected by real-time PCR and western blotting, respectively.

Results

Berberine displayed a cytotoxic effect on all the cell lines tested. The IC50 values were determined (Tca8113, 218.52 ±18.71; CNE2, 249.18 ±18.14; MCF-7, 272.15 ±11.06; Hela, 245.18 ±17.33; and HT29, 52.37 ±3.45). PI staining revealed that berberine treatment resulted in cell cycle arrest at G2/M. The treatment also induced early apoptosis as shown by annexin V staining. In addition, berberine significant elevated gene and protein expression of BAX, which was accompanied by substantial decreases in BCL-2 gene and protein levels. The effects of berberine on BAX and BCL-2 were time-dependent.

Conclusions

Berberine exhibited cytotoxic effects on multiple cancer cell lines by inducing apoptosis and cell cycle arrest. The BCL-2/BAX signaling pathway may be the common pathway underlying the anti-tumor effect of berberine. The findings support the notion that berberine is a dietary compound that can be further developed into a drug candidate for cancer treatment.

Generating a Precision Endoxifen Prediction Algorithm to Advance Personalized Tamoxifen Treatment in Patients with Breast Cancer

Tamoxifen is an endocrine treatment for hormone receptor positive breast cancer. The effectiveness of tamoxifen may be compromised in patients with metabolic resistance, who have insufficient metabolic generation of the active metabolites endoxifen and 4-hydroxy-tamoxifen. This has been challenging to validate due to the lack of measured metabolite concentrations in tamoxifen clinical trials. CYP2D6 activity is the primary determinant of endoxifen concentration. Inconclusive results from studies investigating whether CYP2D6 genotype is associated with tamoxifen efficacy may be due to the imprecision in using CYP2D6 genotype as a surrogate of endoxifen concentration without incorporating the influence of other genetic and clinical variables. This review summarizes the evidence that active metabolite concentrations determine tamoxifen efficacy. We then introduce a novel approach to validate this relationship by generating a precision endoxifen prediction algorithm and comprehensively review the factors that must be incorporated into the algorithm, including genetics of CYP2D6 and other pharmacogenes. A precision endoxifen algorithm could be used to validate metabolic resistance in existing tamoxifen clinical trial cohorts and could then be used to select personalized tamoxifen doses to ensure all patients achieve adequate endoxifen concentrations and maximum benefit from tamoxifen treatment.

Predictive Value of Microdose Pharmacokinetics

Phase 0 microdose trials are exploratory studies to early assess human pharmacokinetics of new chemical entities, while limiting drug exposure and risks for participants. The microdose concept is based on the assumption that microdose pharmacokinetics can be extrapolated to pharmacokinetics of a therapeutic dose. However, it is unknown whether microdose pharmacokinetics are actually indicative of the pharmacokinetics at therapeutic dose. The aim of this review is to investigate the predictive value of microdose pharmacokinetics and to identify drug characteristics that may influence the scalability of these parameters. The predictive value of microdose pharmacokinetics was determined for 46 compounds and showed adequate predictability for 28 of 41 orally administered drugs (68%) and 15 of 16 intravenously administered drugs (94%). Microdose pharmacokinetics were considered predictive if the mean observed values of the microdose and the therapeutic dose were within twofold. Nonlinearity may be caused by saturation of enzyme and transporter systems, such as intestinal and hepatic efflux and uptake transporters. The high degree of success regarding linear pharmacokinetics shows that phase 0 microdose trials can be used as an early human model for determination of drug pharmacokinetics.

Variation in the UGT2B17 genotype, exemestane metabolism and menopause-related toxicities in the CCTG MAP.3 trial

PURPOSE

To examine associations between the UGT2B17 gene deletion and exemestane metabolites, and commonly reported side effects (fatigue, hot flashes, and joint pain) among postmenopausal women participating in the MAP.3 chemoprevention trial.

METHODS

The analytical samples for the UGT2B17 analysis comprised 1752 women on exemestane and 1721 women on placebo; the exemestane metabolite analysis included 1360 women on exemestane with one-year serum samples. Both the UGT2B17 gene deletion and metabolites were measured in blood. The metabolites were conceptualized as a ratio (17-DHE-Gluc:17-DHE). Symptoms were assessed using the CTCAE v4.0 at approximately 1-year intervals. Log-binomial regression was used to examine the associations between UGT2B17 deletion, exemestane metabolites and each side effect at 1 and up to 5-year follow-up, adjusting for potential confounders.

RESULTS

Among individuals on exemestane with the UGT2B17 gene deletion (i.e., lower detoxification), a higher risk of severe fatigue (RR = 2.59 95% CI: 1.14-5.89) was observed at up to 5-year follow-up. Among individuals on placebo, those with the UGT2B17 gene deletion had a higher risk of any fatigue (RR = 1.39, 95% CI: 1.02-1.89) at year 1. A lower metabolite ratio (poor detoxification) was associated with a higher risk of any fatigue, hot flashes and joint pain at year 1 (fatigue: RR = 1.89, 95% CI: 1.16-3.09; hot flashes: RR = 1.77, 95% CI: 1.40-2.24; joint pain: RR = 2.05, 95% CI: 1.35-3.12); similar associations were observed at 5-year follow-up.

CONCLUSION

Variation in the metabolism of exemestane through the UGT2B17-mediated pathway is associated with subsequent risk of commonly reported symptoms in MAP.3.

Representation of CYP3A4, CYP3A5 and UGT1A4 Polymorphisms within Croatian Breast Cancer Patients' Population

Single nucleotide polymorphism (SNP) in genes encoding drug-metabolizing enzymes (DME) could have a critical role in individual responses to anastrozole. Frequency of CYP3A4*1B, CYP3A5*3 and UGT1A4*2 SNPs in 126 Croatian breast cancer (BC) patients and possible association with anastrozole-induced undesirable side effects were analyzed. Eighty-two postmenopausal patients with estrogen receptor (ER)-positive BC treated with anastrozole and 44 postmenopausal ER-positive BC patients before hormonal adjuvant therapy were included in the study. Genomic DNA was genotyped by TaqMan Real-Time PCR. Bone mineral density (BMD) was measured by dual-energy X-ray absorptiometry. The homozygotes for the variant G allele of CYP3A5*3 were predominant (88%), and the homozygotes for the reference A allele were not detected. While homozygotes for the variant G allele of CYP3A4*1B were not detected, predominantly wild type homozygotes for A allele (94%) were present. CYP3A4*1B and CYP3A5*3 SNPs were in 84.3% linkage disequilibrium (D’ = 0.843) and 95.1% (D’ = 0.951) in group treated with anastrozole and w/o treatment, respectively. Homozygotes for the A allele of UGT1A4*2 were not detected in our study groups. Although the variant CYP3A5*3 allele, which might result in poor metabolizer phenotype and more pronounced side effects, was predominant, significant association with BMD changes induced by anastrozole were not confirmed.

Association between ESR1, ESR2, HER2, UGT1A4, and UGT2B7 polymorphisms and breast Cancer in Jordan: a case-control study

BACKGROUND

Breast cancer risk, development, and treatment are influenced by genetic variation in certain genes, namely those involved in cell proliferation, tumor suppression, and drug metabolism. In turn, the relevance of the aforementioned genetic variation to cancer depends on the ethnic group in question, highlighting the need for population-specific association studies. Therefore, the objective of the present study was to investigate the association between certain ESR1, ESR2, HER2, UGT1A4, and UGT2B7 single nucleotide polymorphisms and breast cancer.

METHODS

Blood samples were collected from 437 Jordanian-Arab breast cancer patients and healthy volunteers and subject to genotyping using the Sequenom MassARRAY® system (iPLEX GOLD).

RESULTS

Our findings show a significant association between breast cancer and the allelic (P = 0.02486879) and genotypic (P = 0.04793066) frequencies of the ESR1 polymorphism rs3798577, a result which was confirmed in different genetic models. No other investigated polymorphism showed a significant association with breast cancer itself in Jordanian Arabs, but the Rare Hz (GG) vs Het (AG) genetic model revealed an association of the disease with the ESR1 polymorphism rs3798577. However, several associations were found between certain polymorphisms and breast cancer's prognostic factors.

CONCLUSION

This study suggests that certain polymorphisms may increase the risk of breast cancer in the Jordanian-Arab population. Future research and clinical translation could incorporate the current results in preventative breast cancer approaches tailored for Jordanian-Arab patients.

Genetic Underpinnings of Musculoskeletal Pain During Treatment With Aromatase Inhibitors for Breast Cancer: A Biological Pathway Analysis

BACKGROUND

Musculoskeletal pain (MSKP) is the most reported symptom during treatment with aromatase inhibitors (AIs) for breast cancer. The mechanisms underlying MSKP are multidimensional and not well understood. The goals of this biological pathway analysis were to (1) gain an understanding of the genetic variation and biological mechanisms underlying MSKP with AI therapy and (2) identify plausible biological pathways and candidate genes for future investigation.

METHOD

Genes associated with MSKP during AI therapy or genes involved in drug metabolism of and response to AIs were identified from the literature. Studies published through February 2019 were queried in PubMed®. The genes identified from the literature were entered into QIAGEN's Ingenuity® Pathway Analysis (IPA) software to generate canonical pathways, upstream regulators, and networks through a core analysis.

RESULTS

The 17 genes identified were ABCB1, ABCG1, CYP17A1, CYP19A1, CYP27B1, CYP2A6, CYP3A4, CYP3A5, ESR1, OATP1B1, OPG, RANKL, SLCO3A1, TCL1A, UGT2A1, UGT2B17, and VDR. These genes are involved in encoding bone-remodeling regulators, drug-metabolizing enzymes (cytochrome P450 family, UDP-glucuronosyltransferases family), or drug transporters (ATP-binding cassette transporters, organic anion transporters). Multiple plausible biological pathways (e.g., nicotine degradation, melatonin degradation) and candidate genes (e.g., NFKB, HSP90, AKT, ERK1/2, FOXA2) are proposed for future investigation based on the IPA results.

CONCLUSION

Multiple genes and molecular-level etiologies may contribute to MSKP with AI therapy in women with breast cancer. Our innovative combination of gene identification from the literature plus biological pathway analysis allowed for the emergence of novel candidate genes and biological pathways for future investigations.

Prognostic impact of genetic variants of CYP19A1 and UGT2B17 in a randomized trial for endocrine-responsive postmenopausal breast cancer

Polymorphisms of genes involved in estrogen synthesis have been linked to breast cancer risk, prognosis, and treatment response. We investigated the prognostic impact of a deletion spanning the entire UGT2B17 gene (UGT2B17*2) and genetic variants of the aromatase CYP19A1 and estrogen receptor α (ESR1) in 125 postmenopausal women with ER-positive breast cancer enrolled in a randomized pre-surgical trial. The UGT2B17*2 was estimated by copy number variation assays and the CYP19A1 rs10046/rs4646 and ESR1 rs2077647/rs2234693/rs9340799 by TaqMan allelic discrimination assays. Serum exemestane/17-hydroxy exemestane were determined by MS and estrone (E1)/estradiol (E2)/ by GC-MS/MS. The association of genetic polymorphisms with "any event" was assessed by the Cox proportional hazards models adjusted for confounders. The UGT2B17*2 was associated with higher levels of 17-hydroxy exemestane (P = 0.04) and better prognosis (HR = 0.45; 95% CI: 0.20-1.01; P = 0.05) compared with homozygote UGT2B17 wt. The CYP19A1 rs10046 A and rs4646 C alleles were associated with higher estrogen levels: rs10046 AA vs. AG/GG genotypes had median E1 of 35.9 vs. 27.4 pg/mL (P = 0.05) and E2 of 7.57 vs. 3.9 pg/mL (P < 0.004). After a median follow-up of 7 years, women carrying the "low estrogen" alleles rs10046 G and rs4646 A had a better prognosis compared with homozygote wt for both polymorphisms (HR = 0.40; 95% CI: 0.17-0.93; P = 0.03). Our analysis points to an impact of UGT2B17 and CYP19A1 in postmenopausal endocrine responsive breast cancer. Carriers of UGT2B17*2 and CYP19A1 low estrogen variants may have better prognosis, supporting studies addressing the role of these polymorphisms in optimizing endocrine therapy. Trial registration: http://www.isrctn.com/ISRCTN86894592.

Bioequivalence of Oral Formulations of Anastrozole in Healthy Chinese Male Volunteers: A Randomized, Single-Dose, Two-Period, Two-Sequence Crossover Study

Anastrozole is currently used as first-line treatment in locally advanced or metastatic breast cancer. A generic anastrozole tablet was developed to offer an alternative to the marketed tablet formulation. The aim of the current study was to evaluate the bioequivalence between the test and reference formulations of anastrozole in a single-dose, 2-period, 2-sequence crossover study with a 14-day washout interval. A total of 20 healthy male Chinese volunteers were enrolled and completed the study, after oral administration of a single dose of 1.0-mg test and reference formulations of anastrozole. The blood samples were collected at different times and were determined by a fully validated high-pressure liquid chromatography-tandem mass spectrometry method. The evaluated pharmacokinetic parameters, including C_max , AUC_0-t , and AUC_0-∞ , were assessed for bioequivalence based on current guidelines. The observed pharmacokinetic parameters of anastrozole of the test drug were similar to those of the reference formulation. The 90% confidence intervals of test/reference ratios for C_max , AUC_0-t , and AUC_0-∞ were within the bioequivalence acceptance range of 80%-125%. The results obtained from these healthy Chinese subjects in this study suggest that the test formulation of anastrozole 1.0-mg tablet is bioequivalent to the reference formulation (Arimidex 1.0-mg tablet).

Molecular basis of aromatase inhibitor associated arthralgia: known and potential candidate genes and associated biomarkers

INTRODUCTION

Aromatase inhibitors (AIs) are routinely used for the adjuvant treatment of women with hormone receptor-positive early breast cancer. AIs are widely prescribed in the postmenopausal setting, as they are effective at preventing recurrence. However, their use is complicated by significant adverse effects, particularly arthralgia, noted in up to 50% of treated patients, and thereby affects quality of life and AI compliance. The mechanism by which AIs cause arthralgia is largely unknown, although there is a growing body of literature which suggests that there may be multiple intersecting mechanisms. Areas covered: This review describes the evidence for the mechanistic basis of AI arthralgia as well as potential pathways that could contribute to the development of AI associated arthralgia. Expert opinion: Interplay of multiple factors, such as interpatient variability in AI metabolism, possibly related to pharmacogenetic factors, the sudden decline of estrogen synthesis, vitamin D status, as well as upregulation of cytokines and inflammation pathways may precipitate or exacerbate muscle and joint pain are linked during AI therapy. However, much more research is needed in this area given the frequency and severity of AI-associated arthralgia.

Emerging role of NRF2 in chemoresistance by regulating drug-metabolizing enzymes and efflux transporters

Chemoresistance is a disturbing barrier in cancer therapy, which always results in limited therapeutic options and unfavorable prognosis. Nuclear factor E2-related factor 2 (NRF2) controls the expression of genes encoding cytoprotective enzymes and transporters that protect against oxidative stress and electrophilic injury to maintain intrinsic redox homeostasis. However, recent studies have demonstrated that aberrant activation of NRF2 due to genetic and/or epigenetic mutations in tumor contributes to the high expression of phase I and phase II drug-metabolizing enzymes, phase III transporters, and other cytoprotective proteins, which leads to the decreased therapeutic efficacy of anticancer drugs through biotransformation or extrusion during chemotherapy. Therefore, a better understanding of the role of NRF2 in regulation of these enzymes and transporters in tumors is necessary to find new strategies that improve chemotherapeutic efficacy. In this review, we summarized the recent findings about the chemoresistance-promoting role of NRF2, NRF2-regulated phase I and phase II drug-metabolizing enzymes, phase III drug efflux transporters, and other cytoprotective genes. Most importantly, the potential of NRF2 was proposed to counteract drug resistance in cancer treatment.

The implications of genetic variation for the pharmacokinetics and pharmacodynamics of aromatase inhibitors

INTRODUCTION

Breast cancer is the most common female cancer and remains a serious public health concern worldwide. Third-generation aromatase inhibitors (AIs) are widely used in postmenopausal women with estrogen receptor positive breast cancer. However, there is marked interindividual variability in terms of the efficacy and incidence of adverse events following treatment with AIs. Pharmacogenetics has the potential to predict clinical outcomes based on patients' genetic information, paving the way towards personalized treatment.

AREAS COVERED

This article reviews pharmacogenetic studies of AIs, including pharmacokinetic and pharmacodynamic aspects, highlighting those studies where the efficacy and adverse events of AIs have been examined using both candidate gene and genome-wide approaches.

EXPERT OPINION

Pharmacogenetics is a promising approach to develop personalized medicine with AIs. However, the application of pharmacogenetics to predict therapeutic efficacy and adverse events in breast cancer patients is still far from implementation in routine clinical practice. Large, comprehensive, multicenter studies that simultaneously evaluate multiple genes and pathways, including rare variants, are warranted in order to produce reliable and informative results. The ultimate aim is to develop clinically-relevant guidelines for breast cancer therapy.

The influence of genetic polymorphisms on the efficacy and side effects of anastrozole in postmenopausal breast cancer patients

Breast cancer is a common cause of cancer mortality among women. Several genetic factors have been implicated in its development. Current treatment guidelines for estrogen receptor-positive breast cancer recommend that anastrozole [or any of the other two aromatase inhibitors (letrozole and exemestane)] is used as an alternative to tamoxifen or following several years of tamoxifen treatment. Nevertheless, this approach is still associated with many challenges, ranging from the recurrence of breast cancer to considerable interindividual variability in the tolerability of anastrozole, which may cause adverse effects, such as musculoskeletal symptoms, and lead to the withdrawal of many patients from treatment. Variabilities in the genes encoding the drug target (aromatase) or its metabolizing enzymes (CYP3A and UGT1A) contribute toward the interindividual variability in anastrozole's pharmacokinetics and/or pharmacodynamics. This paper reviews the role of genetic polymorphisms of CYP19A1, CYP3A4, and UGT1A4 in the responses of female hormone receptor-positive postmenopausal breast cancer patients to anastrozole. Many reviews in the literature have suggested that the study of functional polymorphisms and investigation of relevant genetic markers may provide valuable information in predicting responses to anastrozole in terms of its therapeutic and adverse effects. Nevertheless, more studies are required before the knowledge of its pharmacogenomics can be applied to the individualization of treatment to ensure that patients receive the maximum benefits. Therefore, future analyses, including but not limited to genome-wide association studies, are encouraged to address some of the gray areas in the pharmacogenomics of anastrozole therapy in postmenopausal breast cancer cases; this will help in providing guidance for future pharmacogenomics protocols when anastrozole is utilized in patients' management.

Influence of multidrug resistance and drug transport proteins on chemotherapy drug metabolism

INTRODUCTION

Chemotherapy involving the use of anticancer drugs remains an important strategy in the overall management of patients with metastatic cancer. Acquisition of multidrug resistance remains a major impediment to successful chemotherapy. Drug transporters in cell membranes and intracellular drug metabolizing enzymes contribute to the resistance phenotype and determine the pharmacokinetics of anticancer drugs in the body.

AREAS COVERED

ATP-binding cassette (ABC) transporters mediate the transport of endogenous metabolites and xenobiotics including cytotoxic drugs out of cells. Solute carrier (SLC) transporters mediate the influx of cytotoxic drugs into cells. This review focuses on the substrate interaction of these transporters, on their biology and what role they play together with drug metabolizing enzymes in eliminating therapeutic drugs from cells.

EXPERT OPINION

The majority of anticancer drugs are substrates for the ABC transporter and SLC transporter families. Together, these proteins have the ability to control the influx and the efflux of structurally unrelated chemotherapeutic drugs, thereby modulating the intracellular drug concentration. These interactions have important clinical implications for chemotherapy because ultimately they determine therapeutic efficacy, disease progression/relapse and the success or failure of patient treatment.

A potential role for human UDP-glucuronosyltransferase 1A4 promoter single nucleotide polymorphisms in the pharmacogenomics of tamoxifen and its derivatives

Tamoxifen (Tam) is a selective estrogen receptor modulator used to inhibit breast tumor growth. Tam can be directly N-glucuronidated via the tertiary amine group or O-glucuronidated after cytochrome P450-mediated hydroxylation. In this study, the glucuronidation of Tam and its hydroxylated and/or chlorinated derivatives [4-hydroxytamoxifen (4OHTam), toremifene (Tor), and 4-hydroxytoremifene (4OHTor)] was examined using recombinant human UDP-glucuronosyltransferases (UGTs) from the 1A subfamily and human hepatic microsomes. Recombinant UGT1A4 catalyzed the formation of N-glucuronides of Tam and its derivatives and was the most active UGT enzyme toward these compounds. Therefore, it was hypothesized that single nucleotide polymorphisms (SNPs) in the promoter region of UGT1A4 have the ability to significantly decrease the glucuronidation rates of Tam metabolites in the human liver. In vitro activity of 64 genotyped human liver microsomes was used to determine the association between the UGT1A4 promoter and coding region SNPs and the glucuronidation rates of Tam, 4OHTam, Tor, and 4OHTor. Significant decreases in enzymatic activity were observed in microsomes for individuals heterozygous for -163G/A and -217T/G. These alterations in glucuronidation may lead to prolonged circulating half-lives and may potentially modify the effectiveness of these drugs in the treatment of breast cancer.

Impact of metabolizing enzymes on drug response of endocrine therapy in breast cancer

Estrogen-receptor positive breast cancer accounts for 75% of diagnosed breast cancers worldwide. There are currently two major options for adjuvant treatment: tamoxifen and aromatase inhibitors. Variability in metabolizing enzymes determines their pharmacokinetic profile, possibly affecting treatment response. Therefore, prediction of therapy outcome based on genotypes would enable a more personalized medicine approach, providing optimal therapy for each patient. In this review, the authors will discuss the current evidence on the most important metabolizing enzymes in endocrine therapy, with a special focus on CYP2D6 and its role in tamoxifen metabolism.

The letrozole phase 1 metabolite carbinol as a novel probe drug for UGT2B7

Carbinol [4,4'-(hydroxymethylene)dibenzonitrile] is the main phase 1 metabolite of letrozole, a nonsteroidal aromatase inhibitor used for endocrine therapy in postmenopausal breast cancer. We elucidated the contribution of UDP-glucuronosyltransferase (UGT) isoforms on the glucuronidation of carbinol. Identification of UGT isoforms was performed using a panel of recombinant human UGT enzymes. Kinetic studies were done in recombinant human UGT2B7 and pooled human liver microsomes (HLMs). A liquid chromatography-tandem mass spectrometry method was used for detection of metabolites. To assess the impact of UGT2B7*2, we determined the carbinol glucuronidation activity using HLM as well as UGT2B7 protein expression in 148 human livers. Moreover, we analyzed the plasma concentrations of 60 letrozole-treated breast cancer patients. We identified UGT2B7 as the predominant UGT isoform involved in carbinol glucuronidation. In HLMs and recombinant UGT2B7, we determined K(m) values (9.99 and 9.56 µM) and V(max) values (3430 and 2399 pmol/min per milligram of protein), respectively. In the set of 148 human livers, carbinol glucuronidation activity significantly correlated with UGT2B7 protein as determined by Western blotting (r(s) = 0.5088, P < 0.0001). Neither carbinol glucuronidation activity (*1/*1: n = 25, 2434 ± 1018; *1/*2: n = 80, 2356 ± 1372; *2/*2: n = 43, 2251 ± 1421 pmol/min per milligram of protein) nor UGT2B7 protein expression was altered by the UGT2B7*2 genotype. No impact of UGT2B7*2 on plasma levels of carbinol and carbinol-gluc [bis(4-cyanophenyl)methyl hexopyranosiduronic acid] in 60 letrozole-treated patients was found. Taken together, these findings suggest carbinol as a novel in vitro probe substrate for UGT2B7. In vitro and in vivo data suggest a lack of influence of the UGT2B7*2 polymorphism on carbinol glucuronidation.

Potential role of UGT1A4 promoter SNPs in anastrozole pharmacogenomics

Anastrozole belongs to the nonsteroidal triazole-derivative group of aromatase inhibitors. Recently, clinical trials demonstrated improved antitumoral efficacy and a favorable toxicity with third-generation aromatase inhibitors, compared with tamoxifen. Anastrozole is predominantly metabolized by phase I oxidation with the potential for further phase II glucuronidation. It also, however, is subject to direct N-glucuronidation by UDP-glucuronosyltransferase 1A4 (UGT1A4). Anastrozole pharmacokinetics vary widely among patients, but pharmacogenomic studies of patients treated with anastrozole are sparse. In this study, we examined individual variability in the glucuronidation of anastrozole and its association with UGT1A4 promoter and coding region polymorphisms. In vitro assays using liver microsomal preparations from individual subjects (n = 96) demonstrated 235-fold variability in anastrozole glucuronidation. Anastrozole glucuronidation was correlated (r = 0.99; P < 0.0001) with lamotrigine glucuronidation (a diagnostic substrate for UGT1A4) and with UGT1A4 mRNA expression levels in human liver microsomes (r = 0.99; P < 0.0001). Recombinant UGT1A4 catalyzed anastrozole glucuronidation, which was inhibited by hecogenin (IC50 = 15 µM), a UGT1A4 specific inhibitor. The promoter region of UGT1A4 is polymorphic, and compared with those homozygous for the common allele, lower enzymatic activity was observed in microsomes from individuals heterozygous for -163G<A, -219T<G, and -217C<T (P = 0.009, P = 0.014, and P = 0.009, respectively). These results indicate that variability in glucuronidation could contribute to response to anastrozole in the treatment of breast cancer.

Accurate prediction of glucuronidation of structurally diverse phenolics by human UGT1A9 using combined experimental and in silico approaches

PURPOSE

Catalytic selectivity of human UGT1A9, an important membrane-bound enzyme catalyzing glucuronidation of xenobiotics, was determined experimentally using 145 phenolics and analyzed by 3D-QSAR methods.

METHODS

Catalytic efficiency of UGT1A9 was determined by kinetic profiling. Quantitative structure activity relationships were analyzed using CoMFA and CoMSIA techniques. Molecular alignment of substrate structures was made by superimposing the glucuronidation site and its adjacent aromatic ring to achieve maximal steric overlap. For a substrate with multiple active glucuronidation sites, each site was considered a separate substrate.

RESULTS

3D-QSAR analyses produced statistically reliable models with good predictive power (CoMFA: q2 = 0.548, r2 = 0.949, r pred 2 = 0.775; CoMSIA: q2 = 0.579, r2 = 0.876, r pred 2 = 0.700). Contour coefficient maps were applied to elucidate structural features among substrates that are responsible for selectivity differences. Contour coefficient maps were overlaid in the catalytic pocket of a homology model of UGT1A9, enabling identification of the UGT1A9 catalytic pocket with a high degree of confidence.

CONCLUSION

CoMFA/CoMSIA models can predict substrate selectivity and in vitro clearance of UGT1A9. Our findings also provide a possible molecular basis for understanding UGT1A9 functions and substrate selectivity.

Role of human UDP-glucuronosyltransferases in the biotransformation of the triazoloacridinone and imidazoacridinone antitumor agents C-1305 and C-1311: highly selective substrates for UGT1A10

5-Diethylaminoethylamino-8-hydroxyimidazoacridinone, C-1311 (NSC-645809), is an antitumor agent shown to be effective against breast cancer in phase II clinical trials. A similar compound, 5-dimethylaminopropylamino-8-hydroxytriazoloacridinone, C-1305, shows high activity against experimental tumors and is expected to have even more beneficial pharmacological properties than C-1311. Previously published studies showed that these compounds are not substrates for cytochrome P450s; however, they do contain functional groups that are common targets for glucuronidation. Therefore, the aim of this work was to identify the human UDP-glucuronosyltransferases (UGTs) able to glucuronidate these two compounds. High-performance liquid chromatography analysis was used to examine the activities of human recombinant UGT1A and UGT2B isoforms and microsomes from human liver [human liver microsomes (HLM)], whole human intestinal mucosa [human intestinal microsomes (HIM)], and seven isolated segments of human gastrointestinal tract. Recombinant extrahepatic UGT1A10 glucuronidated 8-hydroxyl groups with the highest catalytic efficiency compared with other recombinant UGTs, V(max)/K(m) = 27.2 and 8.8 μl · min⁻¹ · mg protein⁻¹, for C-1305 and C-1311, respectively. In human hepatic and intestinal microsomes (HLM and HIM, respectively), high variability in UGT activities was observed among donors and for different regions of intestinal tract. However, both compounds underwent UGT-mediated metabolism to 8-O-glucuronides by microsomes from both sources with comparable efficiency; V(max)/K(m) values were from 4.0 to 5.5 μl · min⁻¹ · mg protein⁻¹. In summary, these studies suggest that imid azoacridinone and triazoloacridinone drugs are glucuronidated in human liver and intestine in vivo and may form the basis for future translational studies of the potential role of UGTs in resistance to these drugs.

Pharmacogenomics of third-generation aromatase inhibitors

INTRODUCTION

Breast cancer is a common, life-threatening disease among women. Contemporary hormonal therapy with third-generation aromatase inhibitors for estrogen-receptor-positive breast cancers in postmenopausal women is still facing the challenge of interpatient variability in therapeutic response and intensity of adverse effects.

AREAS COVERED

This review highlights up-to-date literature regarding genomic findings in the literature pertaining to anastrozole, exemestane and letrozole metabolism, as well as the drug target aromatase. Genetic polymorphisms in phase I and II aromatase inhibitor metabolizing enzymes that contribute to altered responses among different patient genotypes are discussed. Similarly, aromatase CYP19A1 functional genetic polymorphisms are presented in correlation to altered aromatase activity, disease prognosis and severity of aromatase inhibitor adverse effects.

EXPERT OPINION

The field of pharmacogenomics has shown remarkable progress over the last few years, notably in cancer. However, large comprehensive genotyping studies, evaluated under clinical settings, are still needed to unravel the potential impact of aromatase inhibitor pharmacogenomics on breast cancer treatment, monitoring and predicting adverse effects.

In vitro and in vivo oxidative metabolism and glucuronidation of anastrozole

AIMS

Little information is available regarding the metabolic routes of anastrozole and the specific enzymes involved. We characterized anastrozole oxidative and conjugation metabolism in vitro and in vivo.

METHODS

A sensitive LC-MS/MS method was developed to measure anastrozole and its metabolites in vitro and in vivo. Anastrozole metabolism was characterized using human liver microsomes (HLMs), expressed cytochrome P450s (CYPs) and UDP-glucuronosyltransferases (UGTs).

RESULTS

Hydroxyanastrozole and anastrozole glucuronide were identified as the main oxidative and conjugated metabolites of anastrozole in vitro, respectively. Formation of hydroxyanastrozole from anastrozole was markedly inhibited by CYP3A selective chemical inhibitors (by >90%) and significantly correlated with CYP3A activity in a panel of HLMs (r= 0.96, P= 0.0005) and mainly catalyzed by expressed CYP3A4 and CYP3A5. The K(m) values obtained from HLMs were also close to those from CYP3A4 and CYP3A5. Formation of anastrozole glucuronide in a bank of HLMs was correlated strongly with imipramine N-glucuronide, a marker of UGT1A4 (r= 0.72, P < 0.0001), while expressed UGT1A4 catalyzed its formation at the highest rate. Hydroxyanastrozole (mainly as a glucuronide) and anastrozole were quantified in plasma of breast cancer patients taking anastrozole (1 mg day⁻¹); anastrozole glucuronide was less apparent.

CONCLUSION

Anastrozole is oxidized to hydroxyanastrozole mainly by CYP3A4 (and to some extent by CYP3A5 and CYP2C8). Once formed, this metabolite undergoes glucuronidation. Variable activity of CYP3A4 (and probably UGT1A4), possibly due to genetic polymorphisms and drug interactions, may alter anastrozole disposition and its effects in vivo.

Aromatase inhibitors: prediction of response and nature of resistance

IMPORTANCE OF THE FIELD

Aromatase inhibitors (AIs) are recommended for and central to endocrine management of breast cancer patients. Response rates can be high, but resistance is a major obstacle. Optimal management therefore requires accurate prediction of response and an understanding of the nature by which resistance occurs. These are the subjects of this review.

AREAS COVERED IN THIS REVIEW

The complications of assessing response in different clinical settings and the types of response in terms of clinical, pathological, proliferative and molecular endpoints are reviewed. The current status of predictors of response such as estrogen receptors (ERs), progesterone receptors, other markers of estrogen action, ER phosphorylation, ER coregulators and multigene signatures are assessed. Different types of resistance to AIs, their heterogeneity, diversity in mechanisms of resistance and their identification are also considered.

WHAT THE READER WILL GAIN

The review provides fundamental information on response and predictors of response to AIs as well as an understanding of the diversity of resistance mechanisms to such endocrine agents.

TAKE HOME MESSAGES

ER status is the only factor used routinely for treatment selection, but additional markers are needed to predict response. Other markers have some predictive powers, but are of limited utility. The hope is, therefore, that discovery strategies based on genome-wide searches will identify new markers. Assessments may be required both before and after a short period of treatment so that early changes can be used to predict subsequent clinical response. Mechanisms of resistance to AIs are diverse. Knowledge of specific resistance mechanisms in individual cases will be necessary if strategies to circumvent resistance are to be developed rationally. A future can be envisaged in which molecular phenotyping of individual tumors is used to decide not only which patients should be treated with AIs but also whether AIs should be used alone or in combination/sequence with other drug regimes.

Characterization of 17-dihydroexemestane glucuronidation: potential role of the UGT2B17 deletion in exemestane pharmacogenetics

OBJECTIVE

Exemestane is a third-generation aromatase inhibitor used in the treatment of breast cancer in postmenopausal women. Reduction to form 17-dihydroexemestane and subsequent glucuronidation to exemestane-17-O-glucuronide is a major pathway for exemestane metabolism. The goal of this study was to analyze 17-dihydroexemestane anti-aromatase activity, characterize the 17-dihydroexemestane glucuronidation pathway, and determine whether the functional polymorphisms in active UGTs could play a role in altered 17-dihydroexemestane glucuronidation.

METHODS

Homogenates from a HEK293 aromatase-overexpressing cell line (HEK293-aro) were used to examine exemestane versus 17-dihydroexemestane anti-aromatase activities. UGT-overexpressing cell lines and a panel (n=110) of human liver microsome (HLM) were screened for glucuronidation activity against 17-dihydroexemestane. UGT2B17 genotyping and liver mRNA expression were performed by real-time PCR.

RESULTS

The inhibition of estrone formation from androst-4-ene-3,17-dione in HEK293-aro cell homogenates was similar for 17-dihydroexemestane (IC(50)=2.3±0.83 μmol/l) and exemestane (IC(50)=1.4±0.42 μmol/l). UGTs 2B17 and 1A4 were high-expression hepatic UGTs that exhibited activity against 17-dihydroexemestane, with UGT2B17 exhibiting a 17-fold higher V(max)/K(M) than UGT1A4. The rate of exemestane-17-O-glucuronide formation was shown to be significantly (P<0.001) decreased (14-fold) in HLMs exhibiting the UGT2B17(*2/*2) deletion genotype versus wild-type UGT2B17(*1/*1) HLMs; a 36-fold lower V(max)/K(M) (P=0.023) was observed in UGT2B17(*2/*2) versus UGT2B17(*1/*1) HLMs. A significant (P<0.0001, R(2)=0.72) correlation was observed between HLM exemestane-17-O-glucuronide formation and liver UGT2B17 expression.

CONCLUSION

These data suggest that 17-dihydroexemestane is an active metabolite of exemestane and that the UGT2B17 deletion polymorphism could play an important role in determining levels of excretion of 17-dihydroexemestane and overall exemestane metabolism.

Function of UDP-glucuronosyltransferase 2B17 (UGT2B17) is involved in endometrial cancer

Endometrial cancer (EC) is a steroid hormone-dependent cancer. Uridine 5'-diphospho-glucuronosyltransferase enzymes conjugate and detoxify endogenous and exogenous steroid hormones and environmental carcinogens. Among these enzymes, the function of UGT2B17 is unknown except for glucuronidation. The messenger RNA expression of UGT2B17 and myeloid cell leukemia-1 (Mcl-1) was significantly increased in EC tissues compared with matched normal endometrial tissues. Therefore, we focused on the function of UGT2B17 in EC. A total of nine patients with confirmed EC were enrolled in this study to investigate the expression of UGT2B17 and target genes. EC cell lines were used for functional tests including cell growth, invasion, apoptosis and cell cycle analyses. To find the target genes of UGT2B17, we performed microarray analysis to see which genes were upregulated or downregulated by UGT2B17-transfected cells. Functional analysis showed decreased numbers of viable cells and increased numbers of apoptotic cells in si-UGT2B17-transfected Ishikawa cells. Among microarray target genes, Mcl-1 was significantly downregulated in si-UGT2B17-transfected cells. We also found upregulation of Puma protein, a target of Mcl-1, in si-UGT2B17-transfected cells. This is the first report to show that UGT2B17 and Mcl-1 expression are upregulated in EC tissues and that UGT2B17 depletion induces inhibition of cell growth and apoptosis in EC cells through Mcl-1 downregulation.

Uridine 5'-diphospho-glucuronosyltransferase genetic polymorphisms and response to cancer chemotherapy

Pharmacogenetics aims to elucidate how genetic variation affects the efficacy and side effects of drugs, with the ultimate goal of personalizing medicine. Clinical studies of the genetic variation in the uridine 5'-diphosphoglucuronosyltransferase gene have demonstrated how reduced-function allele variants can predict the risk of severe toxicity and help identify cancer patients who could benefit from reduced-dose schedules or alternative chemotherapy. Candidate polymorphisms have also been identified in vitro, although the functional consequences of these variants still need to be tested in the clinical setting. Future approaches in uridine 5'-diphosphoglucuronosyltransferase pharmacogenetics include genetic testing prior to drug treatment, genotype-directed dose-escalation studies, study of genetic variation at the haplotype level and genome-wide studies.