Current Drug Metabolism - Volume 12, Issue 10, 2011

For several years, 5-fluorouracil (5-FU) has represented the backbone in the majority of regiments for colorectal cancer (CRC) treatment; thanks to the introduction in the clinical practice of new drugs such as irinotecan and oxaliplatin and modern biological drugs such as bevacizumab and cetuximab the treatment of CRC evolved substantially during the past decade. Despite this progress, many questions remain unsolved highlighting the urgent need to identify predictor factors of response and toxicity that could help clinicians in selecting patients to be treated with these drugs in order to obtain better treatment responses. In this issue, our aim was to provide readers a critical and updated review of data regarding the status of treatment with chemotherapeutics or biological agents and possible predictor factors of response and toxicity that can give a general view of therapeutic options in selected populations of CRC patients. The issue starts with Gnoni et al. analysis of the pharmacokinetic and metabolism determinants of oxaliplatin and fluoropyrimidines activity in treatment of colorectal patients. They reported, through an extensive revision of literature data, that predictive markers of these “ancient” drugs effectiveness and toxicity are thymidylate synthase, dihydropyrimidine dehydrogenase, orotate phosphoribosyl transferase, methylenetetrahydrofolate reductase, deoxyuridine triphosphate nucleotidohydrolase and microsatellite instability for fluoropyrimidines and excision repair cross-complementation group 1, X-ray cross-complementing group 1, xeroderma pigmentosum group D and glutathione Stransferase for oxaliplatin. Di Paolo et al. critically revised clinical pharmacology and pharmacogenetics of irinotecan enlightening that more than cellular markers also genetic polymorphisms seem to offer a more reliable and safer approach for the identification of patients at risk...... In recent years, target therapy has been validated in the treatment of CRC patients; in their report, Caraglia et al. revised the effectiveness and reliable markers of efficacy and toxicity for drugs acting on EGFR and VEGF pathways. A deepening on the relevance of genetic aberrations underlying CRC development has been provided by Leon et al. and Yi and Tang who summarized updated insights pointing out a correlation between DNA copy-number profiles and response to fluoropyrimidine-based regimens and the relationship between imbalance alternative splicing with cancer, respectively. Moreover, Porcelli et al. reported that while folate supplementation is an essential part of several current chemotherapeutic regimens the increasing folate fortification of population may interfere with the effectiveness of chemotherapeutics. In their review, the authors reported the cellular and molecular mechanisms of cellular adaptation to folate status that may alter the pharmacology of chemotherapeutics mainly used in the treatment of CRC pointing out the importance of a preliminary evaluation of folate in tissue and blood of patients to maximize treatment efficacy while reducing a long-term impact on drug resistance. Finally, stressing the concept that chemotherapy could be responsible for MultiDrug Resistance through the involvement of ABC transporters, Colabufo et al. revised literature data on the possibility to use ABC transporters' ligands as PET radiotracers, emphasizing the dual role of these compounds as drugs and radiotracers. I and all authors involved in the preparation of this issue hope that it could be a useful tool for updating on the current biomarkers of chemotherapeutics efficacy and toxicity in CRC and we wish “good reading” to all. I think that a special thanks is due to our reviewers for their strong effort.

Fluoropyrimidines and oxaliplatin continued to be the mainstay of therapeutic regimens in the treatment of colorectal cancer (CRC). For this reason, pharmacokinetic and metabolism of these drugs were analyzed and the identification of accurate and validated predictive, prognostic and toxicity markers became necessary to develop an effective therapy adapted to the patient's molecular profile, while minimizing life-threatening toxicities. In this review, we discuss literature data, defining predictive and prognostic markers actually identified in the treatment of CRC. We analyzed predictive markers of fluoropyrimidines effectiveness, principally for 5-Fluorouracil (5- FU) and also for oral fluoropyrimidines, as thymidylate Synthase (TS), dihydropyrimidine dehydrogenase (DPD), orotate phosphoribosyl transferase (OPRT), methylenetetrahydrofolate reductase (MTHFR), deoxyuridine triphosphate nucleotidohydrolase (dUTPase), microsatellite instability. DPD represent the more studied 5-FU toxicity marker, followed by TS and OPRT. Oxaliplatin effectiveness is principally regulated by nucleotide excision repair (NER) pathway, including excision repair cross-complementation group 1 (ERCC1), X-ray cross-complementing group 1 (XRCC1) and xeroderma pigmentosum group D (XDP). The major oxaliplatin toxicity marker is represented by glutathione S-transferase (GST). All these results are based principally on retrospective studies. The future challenge became to validate molecular markers and their association with clinical outcomes in prospective trials, refining technologic platforms and bioinformatics to accommodate the complexity of the multifaceted molecular map that may determine outcome, and determining CRC patients most likely to benefit from therapeutic interventions tailored specifically for them.

After the rapid development of new classes of antineoplastic drugs, research activities have focused their efforts to the identification of predictive markers of drug activity and tolerability. Irinotecan (CPT-11) may induce severe toxicities (diarrhea, neutropenia) that limit its clinical use, but the increasing knowledge of its pharmacokinetics offered a potential approach to treatment optimization. Pharmacokinetics, the first area of investigation, has identified markers such as biliary index, the relative extent of conversion and the glucuronidation ratio, which are capable to define the risk for severe adverse effects. Because of the existence of some issues concerning the adoption of pharmacokinetic strategies to optimize CPT-11 dose and schedule, analyses of genetic polymorphisms seemed to offer a more reliable and safer approach for the identification of patients at risk than pharmacokinetics. In this view, the uridine diphosphate glucuronosil transferase isoform 1A1 (UGT1A1) was associated with significant changes in disposition of CPT-11 and its metabolites, and consequently with treatment-induced toxicities. However, the complex pharmacokinetics of irinotecan and the involvement of several enzymes other than UGT (i.e., carboxyl estherases, CYP450 isoforms), and transmembrane transporters (ABCB1, ABCC1, ABCG2, SLCO1B1) make difficult the identification of patients with an optimal sensitivity and specificity, and a large part of variability among patients still remains unexplained. Furthermore, prospective clinical studies that should demonstrate the reliability of those pharmacokinetic and pharmacogenetic markers are still lacking. In the present review, pharmacokinetic and pharmacogenetic markers will be discussed.

The treatment of solid malignancies includes various target drugs, such as monoclonal antibodies and tyrosine kinase inhibitors, which exert their effect alone or in combination with chemotherapy. The main part of these molecules have a target on proteins of EGFR and VEGF pathways. The particular toxicity profile and the financial impact, deriving from the application of these agents in cancer treatment, prompted a lot of researches to define predictive factors of their efficacy. Various biomarker were identified among the components of the targeted pathways. However just few studies allowed to identify specific factors to predict the toxicity of these drugs. In this review EGFR and VEGF-related pathways are described, most relevant clinical findings about target therapy applications are exposed and the clinical impact of predictive factors of efficacy and toxicity are discussed.

For decades 5-fluorouracil (5-FU) has remained the treatment of choice in the adjuvant and palliative setting of colorectal cancer (CRC). The combinations of 5-FU or its oral prodrug capecitabine with irinotecan/oxaliplatin and the novel agents bevacizumab/cetuximab increased responses. However, the overall prognosis is poor, and predictive biomarkers of cytotoxic drugs activity are missing. Pharmacogenetic studies focused on candidate determinants of drug activity/metabolism, such as thymidylate synthase or dihydropyrimidine dehydrogenase, but reported controversial results. Given the heterogeneous and complex nature of CRC, it is likely that many aberrations underlying its progression can also affect therapeutic response. Therefore, high-throughput arrays for genome-wide-DNA aberrations play a pivotal role for new markers discovery by moving from hypothesis-driven, targeted-research to unbiased screening of the whole genetic spectrum. Chromosomal aberrations are critical events in tumorigenesis, and genomic regions harbouring DNA gains/losses have been identified in 85% of CRC patients. These aberrations change the expression of many genes, which might explain the differential effects of specific chemotherapeutic agents. In particular, recent studies reported correlations between DNA copy-number profiles and response to fluoropyrimidine-based regimens, such as leucovorin-modulated-5-FU&plusirinotecan (FOLFIRI), capecitabine+irinotecan (CAPIRI) and pemetrexed+irinotecan (ALIRI). Genome-wide profiling by oligonucleotide-based array-comparative-genomic-hybridization (aCGH) revealed genomic loci, of which the copy-number status may serve as marker for outcome after FOLFIRI and CAPIRI. Larger randomized and prospective trials of these aCGH platforms in CRC patients treated with fluoropyrimidine-based regimens are ongoing, and will ultimately demonstrate whether these findings can be of actual value to predict clinical outcome and direct the choice of therapy.

Surgical resection and adjuvant therapy, which mainly involves 5-fluorouracil (5-FU), irinotecan (CPT-11), oxaliplatin (LOHP) chemotherapy and recently targeted therapy, are the most common treatments of colorectal cancer (CRC). As to improve the therapeutic efficacy and assist with therapeutic decisions, there is an urgent need for prognostic and predictive molecular biomarkers. Recent evidence demonstrates that aberrations in alternative splicing process of cancer will provide candidate biomarkers for cancers to meet this need. In this review, we outline the fundamental mechanism of alternative splicing that plays a major role in protein diversity, and summarize the relationship between imbalance alternative splicing with cancer. Moreover, several alternative spliced variants and cancerspecific splicing events at the mRNA level in CRC, which may serve as diagnostic, predictive, prognostic markers of CRC, are also discussed. These specific splice variants or the RNA splicing machinery will be new, potential targets for the treatment of CRC that offers a specific site of anti-cancer chemotherapy.

Over the past three decades, numerous reports have addressed several aspects of drug resistance phenomena. However, little is known regarding the impact that dietary components and nutritional supplements have on the mechanisms of resistance that malignant cells develop to chemotherapeutic agents. The increased fortification of cereals, grains and bread with folic acid (FA) has resulted in a marked rise in folate levels in blood and tissues. Vitamin fortification that includes FA is rather commonly used by cancer patients, but FA is also used to protect against pemetrexed induced side effects in the treatment of non-small cell lung cancer and mesothelioma or that of the antifolate methotrexate in rheumatoid arthritis. Moreover, the reduced folate leucovorin (LV, 5-formyltetrahydrofolate) is also used along with 5-fluorouracil in the treatment of colorectal cancer. Likewise, LV is used to reduce toxicity of methotrexate in the treatment of leukemia. FA can also increase efficacy of unrelated regimens, containing cisplatin. Hence there is growing evidence that dietary supplements as folic acid, can mimic, intensify, or attenuate the effects of unrelated chemotherapeutic agents. The aim of this review is to highlight some new insights in the cellular and molecular mechanisms affected by folate status, leading to chemotherapy resistance, especially towards antifolates in colorectal cancer treatment. This encompasses the effect of folate status on drug export, as well as on the increased expression of mutated target enzymes involved in folate metabolism and on the augmentation of cellular folate pools that impair polyglutamylation of antifolates, ultimately affecting treatment efficacy.

The pharmacological treatment of colorectal tumour leads to MultiDrug Resistance due to overexpression of several ABC transporters such as P-glycoprotein and some Multidrug associated Resistance Proteins (MRPs) that are able to efflux the chemotherapeutic agent out of the cell. A strategy to reverse MDR is the co-administration of antineoplastic agent with a P-glycoprotein inhibitor. These inhibitors are an useful tool for investigating, by PET, the expression and the activity of P-gp and MRPs that are overexpressed in chemoresistant colorectal tumor cells. In this review will be focused the aspect on P-gp and MRPs ligands employed as PET radiotracers considering their pharmacokinetic pharmacodynamic profile and their selectivity towards ABC transporters involved in chemoresistant cell of colorectal tumour.

It is estimated that three quarters of the world population rely on herbal and traditional medicine as a basis for primary health care. Therefore, it is one of the most important and challenging tasks for scientists working in drug research to investigate the efficacy of herbal medicine, to dissect favorable from adverse effects, to identify active principles in medicinal plants and to ban poisonous plants or contaminations from herbal mixtures. In the present review, some problems are critically discussed. Botanical misidentification or mislabeling of plant material can play a role for toxic reactions in humans. Some plant descriptions in traditional herbal medicine (e.g. traditional Chinese medicine) have changed over time, which may lead to unintended intoxication by using wrong plants. A problem is also the contamination of herbals with microorganisms, fungal toxins such as aflatoxin, with pesticides and heavy metals. Unprofessional processing, which differs from safe traditional preparation represents another potential source for herbal poisoning. Unwanted effects of herbal products may also develop by the interaction of herbs with conventional drugs upon concomitant intake. The art of herbal medicine is to dissect pharmacologically and therapeutically valuable herbal drugs from harmful and toxic ones and to develop combinations of medicinal plants as safe and efficient herbal remedies. Standardization and strict control measures are necessary to monitor sustainable high quality of herbal products and to exclude contaminations that badly affect patients consuming herbal medicine.

Extrapolation of the metabolic, pharmacokinetic and toxicological data obtained from animals to humans is not always straightforward, given the remarkable species difference in drug metabolism that is due in large part to the differences in drugmetabolizing enzymes between animals and humans. Furthermore, genetic variations in drug-metabolizing enzymes may significantly alter pharmacokinetics, drug efficacy and safety. Thus, humanized transgenic mouse lines, in which the human drug-metabolizing enzymes are expressed in mouse tissues in the presence or absence of mouse orthologues, have been developed to address such challenges. These humanized transgenic mice are valuable animal models in understanding the significance of specific human drug-metabolizing enzymes in drug clearance and pharmacokinetics, as well as in predicting potential drug-drug interactions and chemical toxicity in humans. This review, therefore, aims to summarize the development and application of some humanized transgenic mouse models expressing human drug-metabolizing enzymes. The limitations of these genetically modified mouse models are also discussed.