Rare Dihydropyrimidine Dehydrogenase Variants and Toxicity by Floropyrimidines: A Case Report

Physiologically Based Pharmacokinetic Modeling for Prediction of 5-FU Pharmacokinetics in Cancer Patients with Hepatic Impairment After 5-FU and Capecitabine Administration

PURPOSE

5-fluorouracil (5-FU) and its prodrug capecitabine are commonly prescribed anti-tumor medications. We aimed to establish physiologically based pharmacokinetic (PBPK) models of capecitabine-metabolites and 5-FU-metabolites to describe their pharmacokinetics in tumor and plasma of cancer patients with liver impairment.

METHODS

Models including the cancer compartment were developed in PK-Sim® and MoBi® and evaluated by R programming language with 25 oral capecitabine and 18 intravenous 5-FU studies for cancer patients with and without liver impairment.

RESULTS

The PBPK models were constructed successfully as most simulated Cmax and AUClast were within two-fold error of observed values. The simulated alterations of tumor 5-FU Cmax and AUClast in cancer patients with severe liver injury compared with normal liver function were 1.956 and 3.676 after oral administration of capecitabine, but no significant alteration was observed after intravenous injection of 5-FU. Besides, 5-FU concentration in tumor tissue increases with higher tumor blood flow but not tumor size. Sensitivity analysis revealed that dihydropyrimidine dehydrogenase (DPD) and other metabolic enzymes' activity, capecitabine intestinal permeability and plasma protein scale factor played a vital role in tumor and plasma 5-FU pharmacokinetics.

CONCLUSIONS

PBPK model prediction suggests no dosage adaption of capecitabine or 5-FU is required for cancer patients with hepatic impairment but it would be reduced when the toxic reaction is observed. Furthermore, tumor blood flow rate rather than tumor size is critical for 5-FU concentration in tumor. In summary, these models could predict pharmacokinetics of 5-FU in tumor in cancer patients with varying characteristics in different scenarios.

Pharmacogenetic Practice of Anticancer Drugs: Multiple Approaches for an Accurate and Comprehensive Genotyping

The application of pharmacogenetics in oncology is part of the routine clinical practice. In particular, genotyping of dihydropyrimidine dehydrogenase (DPYD) and UDP-glucuronosyltransferase (UGT1A1) is crucial to manage the treatment of patients taking fluoropyrimidines and irinotecan. The unique approach of our laboratory to the pharmacogenetic diagnostic service in oncology is to combine two real-time PCR methods, LightSNiP assay (TIB MOLBIOL), and more recently FRET (Fluorescent Resonance Energy Transfer) probes technology (Nuclear Laser Medicine), plus TaqMan assay (Thermo Fisher) for the confirmation of the presence of variant alleles on DNA from a second extraction. We found that both the FRET and LightSNiP assays, where detection occurs by melting curve analysis, offer an advantage over the competing TaqMan technology. Whereas unexpected genetic variants may be missed using a mutation-specific TaqMan assay, the information thus obtained can be useful to adjust the therapy in case of unexpected post-treatment toxicity. The combination of TaqMan and FRET assays helped us to achieve more accurate genotyping and a correct result for the patient. The added value of the DPYD FRET assay is the possibility of detecting, with the same amplification profile of the polymorphisms detailed in the guidelines, also the c.2194G>A (*6 rs1801160), cited in the recommendations as a variant to be investigated in case of severe toxicity. Regarding the UGT1A1 (TA)n promoter polymorphism (rs3064744), the distinctive and positive feature of the FRET assay is to allow clearly identifying all those potential variant alleles, including the (TA)5 and (TA)8 alleles, that are frequent in African Americans. Our clinical practice emphasizes the importance of not only rapid and easy-to-use assays, such as the new FRET ones, but also of accurate and comprehensive genotyping for good pharmacogenetic diagnostic activity.

Rare genetic variant burden in DPYD predicts severe fluoropyrimidine-related toxicity risk

Preemptive targeted pharmacogenetic testing of candidate variations in DPYD is currently being used to limit toxicity associated with fluoropyrimidines. The use of innovative next generation sequencing (NGS) approaches could unveil additional rare (minor allele frequency <1%) genetic risk variants. However, their predictive value and management in clinical practice are still controversial, at least partly due to the challenges associated with functional analyses of rare variants. The aim of this study was to define the predictive power of rare DPYD variants burden on the risk of severe fluoropyrimidine-related toxicity.

The DPYD coding sequence and untranslated regions were analyzed by NGS in 120 patients developing grade 3–5 (NCI-CTC vs3.0) fluoropyrimidine-related toxicity and 104 matched controls (no-toxicity). The functional impact of rare variants was assessed using two different in silico predictive tools (i.e., Predict2SNP and ADME Prediction Framework) and structural modeling. Plasma concentrations of uracil (U) and dihydrouracil (UH2) were quantified in carriers of the novel variants.

Here, we demonstrate that the burden of rare variants was significantly higher in patients with toxicity compared to controls (p = 0.007, Mann-Whitney test). Carriers of at least one rare missense DPYD variant had a 16-fold increased risk in the first cycle and an 11-fold increased risk during the entire course of chemotherapy of developing a severe adverse event compared to controls (p = 0.013 and p = 0.0250, respectively by multinomial regression model). Quantification of plasmatic U/UH2 metabolites and in silico visualization of the encoded protein were consistent with the predicted functional effect for the novel variations.

Analysis and consideration of rare variants by DPYD-sequencing could improve prevention of severe toxicity of fluoropyrimidines and improve patients’ quality of life.

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DPYD genotype-guided dosing reduces fluoropyrimidine (FP) toxicity risk.

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Rare DPYD variants associate with severe FP toxicities.

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Carriers of rare DPYD variants have 11-fold increased risk of toxicity.

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DPYD sequencing and in silico functional prediction could prevent FP toxicity events.

Predicting drug response and toxicity in metastatic colorectal cancer: the role of germline markers

INTRODUCTION

Despite the introduction of targeted agents leading to therapeutic advances, clinical management of patients with metastatic colorectal cancer (mCRC) is still challenged by significant interindividual variability in treatment outcomes, both in terms of toxicity and therapy efficacy. The study of germline genetic variants could help to personalize and optimize therapeutic approaches in mCRC.

AREAS COVERED

A systematic review of pharmacogenetic studies in mCRC patients published on PubMed between 2011 and 2021, evaluating the role of germline variants as predictive markers of toxicity and efficacy of drugs currently approved for treatment of mCRC, was perfomed.

EXPERT OPINION

Despite the large amount of pharmacogenetic data published to date, only a few genetic markers (i.e., DPYD and UGT1A1 variants) reached the clinical practice, mainly to prevent the toxic effects of chemotherapy. The large heterogeneity of available studies represents the major limitation in comparing results and identifying potential markers for clinical use, the role of which remains exploratory in most cases. However, the available published findings are an important starting point for future investigations. They highlighted new promising pharmacogenetic markers within the network of inflammatory and immune response signaling. In addition, the emerging role of previously overlooked rare variants has been pointed out.

Dihydropyrimidine Dehydrogenase Deficiency and Implementation of Upfront DPYD Genotyping

Fluoropyrimidines (FP; 5-fluorouracil, capecitabine, and tegafur) are a commonly prescribed class of antimetabolite chemotherapies, used for various solid organ malignancies in over 2 million patients globally per annum. Dihydropyrimidine dehydrogenase (DPD), encoded by the DPYD gene, is the critical enzyme implicated in FP metabolism. DPYD variant genotypes can result in decreased DPD production, leading to the development of severe toxicities resulting in hospitalization, intensive care admission, and even death. Management of toxicity incurs financial burden on both patients and healthcare systems alike. Upfront DPYD genotyping to identify variant carriers allows an opportunity to identify patients who are at high risk to suffer from serious toxicities and allow prospective dose adjustment of FP treatment. This approach has been shown to reduce patient morbidity, as well as improve the cost-effectiveness of managing FP treatment. Upfront DPYD genotyping has been recently endorsed by several countries in Europe and the United Kingdom. This review summarizes current knowledge about DPD deficiency and upfront DPYD genotyping, including clinical and cost-effectiveness outcomes, with the intent of supporting implementation of an upfront DPYD genotyping service with individualized dose-personalization.

Recent Updates on Mechanisms of Resistance to 5-Fluorouracil and Reversal Strategies in Colon Cancer Treatment

5-Fluorouracil (5-FU) plus leucovorin (LV) remain as the mainstay standard adjuvant chemotherapy treatment for early stage colon cancer, and the preferred first-line option for metastatic colon cancer patients in combination with oxaliplatin in FOLFOX, or irinotecan in FOLFIRI regimens. Despite treatment success to a certain extent, the incidence of chemotherapy failure attributed to chemotherapy resistance is still reported in many patients. This resistance, which can be defined by tumor tolerance against chemotherapy, either intrinsic or acquired, is primarily driven by the dysregulation of various components in distinct pathways. In recent years, it has been established that the incidence of 5-FU resistance, akin to multidrug resistance, can be attributed to the alterations in drug transport, evasion of apoptosis, changes in the cell cycle and DNA-damage repair machinery, regulation of autophagy, epithelial-to-mesenchymal transition, cancer stem cell involvement, tumor microenvironment interactions, miRNA dysregulations, epigenetic alterations, as well as redox imbalances. Certain resistance mechanisms that are 5-FU-specific have also been ascertained to include the upregulation of thymidylate synthase, dihydropyrimidine dehydrogenase, methylenetetrahydrofolate reductase, and the downregulation of thymidine phosphorylase. Indeed, the successful modulation of these mechanisms have been the game plan of numerous studies that had employed small molecule inhibitors, plant-based small molecules, and non-coding RNA regulators to effectively reverse 5-FU resistance in colon cancer cells. It is hoped that these studies would provide fundamental knowledge to further our understanding prior developing novel drugs in the near future that would synergistically work with 5-FU to potentiate its antitumor effects and improve the patient's overall survival.

[Analysis of carrying clinically significant allelic variants of TPMT and DPYD genes associated with the response to drug therapy in cancer practice among 9 ethnic groups of the Russian Federation]

AIM

To study the peculiarities of carrying clinically significant allelic variants of TPMT and DPYD genes associated with the response to drug therapy in cancer practice among 9 ethnic groups of the Russian Federation.

MATERIALS AND METHODS

The study included 1446 conditionally healthy volunteers from 9 ethnic groups. Carriage of polymorphic TPMT and DPYD gene markers was detected by the Real-Time PCR (polymerase chain reaction) method.

RESULTS

In all ethnic groups, the distribution of genotypes and alleles matched the equilibrium of Hardy-Weinberg. TPMT*3A (rs1800460) and TPMT*3C (rs1142345) were observed in heterozygous state in all investigated ethnic groups. In the Kabardinian group (n=204) the frequency of the TPMT*3A minor allele (MAF, %) was 2.94%; Balkars (n=200) 1.25%; Ossetians (n=239) 1.67%; Chuvashes (n=238) 1.89%: Mari (n=206) 1.21%; Tatars (n=141) 1.77%; Russians (n=134) 4.85%. The frequency of the TPMT*3C minor allele (MAF, %) in the Kabardinian group (n=204) MAF was 4.90%; Balkars (n=200) 1. 75%; Buryats (n=114) 0.44%; Ossetians (n=239) 1.88%; Chuvashes (n=238) 1.68%: Mari (n=206) 1.21%; Tatars (n=141) 1.42%; Russians (n=134) 4.48%. The results of the analysis of DPYD*2A polymorphism (rs3918290) demonstrated ethnic peculiarities of distribution. In the heterozygous state it was found only in the groups of Kabardins (n=204, MAF 1.22%), Balkars (n=200, MAF 2.00%), and Ossetians (n=239, MAF 0.63%).

CONCLUSION

The results obtained in the study will be useful for developing personalized algorithms of antitumor therapy in cancer practice, including those aimed at increasing the safety of chemotherapy.