The effect of sorivudine on dihydropyrimidine dehydrogenase activity in patients with acute herpes zoster

Testing for Dihydropyrimidine Dehydrogenase Deficiency to Individualize 5-Fluorouracil Therapy

Simple Summary

5-Fluorouracil (5-FU) is a chemotherapy drug that is commonly used to treat multiple cancers. Many people who are treated with 5-FU experience severe toxicity to the drug, and in severe cases, patients can die. This review discusses current methods for identifying people who are at high risk for severe side effects to 5-FU therapy.

Abstract

Severe adverse events (toxicity) related to the use of the commonly used chemotherapeutic drug 5-fluorouracil (5-FU) affect one in three patients and are the primary reason cited for premature discontinuation of therapy. Deficiency of the 5-FU catabolic enzyme dihydropyrimidine dehydrogenase (DPD, encoded by DPYD) has been recognized for the past 3 decades as a pharmacogenetic syndrome associated with high risk of 5-FU toxicity. An appreciable fraction of patients with DPD deficiency that receive 5-FU-based chemotherapy die as a result of toxicity. In this manuscript, we review recent progress in identifying actionable markers of DPD deficiency and the current status of integrating those markers into the clinical decision-making process. The limitations of currently available tests, as well as the regulatory status of pre-therapeutic DPYD testing, are also discussed.

Review: Mechanisms of How the Intestinal Microbiota Alters the Effects of Drugs and Bile Acids

Information on the intestinal microbiota has increased exponentially this century because of technical advancements in genomics and metabolomics. Although information on the synthesis of bile acids by the liver and their transformation to secondary bile acids by the intestinal microbiota was the first example of the importance of the intestinal microbiota in biotransforming chemicals, this review will discuss numerous examples of the mechanisms by which the intestinal microbiota alters the pharmacology and toxicology of drugs and other chemicals. More specifically, the altered pharmacology and toxicology of salicylazosulfapridine, digoxin, l-dopa, acetaminophen, caffeic acid, phosphatidyl choline, carnitine, sorivudine, irinotecan, nonsteroidal anti-inflammatory drugs, heterocyclic amines, melamine, nitrazepam, and lovastatin will be reviewed. In addition, recent data that the intestinal microbiota alters drug metabolism of the host, especially Cyp3a, as well as the significance and potential mechanisms of this phenomenon are summarized. The review will conclude with an update of bile acid research, emphasizing the bile acid receptors (FXR and TGR5) that regulate not only bile acid synthesis and transport but also energy metabolism. Recent data indicate that by altering the intestinal microbiota, either by diet or drugs, one may be able to minimize the adverse effects of the Western diet by altering the composition of bile acids in the intestine that are agonists or antagonists of FXR and TGR5. Therefore, it may be possible to consider the intestinal microbiota as another drug target.

Antiviral drug development--success and failure: a personal perspective with a Japanese connection

At the 25th International Conference on Antiviral Research, I received a special recognition for my contribution to the International Society of Antiviral Research over a period of 25 years (from 1987 until 2012). This review follows the theme of my presentation at that event, which comprised 10 reminiscences, all with a Japanese connection concerning the success, or otherwise, in the clinical development of: double- and single-stranded polynucleotides; suramin, a polysulfonate; dextran sulfate, a polysulfate; brivudin; BVaraU; 2',3'-dideoxynucleoside analogues; HEPT; adefovir and tenofovir; CXCR4 antagonists; and elvitegravir.

Alterations of chemotherapeutic pharmacokinetic profiles by drug-drug interactions

BACKGROUND

Drug interactions in oncology are common place and largely ignored as we tolerate high thresholds of 'toxic' drug responses in these patients. However, in the era of 'targeted' or seemingly 'less toxic' therapy, these interactions are more commonly flagged and contribute significantly towards poor 'quality of life' and medical fatalities.

OBJECTIVE

This review and opinion article focuses on alteration of chemotherapeutic pharmacokinetic profiles by drug interactions in the setting of polypharmacy. The assumption is that the drugs, with changes in their pharmacokinetics, will contribute towards changes in their pharmacodynamics.

METHODS

The examples cited for such drug-drug interactions are culled from published literature with an emphasis on those interactions that have been well characterized at the molecular level.

RESULTS

Although very few drug interaction studies have been performed on approved oncology based drugs, it is clear that drugs whose pharmacokinetics profiles are closely related to their pharmacodynamics will indeed result in clinically important drug interactions. Some newer mechanisms are described that involve interactions at the level of gene transcription, whereby, drug metabolism is significantly altered. However, for any given drug interaction, there does not seem to be a comprehensive model describing interactions.

CONCLUSIONS

Mechanisms based drug interactions are plentiful in oncology; however, there is an absolute lack of a comprehensive model that would predict drug-drug interactions.

(E)-5-(2-bromovinyl)-2?-deoxyuridine (BVDU)

(E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU, Brivudin, Zostex, Zerpex, Zonavir), now more than 20 years after its discovery, still stands out as a highly potent and selective inhibitor of herpes simplex virus type 1 (HSV-1) and varicella-zoster virus (VZV) infections. It has been used in the topical treatment of herpetic keratitis and recurrent herpes labialis and the systemic (oral) treatment of herpes zoster (zona, shingles). The high selectivity of BVDU towards HSV-1 and VZV depends primarily on a specific phosphorylation of BVDU to its 5'-diphosphate (DP) by the virus-encoded thymidine kinase (TK). After further phosphorylation (by cellular enzymes), to the 5'-triphosphate (TP), the compound interferes as a competitive inhibitor/alternate substrate with the viral DNA polymerase. The specific phosphorylation by the HSV- and VZV-induced TK also explains the marked cytostatic activity of BVDU against tumor cells that have been transduced by the viral TK genes. This finding offers considerable potential in a combined gene therapy/chemotherapy approach for cancer. To the extent that BVDU or its analogues (i.e., BVaraU) are degraded (by thymidine phosphorylase) to (E)-5-(2-bromovinyl)uracil (BVU), they may potentiate the anticancer potency, as well as toxicity, of 5-fluorouracil. This ensues from the direct inactivating effect of BVU on dihydropyrimidine dehydrogenase, the enzyme that initiates the degradative pathway of 5-fluorouracil. The prime determinant in the unique behavior of BVDU is its (E)-5-(2-bromovinyl) substituent. Numerous BVDU analogues have been described that, when equipped with this particular pharmacophore, demonstrate an activity spectrum characteristic of BVDU, including selective anti-VZV activity.

Discovery and development of BVDU (brivudin) as a therapeutic for the treatment of herpes zoster

This Commentary is dedicated to the memory of Dr. Jacques Gielen, the late Editor of Biochemical Pharmacology, whom I have known as both an author and reviewer for the Journal for about 25 years. This is, quite incidentally, about the time it took for bringing brivudin (BVDU) [(E)-5-(2-bromovinyl)-2'-deoxyuridine] from its original description as an antiviral agent to the market place (in a number of European countries, including Germany and Italy) for the treatment of herpes zoster in immunocompetent persons. BVDU is exquisitely active and selective against varicella-zoster virus (VZV) and herpes simplex virus type 1 (HSV-1). BVDU owes this high selectivity and activity profile to a specific phosphorylation by the virus-encoded thymidine kinase, followed by a potent interaction with the viral DNA polymerase. The (E)-5-(2-bromovinyl)-substituent can be considered as the hallmark for the activity of BVDU against VZV and HSV-1. Extensive clinical studies have indicated that BVDU as a single (oral) daily dose of 125 mg (for no more than 7 days) is effective in the treatment of herpes zoster, as regards both short-term (suppression of new lesion formation) and long-term effects (prevention of post-herpetic neuralgia). In this sense, BVDU is as efficient and/or convenient, if not more so, than the other drugs (acyclovir, valaciclovir, famciclovir) that have been licensed for the treatment of herpes zoster. There is one caveat; however, BVDU should not be given to patients under 5-fluorouracil therapy, as the degradation product of BVDU, namely (E)-5-(2-bromovinyl)uracil (BVU), may potentiate the toxicity of 5-fluorouracil, due to inhibition of dihydropyrimidine dehydrogenase, the enzyme involved in the catabolism of 5-fluorouracil.

The effect of dihydropyrimidine dehydrogenase deficiency on outcomes with fluorouracil

The use of fluorouracil has been complicated by unpredictable pharmacokinetics, low response rates and seemingly random toxicity. The variable pharmacology is largely due to inherited differences in expression of the metabolising enzyme dihydropyrimidine dehydrogenase (DPD). This converts fluorouracil to inactive metabolites (catabolic pathway) and ultimately dictates the amount of fluorouracil that is available to be metabolised to cytotoxic nucleotides (anabolic pathway). Absolute and partial DPD deficiency affect around 0.1 and 3% of the Caucasian population, respectively. Administration of conventional doses of fluorouracil to these individuals has resulted in profound bone marrow and gastrointestinal toxicity, especially in those with absolute DPD deficiency. Other forms of toxicity such as myocardial ischaemia have been difficult to attribute directly to DPD deficiency. Efforts to improve outcomes with fluorouracil have included monitoring of fluorouracil concentrations and modifying fluorouracil administration techniques (e.g. from bolus injections to protracted intravenous infusions). In general, these moves have met with limited therapeutic advancement. The recognition that DPD deficiency increases toxicity has lead to the suggestion that genotypic or phenotypic assessment of DPD status prior to initiating fluorouracil may help predict outcomes. The gene that encodes for DPD expression is called DPYD. Approximately 1% of Caucasians are heterozygotes for the DPYD*2A mutation which is the variant allele that is most frequently implicated in DPD deficiency. Screening for this mutation may identify around 60% of individuals with absolute DPD deficiency who are at the greatest risk of toxicity. Another approach is to determine DPD activity in peripheral blood mononuclear cells, with low activity suggesting an increased risk of toxicity. Intratumoral DPD activity may also be assessed with high activity suggesting a poorer response to fluorouracil. Recently, drugs that inhibit DPD (e.g. eniluracil) have become available. These remove much of the variability in fluorouracil pharmacokinetics and may make assessment of DPD activity redundant. Despite the considerable inroads that have been made, further study is needed before the best means of optimising fluorouracil treatment is determined.

Phase I and pharmacokinetic trial of weekly oral fluorouracil given with eniluracil and low-dose leucovorin to patients with solid tumors

PURPOSE

Fluorouracil (5-FU) given as a weekly, high-dose 24-hour infusion is active and tolerable. We evaluated an oral regimen of eniluracil (which inactivates dihydropyrimidine dehydrogenase [DPD]), 5-FU, and leucovorin to simulate this schedule.

PATIENTS AND METHODS

Patients received a single 24-hour infusion of 5-FU (2,300 mg/m(2) on day 2) with leucovorin (15 mg orally [PO] bid on days 1 through 3) to provide reference pharmacokinetic data. Two weeks later, patients began treatment with eniluracil (20 mg) and leucovorin (15 mg) (PO bid on days 1 through 3) and 5-FU (10 to 15 mg/m(2) PO bid on day 2).

RESULTS

Dose-limiting toxicity (diarrhea, neutropenia, and fatigue) was seen with 5-FU 15 mg/m(2) PO bid on day 2 given weekly for either 6 of 8 weeks or 3 of 4 weeks, whereas five of seven patients tolerated 5-FU 10 mg/m(2) PO bid given weekly for 3 of 4 weeks. Eniluracil led to a 35-fold reduction in 5-FU clearance. Fluoro-beta-alanine, a 5-FU catabolite, was not detected in plasma during oral 5-FU-eniluracil therapy. DPD activity was markedly suppressed in all patients during eniluracil therapy; the inactivation persisted after the last eniluracil dose; percentages of baseline values were 1.8% on day 5, 4.5% on day 12, and 23.6% on day 19.

CONCLUSION

The recommended oral dosage of 5-FU (10 mg/m(2) PO bid) given with eniluracil and leucovorin is approximately 115-fold lower than the reference dosage for 24-hour infusional 5-FU. This difference is greater than expected given the reduction in 5-FU clearance. DPD inactivation persisted for several weeks after completion of eniluracil therapy.

Preclinical development of eniluracil: enhancing the therapeutic index and dosing convenience of 5-fluorouracil

Eniluracil (5-ethynyluracil, GW 776, 776C85) is being developed as a novel modulator of 5-fluorouracil (5-FU) for the treatment of cancer. Eniluracil is an effective mechanism-based inactivator of dihydropyrimidine dehydrogenase (DPD), the first enzyme in the catabolic pathway of 5-FU. By temporarily eliminating this prevalent enzyme, eniluracil provides predictable dosing of 5-FU and enables oral administration of 5-FU to replace intravenous bolus and continuously infused dosing. New DPD is synthesized with a half-life of 2.6 days. It also eliminates the formation of problematic 5-FU catabolites. Most importantly, in laboratory animals, eniluracil increases the therapeutic index and absolute efficacy of 5-FU. Accompanying reports in this journal indicate that eniluracil has promising clinical potential.

Measurement of plasma uracil using gas chromatography-mass spectrometry in normal individuals and in patients receiving inhibitors of dihydropyrimidine dehydrogenase

A sensitive gas chromatographic-mass spectrometric method is described for reliably measuring endogenous uracil in 100 microl of human plasma. Validation of this assay over a wide concentration range, 0.025 microM to 250 microM (0.0028 microg/ml to 28 microg/ml), allowed for the determination of plasma uracil in patients treated with agents such as eniluracil, an inhibitor of the pyrimidine catabolic enzyme, dihydropyrimidine dehydrogenase. Calibration standards were prepared in human plasma using the stable isotope, [15N2]uracil, to avoid interference from endogenous uracil and 10 microM 5-chlorouracil was added as the internal standard.

Varicella zoster virus

Because of its ability to produce two clinically distinct disease entities (chickenpox and shingles), varicella zoster virus (VZV) is an unusual etiologic agent. Although in the past viral exanthems were mostly only of academic interest to the practitioner, the development of antiviral agents and the newly approved varicella (OKA) vaccine have increased the clinical significance. Also, with the increasing seroprevalence of HIV infection, more patients will be stricken with zoster (at a younger age) and disseminated varicella. In this review, the history, incidence, pathogenesis, clinical manifestations, and treatment options (of VZV infection and postherpetic neuralgia) will be discussed.

Development and validation of a bioanalytical assay for (E)-5-(2-bromovinyl)-2'deoxyuridine in plasma by capillary zone electrophoresis

A capillary zone electrophoretic method for the quantification of (E)-5-(2-bromovinyl)-2'-deoxyuridine in plasma has been developed and validated. Separation was performed with a 25 mmol/l borate buffer, pH 9.0, after an initial rinsing step with sodium hydroxide. The rinsing step was necessary for reproducible analyses of aqueous samples and plasma extracts obtained by C18 solid-phase extraction after deproteination with perchloric acid. No interferences with plasma compounds were observed. The calibration graph was linear over the range of 30 to 3000 ng/ml using 5-fluorouracil as external standard. The limit of quantification was 24 ng/ml. The CZE method is fast, reproducible, linear and is therefore a good alternative for the already established HPLC methods.

Sorivudine and 5-fluorouracil; a clinically significant drug-drug interaction due to inhibition of dihydropyrimidine dehydrogenase

Sorivudine (1-beta-D-arabinofuranosyl-E-5-[2-bromovinyl] uracil; BV-araU; SQ32,756) is an antimetabolite which is a synthetic analogue of thymidine. This drug has demonstrated antiviral activity against varicella zoster virus, herpes simplex type 1 virus, and Epstein-Barr virus. Clinical studies in Japan and subsequently worldwide showed this drug to be a potent agent for treating varicella zoster infections. Although in general well tolerated, a fatal drug interaction with fluoropyrimidine drugs was subsequently observed. While three deaths resulting from this interaction were recognized to have occurred during the initial clinical evaluation in Japan, the full impact of the interaction was not recognized in Japan until post-marketing when an additional 23 cases of severe toxicity were reported including 16 patients who subsequently died from fluoro-pyrimidine toxicity. Worldwide recognition of this potentially fatal drug-drug interaction led to subsequent disapproval in the US and elsewhere. The interaction has been shown to be due to suppression of 5-fluorouracil (5-FU) catabolism, resulting in higher levels of 5-FU than would normally be observed. The mechanism of this interaction is mediated through inhibition of the 5-FU rate-limiting catabolizing enzyme dihydropyrmidine dehydrogenase (DPD) by the BV-araU metabolite BVU. This drug-drug interaction of sorivudine and 5-FU further emphasizes the critical importance of DPD on the clinical pharmacology of 5-FU.