Mechanism-based inactivation of dihydropyrimidine dehydrogenase by 5-ethynyluracil

Cyanine- and Rhodamine-Derived Alkynes for the Selective Targeting of Cancerous Mitochondria through Radical Thiol-Yne Coupling in Live Cells

Despite their long history and their synthetic potential underlined by various recent advances, radical thiol-yne coupling reactions have so far only rarely been exploited for the functionalization of biomolecules, and no examples yet exist for their application in live cells - although natural thiols show widespread occurrence therein. By taking advantage of the particular cellular conditions of mitochondria in cancer cells, we have demonstrated that radical thiol-yne coupling represents a powerful reaction principle for the selective targeting of these organelles. Within our studies, fluorescently labeled reactive alkyne probes were investigated, for which the fluorescent moiety was chosen to enable both mitochondria accumulation as well as highly sensitive detection. After preliminary studies under cell-free conditions, the most promising alkyne-dye conjugates were evaluated in various cellular experiments comprising analysis by flow cytometry and microscopy. All in all, these results pave the way for improved future therapeutic strategies relying on live-cell compatibility and selectivity among cellular compartments.

The unusual chemical sequences of mammalian dihydropyrimidine dehydrogenase revealed by transient-state analysis

Dihydropyrimidine dehydrogenase (DPD) catalyzes the reduction of the 5,6-vinylic bond of uracil and thymine with electrons from NADPH. The complexity of the enzyme belies the simplicity of the reaction catalyzed. To accomplish this chemistry DPD has two active sites that are ∼60Å apart, both of which house flavin cofactors, FAD and FMN. The FAD site interacts with NADPH, while the FMN site with pyrimidines. The distance between the flavins is spanned by four Fe₄S₄ centers. Though DPD has been studied for nearly 50years, it is only recently that the novel apects of its mechanism have been described. The primary reason for this is that the chemistry of DPD is not portrayed adequately by known descriptive steady-state mechanism categories. The highly chromophoric nature of the enzyme has recently been exploited in transient-state to document unexpected reaction sequences. Specifically, DPD undergoes reductive activation prior to catalytic turnover. Two electrons are taken up from NADPH and transmitted via the FAD and Fe₄S₄ centers to form the FAD•4(Fe₄S₄)•FMNH₂ form of the enzyme. This form of the enzyme will only reduce pyrimidine substrates in the presence NADPH, establishing that hydride transfer to the pyrimidine precedes reductive reactivation that reinstates the active form of the enzyme. DPD is therefore the first flavoprotein dehydrogenase known to complete the oxidative half-reaction prior to the reductive half-reaction. Here we describe the methods and deduction that led to this mechanistic assignment.

Dihydropyrimidine Dehydrogenase-Mediated Resistance to 5-Fluorouracil: Mechanistic Investigation and Solution

5-Fluorouracil (5-FU) is one of the most widely used chemotherapeutics for the treatment of cancers associated with the aerodigestive tract, breast, and colorectal system. The efficacy of 5-FU is majorly affected by dihydropyrimidine dehydrogenase (DPD) as it degrades more than 80% of administered 5-FU into an inactive metabolite, dihydrofluorouracil. Herein we discuss the molecular mechanism of this inactivation by analyzing the interaction pattern and electrostatic complementarity of the DPD-5-FU complex. The basis of DPD overexpression in cancer cell lines due to significantly distinct levels of the miRNAs (miR-134, miR-27b, and miR-27a) compared to normal cells has also been outlined. Additionally, some kinases including sphingosine kinase 2 (SphK2) have been reported to correlate with DPD expression. Currently, to address this problem various strategies are reported in the literature, including 5-FU analogues (bypass the DPD-mediated inactivation), DPD downregulators (regulate the DPD expression levels in tumors), inhibitors (as promising adjuvants), and formulation development loaded with 5-FU (liposomes, nanoparticles, nanogels, etc.), which are briefly discussed in this Review.

Drug Resistance in Colorectal Cancer: From Mechanism to Clinic

Colorectal cancer (CRC) is one of the leading causes of death worldwide. The 5-year survival rate is 90% for patients with early CRC, 70% for patients with locally advanced CRC, and 15% for patients with metastatic CRC (mCRC). In fact, most CRC patients are at an advanced stage at the time of diagnosis. Although chemotherapy, molecularly targeted therapy and immunotherapy have significantly improved patient survival, some patients are initially insensitive to these drugs or initially sensitive but quickly become insensitive, and the emergence of such primary and secondary drug resistance is a significant clinical challenge. The most direct cause of resistance is the aberrant anti-tumor drug metabolism, transportation or target. With more in-depth research, it is found that cell death pathways, carcinogenic signals, compensation feedback loop signal pathways and tumor immune microenvironment also play essential roles in the drug resistance mechanism. Here, we assess the current major mechanisms of CRC resistance and describe potential therapeutic interventions.

Mammalian dihydropyrimidine dehydrogenase

Dihydropyrimidine dehydrogenase (DPD) catalyzes the two-electron reduction of pyrimidine bases uracil and thymine as the first step in pyrimidine catabolism. The enzyme achieves this simple chemistry using a complex cofactor set including two flavins and four Fe₄S₄ centers. The flavins, FAD and FMN, interact with respective NADPH and pyrimidine substrates and the iron-sulfur centers form an electron transfer wire that links the two active sites that are separated by 56 Å. DPD accepts the common antineoplastic agent 5-fluorouracil as a substrate and so undermines the establishment of efficacious toxicity. Though studied for multiple decades, a precise description of the behavior of the enzyme had remained elusive. It was recently shown that the active form of DPD has the cofactor set of FAD-4(Fe₄S₄)-FMNH₂. This two-electron reduced state is consistent with fewer mechanistic possibilities and data suggests that the instigating and rate determining step in the catalytic cycle is reduction of the pyrimidine substrate that is followed by relatively rapid oxidation of NADPH at the FAD that, via the electron conduit of the 4(Fe₄S₄) centers, reinstates the FMNH₂ cofactor for subsequent catalytic turnover.

Perturbing the Movement of Hydrogens to Delineate and Assign Events in the Reductive Activation and Turnover of Porcine Dihydropyrimidine Dehydrogenase

The native function of dihydropyrimidine dehydrogenase (DPD) is to reduce the 5,6-vinylic bond of pyrimidines uracil and thymine with electrons obtained from NADPH. NADPH and pyrimidines bind at separate active sites separated by ∼60 Å that are bridged by four Fe₄S₄ centers. We have shown that DPD undergoes reductive activation, taking up two electrons from NADPH [Beaupre, B. A., et al. (2020) Biochemistry 59, 2419-2431]. pH studies indicate that the rate of turnover is not controlled by the protonation state of the general acid, cysteine 671. The activation of the C671 variants is delineated into two phases particularly at low pH values. Spectral deconvolution of the delineated reductive activation reaction reveals that the initial phase results in the accumulation of charge transfer absorption added to the binding difference spectrum for NADPH. The second phase results in reduction of one of the two flavins. X-ray crystal structure analysis of the C671S variant soaked with NADPH and the slow substrate, thymine, in a low-oxygen atmosphere resolved the presumed activated form of the enzyme that has the FMN cofactor reduced. These data reveal that charge transfer arises from the proximity of the NADPH and FAD bases and that the ensuing flavin is a result of rapid transfer of electrons to the FMN without accumulation of reduced forms of the FAD or Fe₄S₄ centers. These data suggest that the slow rate of turnover of DPD is governed by the movement of a mobile structural feature that carries the C671 residue.

The Interaction of Porcine Dihydropyrimidine Dehydrogenase with the Chemotherapy Sensitizer: 5-Ethynyluracil

Dihydropyrimidine dehydrogenase (DPD) is a complex enzyme that reduces the 5,6-vinylic bond of pyrimidines, uracil, and thymine. 5-Fluorouracil (5FU) is also a substrate for DPD and a common chemotherapeutic agent used to treat numerous cancers. The reduction of 5FU to 5-fluoro-5,6-dihydrouracil negates its toxicity and efficacy. Patients with high DPD activity levels typically have poor outcomes when treated with 5FU. DPD is thus a central mitigating factor in the treatment of a variety of cancers. 5-Ethynyluracil (5EU) covalently inactivates DPD by cross-linking with the active-site general acid cysteine in the pyrimidine binding site. This reaction is dependent on the simultaneous binding of 5EU and nicotinamide adenine dinucleotide phosphate (NADPH). This ternary complex induces DPD to become activated by taking up two electrons from the NADPH. The covalent inactivation of DPD by 5EU occurs concomitantly with this reductive activation with a rate constant of ∼0.2 s-1. This kinact value is correlated with the rate of reduction of one of the two flavin cofactors and the localization of a mobile loop in the pyrimidine active site that places the cysteine that serves as the general acid in catalysis proximal to the 5EU ethynyl group. Efficient cross-linking is reliant on enzyme activation, but this process appears to also have a conformational aspect in that nonreductive NADPH analogues can also induce a partial inactivation. Cross-linking then renders DPD inactive by severing the proton-coupled electron transfer mechanism that transmits electrons 56 Å across the protein.

Transient-State Analysis of Porcine Dihydropyrimidine Dehydrogenase Reveals Reductive Activation by NADPH

Dihydropyrimidine dehydrogenase (DPD) catalyzes the initial step in the catabolism of the pyrimidines uracil and thymine. Crystal structures have revealed an elaborate subunit architecture consisting of two flavin cofactors, apparently linked by four Fe₄S₄ centers. Analysis of the DPD reaction(s) equilibrium position under anaerobic conditions revealed a reaction that favors dihydropyrimidine formation. Single-turnover analysis shows biphasic kinetics. The serine variant of the candidate general acid, cysteine 671, provided enhanced kinetic resolution for these phases. In the first event, one subunit of the DPD dimer takes up two electrons from NADPH in a reductive activation. Spectrophotometric deconvolution suggests that these electrons reside on one of the two flavins. The fact that oxidation of the enzyme by dioxygen can be suppressed by the addition of pyrimidine is consistent with these electrons residing on the FMN. The second phase involves further oxidation of NADPH and concomitant reduction of the pyrimidine substrate. During this phase no net reduction of DPD cofactors is observed, indicating that the entire cofactor set acts as a wire, transmitting electrons from NADPH to the pyrimidine rapidly. This indicates that the availability of the proton from the C671 general acid controls the transmittance of electrons from NADPH to the pyrimidine. Acid quench and high-performance liquid chromatography product analysis of single-turnover reactions with limiting NADPH confirmed a 2:1 NADPH:pyrimidine stoichiometry for the enzyme, accounting for successive activation and pyrimidine reduction. These data support an alternating subunit model in which one protomer is activated and turns over before the other subunit can be activated and enter catalysis.

Acetylene Group, Friend or Foe in Medicinal Chemistry

The use of an acetylene (ethynyl) group in medicinal chemistry coincides with the launch of the Journal of Medicinal Chemistry in 1959. Since then, the acetylene group has been broadly exploited in drug discovery and development. As a result, it has become recognized as a privileged structural feature for targeting a wide range of therapeutic target proteins, including MAO, tyrosine kinases, BACE1, steroid receptors, mGlu5 receptors, FFA1/GPR40, and HIV-1 RT. Furthermore, a terminal alkyne functionality is frequently introduced in chemical biology probes as a click handle to identify molecular targets and to assess target engagement. This Perspective is divided into three parts encompassing: (1) the physicochemical properties of the ethynyl group, (2) the advantages and disadvantages of the ethynyl group in medicinal chemistry, and (3) the impact of the ethynyl group on chemical biology approaches.

Stimuli-Responsive Therapeutic Metallodrugs

The success of platinum-based anticancer agents has motivated the exploration of novel metal-based drugs for several decades, whereas problems such as drug-resistance and systemic toxicity hampered their clinical applications and efficacy. Stimuli-responsiveness of some metal complexes offers a good opportunity for designing site-specific prodrugs to maximize the therapeutic efficacy and minimize the side effect of metallodrugs. This review presents a comprehensive and up-to-date overview on the therapeutic stimuli-responsive metallodrugs that have appeared in the past two decades, where stimuli such as redox, pH, enzyme, light, temperature, and so forth were involved. The compounds are classified into three major categories based on the nature of stimuli, that is, endo-stimuli-responsive metallodrugs, exo-stimuli-responsive metallodrugs, and dual-stimuli-responsive metallodrugs. Representative examples of each type are discussed in terms of structure, response mechanism, and potential medical applications. In the end, future opportunities and challenges in this field are tentatively proposed. With diverse metal complexes being introduced, the foci of this review are pointed to platinum and ruthenium complexes.

Pharmacokinetic Enhancers (Boosters)-Escort for Drugs against Degrading Enzymes and Beyond

Pharmacokinetic enhancers (boosters) are compounds used in combination with a primary therapeutic agent (drug) and are not used for their direct effects on the disease but because they enhance or restore the activity of the primary agent. Hence, in certain cases, they represent an indispensable escort for enzyme-labile drugs. Pharmacokinetic enhancers can exert their activity on different ways. In the most common case, they inhibit enzymes such as human cytochrome P450 enzymes in the liver or other organs and, thereby, block or reduce undesired metabolism and inactivation of the primary drug. In this review, an overview will be given on the therapeutically most important classes of pharmacokinetic enhancers like β-lactamase inhibitors, inhibitors of CYP (cytochrome P450) enzymes in HIV therapy and hepatitis C, boosters for fluoropyrimidine-type anticancer agents, compounds utilized for enabling therapy of Parkinson's disease with levodopa, and others. Inhibitors of efflux pumps in both pathogenic bacteria and tumor cells will be addresses shortly.

Cellular toxicity induced by the photorelease of a caged bioactive molecule: design of a potential dual-action Ru(II) complex

The series [Ru(tpy)(CH3CN)3](2+) (1), cis-[Ru(tpy)(CH3CN)2Cl](+) (2), and [Ru(tpy)(5CNU)3](2+) (3), where tpy = 2,2':6',2″-terpyridine and 5CNU = 5-cyanouracil, was synthesized, and their photochemical properties were investigated for use as potential photodynamic therapy (PDT) agents. When irradiated with visible light, 1-3 exhibit efficient exchange of the axial CH3CN or 5CNU ligand with H2O solvent molecules. Complexes 1-3 also exhibit photoinitiated binding to DNA when irradiated with λirr ≥ 395 nm light, and DNA binding can be accessed for 2 with λirr > 645 nm, well within the PDT window. Since 3 binds DNA and simultaneously releases biologically active 5CNU, it has the potential to be a dual-action therapeutic agent. Indeed, 3 is cytotoxic upon irradiation with visible light, whereas 1 is not under similar experimental conditions. The lack of toxicity imparted by 1 is explained by the exchange of only one CH3CN ligand in the complex under the irradiation conditions used for the cellular studies. Strategies are being sought to increase the quantum yields of ligand exchange and the cellular penetration of these compounds.

[Ru(bpy)2(5-cyanouracil)2]2+ as a potential light-activated dual-action therapeutic agent

The cation cis-[Ru(bpy)(2)(5CNU)(2)](2+) (bpy = 2,2'-bipyridine; 5CNU = 5-cyanouracil) was synthesized and investigated for use as a potential light-activated dual-action therapeutic agent. The complex undergoes efficient photoinduced 5CNU ligand exchange for solvent water molecules, thus simultaneously releasing biologically active 5CNU and generating [Ru(bpy)(2)(H(2)O)(2)](2+). The latter binds covalently to ds-DNA, such that photolysis results in the generation of 3 equiv of potential therapeutic agents from a single molecule.

The dual role of thymidine phosphorylase in cancer development and chemotherapy

Thymidine phosphorylase (TP), also known as "platelet-derived endothelial cell growth factor" (PD-ECGF), is an enzyme, which is upregulated in a wide variety of solid tumors including breast and colorectal cancers. TP promotes tumor growth and metastasis by preventing apoptosis and inducing angiogenesis. Elevated levels of TP are associated with tumor aggressiveness and poor prognosis. Therefore, TP inhibitors are synthesized in an attempt to prevent tumor angiogenesis and metastasis. TP is also indispensable for the activation of the extensively used 5-fluorouracil prodrug capecitabine, which is clinically used for the treatment of colon and breast cancer. Clinical trials that combine capecitabine with TP-inducing therapies (such as taxanes or radiotherapy) suggest that increasing TP expression is an adequate strategy to enhance the antitumoral efficacy of capecitabine. Thus, TP plays a dual role in cancer development and therapy: on the one hand, TP inhibitors can abrogate the tumorigenic and metastatic properties of TP; on the other, TP activity is necessary for the activation of several chemotherapeutic drugs. This duality illustrates the complexity of the role of TP in tumor progression and in the clinical response to fluoropyrimidine-based chemotherapy.

A phase I study of eniluracil/5-FU in combination with radiation therapy for potentially resectable and/or unresectable cancer of the pancreas and distal biliary tract

PURPOSE

Eniluracil is an effective inactivator of dihydropyrimidine dehydrogenase (DPD). It allows for oral dosing of 5-fluorouracil (5-FU), which may potentially improve the antitumor activity of 5-FU when delivered concurrently with radiotherapy while avoiding the inconvenience and morbidity of continuous infusion (CI) 5-FU. We addressed the safety of oral eniluracil/5-FU combined with radiation therapy and determined the profile of dose-limiting toxicities and recommended Phase II dose (RPTD) in patients with pancreatic and hepatobiliary cancers.

METHODS AND MATERIALS

Patients with resectable or locally advanced pancreatic and biliary cancer received eniluracil (starting at 6.0 mg/m(2) q12h)/5-FU (starting at 0.6 mg/m(2) q12h). Eniluracil/5-FU were given concurrently with preoperative radiation to 4500 cGy followed by 540 cGy by reduced fields. Surgery was considered 4 weeks after completion of therapy.

RESULTS

Thirteen patients were enrolled. Chemoradiotherapy was completed in all patients. The MTD was not reached and, thus, the RPTD of eniluracil/5-FU was determined to be 10 mg/m(2) q12h/1 mg/m(2) q12h. Two patients with locally advanced disease had a 30-45 percent cross-sectional tumor reduction, one of which underwent margin-negative resection. Two of 5 patients with pancreatic cancer, and 1 of 3 patients with cholangiocarcinoma, with underwent exploratory surgery had margin-negative resections. One patient had a pathologic complete response (pCR). Patient 5-FU plasma exposure increased slightly from Day 8 to Day 31.

CONCLUSION

Preoperative chemoradiation with oral eniluracil/5-FU is feasible, well tolerated, and potentially effective in the neoadjuvant setting. Further investigation of oral fluoropyrimidines as radiosensitizers for pancreaticobiliary malignancies is warranted.

Preparation of C-5 Substituted Cidofovir Derivatives

1-[(S)-3-Hydroxy-2-(phosphonomethoxy)propyl]cytosine (HPMPC, cidofovir) was modified by substitution on the base moiety in positions C-5 andN4. Key intermediates of these syntheses, diisopropyl esters of (S)-1-[2-(phosphonomethoxy)-3-(triphenylmethoxy)propyl]-5-alkylcytosines (6and7) prepared from 5-alkyl-4-methoxypyrimidin-2(1H)-ones were transformed to the corresponding 5-substituted cytosine orN4-alkylcytosine derivatives by the action of ammonia or primary amines, respectively. These fully protected phosphonate esters gave by treatment with bromotrimethylsilane followed by hydrolysis free phosphonic acids: 1-[(S)-3-hydroxy-2-(phosphonomethoxy)propyl]-5-methylcytosine (5-methyl-HPMPC,10), 5-ethyl-1-[(S)-3-hydroxy-2-(phosphonomethoxy)propyl]cytosine (5-ethyl-HPMPC,11) and a series of 5-ethyl-HPMPC analogues17-21bearing various substituents inN4position (cyclopropyl, cyclopentyl, 2-hydroxyethyl, allyl, 2-(dimethylamino)ethyl). 5-Ethynyl-1-[(S)-3-hydroxy-2-(phosphonomethoxy)propyl]cytosine (5-ethynyl-HPMPC,26) was prepared from 5-iodocytosine derivative23using Sonogashira coupling with (trimethylsilyl)acetylene, CuI and [PdCl2(Ph3P)2]. None of the prepared compounds exhibited antiviral activity in vitro.

Dihydropyrimidine dehydrogenase: a flavoprotein with four iron–sulfur clusters

Dihydropyrimidine dehydrogenase (DPD) is the first and rate-limiting enzyme in the pathway for degradation of pyrimidines, responsible for the reduction of the 5,6-double bond to give the dihydropyrimidine using NADPH as the reductant. The enzyme is a dimer of 220 kDa, and each monomer contains one FAD, one FMN, and four FeS clusters. The FAD is situated at one end of the protein, the FMN is at the other, and four FeS clusters form a conduit for electron transfer between the two sites comprised of two FeS clusters from each monomer. The enzyme has a two-site ping-pong mechanism with NADPH reducing FAD and reduced FMN responsible for reducing the pyrimidine. Solvent deuterium kinetic isotope effects indicate a rate-limiting reduction of FAD accompanied by pH-dependent structural rearrangement for proper orientation of the nicotinamide ring. Transfer of electrons from site 1 to site 2 is downhill with FMN rapidly reduced by FADH(2) via the FeS conduit. The reduction of the pyrimidine at site 2 proceeds using general acid catalysis with protonation at N5 of FMN carried out by K574 as FMN is reduced and protonation at C5 of the pyrimidine by C671 as it is reduced. Kinetic isotope effects indicate a stepwise reaction for reduction of the pyrimidine with hydride transfer at C6 preceding proton transfer at C5, with a late transition state for the proton transfer step.

The Effect of Food on the Pharmacokinetics of S-1 after Single Oral Administration to Patients with Solid Tumors

PURPOSE

The purpose is to determine the effect of food on the bioavailability of S-1, an oral formulation of the 5-fluorouracil (5FU) prodrug Ftorafur (FT), 5-chloro-2,4-dihydroxypyridine (CDHP), a dihydropyrimidine dehydrogenase inhibitor, and oxonic acid (an inhibitor of 5FU phosphoribosylation in normal gut mucosa) in a molar ratio of 1:0.4:1.

EXPERIMENTAL DESIGN

Eighteen patients received a single dose of S-1 of 35 mg/m(2) with (535-885 kcal) or without food in a crossover study design: in arm A without breakfast on day -7 and with breakfast on day 0 and in arm B the reversed sequence. Blood samples were taken before and after S-1 administration. This food effect was evaluated according to the Food and Drug Administration guidelines using log-transformed data.

RESULTS

Pharmacokinetic parameters for 5FU without breakfast were as follows: Tmax, 107 min; Cmax, 1.60 microm; area under the plasma concentration-time curve (AUC) 441 microm x min; and T(1/2), 104 min. Fasting decreased Tmax of FT, 5FU, CDHP, and oxonic acid significantly (P < 0.006) and increased the Cmax (P < 0.013). The food/fast ratio for the AUC of FT was not different, which for 5FU was 0.84 (P = 0.041), for CDHP was 0.89 (P = 0.191), for oxonic acid was 0.48 (P < 0.0005), and for cyanuric acid, the breakdown product of oxonic acid, was 5.1 (P = 0.019). Accumulation of uracil, indicative for dihydropyrimidine dehydrogenase inhibition, was not affected, as well as the T(1/2) of FT, 5FU, CDHP, and oxonic acid. Evaluation of the log-transformed data demonstrated that the 90% confidence interval for the food/fast ratio for the Cmax and AUC of FT, 5FU, CDHP, and uracil were within 70-143% and 80-125%, respectively, indicating no food effect. Only for oxonic acid and cyanuric acid were these values outside this interval.

CONCLUSIONS

Food intake affected only the pharmacokinetics of the S-1 constituent oxonic acid but not of FT, CDHP, and 5FU. Because oxonic acid is included to protect against gastrointestinal toxicity, this observation might affect the gastrointestinal toxicity and thus the efficacy of S-1.

A Phase I trial of preoperative eniluracil plus 5-fluorouracil and radiation for locally advanced or unresectable adenocarcinoma of the rectum and colon

PURPOSE

Eniluracil, an effective inactivator of dihydropyrimidine dehydrogenase, allows for oral dosing of 5-fluorouracil (5-FU), which avoids the morbidity of continuous infusion 5-FU. We addressed the safety of oral eniluracil and 5-FU combined with preoperative radiotherapy and determined the recommended Phase II dose and dose-limiting toxicity in patients with locally advanced rectal and colon cancer.

METHODS AND MATERIALS

Patients with TNM Stage II or III rectal cancer and residual or recurrent colon cancer received eniluracil (starting at 6.0 mg/m(2) every 12 h) and 5-FU (starting at 0.6 mg/m(2) every 12 h). Eniluracil and 5-FU were given with a 5-week course of preoperative radiotherapy of 4500 cGy, with a possible 540-cGy boost. Surgery was performed approximately 4 weeks after completion of chemoradiotherapy.

RESULTS

Twenty-two patients were enrolled; 1 patient was withdrawn owing to noncompliance. Chemotherapy was completed in all patients; radiotherapy was completed in 20 patients. The recommended Phase II dose of eniluracil and 5-FU was 8 mg/m(2) every 12 h and 0.8 mg/m(2) every 12 h, respectively. Diarrhea was the dose-limiting toxicity. Eleven of the 17 patients with primary rectal cancer underwent a sphincter-sparing procedure. One patient had a pathologic complete response.

CONCLUSION

Preoperative chemoradiotherapy with oral eniluracil and 5-FU is feasible and well tolerated. Additional investigation is warranted.

Pharmacokinetics of S-1, an oral formulation of ftorafur, oxonic acid and 5-chloro-2,4-dihydroxypyridine (molar ratio 1:0.4:1) in patients with solid tumors

S-1 is an oral formulation of ftorafur (FT), oxonic acid and 5-chloro-2,4-dihydroxypyridine (CDHP) at a molar ratio of 1:0.4:1. FT is a 5-fluorouracil (5-FU) prodrug, CDHP is a dihydropyrimidine dehydrogenase (DPD) inhibitor and oxonic acid is an inhibitor of 5-FU phosphoribosylation in the gastrointestinal mucosa and was included to prevent gastrointestinal toxicity. We determined the pharmacokinetics of S-1 in 28 patients at doses of 25, 35, 40 and 45 mg/m(2). The plasma C(max) values of FT, 5-FU, oxonic acid and CDHP increased dose-dependently and after 1-2 h were in the ranges 5.8-13 microM, 0.4-2.4 microM, 0.026-1.337 microM, and 1.1-3.6 microM, respectively. Uracil levels, indicative of DPD inhibition, also increased dose-dependently from basal levels of 0.03-0.25 microM to 3.6-9.4 microM after 2-4 h, and 0.09-0.9 microM was still present after 24 h. The pharmacokinetics of CDHP and uracil were linear over the dose range. The areas under the plasma concentration curves (AUC) for CDHP and uracil were in the ranges 418-1735 and 2281-8627 micromol x min/l, respectively. The t(1/2) values were in the ranges 213-692 and 216-354 min, respectively. Cumulative urinary excretion of FT was predominantly as 5-FU and was 2.2-11.9%; the urinary excretion of both fluoro-beta-alanine and uracil was generally maximal between 6 and 18 h. During 28-day courses with twice-daily S-1 administration, 5-FU and uracil generally increased. Before each intake of S-1, 5-FU varied between 0.5 and 1 microM and uracil was in the micromolar range (up to 7 microM), indicating that effective DPD inhibition was maintained during the course. In a biopsy of an esophageal adenocarcinoma metastasis that had regressed, thymidylate synthase, the target of 5-FU, was inhibited 50%, but increased four- to tenfold after relapse in subsequent biopsies. In conclusion, oral S-1 administration resulted in prolonged exposure to micromolar 5-FU concentrations due to DPD inhibition, and the decrease in uracil levels after 6 h followed the pattern of CDHP and indicates reversible DPD inhibition.

5-fluorouracil: mechanisms of action and clinical strategies

5-fluorouracil (5-FU) is widely used in the treatment of cancer. Over the past 20 years, increased understanding of the mechanism of action of 5-FU has led to the development of strategies that increase its anticancer activity. Despite these advances, drug resistance remains a significant limitation to the clinical use of 5-FU. Emerging technologies, such as DNA microarray profiling, have the potential to identify novel genes that are involved in mediating resistance to 5-FU. Such target genes might prove to be therapeutically valuable as new targets for chemotherapy, or as predictive biomarkers of response to 5-FU-based chemotherapy.

Design and synthesis of the tumor-activated prodrug of dihydropyrimidine dehydrogenase (DPD) inhibitor, RO0094889 for combination therapy with capecitabine

A series of tumor-activated prodrugs of the inhibitors of dihydropyrimidine dehydrogenase (DPD), an enzyme catabolizing 5-fluorouracil (5-FU: 4g), has been designed and synthesized. RO0094889 (11c) is a prodrug of 5-vinyluracil (4c), a known DPD inhibitor, and was designed to generate 4c selectively in tumor tissues by sequential conversion of 11c by three enzymes: esterase, cytidine deaminase and thymidine phosphorylase, the latter two of which are known to be highly expressed in various tumor tissues. When capecitabine (1), a tumor-activated prodrug of 5-FU, was co-administered orally with 11c, 5-FU in tumor tissues was significantly increased with only a slight increase of 5-FU in plasma as compared with oral capecitabine alone.

A phase I trial of weekly paclitaxel plus prolonged oral eniluracil/5-fluorouracil in patients with refractory malignancies

PURPOSE

This phase I study was conducted to determine the dose-limiting toxicity (DLT), maximum-tolerated doses, and recommended phase II doses of the combination of weekly intravenous paclitaxel and oral eniluracil/5-fluorouracil (5-FU).

PATIENTS AND METHODS

Patients received paclitaxel i.v. over 1 hour weekly for four consecutive weeks of each cycle. Oral eniluracil/5-FU was administered orally twice daily for 28 consecutive days starting on day 1 of each cycle. Cycles were repeated every 35 days. Patients were accrued at six different dosing combinations. Weekly paclitaxel doses ranged from 60 mg/m(2) to 80 mg/m(2), and oral eniluracil/5-FU doses ranged from 8.0/0.8 mg/m(2) to 11.5/1.15 mg/m(2) twice daily.

RESULTS

Thirty-seven patients received 126 cycles of therapy. Myelosuppression was minimal at all dose levels, with no grade 4 neutropenia or thrombocytopenia reported. DLT was reported in three out of six patients enrolled at the highest dose level and consisted of grade 3 diarrhea (two patients) and grade 3 mucositis (one patient). No DLTs were reported in patients enrolled at lower dose levels. One complete response and three partial responses were reported in patients with taxane-resistant metastatic breast cancer.

CONCLUSION

The combination of paclitaxel and eniluracil/5-FU was generally well tolerated. The recommended doses for further phase II testing are paclitaxel 80 mg/m(2) i.v. weekly for 4 weeks plus eniluracil/ 5-FU 10.0/1.0 mg/m(2) orally twice daily on days 1-28 with cycles repeated every 35 days.

Future potential of thymidylate synthase inhibitors in cancer therapy

Thymidylate synthase (TS) catalyses the de novo synthesis of deoxythymidylate and is a key rate-limiting enzyme of DNA synthesis. The primary site of action of the classic antifolate methotrexate is direct inhibition of dihydrofolate reductase, but it also inhibits TS indirectly by diminishing levels of the TS cosubstrate 5,10-methylenetetrahydrofolate. Polyglutamated metabolites of methotrexate also directly bind and inhibit TS. The prototype fluoropyrimidine fluorouracil is metabolised to an irreversible inhibitor of TS and is the standard chemotherapy for gastrointestinal carcinomas. It is also frequently used in combination with other anticancer drugs against breast cancer and head and neck cancers. The clinical efficacy of fluorouracil is routinely increased by concomitant administration of the biomodulating compound leucovorin (folinic acid). Both the success and limitations of these early drugs led to a search for new, more efficacious TS inhibitors active against a broader range of neoplasms. Raltitrexed (ZD1694, Tomudex) is an antifolate TS inhibitor developed over the last decade that is similarly effective, yet better tolerated, than fluorouracil against colorectal cancer. Additional antifolate and fluoropyrimidine-based TS inhibitors continue to be developed. Many of these experimental drugs have been designed to exploit or thwart selective metabolism in neoplasms, including specific mechanisms of resistance. As the curative potential of relatively non-selective antiproliferative drugs like TS inhibitors is limited against most neoplasms, the future role of TS inhibitors will likely continue to be adjunctive in surgically resectable tumours and palliative in combination with other agents for non-resectable disease. Although TS inhibitors will eventually be supplanted by yet to be discovered agents targeting more tumour-specific cellular signalling pathways, they will probably remain important for the above uses for some time. Future advances in the effective use of TS inhibitors may be forthcoming in the form of improved dosing, fewer untoward effects and increased tumour selectivity with novel fluorouracil prodrug formulations. Furthermore, there is emerging evidence that some novel antifolate TS inhibitors are active against a broader range of neoplams, including lung carcinomas and mesothelioma, compared to classical TS inhibitors. Other possible advances to come include effective biomodulation of antifolate TS inhibitors with nucleoside transport inhibitors and individualised patient therapy based on tumour gene expression and resistance patterns (pharmacogenetics).

Chemotherapy for colorectal cancer

With effective chemotherapy as adjuvant treatment, the survival benefit is clearly achieved for certain (stage III) colorectal cancer patients, though there still exist many unsettled issues including the controversies in the treatment of stage II disease. Advances in the development of a new generation of cytotoxic agents in the past several years have allowed us to move forward from the "fluorouracil-only era" in the treatment of advanced/metastatic colorectal cancer. It is still not very clear how best to minimize toxicity without compromising efficacy of the combination therapy with newer agents, or how to maximize the benefit of chemotherapy (concurrent versus sequential). There are many current ongoing clinical trials designed to address these issues. With better understanding of the signal transduction and molecular biology characteristics of colorectal cancer, and the development of biologic and molecular target agents, the outcomes of patients with colorectal cancer will be improved further. Future clinical trials should be focused on optimizing and individualizing therapy for patients based on their molecular profiles to achieve maximal clinical benefit.

Intratumoral dihydropyrimidine dehydrogenase messenger RNA level reflects tumor progression in human colorectal cancer

BACKGROUND

Determination of intratumoral dihydropyrimidine dehydrogenase (DPD) is of clinical interest because increased DPD levels can influence the tumor response to 5-fluorouracil-based chemotherapy through increased inactivation of the agent in tumor cells.

METHODS

DPD messenger RNA (mRNA) levels were evaluated in 80 consecutive patients undergoing surgery for primary colorectal cancer and 12 cases of liver metastasis.

RESULTS

Higher DPD mRNA levels were associated with higher pathologic classification, corresponding to the T categories (r =.267; P =.003). The DPD mRNA level was statistically higher in tumors with microscopic lymph node metastasis than in those without (P =.002). Hence, the DPD mRNA level increased in accordance with Dukes' classification (r =.387; P =.0001). The DPD mRNA level of the liver metastasis from colorectal cancer was significantly higher than that of primary lesions (P =.002). In eight patients, the DPD mRNA level of the liver metastasis was significantly higher than that of the matched primary tumor (P =.017).

CONCLUSIONS

Increases of the DPD mRNA level in cancerous tissue seem to reflect tumor progression. High DPD mRNA levels in liver metastasis and advanced colorectal cancer may have clinical importance for 5-fluorouracil-based chemosensitivity.

Enhancement of the antitumour activity of 5-fluorouracil (5-FU) by inhibiting dihydropyrimidine dehydrogenase activity (DPD) using 5-chloro-2,4-dihydroxypyridine (CDHP) in human tumour cells

The purpose of this study was to evaluate the use of 5-chloro-2,4-dihydroxypyridine (CDHP), a potent inhibitor of dihydropyrimidine dehydrogenase (DPD), to enhance the antitumour activity of the fluoropyrimidines. In an in vitro study, CDHP did not influence cell proliferation by itself. However, CDHP did inhibit 5-fluorouracil (5-FU) degradation and enhanced 5-FU cytotoxicity in a concentration-dependent manner in two human tumour cell lines (MIAPaCa-2 and HuTu80) with relatively high basal DPD activity. CDHP exhibited a maximum effect at a molar ratio (CDHP:5-FU) of more than 0.2. However, CDHP did not have any effect on 5-FU cytotoxicity in the CAL27 tumour cell line, which has a relatively low basal DPD activity, even at concentrations where the DPD activity is almost completely inhibited. In an in vivo study, the maximal tolerable doses (MTD) of tegafur (FT) and a combination of FT and CDHP at a molar ratio of 1:0.4 (FT/CDHP) for nude mice were determined by oral administration for 14 consecutive days. After a single oral administration of either FT or FT/CDHP at the MTD, the 5-FU serum concentration-time profiles were almost the same for both treatment strategies. When nude mice bearing subcutaneous (s.c.) MIAPaCa-2 cells were treated with either FT or FT/CDHP at the MTD, the FT/CDHP treatment showed a significantly higher antitumour effect than the FT treatment (tumour growth inhibition: FT/CDHP, 51+/-12%; FT, 21+/-25%; P<0.05). However, the host-body weight suppression induced by FT/CDHP and FT was equivalent. These findings suggest that the combination of fluoropyrimidine and CDHP for the treatment of tumours with a high basal DPD elicits a greater antitumour effect than treatment with fluoropyrimidines alone and we suggest that CDHP inhibits the degradation of 5-FU in the tumour.

Phase I study of eniluracil and oral 5-fluorouracil in combination with docetaxel in the treatment of patients with metastatic breast carcinoma

PURPOSE

The authors conducted a single-institution Phase I clinical trial to determine the maximum tolerated doses and to define the toxic effects of oral eniluracil and oral 5-fluorouracil (5-FU) combined with docetaxel in patients with metastatic breast carcinoma.

PATIENTS AND METHODS

Patients with metastatic breast carcinoma were eligible if they had disease progression after anthracycline-based therapy and had never been exposed to taxanes. The starting doses of oral eniluracil and oral 5-FU were 11.5 mg/m(2) and 1.15 mg/m(2), respectively, twice daily on Days 1-14. Docetaxel was given intravenously at a starting dose of 50 mg/m(2) on Day 1 only. The dose of docetaxel was escalated among cohorts until a maximum tolerated dose was reached. Courses were repeated every 21 days.

RESULTS

The authors treated 19 patients with Stage IV breast carcinoma, of whom 5 had received prior chemotherapy for their metastatic disease. Fifty-three percent had a performance status of 1, and 53% had bone or soft tissue involvement as the dominant site of disease. All patients had received prior therapy with doxorubicin. The dose-limiting toxicity was neutropenic fever. No episodes of sepsis were observed. Significant antitumor activity was observed with a total of two complete and nine partial responses. The recommended doses for Phase II studies are 72 mg/m(2) docetaxel on Day 1 and 10.0/1.0 mg/m(2) oral eniluracil/5-FU twice daily for a total of 14 days, with courses being repeated every 21 days.

CONCLUSIONS

The combination of oral eniluracil/5-FU and intravenous docetaxel is a safe and well tolerated regimen. Significant antitumor activity is associated with this combination.

Comparative pharmacokinetic study of continuous venous infusion fluorouracil and oral fluorouracil with eniluracil in patients with advanced solid tumors

PURPOSE

To compare the pharmacokinetics of continuous venous infusion (CVI) fluorouracil (5-FU) with that of oral eniluracil/5-FU and to describe toxicities and clinical activity of prolonged oral administration of eniluracil/5-FU.

PATIENTS AND METHODS

A randomized, open-label, cross-over study compared CVI 5-FU to an oral 5-FU/eniluracil combination. Seventeen patients (arm A) were randomly assigned to receive eniluracil/5-FU combination tablets (10:1 mg/m(2) BID for 7 days) during the first study period, followed by 5-FU (300 mg/m(2) CVI for 7 days) during period 2, with a 14-day washout between periods. Sixteen patients (arm B) received treatment in the opposite sequence. In period 3, all patients received eniluracil/5-FU tablets BID for 28 days. Plasma levels of 5-FU during CVI and oral administration were analyzed in periods 1 and 2. Dihydropyrimidine dehydrogenase (DPD) activity was determined by measuring plasma uracil, urinary alpha-fluoro-beta-alanine, and peripheral-blood mononuclear cell (PBMC) DPD activity.

RESULTS

There were no grade 3 or 4 toxicities in either arm. Partial responses were observed in three patients. Another three patients had stable disease for > or = 3 months. Eniluracil and 5-FU pharmacokinetics were similar to those observed in previous studies and were unaffected by administration sequence. The mean +/- SD steady-state plasma concentration (C(P)) and area under the curve (AUC)(144-168h) for CVI 5-FU (104 +/- 45 ng/mL and 2,350 +/- 826 ng x h/mL, respectively) were three-fold greater than those for oral 5-FU (38.1 +/- 7.7 ng/mL and 722 +/- 182 ng x h/mL, respectively [P <.00001]). Individual 5-FU concentrations during CVI were highly variable, whereas those after eniluracil/5-FU were very reproducible. DPD activity in PBMCs before each study period was normal.

CONCLUSION

Both CVI 5-FU and oral eniluracil/5-FU were well tolerated, with moderate activity in these heavily pretreated patients. However, 5-FU steady-state C(P) and AUCs achieved with oral eniluracil/5-FU were significantly less than with CVI 5-FU.

Randomized, open-label, phase III study of a 28-day oral regimen of eniluracil plus fluorouracil versus intravenous fluorouracil plus leucovorin as first-line therapy in patients with metastatic/advanced colorectal cancer

PURPOSE

To compare the efficacy and tolerability of eniluracil (EU)/fluorouracil (5-FU) with that of 5-FU/leucovorin (LV) as first-line therapy for patients with metastatic/advanced colorectal cancer.

PATIENTS AND METHODS

This multicenter, randomized, open-label, phase III study (FUMA3008) conducted in the United States and Canada compared the safety and efficacy of EU/5-FU (11.5 mg/m(2)/1.15 mg/m(2) twice daily for 28 days every 35 days) with that of intravenous 5-FU/LV (425 mg/m(2)/20 mg/m(2) once daily for 5 days every 28 days) in patients with previously untreated metastatic colorectal cancer. Overall survival (OS) was the primary end point.

RESULTS

A total of 981 patients were randomized and 964 patients received treatment (485 EU/5FU, 479 5FU/LV). Survival for EU/5-FU was not statistically equivalent (but not statistically inferior) to that for 5-FU/LV (hazard ratio, 0.880; 95% confidence interval [CI], 0.75 to 1.03). Median duration of survival was 13.3 months in the EU/5-FU group and 14.5 months in the 5-FU/LV group. Median duration of progression-free survival for EU/5-FU was statistically inferior to that of the control group (20.0 weeks [95% CI, 19.1 to 20.9 weeks] v 22.7 weeks [95% CI, 18.3 to 24.6 weeks]; P =.01). Both treatments were well tolerated. Diarrhea was the most common nonhematologic toxicity in both groups; treatment-related grade 3 or 4 diarrhea occurred in 19% of patients treated with EU/5-FU and 16% of patients receiving 5-FU/LV (P =.354). Grade 3 or 4 granulocytopenia occurred in 5% of EU/5-FU patients and 47% of 5-FU/LV patients.

CONCLUSION

Safety profiles of both treatments were acceptable. Although antitumor activity was observed, EU/5-FU did not meet the protocol-specified statistical criteria for equivalence to 5-FU/LV in terms of OS.

New drugs and combinations in the palliative treatment of colon and rectal cancer

Colorectal cancer (CRC) is a common disease. The overall survival has improved only marginally in recent decades despite advances in surgery and early detection. Potentially curative resection at disease presentation can be performed only in 70-80% of the patients, and overall survival at 5 years is less than 60%. Advanced disease is associated with a poor prognosis. Treatment for advanced colorectal cancer has nevertheless made progress in the last few years. Systemic chemotherapy doubles the survival of these patients compared to untreated controls. Chemotherapy has demonstrated effective palliation, improvement of quality of life (QoL) and symptom improvement in such patients. For nearly four decades, fluorouracil (5FU) has been the mainstay of treatment. New compounds active against colorectal cancer are now available. Several studies on this topic are ongoing.

Capecitabine: a novel agent for the treatment of solid tumors

Although 5-fluorouracil (5-FU) has been used to treat breast and colorectal cancers for several decades, bolus 5-FU has disappointing efficacy. Prolonged infusion schedules and biomodulation with leucovorin have resulted in improved response rates, but these have not translated into significant improvements in survival in patients with metastatic disease. Furthermore, prolonged infusion is inconvenient for patients and can result in medical complications. New oral fluoropyrimidines, including capecitabine, are promising alternatives to i.v. 5-FU. Capecitabine generates 5-FU preferentially within tumors through exploitation of the high intratumoral activity of thymidine phosphorylase. The tumor selectivity of capecitabine has been confirmed in a clinical study of colorectal cancer patients. Clinical trials have shown that capecitabine is an effective, well-tolerated treatment for breast and colorectal cancer, with response rates of 20-26% in anthracycline- and taxane-pretreated metastatic breast cancer. As first-line monotherapy, capecitabine produces response rates of 25-27% in metastatic colorectal cancer and 30% in metastatic breast cancer. In all studies to date, capecitabine has been well tolerated, with adverse events typical of infusional 5-FU and manageable with treatment interruption/dose modification. Myelosuppression and alopecia are rare. Capecitabine is also being investigated in other solid tumors (including ovarian, pancreatic and gastric cancers) as adjuvant monotherapy in breast and colorectal cancer, and in combination with other cytotoxic agents. Results of ongoing trials are eagerly awaited.

Novel chemotherapy agents for colorectal cancer: oral fluoropyrimidines, oxaliplatin, and raltitrexed

Agents now under investigation for treatment of advanced colorectal cancer (CRC) include the oral fluoropyrimidines, oxaliplatin, and raltitrexed. Research efforts directed at finding agents that conveniently and effectively deliver 5-fluorouracil (5-FU) in a protracted fashion have led to the development of several oral fluoropyrimidines. These agents, which include capecitabine; tegafur and uracil plus leucovorin (UFT/LV); eniluracil plus oral 5-FU; and S-1, are convenient and less toxic than intravenous bolus 5-FU. Oxaliplatin has a uniquely different mechanism of action compared with that of 5-FU and has demonstrated activity not only in the first-line treatment setting but also in patients whose disease has progressed during or following 5-FU treatment. In the first-line setting, when oxaliplatin is combined with 5-FU plus LV, response rates and time to disease progression are remarkably improved compared with 5-FU/LV alone. Raltitrexed, a unique thymidylate synthase inhibitor, has undergone extensive phase III evaluation in CRC. The advent of these novel agents has led to development of combined chemotherapy regimens now being introduced into the adjuvant setting.

Phase II study of oral eniluracil, 5-fluorouracil, and leucovorin in patients with advanced colorectal carcinoma

BACKGROUND

The oral administration of 5-fluorouracil (5-FU) is hindered by erratic bioavailability due to catabolism of 5-FU by the enzyme dihydropyrimidine dehydrogenase (DPD) in the gastrointestinal tract. Eniluracil is a potent inactivator of DPD which results in 100% oral bioavailability of 5-FU. Leucovorin (LV) is another biochemical modulator of 5-FU that potentiates inhibition of thymidylate synthase, the primary target of 5-FU. The goal of this study was to determine the antitumor activity and toxicity of an oral regimen containing eniluracil, 5-FU, and LV in patients with colorectal carcinoma.

METHODS

Sixty eligible patients who had previously untreated, measurable, metastatic colorectal carcinoma were treated with oral eniluracil 50 mg on Days 1-7, 5-FU 20 mg/m(2) on Days 2-6, and LV 50 mg on Days 2-6. Cycles were repeated at 28-day intervals.

RESULTS

The overall response rate was 13% (95% confidence interval [CI] = 6, 25%), with 1 complete response and 7 partial responses. Three additional patients had partial responses that were not confirmed at subsequent evaluations. The median time to progression of disease was 4.4 months (95% CI = 3.45, 7.69) and the median survival time was 12.6 months (95% CI = 9.1, 14.75). Grade 3-5 toxicity (1 toxic death) occurred in 51 patients (85%). Grade 4 neutropenia occurred in 25 patients (42%), and 18 patients (30%) had Grade 3-4 diarrhea. Twenty-one patients (35%) were hospitalized for toxicity, and 12 (20%) had febrile neutropenia. Baseline creatinine clearance was associated inversely with severe toxicity (P = 0.001).

CONCLUSIONS

Although antitumor activity was observed, the frequent occurrence of severe toxicity with this regimen limited its clinical utility. Alternate schedules with a more favorable therapeutic index are undergoing clinical testing and should be pursued. The high level of toxicity observed with orally administered low dose 5-FU underscored the potency of eniluracil as a biochemical modulator.

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.

Phase I assessment of the pharmacokinetics, metabolism, and safety of emitefur in patients with refractory solid tumors

PURPOSE

To determine the toxicities, dose-limiting toxicities (DLT), maximum-tolerated dose, and pharmacokinetic profile of emitefur (BOF-A2) in patients with advanced solid tumors.

METHODS

This was a phase I dose-escalating trial in which cohorts of patients received BOF-A2 (cohort 1, 300 mg/m(2) orally [PO] tid; cohort 2, 200 mg/m(2) PO tid; cohort 3, 200 mg/m(2) bid; and cohort 4, 250 mg/m(2) bid) for 14 consecutive days followed by 1 week of rest (cycle 1). Pharmacokinetics, toxicity, and tumor response were monitored.

RESULTS

Nineteen patients received 110 cycles (three patients in cohort 1, three patients in cohort 2, 10 patients in cohort 3, and three patients in cohort 4). DLT (grade 3 stomatitis, diarrhea, leukopenia) was observed in cohorts 1, 2, and 4. Pharmacokinetics indicated that prolonged systemic expression of fluorouracil (5-FU) is maintained after administration of BOF-A2 at a dose of 200 mg bid for 14 days. The mean steady-state concentration of plasma 5-FU was > or = 24 ng/mL, which was 184-fold greater than the minimum effective cytotoxic concentration in vitro. Lack of variation of 5-FU trough levels within a day at steady-state indicates suppression of circadian variation. One patient in cohort 3 achieved a partial response and five patients maintained stable disease in excess of 6 months.

CONCLUSION

BOF-A2 at a dose of 200 mg PO bid for 14 days followed by 7 days of rest is well tolerated. Prolonged exposure to 5-FU above the predicted preclinical minimum effective concentration is maintained, without evidence of circadian variation. Furthermore, evidence of antitumor activity is suggested.

Pharmacokinetics and bioequivalence of a combined oral formulation of eniluracil, an inactivator of dihydropyrimidine dehydrogenase, and 5-fluorouracil in patients with advanced solid malignancies

BACKGROUND

This study was performed to evaluate the pharmacokinetics, bioequivalence, and feasibility of a combined oral formulation of 5-flurouracil (5-FU) and eniluracil (Glaxo Wellcome Inc., Research Triangle Park, North Carolina), an inactivator of dihydropyrimidine dehydrogenase (DPD). The rationale for developing a combined eniluracil/5-FU formulation oral dosing form is to simplify treatment with these agents, which has been performed using separate dosing forms, and decrease the probability of severe toxicity and/or suboptimal therapeutic results caused by inadvertently high or conversely insufficient 5-FU dosing.

PATIENTS AND METHODS

The trial was a randomized, three-way crossover bioequivalence study of three oral dosing forms of eniluracil/5-FU tablets in adults with solid malignancies. Each period consisted of two days of treatment and a five- to seven-day washout phase. Eniluracil at a dose of 20 mg, which results in maximal DPD inactivation, was administered twice daily on the first day and in the evening on the second day of each of the three treatments. On the morning of the second day, all patients received a total eniluracil dose of 20 mg orally and a total 5-FU dose of 2 mg orally as either separate tablets (treatment A) or combined eniluracil/5-FU tablets in two different strengths (2 tablets of eniluracil/5-FU at a strength (mg/mg) of 10/1 (treatment B) or 8 tablets at a strength of 2.5/0.25 (treatment C)). The pharmacokinetics of plasma 5-FU, eniluracil, and uracil, and the urinary excretion of eniluracil, 5-FU, uracil, and alpha-fluoro-beta-alanine (FBAL), were studied. To determine the bioequivalence of the combined eniluracil/5-FU dosing forms compared to the separate tablets, an analysis of variance on pharmacokinetic parameters reflecting eniluracil and 5-FU exposure was performed.

RESULTS

Thirty-nine patients with advanced solid malignancies had complete pharmacokinetic studies performed during treatments A, B, and C. The pharmacokinetics of eniluracil and 5-FU were similar among the three types of treatment. Both strengths of the combined eniluracil/5-FU dosing form and the separate dosing forms were bioequivalent. Mean values for terminal half-life, systemic clearance, and apparent volume of distribution for oral 5-FU during treatments A/B/C were 5.5/5.6/5.6 hours, 6.6/6.6/6.5 liters/hour, and 50.7/51.5/50.0 liters, respectively. The intersubject coefficient of variation for pharmacokinetic variables reflecting 5-FU exposure and clearance in treatments ranged from 23% to 33%. The urinary excretion of unchanged 5-FU over 24 hours following treatments A, B, and C averaged 52.2%, 56.1%, and 50.8'%, of the administered dose of 5-FU, respectively. Parameters reflecting DPD inhibition, including plasma uracil and urinary FBAL excretion following treatments A, B, and C were similar. Toxicity was generally mild and similar following all three types of treatments.

CONCLUSIONS

The pharmacokinetics of 5-FU and eniluracil were similar and met bioequivalence criteria following treatment with the separate oral formulations of 5-FU and eniluracil and two strengths of the combined formulation. The availability of a combined eniluracil/5-FU oral dosing form will likely simplify dosing and decrease the probability of severe toxicity or suboptimal therapeutic results caused by an inadvertent 5-FU overdose or insufficient 5-FU dosing in the case of separate oral formulations, thereby enhancing the overall feasibility and 0therapeutic index of oral 5-FU therapy.

Progress in colorectal cancer chemotherapy: how far have we come, how far to go?

Fluorouracil has been the mainstay of treatment for colorectal cancer (CRC) for almost 40 years. Various schedules and biochemical modulators have been investigated in an attempt to improve the therapeutic efficacy of fluorouracil. To date, fluorouracil plus folinic acid represents the standard therapy in CRC for the adjuvant treatment of patients at high risk for relapse and for the first-line treatment of metastatic disease. To gain clinical acceptance, however, oral fluoropyrimidines must confer at least the same survival advantages associated with the optimal intravenous fluorouracil regimens. Irinotecan and oxaliplatin are 2 other novel agents that have mechanisms of action that are uniquely different from those of fluorouracil, with demonstrated activity in patients with fluorouracil-refractory disease. Recent randomised trials comparing fluorouracil plus folinic acid with combinations of either irinotecan or oxaliplatin and fluorouracil plus folinic acid have shown that response rates are improved and time to progression is increased in patients receiving the combination regimens. These regimens are being rapidly introduced in the adjuvant setting, and the role and acceptance of these combination regimens as first-line therapy needs to be defined. Other novel agents being evaluated in the treatment of patients with advanced CRC include oral edrecolomab (monoclonal antibody 17-1A) and tumour vaccines. Future research is focused on enabling clinicians to individualise treatment strategies in patients with CRC, so as to improve clinical outcomes and reduce drug toxicity.

Multicenter phase II study to evaluate a 28-day regimen of oral fluorouracil plus eniluracil in the treatment of patients with previously untreated metastatic colorectal cancer

PURPOSE

To determine the efficacy of fluorouracil (5-FU) plus eniluracil when administered to patients with previously untreated metastatic colorectal cancer.

PATIENTS AND METHODS

In this single-arm phase II study, patients with previously untreated metastatic colorectal cancer received oral eniluracil plus 5-FU (10:1 dose ratio), at 5-FU doses of 1.00 mg/m(2) or 1.15 mg/m(2) twice daily (every 12 hours) for 28 consecutive days repeated every 5 weeks (one cycle). Treatment continued until there was documented disease progression or unacceptable toxicity.

RESULTS

Thirty and 25 patients were enrolled at a starting dose of 1.00 mg/m(2) and 1.15 mg/m(2), respectively. Fourteen (25%) of 55 patients (95% confidence interval, 15% to 39%) had a partial response, and 20 patients (36%) had stable disease. The median durations of the partial responses and stable disease were 23.9 weeks (range, 12.3 to 52.1+ weeks) and 24.1 weeks (range, 17.1 to 55.6+ weeks), respectively. The median durations of progression-free and overall survival were 22.6 weeks (range, 21.0 to 29.0 weeks) and 59 weeks (range, 4 to 84+ weeks), respectively. The response rate in the 1.15 mg/m(2)-dose group was similar to the 1.00 mg/m(2)-dose group (28% v 23%, respectively). Severe (grade 3/4) nonhematologic treatment-related toxicity included diarrhea (nine patients), nausea/vomiting (one patient each), mucositis (two patients), and anorexia (one patient). Severe hematologic toxicities were rare. At the 1.15 mg/m(2)-dose level, two patients exhibited grade 3 granulocytopenia, and two patients had grade 3 anemia.

CONCLUSION

The response rate with oral 5-FU plus eniluracil is comparable with that observed with infusional 5-FU or bolus 5-FU and leucovorin. The toxicity profile of this oral regimen is acceptable for use in an outpatient home-based setting.