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

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.

Oral drugs in the treatment of metastatic colorectal cancer

Colorectal cancer (CRC) is one of the most common forms of cancer, with an estimated 1.36 million new cases and almost 700,000 deaths annually. Approximately 21% of patients with CRC have metastatic disease at diagnosis. The objective of this article is to review the literature on the efficacy and safety of oral drugs available for the treatment of metastatic colorectal cancer (mCRC). Several such drugs have been developed, and fluoropyrimidines are the backbone of chemotherapy in this indication. They exert their antitumour activity by disrupting the synthesis and function of DNA and RNA. Oral fluoropyrimidines include prodrugs capecitabine, tegafur, eniluracil/5-fluorouracil, tegafur/uracil, tegafur/gimeracil/oteracil and trifluridine/tipiracil (FTD/TPI). Oral drugs offer several advantages over injectable formulations, including convenience, flexibility, avoidance of injection-related adverse events (AEs) and, in some circumstances, lower costs. However, oral drugs may not be suitable for patients with gastrointestinal obstruction or malabsorption, they may result in reduced treatment adherence and should not be co-administered with drugs that interfere with absorption or hepatic metabolism. Oral fluoropyrimidines such as capecitabine, as monotherapy or in combination with oxaliplatin, irinotecan or bevacizumab, are as effective as intravenous 5-fluorouracil (5-FU) in first-line treatment of mCRC. Other oral fluoropyrimidines, such as FTD/TPI, are effective in patients with mCRC who are refractory, intolerant or ineligible for 5-FU. In addition, oral fluoropyrimidines are used in adjuvant treatment of mCRC. Regorafenib is an oral multikinase inhibitor used in patients in whom several previous lines of therapy have failed. Frequent AEs associated with oral drugs used in the treatment of CRC include hand-foot syndrome and gastrointestinal and haematological toxicities.

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.

Therapeutic synergy between irinotecan and 5-fluorouracil against human tumor xenografts

PURPOSE

Although the combination of irinotecan and 5-Fluorouracil is clinically active, it is associated with significant toxicity and resistance. Studies were carried out to define the optimal dosage, sequence, and timing for the combination in mice bearing xenografted human tumors.

EXPERIMENTAL DESIGN

The maximum tolerated dose of irinotecan and 5-Fluorouracil in combination was determined in nude mice. Therapeutic efficacy against established human colon carcinoma xenografts, HCT-8 and HT-29, and human head and neck squamous cell carcinoma xenografts, FaDu and A253, was determined using the rugs individually, simultaneously, and in sequence with various intervals in between. Treatments were i.v. weekly x 4. Immunohistochemical and reverse transcription-PCR measurements of relevant drug-metabolizing enzymes, apoptosis-related proteins, cell cycle distribution, cyclin A, and S phase fraction expression were carried out and compared with the therapeutic outcome.

RESULTS

The maximum tolerated dose of irinotecan resulted in cure rates of 30% or less in all xenografts. No cures were achieved with FUra alone. Concurrent administration of irinotecan and FUra, or of FUra 24 h before irinotecan, resulted in cure rates of <20%, except for FaDu (60%). Administration of irinotecan 24 h before FUra resulted in the highest cure rates, 80% in HCT-8, 0% in HT-29, 100% in FaDu, and 10% in A253.

CONCLUSIONS

The optimal therapeutic synergy was achieved when irinotecan was administered 24 h before 5-Flurouracil. Sensitivity to this combination was associated with poor differentiation status, higher cyclin A index, recruitment of cells into S phase, and induction of Bax expression and apoptosis.

First-Line Treatment Strategies to Improve Survival in Patients with Advanced Colorectal Cancer

Advanced colorectal cancer is a significant cause of worldwide cancer-related mortality. For the majority of patients, palliative chemotherapy can yield substantial improvements in survival. Fluorouracil has been the mainstay of treatment in this setting for the past few decades. The relatively recent availability of new combinations with active agents such as irinotecan and oxaliplatin makes this a promising and hopeful time for the treatment of metastatic colorectal cancer, with median survivals now approaching 18-21 months. For patients presenting with resectable metastases, the goal of therapy is surgery with a curative intent. There exists the potential for this approach to be extended also to a greater proportion of patients whose cancer may be rendered resectable following effective neoadjuvant chemotherapy. In addition, an improved understanding of molecular predictors for treatment response and toxicity may facilitate the future selection of individualised treatments for a given tumour profile. Further improvements in the management of advanced disease will continue to be pursued through the ongoing development of multimodality approaches and the incorporation of novel targeted agents with innovative chemotherapy combinations.