A novel class of antitumor prodrug, 1-(2'-oxopropyl)-5-fluorouracil (OFU001), that releases 5-fluorouracil upon hypoxic irradiation

Radiotherapy activates picolinium prodrugs in tumours

Radiotherapy-induced prodrug activation provides an ideal solution to reduce the systemic toxicity of chemotherapy in cancer therapy, but the scope of the radiation-activated protecting groups is limited. Here we present that the well-established photoinduced electron transfer chemistry may pave the way for developing versatile radiation-removable protecting groups. Using a functional reporter assay, N-alkyl-4-picolinium (NAP) was identified as a caging group that efficiently responds to radiation by releasing a client molecule. When evaluated in a competition experiment, the NAP moiety is more efficient than other radiation-removable protecting groups discovered so far. Leveraging this property, we developed a NAP-derived carbamate linker that releases fluorophores and toxins on radiation, which we incorporated into antibody-drug conjugates (ADCs). These designed ADCs were active in living cells and tumour-bearing mice, highlighting the potential to use such a radiation-removable protecting group for the development of next-generation ADCs with improved stability and therapeutic effects.

Synthesis and DNA/RNA complementation studies of peptide nucleic acids containing 5-halouracils

The monomers of peptide nucleic acids containing 5-halouracils (5-XU-PNA), incorporated into heptameric PNA in the middle position, have been synthesized. Thermodynamic analyses revealed that the heptameric PNA oligomer with DNA and RNA showed higher duplex stability compared to the unmodified PNA counterpart. NMR studies suggested that the electron withdrawing effect of the halogen atom increased the strength of the XU-A hydrogen bond.

Radiolytic one-electron reduction characteristics of tyrosine derivative caged by 2-oxopropyl group

We employed X-irradiation to activate a caged amino acid with a 2-oxoalkyl group. We designed and synthesized tyrosine derivative caged by a 2-oxoalkyl group (Tyr(Oxo)) to evaluate its radiolytic one-electron reduction characteristics in aqueous solution. Upon hypoxic X-irradiation, Tyr(Oxo) released a 2-oxopropyl group to form the corresponding uncaged tyrosine. In addition, radiolysis of dipeptides containing Tyr(Oxo) revealed that the efficiency of radiolytic removal of 2-oxopropyl group increased significantly by the presence of neighboring aromatic amino acids.

Radiation- and photo-induced activation of 5-fluorouracil prodrugs as a strategy for the selective treatment of solid tumors

5-Fluorouracil (5-FU) is used widely as an anticancer drug to treat solid cancers, such as colon, breast, rectal, and pancreatic cancers, although its clinical application is limited because 5-FU has gastrointestinal and hematological toxicity. Many groups are searching for prodrugs with functions that are tumor selective in their delivery and can be activated to improve the clinical utility of 5-FU as an important cancer chemotherapeutic agent. UV and ionizing radiation can cause chemical reactions in a localized area of the body, and these have been applied in the development of site-specific drug activation and sensitization. In this review, we describe recent progress in the development of novel 5-FU prodrugs that are activated site specifically by UV light and ionizing radiation in the tumor microenvironment. We also discuss the chemical mechanisms underlying this activation.

Emission under hypoxia: one-electron reduction and fluorescence characteristics of an indolequinone-coumarin conjugate

A characteristic feature of the reactivity of indolequinone derivatives, substituents of which can be removed by one-electron reduction under hypoxic conditions, was applied to the development of a new class of fluorescent probes for disease-relevant hypoxia. A reducing indolequinone parent molecule conjugated with fluorescent coumarin chromophores could suppress efficiently the fluorescence emission of the coumarin moieties by an intramolecular electron-transfer quenching mechanism and a conventional internal-filter effect. Under hypoxic conditions, however, the conjugate, denoted IQ-Cou, underwent a one-electron reduction triggered by X irradiation or the action of a reduction enzyme to release a fluorescent coumarin chromophore, whereupon an intense fluorescence emission with a maximum intensity at 420 nm was observed. The one-electron reduction of IQ-Cou was suppressed by molecular oxygen under aerobic conditions. IQ-Cou also showed intense fluorescence in a hypoxia-selective manner upon incubation with a cell lysate of the human fibrosarcoma cell line HT-1080. The IQ-Cou conjugate has several unique properties that are favorable for a fluorescent probe of hypoxia-specific imaging.

Recent trends in targeted anticancer prodrug and conjugate design

Anticancer drugs are often nonselective antiproliferative agents (cytotoxins) that preferentially kill dividing cells by attacking their DNA at some level. The lack of selectivity results in significant toxicity to noncancerous proliferating cells. These toxicities along with drug resistance exhibited by the solid tumors are major therapy limiting factors that result into poor prognosis for patients. Prodrug and conjugate design involves the synthesis of inactive drug derivatives that are converted to an active form inside the body and preferably at the site of action. Classical prodrug and conjugate design have focused on the development of prodrugs that can overcome physicochemical (e.g., solubility, chemical instability) or biopharmaceutical problems (e.g., bioavailability, toxicity) associated with common anticancer drugs. The recent targeted prodrug and conjugate design, on the other hand, hinge on the selective delivery of anticancer agents to tumor tissues thereby avoiding their cytotoxic effects on noncancerous cells. Targeting strategies have attempted to take advantage of low extracellular pH, elevated enzymes in tumor tissues, the hypoxic environment inside the tumor core, and tumor-specific antigens expressed on tumor cell surfaces. The present review highlights recent trends in prodrug and conjugate rationale and design for cancer treatment. The various approaches that are currently being explored are critically analyzed and a comparative account of the advantages and disadvantages associated with each approach is presented.

Review: implications of in vitro research on the effect of radiotherapy and chemotherapy under hypoxic conditions

As it is now well established that human solid tumors frequently contain a substantial fraction of cells that are hypoxic, more and more in vitro research is focusing on the impact of hypoxia on the outcome of radiotherapy and chemotherapy. Indeed, the efficacy of irradiation and many cytotoxic drugs relies on an adequate oxygen supply. Consequently, hypoxic regions in solid tumors often contain viable cells that are intrinsically more resistant to treatment with radiotherapy or chemotherapy. Moreover, efforts have been made to exploit hypoxia as a potential difference between malignant and normal tissues.Nowadays, a body of evidence indicates that oxygen deficiency clearly influences some major intracellular pathways such as those involved in cell proliferation, cell cycle progression, apoptosis, cell adhesion, and others. Obviously, when investigating the effects of radiotherapy or chemotherapy or both combined under hypoxic conditions, it is essential to consider the influences of hypoxia itself on the cell. In this review, we first focus on the effects of hypoxia per se on some critical biological pathways. Next, we sketch an overview of preclinical and clinical research on radiotherapy, chemotherapy, and chemoradiation under hypoxic conditions.

2-Oxoalkyl caged oligonucleotides: one-electron reductive activation into emergence of ordinary hybridization property by hypoxic X-irradiation

Ionizing radiation triggers the activation of caged oligodeoxynucleotides (ODNs) with a 2-oxoalkyl leaving group to give the corresponding normal uncaged strands. We designed and synthesized ODNs caged by a 2-oxopropyl group at a given thymine N(3) position (d(oxo)T) to evaluate their one-electron reduction characteristics. Upon hypoxic X-radiolysis in aqueous solution, the caged ODNs released the 2-oxopropyl group to produce the corresponding uncaged ODNs. Digestion by a restriction enzyme Swa I revealed that caged ODN pre-irradiated in hypoxia could form an ordinary duplex with its complementary strand.

Hypoxia-selective activation of 5-fluorodeoxyuridine prodrug possessing indolequinone structure: radiolytic reduction and cytotoxicity characteristics

We synthesized a 5-fluorodeoxyuridine (5-FdUrd) derivative possessing an indolequinone structure (IQ-FdUrd) to characterize the radiolytic reduction in aqueous solution and the radiation-activated cytotoxicity against EMT6/KU cells under hypoxic conditions. IQ-FdUrd released antitumor agent 5-FdUrd upon hypoxic, but not aerobic, irradiation with the G value of 0.38 x 10(-7) mol J(-1). Laser flash photolysis of IQ-FdUrd in Ar-purged aqueous solution with dimethylaniline as an electron donor gave rise to a transient absorption spectrum characteristic of semiquinone radical anion, which decayed via second order kinetics. It is most likely that bimolecular disproportionation of intermediate semiquinone radicals occurs to release 5-FdUrd. IQ-FdUrd showed enhanced cytotoxicity against EMT6/KU cells in a radiation dose-dependent manner upon hypoxic irradiation. IQ-FdUrd is potentially a prototype compound for new class of radiation-activated antitumor prodrugs that are useful for radiation treatment of hypoxic tumors.

Tumor-activated prodrugs--a new approach to cancer therapy

Systemic cytotoxic (antiproliferative) anticancer drugs rely primarily for their therapeutic effect on cytokinetic differences between cancer and normal cells. One approach aimed at improving the selectivity of tumor cell killing by such compounds is the use of less toxic prodrug forms that can be selectively activated in tumor tissue (tumor-activated prodrugs; TAP). There are several mechanisms potentially exploitable for the selective activation of TAP. Some utilize unique aspects of tumor physiology such as selective enzyme expression or hypoxia. Others are based on tumor-specific delivery techniques, including activation of prodrugs by exogenous enzymes delivered to tumor cells via monoclonal antibodies (ADEPT) or generated in tumor cells from DNA constructs containing the corresponding gene (GDEPT). Whichever activating mechanism is used, only a small proportion of the tumor cells are likely to be competent to activate the prodrug. Therefore, TAP need to fully exploit these "activator" cells by being capable of killing activation-incompetent cells as well via a "bystander effect." A wide variety of chemistries have been explored for the selective activation of TAP. Examples are given of the most important-the reduction of quinones, N-oxides, and nitroaromatics by endogenous enzymes or radiation; the cleavage of amides by endogenous peptidases; and hydrolytic metabolism by a variety of exogenous enzymes, including phosphatases, kinases, amidases, and glycosidases.

Biologic premises of combined radiation therapy and chemotherapy in lung cancer

When radiation is combined with concurrent chemotherapy, independent cell kill and enhancement of tumor response may be expected. On the other hand, spatial cooperation may be the main rationale for sequential combination of the two modalities. Among many new agents being investigated, radiation-activated antitumor prodrugs have a therapeutic potential as a new method to effectively combine radiation and concurrent chemotherapy.

In vitro and in vivo evaluation of novel antitumor prodrugs of 5-fluoro-2′-deoxyuridine activated by hypoxic irradiation

PURPOSE

We previously developed a novel antitumor prodrug that has a 2-oxopropyl substituent at the N(1) position of 5-fluorouracil (5-FU) and releases 5-FU via one-electron reduction on hypoxic irradiation. Although the compound was effective in vivo, its activity against murine tumors was not high enough to warrant clinical studies. Therefore, we developed a similar family of radiation-activated prodrugs of 5-fluoro-2'-deoxyuridine (FdUrd), which is generally more potent than 5-FU, and investigated their radiation chemical reactivity and in vitro and in vivo effects.

METHODS AND MATERIALS

Compounds bearing various 2-oxoalkyl substituents at the N(3) position of FdUrd were synthesized and investigated. After aerobic or hypoxic irradiation to the prodrugs dissolved in water or culture medium, release of FdUrd was measured using high-performance liquid chromatography. To investigate in vitro cytotoxicity, SCCVII and EMT6 cells in culture were irradiated in the presence of the prodrug under aerobic or hypoxic conditions, and then kept with the compound for 24 h. Cell survival was then measured using a colony assay. To investigate in vivo effects, the drug was injected intraperitoneally at a dose of 100 or 300 mg/kg into Balb/c mice bearing EMT6 tumors 30 min before irradiation. The tumor growth delay-time was then assessed.

RESULTS

In vitro, the prodrugs released FdUrd at G-values (molar numbers of molecules produced by 1 J of radiation energy) of 1.6-2.0 x 10(-7) mol/J after hypoxic irradiation. The G-values for FdUrd release with hypoxic irradiation were about 100-fold greater than those with aerobic irradiation. Among the prodrugs tested, OFU106 bearing a 2-oxocyclopentyl substituent released the highest amount of FdUrd in the culture medium, and it was subjected to further in vitro and in vivo assays. Although OFU106 administered alone showed no cytotoxicity up to a concentration of 0.2 mM, it produced an enhanced cytotoxic effect when administered before hypoxic irradiation and kept with the cells for 24 h. The enhancement ratios calculated at the surviving fraction of 1% were 1.35-1.4 at 0.04 mM and 1.45-1.5 at 0.2 mM. In vivo, however, administration of OFU106 (100 or 300 mg/kg) before 20 Gy of irradiation did not produce marked growth delays compared with 20 Gy of radiation alone.

CONCLUSION

On hypoxic irradiation in vitro, the prodrugs of FdUrd were activated as efficiently as were the prodrugs of 5-FU, but marked in vivo effects could not be detected. This strategy of prodrug design should be used in further development of radiation-activated prodrugs of more potent anticancer agents.

The Design of Drugs that Target Tumour Hypoxia

The occurrence of hypoxia in solid tumours is increasingly recognized as a limiting factor in the success of both radiotherapy and chemotherapy treatment, but at the same time offers a tumour-specific phenomenon for the activation of prodrugs. However, the design of clinically useful prodrugs that can be selectively activated in hypoxic cells has proved elusive. Specific reasons (activation by oxygen-insensitive two-electron reductases) have been proposed for the failure of quinone-based prodrugs, but a more general contributing factor may be inappropriate clinical trial design, and the failure to understand the critical importance of drug properties, such as efficient extra-vascular diffusion of the prodrug and back-diffusion of the activated drug in the tumour. Activation of prodrugs by therapeutic radiation and the use of hypoxia-selective gene therapy vectors, such as Clostridia, are exciting new mechanisms for prodrug research to explore, but are in much earlier stages of evaluation.

One-Electron reduction characteristics of N(3)-Substituted 5-fluorodeoxyuridines synthesized as radiation-Activated prodrugs

We designed and synthesized N(3)-substituted 5-fluorodeoxyuridines as radiation-activated prodrugs of the antitumor agent, 5-fluorodeoxyuridine (5-FdUrd). A series of 5-FdUrd derivatives possessing a 2-oxoalkyl group at the N(3)-position released 5-FdUrd in good yield via one-electron reduction initiated by hypoxic irradiation. Cytotoxicity of the 5-FdUrd derivative possessing the 2-oxocyclopentyl group (3d) was low, but was enhanced by hypoxic irradiation resulting in 5-FdUrd release.

Comparison of 5-fluorouracil and 5-fluoro-2'-deoxyuridine as an effector in radiation-activated prodrugs

The purpose of this study was to clarify whether 5-fluoro-2'-deoxyuridine (FdUrd) is superior to 5-fluorouracil (5-FU) as an effector in the radiation-activated prodrugs which we have been developing. The in vitro cytotoxicity of 5-FU and FdUrd was compared in two murine tumor and four human pancreatic cancer cell lines using a colony assay and in vivo efficacy was compared with SCCVII tumor using a growth delay time assay. FdUrd was slightly more hydrophilic than 5-FU. In vitro, FdUrd was more efficient than 5-FU in two lines, whereas 5-FU was more efficient in two lines and the two drugs were almost equal in efficacy in the remaining two. The concentration to reduce tumor cell survival to 50% after 24-h drug exposure was 5-32 microM for both 5-FU and FdUrd in murine lines, while it was 30-210 microM in human pancreatic cancer cell lines. The difference in relative efficacy of the two drugs among these cell lines could not be attributed to the rate of intracellular uptake of the compounds. FdUrd was less toxic than 5-FU in C3H/He mice, and FdUrd was less efficient than 5-FU in SCCVII tumors in vivo. These results suggest that FdUrd is not necessarily more potent than 5-FU, and development of the FdUrd prodrugs may not necessarily turn out to be fruitful.

In vivo evaluation of a novel antitumor prodrug, 1-(2'-oxopropyl)-5-fluorouracil (OFU001), which releases 5-fluorouracil upon hypoxic irradiation

PURPOSE

We previously proposed that a prodrug of 5-fluorouracil (5-FU), OFU001, is activated through capturing of hydrated electrons produced by hypoxic irradiation. Because hydrated electrons are readily deactivated by oxygen, the 5-FU release occurs specifically upon hypoxic irradiation. In this study, we investigated the in vivo efficacy, pharmacokinetics, and toxicity of OFU001.

METHODS AND MATERIALS

Female 10-week-old C3H/He mice bearing SCCVII tumors were used. To measure release of 5-FU from OFU001 in vivo, the mice were given 100 mg/kg of OFU001 intraperitoneally and irradiated. Thereafter, 5-FU levels in the tumor and serum were measured by high-performance liquid chromatography. To evaluate in vivo efficacy, OFU001 was administered 30 min before irradiation, and radiation-potentiating effects were investigated by means of a tumor growth delay assay and a 50% tumor control dose (TCD-50) assay. The lethal dose of OFU001 was evaluated in the same mice.

RESULTS

Following administration of OFU001 and irradiation at 30 Gy, the average 5-FU levels in the tumor and serum were 179 ng/g and 83 ng/mL, respectively. Administration of OFU001 (100-200 mg/kg) to the tumor-bearing mice before a single dose of 15-Gy irradiation produced a mean tumor growth delay of 1-5 days as compared to radiation alone (although the delay was not significant). However, no additional growth delay was observed when OFU001 was combined with 5 radiation fractions of 4 Gy each. The enhancement ratio of OFU001 in the TCD-50 assay was 1.2. No mice died after administration of 0.6-1.2 g/kg of OFU001.

CONCLUSIONS

OFU001 appears to work in vivo via the proposed mechanism of activation. Although the in vivo effect of this compound was not strong enough for clinical efficacy, these results should encourage further research on the development of prodrugs of more potent anticancer agents activated through the same mechanism.