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.

Current molecular design of intelligent drugs and imaging probes targeting tumor-specific microenvironments.

To address the specific challenges of cancer therapy and diagnosis, a number of approaches have been advocated for the development of tumor-targeting antitumor drugs/prodrugs and non-invasive tumor molecular imaging probes. These intelligent drugs and probes are constructed from multi-functional molecular systems. This review focuses on the molecular design of drugs and imaging probes that target tumor-specific microenvironments such as angiogenesis and hypoxia.

Aminopeptidase N/CD13 targeting fluorescent probes: synthesis and application to tumor cell imaging.

A family of fluorescein-peptide conjugates (CNP1-3) for aminopeptidase N (APN/CD13) targeting fluorescent probes were designed and synthesized. Among the three conjugates, CNP1 bearing tumor-homing cyclic peptide CNGRC, could selectively label APN/CD13 over-expressing on the surface of tumor cells of HT-1080, as identified by means of fluorescent microscopic cell imaging. CNP1 was shown to be a promising fluorescent probe applicable to tumor-targeting molecular imaging.

Bioreduction activated prodrugs of camptothecin: molecular design, synthesis, activation mechanism and hypoxia selective cytotoxicity.

Several water-soluble derivatives (CPT3, CPT3a-d) of camptothecin (CPT) were synthesized, among which CPT3 bearing an N,N'-dimethyl-1-aminoethylcarbamate side-chain was further conjugated with reductively eliminating structural units of indolequinone, 4-nitrobenzyl alcohol and 4-nitrofuryl alcohol to produce novel prodrugs of camptothecin (CPT4-6). All CPT derivatives were of lower cytotoxicity than their parent compound of CPT. In contrast, CPT4 and CPT6 showed higher hypoxia selectivity of cytotoxicity towards tumor cells than CPT. A mechanism by which a representative prodrug CPT4 is activated in the presence of DT-diaphorase to release CPT was also discussed. The bioreduction activated CPT prodrugs including CPT4 and CPT6 are identified to be promising for application to the hypoxia targeting tumor chemotherapy.

A new class of 5-fluoro-2'-deoxyuridine prodrugs conjugated with a tumor-homing cyclic peptide CNGRC by ester linkers: synthesis, reactivity, and tumor-cell-selective cytotoxicity.

PURPOSE: Tumor-targeting prodrugs of 5-fluoro-2'-deoxyuridine (5-FdUrd), which are chemical conjugations of 5-FdUrd with a tumor-homing cyclic peptide CNGRC by succinate and glutarate linkers, were synthesized to investigate the structural effects of linkers on the hydrolytic release of 5-FdUrd and the tumor-cell-selective cytotoxicity. METHODS: A solid phase synthesis method was used to produce 5-FdUrd prodrugs. The kinetics and efficiency of hydrolytic 5-FdUrd release from the prodrugs were investigated in phosphate buffer (PB), fetal bovine serum (FBS), HT-1080 cell lysate, MDA-MB-231 cell lysate, and MEM containing 10% FBS. The tumor-cell-selective cytotoxicity of prodrugs was evaluated by an MTT method. RESULTS: Two tumor-targeting prodrugs CNF1 and CNF2 bearing 5-FdUrd conjugated with a common cyclic peptide CNGRC by succinate and glutarate linkers, respectively, and their control compounds CN1 and CN2 without 5-FdUrd moiety were synthesized and identified. CNF1 underwent hydrolysis to release 5-FdUrd more rapidly and efficiently than CNF2. Both prodrugs were of lower cytotoxicity compared to 5-FdUrd, showing more selective cytotoxicity toward APN/CD13 positive cells (HT-1080) than toward APN/CD13 negative cells (HT-29, MDA-MB-231). CONCLUSIONS: A new class of tumor-targeting 5-FdUrd prodrugs CNF1 and CNF2 were successfully synthesized. These prodrugs targeted a tumor marker APN/CD13 to cause tumor-cell-selective cyctotoxicity due to 5-FdUrd release, the rate of which could be controlled by the structure of ester linker.

Synthesis and photochemical properties of photoactivated antitumor prodrugs releasing 5-fluorouracil.

A new family of antitumor prodrugs (1-3) of 5-fluorouracil (5-FU) possessing photolabile 2-nitrobenzyl chromophores have been designed and synthesized to investigate the efficiency and mechanism of photoactivated 5-FU release upon UV-irradiation at lambda(ex)= 365 nm. The photoactivated prodrug 3 derived from conjugation of 2 with a tumor-homing cyclic peptide Cys-Asn-Gly-Arg-Cys (CNGRC) was so designed as to manifest a tumor-targeting function.

Propargylic sulfones possessing a 2-nitroimidazole function: novel hypoxic-cell radiosensitizers with intracellular non-protein thiol depletion ability.

Propargylic sulfones (1a-c) containing a 2-nitroimidazole structure were synthesized, and their non-protein thiol (NPSH) depletion abilities were investigated. Propargylic sulfones 1a,c containing an electron withdrawing p-nitrophenyl group showed high reactivity toward capturing glutathione (GSH), a typical intracellular NPSH, in phosphate buffer. Among the three propargylic sulfones 1a-c, carboxylic acid derivative 1c showed the most potent radiosensitizing activity toward hypoxic EMT6/KU tumor cells. In view of these results and the partition coefficients between 1-octanol and water, we concluded that appropriate NPSH-depletion ability and lipophilicity are both important in achieving potent hypoxic-cell radiosensitization by propargylic sulfones possessing a 2-nitroimidazole function in biological systems.

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.

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.

Stereoisomeric C5-C5'-Linked Dihydrothymine Dimers Produced by Radiolytic One-Electron Reduction of Thymine Derivatives in Anoxic Aqueous Solution: Structural Characteristics in Reference to Cyclobutane Photodimers.

Radiolytic one-electron reduction of 1-methylthymine (1a) and 1,3-dimethylthymine (1b) in anoxic aqueous solution afforded stereoisomeric C5-C5'-linked dihydrothymine dimers, fractionated into the meso forms of (5R,5'S)- and (5S,5'R)-bi-5,6-dihydrothymine (3a,b[meso]) and a racemic mixture of (5R,5'R)- and (5S,5'S)-bi-5,6-dihydrothymines (3a,b[rac]), along with 5,6-dihydrothymines (2a,b). The meso and racemic dimers were produced in almost equivalent yields, possessing structural similarity with cis-syn-cyclobutane pyrimidine photodimers that are identified as highly mutagenic and carcinogenic photolesions induced by UV light. Similar radiolytic one-electron reduction of thymidine (1c) resulted in the pseudo-meso form of (5R,5'S)- and (5S,5'R)-bi-5,6-dihydrothymidine (3c[RS]) and two diastereormers of (5R,5'R)- and (5S,5'S)-bi-5,6-dihydrothymidine (3c[RR] and 3c[SS]). X-ray crystal structures indicated that two pyrimidine rings of the stereoisomeric dimers except 3a[rac] overlap with each other to a considerable extent, as in the cis-syn-cyclobutane photodimers. The pyrimidine rings of the dimers were twisted around 5-Me-C5-C5'-5'-Me by 51.1(2)° for 3a[meso], -85.4(4)° for 3a[rac], -65(1)° for 3b[meso], 43(2)° for 3b[rac], and 64.9(4)° for 3c[RS], respectively. It was predicted that the C5-C5'-linked dihydrothymine dimers may cause some distortion within a DNA duplex if they were incorporated. The pH dependence of the reactivities was in accord with a mechanism of the C5-C5'-linked dimerization by which electron adducts of 1a-c are irreversibly protonated at C6 and the resulting 5,6-dihydrothymin-5-yl radicals undergo bimolecular coupling.