FV-100 versus valacyclovir for the prevention of post-herpetic neuralgia and the treatment of acute herpes zoster-associated pain: A randomized-controlled trial.

This prospective, parallel-group, randomized, double-blind, multicenter study compared the efficacy and safety of FV-100 with valacyclovir for reducing pain associated with acute herpes zoster (HZ). Patients, ≥50 years of age, diagnosed with HZ within 72 h of lesion appearance who had HZ-associated pain, were randomized 1:1:1 to a 7-day course of either FV-100 200 mg QD (n = 117), FV-100 400 mg QD (n = 116), or valacyclovir 1000 mg TID (n =117). Efficacy was evaluated on the basis of the burden of illness (BOI; Zoster Brief Pain Inventory scores); incidence and duration of clinically significant pain (CSP); pain scores; incidence and severity of post-herpetic neuralgia (PHN); and times to full lesion crusting and to lesion healing. Safety was evaluated on the basis of adverse event (AE)/SAE profiles, changes in laboratory and vital signs values, and results of electrocardiograms. The burden of illness scores for pain through 30 days were 114.5, 110.3, and 118.0 for FV-100 200 mg, FV-100 400 mg, and valacyclovir 3000 mg, respectively. The incidences of PHN at 90 days for FV-100 200 mg, FV-100 400 mg, and valacyclovir 3000 mg were 17.8%, 12.4%, and 20.2%, respectively. Adverse event and SAE profiles of the two FV-100 and the valacyclovir groups were similar and no untoward signals or trends were evident. These results demonstrate a potential for FV-100 as an antiviral for the treatment of shingles that could both reduce the pain burden of the acute episode and reduce the incidence of PHN compared with available treatments.

The alpha-herpesviridae in dermatology : Varicella zoster virus.

The second part of this publication deals with varicella zoster virus (VZV) and presents an overview of new, rare, and atypical clinical manifestations, including photolocalized varicella, hemorrhagic bullae during varicella, the implication of VZV in immunoglobulin A vasculitis, VZV-related alopecia, ulcerative varicella skin lesions, childhood herpes zoster (HZ), prolonged prodromal pains, recurrent HZ, VZV implication in burning mouth syndrome, verruciform VZV lesions, the significance of satellite lesions during HZ, and late HZ complications, either neurological or internal. Furthermore, certain associations between the occurrence of HZ and subsequent internal pathologies, as well as risk factors for HZ and new developments in vaccination against HZ will be addressed.

Nucleotide precursors prevent folic acid-resistant neural tube defects in the mouse.

Closure of the neural tube during embryogenesis is a crucial step in development of the central nervous system. Failure of this process results in neural tube defects, including spina bifida and anencephaly, which are among the most common birth defects worldwide. Maternal use of folic acid supplements reduces risk of neural tube defects but a proportion of cases are not preventable. Folic acid is thought to act through folate one-carbon metabolism, which transfers one-carbon units for methylation reactions and nucleotide biosynthesis. Hence suboptimal performance of the intervening reactions could limit the efficacy of folic acid. We hypothesized that direct supplementation with nucleotides, downstream of folate metabolism, has the potential to support neural tube closure. Therefore, in a mouse model that exhibits folic acid-resistant neural tube defects, we tested the effect of specific combinations of pyrimidine and purine nucleotide precursors and observed a significant protective effect. Labelling in whole embryo culture showed that nucleotides are taken up by the neurulating embryo and incorporated into genomic DNA. Furthermore, the mitotic index was elevated in neural folds and hindgut of treated embryos, consistent with a proposed mechanism of neural tube defect prevention through stimulation of cellular proliferation. These findings may provide an impetus for future investigations of supplemental nucleotides as a means to prevent a greater proportion of human neural tube defects than can be achieved by folic acid alone.

Preferential response of cancer cells to zebularine.

The frequent silencing of tumor suppressor genes by altered cytosine methylation and chromatin structural changes makes this process an attractive target for epigenetic therapy. Here we show that zebularine, a stable DNA cytosine methylation inhibitor, is preferentially incorporated into DNA and exhibits greater cell growth inhibition and gene expression in cancer cell lines compared to normal fibroblasts. In addition, zebularine preferentially depleted DNA methyltransferase 1 (DNMT1) and induced expression of cancer-related antigen genes in cancer cells relative to normal fibroblasts. Our results demonstrate that zebularine can be selective toward cancer cells and may hold clinical promise as an anticancer therapy.

Heat shock protein 90 inhibitor 17-AAG down-regulates thymidine phosphorylase expression and potentiates the cytotoxic effect of tamoxifen and erlotinib in human lung squamous carcinoma cells.

Elevated thymidine phosphorylase (TP) levels, a key enzyme in the pyrimidine nucleoside salvage pathway, in cancer cells, are related to a poor prognosis in a variety of cancers. Heat shock protein 90 (Hsp90) is a ubiquitous molecular chaperone that is involved in the stabilization and maturation of many oncogenic proteins. The aim of this study is to elucidate whether Hsp90 inhibitor 17-AAG could enhance tamoxifen- and erlotinib-induced cytotoxicity in nonsmall cell lung cancer (NSCLC) cells via modulating TP expression in two squamous NSCLC cell lines, H520 and H1703. We found that 17-AAG reduced TP expression via inactivating the MKK1/2-ERK1/2-mitogen-activated protein kinase (MAPK) pathway. TP knockdown with siRNA or ERK1/2 MAPK inactivation with the pharmacological inhibitor U0126 could enhance the cytotoxic and growth inhibitory effects of 17-AAG. In contrast, MKK1-CA or MKK2-CA (a constitutively active form of MKK1/2) vector-enforced expression could reduce the cytotoxic and cell growth inhibitory effects of 17-AAG. Furthermore, 17-AAG enhanced the cytotoxic and cell growth inhibitory effects of tamoxifen and erlotinib in NSCLC cells, which were associated with TP expression downregulation and MKK1/2-ERK1/2 signal inactivation. Taken together, Hsp90 inhibition downregulates TP, enhancing the tamoxifen- and erlotinib-induced cytotoxicity in H520 and H1703 cells.

Antiviral efficacy upon administration of a HepDirect prodrug of 2'-C-methylcytidine to hepatitis C virus-infected chimpanzees.

Hepatitis C virus (HCV) infects an estimated 170 million individuals worldwide, and the current standard of care, a combination of pegylated interferon alpha and ribavirin, is efficacious in achieving sustained viral response in ~50% of treated patients. Novel therapies under investigation include the use of nucleoside analog inhibitors of the viral RNA-dependent RNA polymerase. NM283, a 3'-valyl ester prodrug of 2'-C-methylcytidine, has demonstrated antiviral efficacy in HCV-infected patients (N. Afdhal et al., J. Hepatol. 46[Suppl. 1]:S5, 2007; N. Afdhal et al., J. Hepatol. 44[Suppl. 2]:S19, 2006). One approach to increase the antiviral efficacy of 2'-C-methylcytidine is to increase the concentration of the active inhibitory species, the 5'-triphosphate, in infected hepatocytes. HepDirect prodrug technology can increase intracellular concentrations of a nucleoside triphosphate in hepatocytes by introducing the nucleoside monophosphate into the cell, bypassing the initial kinase step that is often rate limiting. Screening for 2'-C-methylcytidine triphosphate levels in rat liver after oral dosing identified 1-[3,5-difluorophenyl]-1,3-propandiol as an efficient prodrug modification. To determine antiviral efficacy in vivo, the prodrug was administered separately via oral and intravenous dosing to two HCV-infected chimpanzees. Circulating viral loads declined by ~1.4 log(10) IU/ml and by >3.6 log(10) IU/ml after oral and intravenous dosing, respectively. The viral loads rebounded after the end of dosing to predose levels. The results indicate that a robust antiviral response can be achieved upon administration of the prodrug.

Inhibition of herpesvirus replication by 5-substituted 4'-thiopyrimidine nucleosides.

A series of 4'-thionucleosides were synthesized and evaluated for activities against orthopoxviruses and herpesviruses. We reported previously that one analog, 5-iodo-4'-thio-2'-deoxyuridine (4'-thioIDU), exhibits good activity both in vitro and in vivo against two orthopoxviruses. This compound also has good activity in cell culture against many of the herpesviruses. It inhibited the replication of herpes simplex virus type 1 (HSV-1), HSV-2, and varicella-zoster virus with 50% effective concentrations (EC(50)s) of 0.1, 0.5, and 2 microM, respectively. It also inhibited the replication of human cytomegalovirus (HCMV) with an EC(50) of 5.9 microM but did not selectively inhibit Epstein-Barr virus, human herpesvirus 6, or human herpesvirus 8. While acyclovir-resistant strains of HSV-1 and HSV-2 were comparatively resistant to 4'-thioIDU, it retained modest activity (EC(50)s of 4 to 12 microM) against these strains. Some ganciclovir-resistant strains of HCMV also exhibited reduced susceptibilities to the compound, which appeared to be related to the specific mutations in the DNA polymerase, consistent with the observed incorporation of the compound into viral DNA. The activity of 4'-thioIDU was also evaluated using mice infected intranasally with the MS strain of HSV-2. Although there was no decrease in final mortality rates, the mean length of survival after inoculation increased significantly (P < 0.05) for all animals receiving 4'-thioIDU. The findings from the studies presented here suggest that 4'-thioIDU is a good inhibitor of some herpesviruses, as well as orthopoxviruses, and this class of compounds warrants further study as a therapy for infections with these viruses.

FV100 as a new approach for the possible treatment of varicella-zoster virus infection.

FV100 is a promising new agent with extreme potency and specificity for varicella-zoster virus (VZV). It is the valyl ester pro-drug of Cf1743, the lead clinical candidate among the highly lipophilic bicyclic nucleoside analogue (BCNA) family discovered in Cardiff/Leuven. Cf1743 is unique amongst antivirals in terms of its structure and lipophilicity. It is exquisitely potent and selective for human VZV. FV100 has recently entered a randomized, controlled Phase II clinical trial for the treatment of shingles, sponsored by Inhibitex.

Emerging therapies for hepatitis C and HIV in drug abusers: drugs and strategies.

New drug therapies to treat hepatitis C (HCV) and HIV infection are being developed with improved understanding of the molecular structures of the viruses themselves, the pathogenesis of infection and the specific immune responses needed to eradicate or control these infections. Interferon and ribavirin based therapies will continue to be a component of HCV therapy for the near future combined with other novel compounds directed at targets of viral replication, immunomodulation or cell entry. The goals of anti-HCV therapy are viral eradication for differing genotypes and prevention of hepatic morbidity such as hepatocellular carcinoma and cirrhosis. Future antiretroviral therapies for HIV will include agents that focus on new classes of inhibitors of viral replication and cell binding. The new treatment choices in HIV will need to ensure effective and durable viral suppression especially against highly resistant virus strains, regimen tolerability and improved toxicity.

Inosine 5'-Monophosphate Dehydrogenase (IMPDH) as a Potential Target for the Development of a New Generation of Antiprotozoan Agents.

Inosine-5'-monophosphate dehydrogenase (IMPDH) is a metabolic enzyme that catalyzes the critical step in guanine nucleotide biosynthesis, and thus is at the center of cell growth and proliferation. However, although this enzyme has been exploited as potential target for the development of immunosuppressive, anticancer, and antiviral agents, the functional importance of IMPDH as a promising antiprotozoan drug target is still in its infancy mainly because of the availability of alternative nucleotides metabolic pathways in many of these parasites. This situation suggests that the inhibition of IMPDH might have little to no effect on the survival of protozoan parasites. As a result, no IMPDH inhibitor is currently commercially available or has advanced to clinical trials as a potential antiprotozoan drug. Nevertheless, recent advances toward the development of selective inhibitors of the IMPDH enzyme from Crystosporidium parvum as potential drug candidates against cryptosporidiosis should revive further investigations of this drug target in other protozoa parasites. The current review examines the chemical structures and biological activities of reported protozoan's IMPDH inhibitors. SciFinder was used to broadly pinpoint reports published on the topic in the chemical literature, with no specific time frame. Opportunities and challenges towards the development of inhibitors of IMPDH enzymes from protozoa parasites as potential chemotherapies toward the respective diseases they cause are also discussed.

5-iodo-2-pyrimidinone-2'-deoxyribose-mediated cytotoxicity and radiosensitization in U87 human glioblastoma xenografts.

PURPOSE: 5-iodo-2-pyrimidinone-2'-deoxyribose (IPdR) is a novel orally administered (p.o.) prodrug of 5-iododeoxyuridine. Because p.o. IPdR is being considered for clinical testing as a radiosensitizer in patients with high-grade gliomas, we performed this in vivo study of IPdR-mediated cytotoxicity and radiosensitization in a human glioblastoma xenograft model, U87. METHODS AND MATERIALS: Groups of 8 or 9 athymic male nude mice (6-8 weeks old) were implanted with s.c. U87 xenograft tumors (4 x 10(6) cells) and then randomized to 10 treatment groups receiving increasing doses of p.o. IPdR (0, 100, 250, 500, and 1000 mg/kg/d) administered once daily (q.d.) x 14 days with or without radiotherapy (RT) (0 or 2 Gy/d x 4 days) on days 11-14 of IPdR treatment. Systemic toxicity was determined by body weight measurements during and after IPdR treatment. Tumor response was assessed by changes in tumor volumes. RESULTS: IPdR alone at doses of > or =500 mg/kg/d resulted in moderate inhibition of tumor growth. The combination of IPdR plus RT resulted in a significant IPdR dose-dependent tumor growth delay, with the maximum radiosensitization using > or =500 mg/kg/d. IPdR doses of 500 and 1000 mg/kg/d resulted in transient 5-15% body weight loss during treatment. CONCLUSIONS: In U87 human glioblastoma s.c. xenografts, p.o. IPdR given q.d. x 14 days and RT given 2 Gy/d x 4 days (days 11-14 of IPdR treatment) results in a significant tumor growth delay in an IPdR dose-dependent pattern. The use of p.o. IPdR plus RT holds promise for Phase I/II testing in patients with high-grade gliomas.

Valopicitabine dihydrochloride:a specific polymerase inhibitor of hepatitis C virus.

Idenix Pharmaceuticals Inc and Novartis AG are codeveloping valopicitabine dihydrochloride, a once-daily oral nucleoside for the potential treatment of HCV infection. In January 2005, a phase IIa clinical trial comparing valopicitabine dihydrochloride with pegylated IFN in treatment-naive HCV patients was ongoing, in addition to a phase IIb trial in patients that had previously failed pegylated IFN and ribavirin combination therapy. In January 2006, an international phase III trial in treatment-refractory patients was planned for the first half of the year, with a phase III trial in treatment-naive individuals planned for the second half of the year.

Enzyme-Driven Chemo-and Radiation-Therapy with 12 Pyrimidine Nucleoside Analogs Not Yet in the Clinic.

Enzymatic activity from tumor and adjacent normal tissue of 200 patients involving deoxycytidine kinase (dCK), uridine/cytidine kinase (U/CK), cytidine deaminase (CD) and deoxycytidylate deaminase (dCMPD) was quantified. Patients with brain (17), colon (24), and breast (30) tumors, 53, 67, and 73%, respectively, had an elevated T/N value (Specific Activity of tumor/ Specific Activity of normal tissue) involving dCK and dCMPD suggesting chemotherapy with 5-fluorodeoxycytidine (5-FdC) alone or in combination with thymidine plus deoxytetrahydrouridine, or with the radiosensitizer, 5-chlorodeoxycytidine (5-CldC) plus tetrahydrouridine (H4U). Among patients with colon (19) and pancreatic tumors (40), 53 and 68 %, respectively, displayed T/N values >4 for CD suggesting chemotherapy with 5-FdC, 4-N-methylamino-5-FdC, 5-trifluoromethyldeoxycytidine and radiosensitization with 5- CldC, 4-N-methylamino-5-CldC, 5-iododeoxycytidine and 5-bromodeoxycytidine. The percent of patients with tumors with a T/N value >4 for U/CK in lung (72), colon (23) and breast (28) was 47, 61 and 68, respectively, suggesting zebularine (plus thymidine) treatment for tumors involving gene silencing. Evidence is presented that the 4-N-alkylamino-dC substituted nucleosides and those with large 5-substitutions are activated only via CD to thymidine kinase (TK) using end-points of cytotoxicity and/or radiosensitization: H4U, the inhibitor of CD is an antagonist, cells with low CD or no TK are resistant to the analogs, the end points are indifferent to the dCK status of cells, they are poor substrates for dCK and good substrates for CD, whereas 5-FdC and 5-CldC are good substrates for both enzymes. The analogs present opportunities for Collateral Sensitivity for 5-azacytidine and gemcitabine resistant tumors.

New therapies: oral inhibitors and immune modulators.

Several new classes of antiviral drugs are undergoing development and should change the way that hepatitis C virus infection is treated in the future. It is likely that combinations of drugs that target different points in the viral replication and disease processes will prove most successful. It is hoped that such combinations will improve the efficacy, tolerability, and duration of antiviral treatment for this disease.

Novel approaches for therapy of chronic hepatitis C.

Currently available anti-HCV therapy is effective in only half of the patients and limited by side effects that often necessitate discontinuation. Therefore, new treatment strategies are being developed including (i) the optimization of current regimens, (ii) the use of additional agents working via novel mechanisms, and (iii) anti-fibrotic strategies. Many new antiviral compounds are now being studied in preclinical and clinical trials. This review will focus on drugs that have already entered the stage of phase 2 or phase 3 studies.

Schedule-dependent drug effects of oral 5-iodo-2-pyrimidinone-2'-deoxyribose as an in vivo radiosensitizer in U251 human glioblastoma xenografts.

PURPOSE: 5-Iodo-2-pyrimidinone-2'-deoxyribose (IPdR) is an oral prodrug of 5-iodo-2'-deoxyuridine (IUdR), an in vitro/in vivo radiosensitizer. IPdR can be rapidly converted to IUdR by a hepatic aldehyde oxidase. Previously, we found that the enzymatic conversion of IPdR to IUdR could be transiently reduced using a once daily (q.d.) treatment schedule and this may affect IPdR-mediated tumor radiosensitization. The purpose of this study is to measure the effect of different drug dosing schedules on tumor radiosensitization and therapeutic index in human glioblastoma xenografts. EXPERIMENTAL DESIGN: Three different IPdR treatment schedules (thrice a day, t.i.d.; every other day, q.o.d.; every 3rd day, q.3.d.), compared with a q.d. schedule, were analyzed using athymic nude mice with human glioblastoma (U251) s.c. xenografts. Plasma pharmacokinetics, IUdR-DNA incorporation in tumor and normal proliferating tissues, tumor growth delay following irradiation, and body weight loss were used as end points. RESULTS: The t.i.d. schedule with the same total daily doses as the q.d. schedule (250, 500, or 1,000 mg/kg/d) improved the efficiency of IPdR conversion to IUdR. As a result, the percentage of IUdR-DNA incorporation was higher using the t.i.d. schedule in the tumor xenografts as well as in normal small intestine and bone marrow. Using a fixed dose (500 mg/kg) per administration, the q.o.d. and q.3.d. schedules also showed greater IPdR conversion than the q.d. schedule, related to a greater recovery of hepatic aldehyde oxidase activity prior to the next drug dosing. In the tumor regrowth assay, all IPdR treatment schedules showed significant increases of regrowth delays compared with the control without IPdR (q.o.d., 29.4 days; q.d., 29.7 days; t.i.d., 34.7 days; radiotherapy alone, 15.7 days). The t.i.d. schedule also showed a significantly enhanced tumor growth delay compared with the q.d. schedule. Additionally, the q.o.d. schedule resulted in a significant reduction in systemic toxicity. CONCLUSIONS: The t.i.d. and q.o.d. dosing schedules improved the efficiency of enzymatic activation of IPdR to IUdR during treatment and changed the extent of tumor radiosensitization and/or systemic toxicity compared with a q.d. dosing schedule. These dosing schedules will be considered for future clinical trials of IPdR-mediated human tumor radiosensitization.

Activity of 2-fluoro-5-methylarabinofuranosyluracil against mouse leukemias sensitive to and resistant to 1-beta-D-arabinofuranosylcytosine.

A new pyrimidine nucleoside, 2'-fluoro-5-methyl-1-beta-D-arabinofuranosyluracil, previously has been shown to be active against the herpes group of viruses in vitro and in vivo. It is also active against mouse and human leukemic cells in culture and against mouse leukemias L1210, P388, and P815 in vivo. In contrast to other 1-beta-D-arabinofuranosylcytosine (ara-C) derivatives, 2'-fluoro-5-methyl-1-beta-D-arabinofuranosyluracil, when given either i.p. or p.o., is highly active against lines of leukemias P815 and L1210 made resistant to ara-C. Against P815/ara-C and L1210/araC, it is more effective than is 5-azacytidine, a drug which has shown definite effectiveness in patients with acute leukemia whose disease has become resistant to ara-C. For these reasons, 2'-fluoro-5-methyl-1-beta-D-arabinofuranosyluracil would seem to merit clinical trial in patients with acute nonlymphocytic leukemia whose disease has become resistant to ara-C.

Synthesis and antiviral properties of 5-vinylpyrimidine nucleoside analogs.

5-Vinylpyrimidine nucleosides can be readily synthesized via organometallic intermediates from commercially available nucleosides. Highly potent and selective inhibitors of herpes simplex virus type 1 (HSV-1) and varicella-zoster virus (VZV) are (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) and some related analogs such as (E)-5-(2-iodovinyl)-2'-deoxyuridine (IVDU), 1-beta-D-arabinofuranosyl-(E)-5-(2-bromovinyl)uracil (BVaraU) and (E)-5-(2-bromovinyl)-2'-deoxycytidine (BVDC). The selective antiviral action of BVDU is based upon a specific phosphorylation by the virus-encoded deoxythymidine kinase (TK), inhibition of the viral DNA polymerase and/or incorporation into viral DNA. The efficacy of BVDU against HSV-1 and VZV infections has been demonstrated in animal models and phase I clinical trials. Possible limitations in the clinical usefulness of 5-vinylpyrimidine nucleosides in general and BVDU in particular are discussed.

Empirical and rational approaches for development of inhibitors of the human immunodeficiency virus--HIV-1.

The human immunodeficiency virus, HIV-1, is generally accepted to be responsible for AIDS. It is imperative that all approaches, empirical and rational, be taken for development of a drug for therapy of this disease. These approaches are discussed, with emphasis on the direction being pursued in our laboratory. Empirically, we found 3'-deoxy-2',3'-didehydrothymidine, a compound first synthesized for potential anticancer activity by J. Horwitz in the 1960s, to be a potent inhibitor of HIV-1. It is now in Phase II/III clinical trials. We have also synthesized several 2,5'-anhydro pyrimidine nucleoside analogs, which have interesting chemical and biological properties. We have evaluated a natural product, gossypol and synthesized various derivatives for anti-HIV-1 activity, but none were appreciably more inhibitory than the parent compound. More recently, we have taken the rational approach and synthesized a boron-modified tetrapeptide, Ac-Thr-Leu-Asn-boro-Phe, which corresponds to the COOH-terminal of the Phe-Pro scissle bond of the gag/pol gene polyprotein product. Potent inhibition of the HIV-1 encoded protease was observed. These approaches and findings will be discussed.

Preclinical evaluation of 5-iodo-2-pyrimidinone-2'-deoxyribose as a prodrug for 5-iodo-2'-deoxyuridine-mediated radiosensitization in mouse and human tissues.

We reported previously that p.o. administered 5-iodo-2-pyrimidinone-2'-deoxyribose (IPdR) was efficiently converted to 5-iodo-2'-deoxyuridine (IUdR) in athymic mice (T. J. Kinsella et al., Cancer Res., 54: 2695-2700, 1994). Here, we further evaluate IPdR metabolism, systemic toxicity, and percentage DNA incorporation in athymic mouse normal tissues and a human colon cancer xenograft (HT29) using higher p.o. doses of IPdR. These data are compared to results using a continuous infusion of IUdR at the maximum tolerable dose. We also evaluate IPdR metabolism in cytosolic extracts from normal human liver, normal human intestine, and human colorectal cancer specimens. Athymic mice tolerated a daily p.o. bolus of up to 2 g/kg IPdR for 6 days with minimal host toxicity (< or = 10% body weight loss). There was rapid conversion of IPdR to IUdR, with peak plasma levels of IUdR of 40-75 microM at 10 min following a p.o. IPdR bolus of 250-1500 mg/kg. The percentage IUdR-DNA in the HT29 s.c. human tumor xenografts increased 1.5 times (2.3-3.6%) with IPdR doses above 1 g/kg/day for 6 days, whereas the percentage IUdR-DNA incorporation in two proliferating normal tissues (4-4.5% in intestine; 1.6-2.2% in bone marrow) and a quiescent normal tissue (< or = 1% in liver) showed < 1.5-fold increases with the IPdR dose escalation between 1-2 g/kg/day for 6 days. In contrast, using a continuous infusion of IUdR at 100 mg/kg/day, significant systemic toxicity (> 20% body weight loss) was found by day 6 of the infusion. Steady-state plasma IUdR levels were 1.0-1.2 microM during the 6-day infusion, and percentage IUdR-DNA incorporations of 2.3, 8, 6, and 1% were measured in s.c. tumors, normal intestine, normal bone marrow, and normal liver, respectively, following the 6-day infusion. Thus, the p.o. IPdR schedule has an improved therapeutic index, based on percentage IUdR-DNA incorporation in normal and tumor tissues, compared to continuous infusion IUdR at the maximum tolerable dose in athymic mice with this human tumor xenograft. Additionally, a tumor regrowth assay to assess the radiation response of HT29 s.c. xenografts showed a 1.5-fold enhancement (time to regrow to 300% initial tumor volume) with IPdR (1000 mg/kg/day for 6 days) plus fractionated irradiation (XRT; 2 Gy/day for 4 days), compared to XRT (2 Gy/day for 4 days) alone. No enhancement in the radiation response of HT29 s.c. xenografts was found with continuous infusion IUdR (100 mg/kg/day for 6 days) plus XRT (2 Gy/day for 4 days), compared to XRT alone. Using cytosolic extracts from normal human liver specimens, we found a rapid (15-min) conversion of IPdR to IUdR. Coincubation of liver cytosol with IPdR and allopurinol, an inhibitor of xanthine oxidase, had no inhibitory effect on IPdR metabolism, whereas coincubation with IPdR and isovanillin or menadione, analogue substrates for aldehyde oxidase, effectively reduced the amount of IPdR oxidized to IUdR. Significantly less metabolism of IPdR to IUdR was seen in cytosolic extracts from normal human intestine specimens, and no metabolism of IPdR was found in cytosolic extracts from colorectal liver metastases in two patients and from the HT29 human colon cancer xenografts in athymic mice. These additional data indicate that IPdR has the potential for clinical use as a p.o. prodrug for IUdR-mediated radiosensitization of resistant human cancers.