Effects of bleomycin with iododeoxyuridine, 5'-amino-5'-deoxythymidine, and radiation in a human bladder cancer cell line

In vitro bleomycin cytotoxicity in 647V, a human bladder cancer cell line, is enhanced when the cells are preincubated for one cell cycle (24 h) with clinically relevant concentrations (2 microM) of iododeoxyuridine (IdUrd). Bleomycin cytotoxicity after a 1-h exposure is further enhanced by a 24-h preincubation with 2 microM IdUrd plus 30 microM 5'-amino-5'-deoxythymidine (5'-AdThd). Chemosensitization of clinically achievable plasma levels of bleomycin (1-10 mU/ml for 1 h) by IdUrd (+/- 5'-AdThd) is associated with the level of DNA incorporation of IdUrd in 647V cells and with enhanced bleomycin-induced damage in IdUrd-substituted DNA compared to control DNA, as measured by rapid alkaline elution. We also studied the in vitro effects of bleomycin (+/- IdUrd) treatment on the subsequent radiation response of 647V cells. We found that a 1-h preirradiation exposure of 647V cells to low concentrations of bleomycin (1 mU/ml) can radiosensitize the cells with an even greater interaction exhibited by pretreatment with 2 microM IdUrd (24 h) plus bleomycin (1 h) vs control. Since the principal normal tissue toxicities of IdUrd (involving bone marrow and gut) and bleomycin (involving lung) do not overlap, these in vitro data suggest that chemosensitization of bleomycin by IdUrd may provide a new therapeutic regimen for chemotherapy and a significant enhancement for radiotherapy in human bladder cancer and possibly other epithelial cancers.

Altering clinical features of recurrent herpes simplex virus-induced keratitis

We studied the alteration in the clinical features of herpes simplex virus-induced keratitis (HSK) after the introduction of acyclovir (ACV) (in 1986). Over a nine-year period, this study investigated 101 patients (52 men and 49 women) whose conditions were diagnosed clinically as HSK. In addition, HSK recurred in 48 (47.5%) of the 101 patients. The distribution of clinical types of primary HSK before 1985 did not differ from that after 1986. Recurrent HSK was significantly more frequent in patients aged 26 to 40 years (75.0%). Although the five-year recurrence rates in primary HSK treated with ACV were significantly lower than those in patients treated with idoxuridine, the recurrence frequency before 1985 was equal to that after 1986. The discordance of the decreased recurrence rate in ACV-treated patients and the high recurrence frequency in patients with recurrent disease after the introduction of ACV were considered to be the result of the incomplete inhibitory effect of this drug.

Sensitizers of photoradiation and ionizing radiation in the management of head and neck cancer

PDT and IUdR-sensitized radiation therapy represent potential advances in the treatment of tumors of the head and neck. Light-activated photosensitizers have definite antitumor activity in both in vitro and in vivo experimental systems. Much of the early clinical work in head and neck cancer involved treatment of patients with advanced, recurrent disease who had not responded to conventional therapy. Because of the limited light penetration in tissue and infiltrative nature of most recurrent lesions, little effective palliation was seen in these advanced cases. More success has been achieved in the treatment of earlier, more superficial lesions, and active investigation continues in this area. Current research is aimed at defining the most appropriate sites and applications for the technique. HpD and DHE are currently only approved for use as investigational compounds in clinical studies. If ongoing trials of PDT in the treatment of superficial bladder cancer, obstructing esophageal cancer, and non-small cell lung cancer show encouraging results, an application will be made to the Food and Drug Administration for approval of DHE as a photosensitizer for general clinical use for these indications. Laboratory work to better understand the mechanism of action of HpD also continues, as well as investigations into alternative photosensitizers with improved tumor localization, less cutaneous photosensitivity, and absorption peaks at deeper penetrating wavelengths of light. A pilot program evaluating IUdR-sensitized radiation therapy for treatment of advanced head and neck cancer is in progress. If encouraging early results continue to be observed, a randomized trial comparing IUdR-sensitized radiation therapy with conventional radiation therapy can be conducted. Hopefully, these developments in the field will improve the therapy for patients with head and neck cancers.

Cross-resistances to antiviral drugs of IUdR-resistant HSV1 in rabbit keratitis and in vitro

The emergence of cross-resistance to various antiviral drugs was investigated both in vivo and in vitro for herpes simplex virus type 1 (HSV1) resistant to idoxuridine (IUdR 0.24%) obtained by seven successive passages (from P0 to P7) in rabbit keratitis treated by IUdR. The viral population obtained at the seventh IUdR passage (P7) showed an activity of the thymidine kinase (TK) reduced to 5.6% of the parental strain (PO); moreover, most of the clones of P7 showed an altered TK phenotype determined by the [125I]iododeoxycytidine (IDC) procedure. In rabbit keratitis, IUdR-resistant viral population P7 showed cross-resistance to bromovinyl desoxyuridine (BVDU) (0.5%) and to acyclovir (ACV) (3%). Under trifluorothymidine (1%) treatment, P7 showed an intermediate sensitivity. HSV1 at P7 remained sensitive to adenine arabinoside (Ara A) (3%) and to dihydroxy-propoxymethylguanine used at high concentration (3%). The in vitro sensitivity determination to various antiviral drugs was investigated by dye-uptake assay for the initial viral population PO and for HSV1 collected under IUdR treatment at the third (P3) and the seventh (P7) passages. Cross-resistance to TK-dependent drugs, such as IDC, BVDU, and ACV were found at P7. P7 remained sensitive to Ara A and to phosphonoformic acids antiviral drugs known not to be dependent on viral TK.

Herpetic keratitis in Tanzania: association with malaria

Of 103 patients who presented to a rural clinic in Africa with corneal ulceration, 62 (60.2%) had corneal ulcers which on clinical diagnosis were attributable to herpes simplex virus. There was a strong association between herpetic ulceration and a history of recent malaria; 37 of 62 (59.7%) herpetic ulcers occurred in the 3 month period from April to June which corresponds to the end of the peak season for malaria compared with 14 of 41 (34%) of the non-herpetic ulcers. Fifty per cent of herpetic ulcers had a geographic morphology, 27.4% were dendritic, and 22.6% presumed herpetic ulcers were stromal: 38.7% of herpetic ulcers occurred in children under 5 years of age. Bilateral herpetic ulcers occurred in 16.1% of patients but were more common in children under 2 years of age. Geographic and stromal ulcers tended to heal more slowly than dendritic ulcers (mean time to healing 12.6, 12.2, and 6.6 days respectively), and were more likely to result in severe corneal scarring (45%, 29%, and 0% respectively). Herpes simplex keratitis is a major cause of corneal scarring in Africa. It is often seen in children, may be bilateral, commonly is geographic in morphology, and has a strong association with malaria infection. Because it is not easily preventable, more effort must be made to ensure early diagnosis and prompt, effective treatment in order to prevent severe scarring and visual loss.

5-Iododeoxyuridine increases the efficacy of the radioimmunotherapy of human tumors growing in nude mice

Recently, there has been much interest in the use of radionuclide conjugated monoclonal antibodies for the treatment of human malignancies. One way to potentially maximize the therapeutic effectiveness of radioimmunotherapy would be to sensitize tumor cells to the radiation dose delivered by the antibody. Since radioimmunotherapy can potentially treat disseminated disease, including micrometastasis, we chose to study a halogenated pyrimidine radiosensitizer, a class of compounds that affect nonhypoxic cells. 5-Iododeoxyuridine, administered with pyrimidine metabolism modulators, increased the therapeutic effectiveness of radioimmunotherapy, resulting in individual cures of human tumors growing in BALB/c nu/nu (nude) mice. 5-Iododeoxyuridine was administered with N-(phosphonacetyl)-L-aspartic acid and 5-fluoro-deoxycytidine plus tetrahydrouridine. This drug treatment was combined with radioimmunotherapy using 131I conjugated to a monoclonal antibody, Mc5. Mc5 binds to a mucin component of the human milk fat globule. This antigen is expressed on the surface of MX-1 cells, the transplantable human tumor used in this study. Tumor-bearing mice treated with both the drug protocol and 131I-Mc5 (540 microCi, 10 microCi/micrograms) showed a regression in average tumor volume. The average tumor volume was reduced below the initial size at treatment for 50 days; two of five cures were obtained. Neither cures nor regressions were observed with either the drug or antibody treatments alone. Our results indicate the potential for increasing the therapeutic effectiveness of radioimmunotherapy of human solid tumors with halogenated pyrimidines.

The potential superiority of bromodeoxyuridine to iododeoxyuridine as a radiation sensitizer in the treatment of colorectal cancer

Although the thymidine analogues 5-bromo-2'-deoxyuridine (BrdUrd) and 5-iodo-2'-deoxyuridine (IdUrd) have been used successfully as radiation sensitizers in clinical trials, it is not clear which of these agents is the more promising to pursue. To begin to assess this question with regard to colorectal cancer metastatic to the liver, a study was carried out using HT29 human colon cancer cells in culture and implanted in nude mice as xenografts. Cells and animals were treated with BrdUrd +/- the thymidylate synthase inhibitor 5-fluoro-2'-deoxyuridine (FdUrd), and the results compared to our previous studies with IdUrd +/- FdUrd (T. S. Lawrence, M. A. Davis, P. E. McKeever, J. Maybaum, P. L. Stetson, D. P. Normolle, and W. D. Ensminger. Cancer Res., 51: 3900-3905, 1991). Using cultured cells, it was found that FdUrd (at concentrations of greater than 10 nM) increased: (a) the incorporation of BrdUrd into the DNA of cultured tumor cells; (b) BrdUrd-mediated radiosensitization; (c) BrdUrd-mediated increase in radiation-induced DNA damage; and (d) BrdUrd-mediated decrease in the repair of radiation-induced damage. The incorporation of BrdUrd was greater than or equal to the incorporation of IdUrd previously determined under the same exposure conditions. Studies using nude mice bearing HT29 xenografts showed that FdUrd increased BrdUrd incorporation more into tumors than into the normal liver. Most tumor cells incorporated BrdUrd (labeling index after a 4-day infusion = 87 +/- 2%; SE); in the liver, labeling was confined chiefly to nonparenchymal cells. In both the presence and absence of FdUrd, the incorporation of BrdUrd into tumors was significantly and consistently greater than the incorporation of IdUrd measured under the same conditions of drug administration (by a factor of 1.2-3.6). Furthermore, the administration of BrdUrd +/- FdUrd tended to produce less weight loss and hematological toxicity than IdUrd +/- FdUrd. These findings suggest that BrdUrd may be superior to IdUrd as a radiation sensitizer in the treatment of colorectal cancer metastatic to the liver.

Long-term follow-up on National Cancer Institute Phase I/II study of glioblastoma multiforme treated with iododeoxyuridine and hyperfractionated irradiation

PURPOSE

We report the results of the final phase I/II program in glioblastoma (GBM) multiforme patients using only hyperfractionated irradiation and intravenous iododeoxyuridine (IdUrd).

METHODS

For a decade we investigated halogenated pyrimidine radiosensitizers in an effort to exploit the potential for differential uptake of thymidine analogs between proliferating tumor and normal brain tissues. Trials began with bromodeoxyuridine (BrdUrd) but were changed to IdUrd when the latter proved less photosensitizing. A series of dose-escalating pilot trials led to treatment at a maximum-tolerated dose (MTD) of IdUrd of 1,000 mg/m2/d for two separate 14-day courses, one during the initial radiation field and one during the cone down. The radiotherapy also evolved over time and was hyperfractionated in all cases reported. Over 5 years we accrued 45 patients into the final hyperfractionated, 1,000 mg/m2/d scheme. We report here results on only the patients with minimum follow-up of 1 year (90% had at least 2 years of follow-up) or until death.

RESULTS

The results do not indicate a significant benefit for use of sensitizers, as compared with other contemporary and aggressive types of radiation treatment. The median survival has been 11 months, with a 2-year actuarial survival of 9%. As yet, there are no survivors at 3 years. Tumor biopsies at craniotomy showed relatively low sensitizer incorporation.

CONCLUSION

The failure of radiosensitizers combined with radiation therapy to show major benefit may be due to patient selection but appears also to be related to the combined problems of poor drug penetration/uptake into tumor, tumor-cell heterogeneity, and a high inherent cellular radioresistance of GBM.

Measurement of thymidine replacement in patients with high grade gliomas, head and neck tumors, and high grade sarcomas after continuous intravenous infusions of 5-iododeoxyuridine

Based upon the radiation sensitization properties of the halogenated pyrimidines, 5-iododeoxyuridine (IdUrd) and 5-bromodeoxyuridine, long term i.v. infusions of halogenated pyrimidines in conjunction with fractionated radiation therapy have been evaluated in the treatment of a variety of human malignancies. While clinical studies have attempted to measure the halogenated pyrimidine incorporation, few have successfully related tumor response to the incorporation of IdUrd by the tumor. The present study reports the continuous IdUrd labeling index (number of cells labeled) and the IdUrd corrected replacement (percentage of thymidine replacement in the labeled cells of the population) from the tumors of 17 patients who received continuous infusions of IdUrd (1000 mg/m2/24 h). The tumors treated included four high grade gliomas, five head and neck tumors, four high grade sarcomas, and five other tumors of varying types. Less than 25% of the cells in three of four gliomas incorporated IdUrd after 5-7-day IdUrd infusion time. Corrected replacement for the gliomas ranged from 0 to 4%. In contrast, 63-85% of the cells in the head and neck biopsies were labeled with IdUrd after 3-7-day IdUrd infusions suggesting that these large tumors (3-12 cm diameter) have a high fraction of dividing cells. Corrected replacements values for the head and neck tumor patients ranged from 2.9 to 26.3%. The high grade sarcomas also demonstrated a high percentage of IdUrd labeled cells (57-79%) with three patients having corrected replacements of 7.5-14.2%. The continuous labeling and thymidine replacement data for four patients from whom serial biopsies were taken during IdUrd infusion demonstrated both an increasing IdUrd replacement and continuous labeling index with an increasing duration of IdUrd infusion. The clinical response of both the high grade glioma and head and neck tumor patients indicate that the IdUrd replacement and labeling data may provide some important predictive information with regard to the successful use of the halogenated pyrimidines in clinical radiation trials.

5-[123I]iodo-2'-deoxyuridine in the radiotherapy of an early ascites tumor model

The extreme biological toxicity of Auger emitters is caused by the decay-associated, highly localized deposition of energy. The antineoplastic capability of an Auger-electron emitter, iodine-123, incorporated into the thymidine analog, 5-iodo-2'-deoxyuridine (IUdR) was evaluated in an intraperitoneal (i.p.) murine ovarian tumor (MOT) in female C3HeB/FeJ mice. Total doses of 0.37 to 8.88 MBq (10-240 microCi) 123IUdR were administered i.p. in five equally divided fractions at 24, 28, 32, 36, and 40 hr after the i.p. inoculation of 0.5 to 1.6 x 10(6) tumor cells per mouse. Control tumor-bearing animals were injected with identical volumes of saline at 4-hr intervals. Biodistribution studies demonstrated a distinct and localized uptake of 123IUdR in the MOT cells (1% of the injected dose was associated with MOT cells 24 hr after the last injection), whereas in animals without tumor there was no radioactivity associated with the peritoneal cells. Analogous results were obtained from scintigraphic images where the focal area of abdominal activity persisted only in MOT-bearing mice while it cleared from the abdomen of the controls. The 50% survival (median survival) of the control group was 19 days for an inoculum of 1.6 x 10(6) MOT cells per animal, whereas the median survival of MOT-bearing animals treated with 123IUdR increased by 11 days for the highest administered dose (8.88 MBq, 240 microCi) and resulted in a 20% absolute survival at 7 weeks. Statistically significant absolute survival prolongation was found with all of the total administered doses. The prolongation of both median and absolute survival time of the tumor-bearing animals treated with 123IUdR conclusively indicates the substantial antineoplastic activity of the Auger-electron emitter iodine-123.

Cutaneous herpes simplex virus infections

Affecting millions of Americans each year, herpes simplex virus infections are among the most common human viral infections. Many clinical forms exist, depending on the site of infection and the patient's age and immune status. Clinical evaluation and laboratory studies help establish the diagnosis. Acyclovir is the drug most often used to treat herpes simplex virus infections, although newer agents, such as phosphonoformate trisodium, may be required for acyclovir-resistant infections.

Potential for tumor therapy with iodine-125 labeled immunoglobulins

Because of the high intranuclear toxicity of Auger-electron emitters, the use of radioiodinated (123I, 125I) 5-iodo-2'-deoxyuridine (IUdR)-antibody conjugates for cancer therapy has been examined. The results have demonstrated that all the conditions necessary for labeling DNA in vivo are present: uptake of the radiolabeled immunoglobulin by target cells, its subsequent internalization, the degradation of the IUdR-protein conjugate by lysosomal enzymes, and the incorporation of the radionucleoside into DNA.

[Herpesvirus infections--indications for chemotherapy in dermato-venereology]

Specific antivirals like acyclovir have ameliorated the outcome of severe herpesvirus infections, especially in immunocompromised patients. Varicella can be prevented in high-risk patients after exposure by therapy with varicella-zoster immunoglobulin. Despite this favorable development, there are many unresolved problems in the management of herpesvirus infections, such as the use of acyclovir during pregnancy, the treatment of both motoric neuropathy and postherpetic neuralgia. Chemotherapy-resistant herpesvirus may cause severe syndromes in patients suffering from HIV infection or from iatrogenic immunosuppression. Isolation of resistant viruses provides the stimulus to establish tests of viral resistance and to use antiviral drugs more carefully.

Analysis of 5-iodo-2'-deoxyuridine incorporation in murine melanoma for photon activation therapy

Quantitative evaluation of the dose enhancement obtained with analog nucleoside agents such as iododeoxyuridine (IdUrd) requires knowledge of the degree to which the thymidine (Thd) in DNA is replaced by IdUrd. In the present investigation, mice were infused with IdUrd using an intravenous infusion apparatus capable of delivering continuous multi-day infusions without restraining the mice. The absolute incorporation of IdUrd in DNA was measured by 125IdUrd label, both in whole tissue and extracted DNA, showing a good correlation between levels observed in DNA and whole tissue. Replacement in a Harding-Passey murine melanoma tumor carried in BALB/c mice approached 10%. In addition, a Neutron Activation Analysis (NAA) technique was developed which showed in vitro, a sensitivity sufficient to evaluate the % replacement of Thd by IdUrd in small biological samples with a sensitivity greater than 0.1 ppm, at 1% replacement in mg samples. This method can provide information on iodine substitution in DNA in humans where the use of a radioactive DNA-seeking substance would be undesirable. Analyses of IdUrd incorporation in cultured cells by NAA and 125I counting showed good agreement.

Intraarterial iododeoxyuridine infusion combined with irradiation. A pilot study

The halogenated pyrimidine, iododeoxyuridine (IUdR), enhances cytotoxicity of ionizing irradiation experimentally. Continuous intraarterial infusion of IUdR was combined with irradiation to maximize drug concentration in tumor and reduce potential systemic toxicity. Percutaneous tumor-specific artery catheterization was utilized in five patients, with delivery of IUdR (20 mg/kg/day) by continuous infusion 5 days prior to irradiation treatments and continued for 10-14 days. Infusion vessels included the internal mammary, the internal iliac, the renal, the common femoral, and the bronchial arteries. Conventional radiotherapy fields, fractionation, and total doses were utilized, and therapy was well tolerated. Low-grade leukopenia and thrombocytopenia was observed several weeks following infusion. A clinically nonsignificant skin reaction was observed within the irradiation fields 2-3 weeks after initiation of irradiation in several patients. No alopecia or stomatitis was observed. This study minimizes initial hepatic dehalogenation of IUdR when given by intraarterial administration. Two patients have been free of disease for over 20 years, with no long-term toxicity from IUdR therapy.

Combination of laser-therapy with 0.5% idoxuridine cream in the treatment of therapy-resistant genital warts in male patients: an open study

Forty heterosexual male patients with therapy resistant penile warts of long duration (mean 12.9 months) were treated with carbon dioxide laser, immediately followed by topical application of 0.5% idoxuridine cream twice daily for 14 days. In case of incomplete or no response to the initial treatment, the treatment procedure was repeated once. All patients had previously been repeatedly treated with podophyllotoxin 0.5% solution and/or carbon dioxide laser surgery. After two weeks of treatment, 32 patients (80%) were completely healed. The remaining eight patients were retreated and four weeks after the start of the study 35 patients (87.5%) showed complete response. Three months after the study had been initiated 34 patients (85%) were still completely healed. No adverse reactions were observed. It was concluded that laser surgery followed by topical application of 0.5% idoxuridine cream for two to four weeks seems to be highly effective in the treatment of longstanding, therapy-resistant genital warts in men. Because of the uncontrolled nature of the present study and the relatively small number of patients treated, it would be important to carry out controlled studies in larger study populations and to carry out a follow-up examination of at least six months after treatment.

Intra-cerebral ventricular infusion of 5-iodo-2-deoxyuridine (IUDR) as a radiosensitizer in the treatment of a rat glioma

The efficacy of 5-iodo-2-deoxyuridine (IUDR) as a radiosensitizer when administered by continuous infusion into the cerebral spinal fluid (CSF) of the lateral cerebral ventricle was evaluated in a 9L gliosarcoma rat brain tumor model. Stereotactic implantation of a 5 x 10(4) tumor cell suspension into the left caudate nucleus was carried out in four groups of 10 rats each. Control animals had a median survival of 16.9 days (range 16-21 days). IUDR, 8.4 mg over 7 days administered by continuous infusion into the left lateral ventricle produced a slight survival advantage (median survival 21.5 days, range 12-56). Irradiation of the entire brain, 8 Gy on days 4, 6 and 7 after tumor cell implantation also produced a slight improvement in survival (median 19.5 days, range 17-34). The combination of radiation and IUDR infusion into the CSF produced a marked survival advantage (median 30.5, range 22-54) compared to the control and single modality treatment groups. This is the first demonstration of the effectiveness of IUDR as a radiosensitizer when administered into the lateral cerebral ventricle in the treatment of an intraparenchymal brain tumor.

Early application of topical 15% idoxuridine in dimethyl sulfoxide shortens the course of herpes simplex labialis: a multicenter placebo-controlled trial

In a double-blind, randomized, patient-initiated treatment study at five medical centers, 301 immunocompetent patients experiencing a recurrence of herpes labialis were treated with topical 15% idoxuridine (IDU) in dimethyl sulfoxide (DMSO), 80% DMSO control solution, or 2% DMSO control solution. IDU did not prevent the development of lesions but significantly accelerated lesion resolution in comparison with the combined control groups. For the total population, the mean duration of pain was reduced by 1.3 days (35%, P = .01) and the mean healing time to loss of crust by 1.7 days (21%, P = .004). Analysis of subpopulations revealed that the beneficial activity of the treatment was concentrated among the patients who began treatment in the prodrome or erythema lesion stage. For these patients, the mean duration of pain was reduced by 1.8 days (42%, P = .08) and the mean healing time to loss of crust by 3.3 days (38%, P less than .001). If only patients with classic herpes lesions (vesicle, ulcer, or crust formation) were considered, there was a greater drug effect on the duration of pain (reduction by 2.6 days, 49%; P = .03) and the mean healing time to normal skin was significantly shortened (reduction by 2.3 days, 23%; P = .004). Adverse reactions to the medication were minimal.

Effects of intratumoral injection of I-125 iododeoxyuridine on Ehrlich ascites carcinoma

Intratumoral injection of I-125 iododeoxyuridine (IUdR), saline solution, and oil suspension was investigated using Ehrlich ascites tumors in the thighs of mice. The oil suspension was more effective in tumor growth delay than was the saline solution. Single injection of the oil suspension at the dose of 12.5 microCi resulted in 21.5 days growth delay, whereas 50 microCi of the saline solution resulted in 11.5 days growth delay relative to control growth delay. At 40 days after treatment, higher radioactivities were observed in the tumor and the skin of the mice treated with the oil suspension, which represented the prolongation of I-125 IUdR oil suspension within the tumor. No normal tissue toxicities were observed.

Radiobiology and clinical application of halogenated pyrimidine radiosensitizers

Halogenated pyrimidines (HP) represent a unique class of non-hypoxic cell radiosensitizers currently under clinical re-investigation. In order for halogenated pyrimidines to sensitize cells to radiation, they must be incorporated into cellular DNA. In the case of human tumors, which have in general rather long cell cycle times, this may require many days of continuous drug infusion to achieve adequate replacement of the DNA base thymidine with HP. In vitro studies support the relationship between the extent of radiosensitization and the percentage of thymidine replacement. Recent clinical studies evaluating the role of iododeoxyuridine (IdUrd) as a radiation sensitizer in large unresectable sarcomas have been extremely encouraging. To support and expand upon these positive clinical findings more information and research is needed regarding: (1) the mechanism of HP-induced radiosensitization; (2) the percentage of HP thymidine replacement in human tumors achievable and how it relates to treatment outcome; (3) the means of increasing HP incorporation in tumor and minimizing incorporation in normal tissues; (4) a better understanding of optimal timing between HP administration and radiation treatment; and (5) methods to evaluate which tumors are appropriate candidates for HP therapy. While presently limited to use in conventional high dose-rate X-ray therapy, laboratory studies suggest that HP might also be effective in low dose-rate brachytherapy and for selected high LET clinical beams. HPs probably will not be 'general' non-hypoxic cell radiosensitizers for all tumor types, but with appropriate tumor-type/anatomical site selection and refinement in their administration, HPs may prove beneficial in cancer treatment.