The dependence of halogenated pyrimidine incorporation and radiosensitization on the duration of drug exposure

Targeting of radio-enhancing drugs

OBJECTIVE

Toxicity to normal tissue is frequently the dose-limiting factor in the chemotherapy and mixed modality treatments of cancer. If the radio-enhancing drug could be localized at the disease site and released slowly over time, then systemic drug toxicities could be decreased while simultaneously maintaining high drug concentrations in the tumor. These considerations support a role for a sustained release intra-tumoral delivery systems for the delivery of radio-enhancing drugs.

METHODS

Two approaches aimed at achieving the end of localizing the radio-enhancing drug to the tumor are described. First, nanoparticles, which have a prolonged circulation time and facility for enhanced tumor targeting. Structural defects in the walls of the tumor vasculature allow the passage of particles too large to pass through the walls of normal blood vessels. This characteristic of tumor blood vessels, referred to as the enhanced permeability and retention (EPR) effect, allows relatively large entities (typically liposomes, nanoparticles, and macromolecular drugs) to pass from the blood vessels to tumor tissue and as a result nanoparticles accumulate in the tumor while being excluded from normal tissue. Second, biodegradable implanted polymers. In these devices, the radio-enhancing drug is physically trapped within the polymer matrix which is implanted in the tumor. The drug is released as the polymer degrades in response to its local environment. The degradation rate of the polymer device can be adjusted to control the rate of drug release. By this means, the level of radio-enhancing drug can be maintained at the tumor site for the duration of radiation treatment.

RESULTS AND CONCLUSIONS

Results of experiments indicate that for both methods tumor control could be optimized by maintaining the radio-enhancing drug at a useful concentration in the tumor over a period of time compatible with the duration of fractionated radiation treatment. These studies have provided proof of principle support for the further development of this approach. To date, while some of the methods and devices for drug delivery described in this paper have been involved in clinical trials, none have so far been developed for routine clinical application.

Enhancement of IUdR Radiosensitization by Low-Energy Photons Results from Increased and Persistent DNA Damage

Low-energy X-rays induce Auger cascades by photoelectric absorption in iodine present in the DNA of cells labeled with 5-iodo-2’-deoxyuridine (IUdR). This photoactivation therapy results in enhanced cellular sensitivity to radiation which reaches its maximum with 50 keV photons. Synchrotron core facilities are the only way to generate such monochromatic beams. However, these structures are not adapted for the routine treatment of patients. In this study, we generated two beams emitting photon energy means of 42 and 50 keV respectively, from a conventional 225 kV X-ray source. Viability assays performed after pre-exposure to 10 μM of IUdR for 48h suggest that complex lethal damage is generated after low energy photons irradiation compared to ¹³⁷Cs irradiation (662KeV). To further decipher the molecular mechanisms leading to IUdR-mediated radiosensitization, we analyzed the content of DNA damage-induced foci in two glioblastoma cell lines and showed that the decrease in survival under these conditions was correlated with an increase in the content of DNA damage-induced foci in cell lines. Moreover, the follow-up of repair kinetics of the induced double-strand breaks showed the maximum delay in cells labeled with IUdR and exposed to X-ray irradiation. Thus, there appears to be a direct relationship between the reduction of radiation survival parameters and the production of DNA damage with impaired repair of these breaks. These results further support the clinical potential use of a halogenated pyrimidine analog combined with low-energy X-ray therapy.

Electron-induced elimination of the bromide anion from brominated nucleobases. A computational study

The enhancement of radiodamage to DNA labeled with halonucleobases is attributed to the reactive radical produced from a halonucleobase by the attachment of an electron. We examined at the B3LYP/6-31++G** level electron capture by four brominated nucleobases (BrNBs): 8-bromo-9-methyladenine, 8-bromo-9-methylguanine, 5-bromo-1-methylcytosine, and 5-bromo-1-methyluracil followed by the release of the bromide anion and a nucleobase radical. We demonstrate that neutral BrNBs in both gas and aqueous phases are better electron acceptors than unsubstituted NBs and that resulting anion radicals, BrNBs(•-), can easily transform into the product complex of the bromide anion and the nucleobase radical ([Br(-)···NB(•)]). The overall thermodynamic stimulus for the process starting with the neutral BrNB and ending with the isolated bromide anion and the NB(•) radical is similar in the case of all four BrNBs studied, which suggests their comparable radiosensitizing capabilities.

Dependence of cell survival on iododeoxyuridine concentration in 35-keV photon-activated Auger electron radiotherapy

PURPOSE

To measure and compare Chinese hamster ovary cell survival curves using monochromatic 35-keV photons and 4-MV x-rays as a function of concentration of the radiosensitizer iododeoxyuridine (IUdR).

METHODS AND MATERIALS

IUdR was incorporated into Chinese hamster ovary cell DNA at 16.6 ± 1.9%, 12.0 ± 1.4%, and 9.2 ± 1.3% thymidine replacement. Cells were irradiated from 1 to 8 Gy with 35-keV synchrotron-generated photons and conventional radiotherapy 4-MV x-rays. The effects of the radiation were measured via clonogenic survival assays. Surviving fraction was plotted vs. dose and fit to a linear quadratic model. Sensitization enhancement ratios (SER(10)) were calculated as the ratio of doses required to achieve 10% surviving fraction for cells without and with DNA-incorporated IUdR.

RESULTS

At 4 MV, SER(10) values were 2.6 ± 0.1, 2.2 ± 0.1, and 1.5 ± 0.1 for 16.6%, 12.0%, and 9.2% thymidine replacement, respectively. At 35 keV, SER(10) values were 4.1 ± 0.2, 3.0 ± 0.1, and 2.0 ± 0.1, respectively, which yielded SER(10) ratios (35 keV:4 MV) of 1.6 ± 0.1, 1.4 ± 0.1, and 1.3 ± 0.1, respectively.

CONCLUSIONS

SER(10) increases monotonically with percent thymidine replacement by IUdR for both modalities. As compared to 4-MV x-rays, 35-keV photons produce enhanced SER(10) values whose ratios are linear with percent thymidine replacement and assumed to be due to Auger electrons contributing to enhanced dose to DNA. Although this Auger effectiveness factor is less than the radiosensitization factor of IUdR, both could be important for the clinical efficacy of IUdR radiotherapy.

IPdR: a novel oral radiosensitizer

IPdR (5-iodo-2-pyrimidinone-2'-deoxyribose) is a novel orally available, halogenated thymidine (TdR) analog and is a potential radiosensitizer for use in human tumors, such as rectal, pancreas, sarcoma and glioma tumors. IPdR is a prodrug that is efficiently converted to IUdR (5-iodo-2'-deoxyuridine), an intravenous radiosensitizer by a hepatic aldehyde oxidase, resulting in high IPdR and IUdR plasma levels in mice for > or = 1 h after oral IPdR. Athymic mice tolerated oral IPdR to doses up to 1500 mg/kg/day t.i.d. for 6 - 14 days without significant systemic toxicities. A number of in vivo preclinical studies have demonstrated that IPdR is a superior radiosensitizer compared with IUdR given as a continuous infusion in terms of safety and efficacy with a significantly lower toxicity profile, including gastrointestinal and hematologic side effects. A preclinical study has shown that IPdR is effective in inducing human colon cancer xenograft radiosensitization in drug-resistant DNA mismatch repair-proficient and -deficient tumor models, as well as in human globlastoma xenograft. In anticipation of performing a clinical Phase I trial in humans, investigators also studied the drug pharmacokinetics and host toxicities in two non-rodent, animal species during a 14-day treatment course. Dose-limiting systemic toxicities (diarrhea, emesis, weight loss and decreased motor activity) were observed in ferrets receiving IPdR at 1500 mg/kg/day on a 14-day schedule that were not found previously in athymic mice. Recently, a once-daily IPdR dosing up to 2000/mg/kg for 28 days in Fischer-344 rats showed reversible mild-to-moderate systemic toxicities without any severe or life-threatening toxicities. However, in all preclinical toxicity studies so far, no significant hematologic, biochemical or histopathologic changes have been found. Hepatic aldehyde oxidase activity was reduced in a dose-dependent fashion in the ferret liver, suggesting partial enzyme inactivation by this IPdwR schedule, but that is not found in Fischer-344 rats. The plasma pharmacokinetic profile in Rhesus monkeys showing biexponential clearance are similar to previously published data in athymic mice. In this paper, the authors review the development, mechanism of action, preclinical data and rationale for clinical studies.

Methoxyamine potentiates iododeoxyuridine-induced radiosensitization by altering cell cycle kinetics and enhancing senescence

We previously reported that methoxyamine (an inhibitor of base excision repair) potentiates iododeoxyuridine (IUdR)-induced radiosensitization in human tumor cells. In this study, we investigated the potential mechanisms of this enhanced cell death. Human colorectal carcinoma RKO cells were exposed to IUdR (3 micromol/L) and/or methoxyamine (3 mmol/L) for 48 hours before ionizing radiation (5 Gy). We found that IUdR/methoxyamine altered cell cycle kinetics and led to an increased G1 population but a decreased S population before ionizing radiation. Immediately following ionizing radiation (up to 6 hours), IUdR/methoxyamine-pretreated cells showed a stringent G1-S checkpoint but an insufficient G2-M checkpoint, whereas a prolonged G1 arrest, containing 2CG1 and 4CG1 cells, was found at later times up to 72 hours. Levels of cell cycle-specific markers [p21, p27, cyclin A, cyclin B1, and pcdc2(Y15)] and DNA damage signaling proteins [gammaH2AX, pChk1(S317), and pChk2(T68)] supported these altered cell cycle kinetics. Interestingly, we found that IUdR/methoxyamine pretreatment reduced ionizing radiation-induced apoptosis. Additionally, the extent of cell death through necrosis or autophagy seemed similar in all (IUdR +/- methoxyamine + ionizing radiation) treatment groups. However, a larger population of senescence-activated beta-galactosidase-positive cells was seen in IUdR/methoxyamine/ionizing radiation-treated cells, which was correlated with the increased activation of the senescence factors p53 and pRb. These data indicate that IUdR/methoxyamine pretreatment enhanced the effects of ionizing radiation by causing a prolonged G1 cell cycle arrest and by promoting stress-induced premature senescence. Thus, senescence, a novel ionizing radiation-induced tumor suppression pathway, may be effectively targeted by IUdR/methoxyamine pretreatment, resulting in an improved therapeutic gain for ionizing radiation.

Differential Radiosensitization in DNA Mismatch Repair-Proficient and -Deficient Human Colon Cancer Xenografts with 5-Iodo-2-pyrimidinone-2′-deoxyribose

PURPOSE

5-iodo-2-pyrimidinone-2'-deoxyribose (IPdR) is a pyrimidinone nucleoside prodrug of 5-iododeoxyuridine (IUdR) under investigation as an orally administered radiosensitizer. We previously reported that the mismatch repair (MMR) proteins (both hMSH2 and hMLH1) impact on the extent (percentage) of IUdR-DNA incorporation and subsequent in vitro IUdR-mediated radiosensitization in human tumor cell lines. In this study, we used oral IPdR to assess in vivo radiosensitization in MMR-proficient (MMR+) and -deficient (MMR-) human colon cancer xenografts.

EXPERIMENTAL DESIGN

We tested whether oral IPdR treatment (1 g/kg/d for 14 days) can result in differential IUdR incorporation in tumor cell DNA and subsequent radiosensitization after a short course (every day for 4 days) of fractionated radiation therapy, by using athymic nude mice with an isogenic pair of human colon cancer xenografts, HCT116 (MMR-, hMLH1-) and HCT116/3-6 (MMR+, hMLH1+). A tumor regrowth assay was used to assess radiosensitization. Systemic toxicity was assessed by daily body weights and by percentage of IUdR-DNA incorporation in normal bone marrow and intestine.

RESULTS

After a 14-day once-daily IPdR treatment by gastric gavage, significantly higher IUdR-DNA incorporation was found in HCT116 (MMR-) tumor xenografts compared with HCT116/3-6 (MMR+) tumor xenografts. Using a tumor regrowth assay after the 14-day drug treatment and a 4-day radiation therapy course (days 11-14 of IPdR), we found substantial radiosensitization in both HCT116 and HCT116/3-6 tumor xenografts. However, the sensitizer enhancement ratio (SER) was substantially higher in HCT116 (MMR-) tumor xenografts (1.48 at 2 Gy per fraction, 1.41 at 4 Gy per fraction), compared with HCT116/3-6 (MMR+) tumor xenografts (1.21 at 2 Gy per fraction, 1.20 at 4 Gy per fraction). No substantial systemic toxicity was found in the treatment groups.

CONCLUSIONS

These results suggest that IPdR-mediated radiosensitization can be an effective in vivo approach to treat "drug-resistant" MMR-deficient tumors as well as MMR-proficient tumors.

Targeted radiosensitisation by pegylated liposome-encapsulated 3', 5'-O-dipalmitoyl 5-iodo-2'-deoxyuridine in a head and neck cancer xenograft model

5-Iodo-2′-deoxyuridine (IUdR) is an effective radiosensitiser but its clinical development has been limited by toxicity. Prolonged intravenous infusions of IUdR are necessary for optimal tumour uptake but cause dose-limiting myelosuppression. The lack of selective tumour uptake can lead to radiosensitisation of adjacent normal tissues and enhanced local radiation toxicity. Liposomal IUdR delivery offers selective targeting of tumour tissues and avoidance of local and systemic toxicity. In these studies, we report the development of a pegylated liposome containing a lipophilic IUdR derivative (3′, 5′-O-dipalmitoyl-5-iodo-2′-deoxyuridine) for use in a head and neck cancer xenograft model. Initial studies confirmed the ability of IUdR to sensitise two head and neck cancer cell lines to single fractions of radiotherapy (SFRT) and this effect was seen to correlate with the thymidine replacement index in KB cells. In vivo delivery of single doses of either unencapsulated IUdR or pegylated liposomal IUdR (PLIUdR) to nude mice bearing KB xenograft tumours did not enhance the effect of SFRT delivered 16 h later. When PLIUdR was delivered by a protracted administration schedule to a dose of 48 mg kg⁻¹ over 7 days, it enhanced the effect of both 4.5 Gy SFRT and fractionated radiotherapy. PLIUdR was at least as effective as unencapsulated IUdR delivered by multiple intravenous injections or continuous subcutaneous infusion. Immunohistochemistry with a specific anti-IUdR monoclonal antibody confirmed greater levels of tumour staining in tumours from animals treated with PLIUdR compared with those treated with unencapsulated IUdR.

Continuous 28-day iododeoxyuridine infusion and hyperfractionated accelerated radiotherapy for malignant glioma: a phase I clinical study

PURPOSE

To investigate the maximal tolerated dose of a continuous 28-day iododeoxyuridine (IUdr) infusion combined with hyperfractionated accelerated radiotherapy (HART); to analyze the percentage of IUdr-thymidine replacement in peripheral granulocytes as a surrogate marker for IUdr incorporation into tumor cells; to measure the steady-state serum IUdr levels; and to assess the feasibility of continuous IUdr infusion and HART in the management of malignant glioma.

METHODS AND MATERIALS

Patients were required to have biopsy-proven malignant glioma. Patients received 100 (n = 4), 200 (n = 3), 300 (n = 3), 400 (n = 6), 500 (n = 4), 625 (n = 5), or 781 (n = 6) mg/m(2)/d of IUdr by continuous infusion for 28 days. HART was started 7 days after IUdr initiation. The total dose was 70 Gy (1.2 Gy b.i.d. for 25 days with a 10-Gy boost [2.0 Gy for 5 Saturdays]). Weekly assays were performed to determine the percentage of IUdr-DNA replacement in granulocytes and serum IUdr levels using standard high performance liquid chromatography methods. Standard Phase I toxicity methods were used.

RESULTS

Between June 1994 and August 1999, 31 patients were enrolled. No patient had Grade 3 or worse HART toxicity. Grade 3 or greater IUdr toxicity predominantly included neutropenia (n = 3), thrombocytopenia (n = 3), and elevated liver function studies (n = 3). The maximal tolerated dose was 625 mg/m(2)/d. Thymidine replacement in the peripheral granulocytes peaked at 3 weeks and increased with the dose (maximal thymidine replacement 4.9%). The steady-state plasma IUdr level increased with the dose (maximum, 1.5 microM).

CONCLUSION

In our study, continuous long-term IUdr i.v. infusion had a maximal tolerated dose of 625 mg/m(2)/d. Granulocyte incorporation data verified the concept that prolonged IUdr infusion results in IUdr-DNA replacement that corresponds to a high degree of cell labeling. IUdr steady-state plasma levels increased with increasing dose and attained levels needed for clinical radiosensitization. Continuous IUdr infusion and HART were both feasible and well tolerated.

Clinical significance of atomic inner shell ionization (ISI) and Auger cascade for radiosensitization using IUdR, BUdR, platinum salts, or gadolinium porphyrin compounds

PURPOSE

Halogenated pyrimidines (iododeoxyuridine [IUdR] and bromodeoxyuridine [BUdR]), platinum salts, and gadolinium porphyrins are heavy atom compounds used as radiosensitizers. For IUdR, it has been hypothesized that iodine inner shell ionizations (ISI) and Auger cascades could be one of the primary radiosensitization mechanisms. The purpose of this paper is to estimate the number of ISI produced per tumor cell and per 2 Gy irradiation in clinically relevant modelings.

MATERIALS AND METHODS

ISI were evaluated using a two-step method. Photon-induced ISI were calculated using the MCNP-4C Monte Carlo code, heavy atom concentrations from clinical data published in the literature, and at various depths in a water phantom irradiated with 6-MV, (60)Co, (137)Cs, or (192)Ir sources. Electron knock-on induced ISI on K, L, and M atomic shells were evaluated with an hybrid method, using simulated electron spectra and cross-sections derived from the Møller formalism. Using a biological dose equivalence of 0.05 Gy per cell ISI, relative biological effectiveness (RBE) values were calculated for each situation.

RESULTS

For platinum and gadolinium, ISI occurs in far less than 0.1% of the cell, whichever is the configuration. For IUdR and BUdR, ISI occurs in between 45% to 483% of the cell. Due to spectrum degradation, about 3 times more photoelectric ISI are generated at greater than shallower depths, and 10 times more for (192)Ir compared with (60)Co or 6-MV X-rays. Photoelectric ISI are about 3 times more frequent for iodine than bromine, but electron knock-on ISI are more frequent on bromine, and at the end about the same number of ISI are generated for both elements. RBEs were found to be between 1.01 and 1.12 for clinically relevant irradiation settings.

CONCLUSIONS

The mechanisms of radiosensitization for platinum and gadolinium are clearly not related to an Auger cascade. For halogenated pyrimidines, however, clinically relevant numbers of ISI are generated within each cell. For IUdR, ISI appears to be strongly tied to the photon spectra. Halogenated pyrimidines should be evaluated again clinically, but using lower energy photons like a (192)Ir implant.

IUdR polymers for combined continuous low-dose rate and high-dose rate sensitization of experimental human malignant gliomas

Local polymeric delivery enhances IUdR radiosensitization of human malignant gliomas (MG). The combined low-dose rate (LDR) (0.03 Gy/h) and fractionated high-dose rate (HDR) treatments result in cures of experimental MGs. To enhance efficacy, we combined polymeric IUdR delivery, LDR, and HDR for treatments of both subcutaneous and intracranial MGs. In vitro: Cells (U251 MG) were trypsinized and replated in triplicate 1 day prior to LDR irradiation in media either without (control) or with 10 microM IUdR. After 72 hr, LDR irradiation cells were acutely irradiated (1.1 Gy/min) with increasing (0, 1.25, 2.5, 5.0, or 10 Gy) single doses. Implantable IUdR polymers [(poly(bis(p-carboxyphenoxy)-propane) (PCPP): sebaic acid (PCPP:SA), 20:80] (50% loading; 10 mg) were synthesized. In vivo: For flank vs. intracranial tumors, mice had 6 x 10(6) subcutaneous vs. 2 x 10(5) intracranial cells. For intracranial or subcutaneous MGs, mice had intratumoral blank (empty) vs. IUdR polymer treatments. One day after implantation, mice had immediate external LDR (3 cGy/h x 3 days total body irradiation) or HDR (2 Gy BID x 4 days to tumor site) or concurrent treatments. For the in vitro IUdR treatments, LDR resulted in a striking increase in cell-killing when combined with HDR. For the in vivo LDR treatments of flank tumors, the growth delay was greater for the IUdR vs. blank polymer treatments. For the combined LDR and HDR, the IUdR treatments resulted in a dramatic decrease in tumor volumes. On day 60 the log V/V0 were -1.7 +/- 0.22 for combined LDR + HDR + IUdR polymer (P < 0.05 vs. combined LDR + HDR + blank polymer). Survival for the intracranial controls was 22.9 +/- 1.2 days. For the blank polymer + LDR vs. blank polymer + LDR + HDR treatments, survival was 25.3 +/- 1.7 (P = NS) vs. 48.1 +/- 3.5 days (P < 0.05). For IUdR polymer + LDR treatment survival was 27.3 +/- 2.3 days (P = NS). The most striking improvement in survival followed the IUdR polymer + LDR + HDR treatment: 66.0 + 6.4 days (P < 0.05 vs. blank polymer + LDR + HDR). The polymeric IUdR delivery plus combined continuous LDR and HDR treatments results in growth delay and improved survival in animals bearing the MG xenografts. This treatment may hold promise for the treatment of human MGs.

Bromodeoxyuridine alternating with radiation for advanced uterine cervix cancer: a phase I and drug incorporation study

PURPOSE

Preclinical studies show a significant increase in the ratio of the radiosensitizer bromodeoxyuridine (BUdR) in tumors versus the intestinal mucosa during the drug elimination period, compared with the ratio during drug infusion. We constructed a phase I study in patients with locally advanced cervix cancer, using alternating cycles of BUdR and radiation therapy (RT).

PATIENTS AND METHODS

Eighteen patients with stage IIB to IVA cervix cancer participated. A treatment cycle consisted of a 4-day BUdR infusion followed by a week of pelvic RT, 15 Gy twice daily in 1.5-Gy fractions. After three cycles, additional BUdR was infused, followed by brachytherapy. The fraction of thymidine replaced by BUdR and the fraction of cells incorporating BUdR were determined in rectal mucosa and tumor biopsies at the end of the first BUdR infusion (day 5), at the middle of the first RT week (day 10), and at the time of brachytherapy.

RESULTS

Dose-limiting toxicity was observed in one of 16 patients receiving 1,000 mg/m2/d x 4 days and in both patients receiving 1,333 mg/m2/d x 4 days each cycle. After a median follow-up of 39 months, 12 patients (66%) were free of pelvic disease and nine (50%) were alive and disease free. The ratio of tumor to rectum BUdR incorporation averaged 1.5 to 1.8 and did not differ significantly between day 5 and day 10. A trend toward reduced ratio was observed at brachytherapy. Drug-containing cells in rectal biopsies migrated from the crypts to the mucosal surface.

CONCLUSION

In this schedule, 1,000 mg/m2/d is the maximum-tolerated dose of BUdR. BUdR incorporation levels in tumors were consistent with clinically significant radiosensitization. The migration of BUdR-containing rectal mucosa cells from the crypts to the surface at the time of RT suggests that this regimen may offer a relative sparing of the mucosa from radiosensitization.

Synthetic, implantable polymers for local delivery of IUdR to experimental human malignant glioma

PURPOSE

Recently, polymeric controlled delivery of chemotherapy has been shown to improve survival of patients with malignant glioma. We evaluated whether we could similarly deliver halogenated pyrimidines to experimental intracranial human malignant glioma. To address this issue we studied the in vitro release from polymers and the in vivo drug delivery of IUdR to experimental human U251 glioblastoma xenografts.

METHODS AND MATERIALS

In vitro: To measure release, increasing (10%, 30%, 50%) proportions of IUdR in synthetic [(poly(bis(p-carboxyphenoxy)-propane) (PCPP):sebacic acid (SA) polymer discs were serially incubated in buffered saline and the supernatant fractions were assayed. In vivo: To compare local versus systemic delivery, mice bearing flank xenografts had intratumoral or contralateral flank IUdR polymer (50% loading) treatments. Mice bearing intracranial (i.c.) xenografts had i.c. versus flank IUdR polymer treatments. Four or 8 days after implantation of polymers, mice were sacrificed and the percentage tumor cells that were labeled with IUdR was measured using quantitative microscopic immunohistochemistry.

RESULTS

In vitro: Increasing percentage loadings of IUdR resulted in higher percentages of release: 43.7 + 0.1, 70.0 + 0.2, and 90.2 + 0.2 (p < 0.001 ANOVA) for the 10%, 30%, and 50% loadings, respectively. In vivo: For the flank tumors, both the ipsilateral and contralateral IUdR polymers resulted in similarly high percentages labeling of the tumors versus time. For the ipsilateral IUdR polymers, the percentage of tumor cellular labeling after 4 days versus 8 days was 45.8 +/- 7.0 versus 40.6 +/- 3.9 (p = NS). For the contralateral polymer implants, the percentage of tumor cellular labeling were 43.9 +/- 10.1 versus 35.9 +/- 5.2 (p = NS) measured 4 days versus 8 days after implantation. For the i.c. tumors treated with extracranial IUdR polymers, the percentage of tumor cellular labeling was low: 13.9 +/- 8.8 and 11.2 +/- 5.7 measured 4 and 8 days after implantation. For the i.c. tumors having the i.c. IUdR polymers, however, the percentage labeling was comparatively much higher: 34.3 +/- 4.9 and 35.3 +/- 4.0 on days 4 and 8, respectively. For the i.c. tumors, examination of the percentage cellular labeling versus distance from the implanted IUdR polymer showed that labeling was highest closest to the polymer disc.

CONCLUSION

Synthetic, implantable biodegradable polymers provide the local, controlled release of IUdR and result in the high, local delivery of IUdR to experimental intracranial human malignant glioma. This technique holds promise for the local delivery of IUdR for radiosensitization of human brain tumors.

Intravenous delivery of 5'-iododeoxyuridine during hyperfractionated radiotherapy for locally advanced head and neck cancers: results of a pilot study

OBJECTIVES

Locally advanced cancers of the head and neck require aggressive treatment, often with limited effectiveness and significant toxicity and morbidity. This pilot study was designed to assess tolerance using combined hyperfractionated radiotherapy and the halogenated pyrimidine radiosensitizer 5'-iododeoxyuridine (IdUrd).

STUDY DESIGN

This was a prospective single-arm study open to patients with advanced head and neck cancers that had a poor chance of control with conventional radiation therapy. Patients were treated with hyperfractionated radiation therapy at standard doses in combination with an IdUrd infusion and observed for tumor response and normal tissue tolerances.

METHODS

Radiation therapy was delivered in fractions of 1.2 Gy or 1.5 Gy twice daily to a total dose in the range of 70 to 76 Gy. IdUrd was delivered as an intravenous infusion (1000 mg/m2 per day) for a maximum of 14 days at the beginning and then again during the middle of the radiotherapy.

RESULTS

Twelve patients with advanced squamous cell lesions were enrolled and 11 were observed to have complete clinical remissions. Seven patients remained clinically free of local disease at the time of death or most recent follow-up. Acute toxicities, usually hematologic or mucosal, were severe and all patients required treatment modifications and considerable supportive care.

CONCLUSIONS

Although a high rate of response was achieved using this regimen, the toxicities are prohibitive. The kinetic profile of IdUrd incorporation suggests the need for future studies using repetitive short courses of IdUrd.

The effect of activation of wild-type p53 function on fluoropyrimidine-mediated radiosensitization

PURPOSE

We have hypothesized that fluoropyrimidine-mediated (FdUrd) radiosensitization occurs in cells that inappropriately enter S phase in the presence of drug, resulting in defective repair of radiation-induced DNA damage (14). This model would predict that prevention of entry into S phase would abrogate sensitization produced by FdUrd. We wished to test this prediction by blocking S phase entry of HT29 human colon cancer cells.

METHODS

We used HT29 cells that had been transduced with a murine temperature-sensitive p53 (ts29G) and, as a control, HT29 cells transduced with a neomycin plasmid (HT29neo). The murine temperature-sensitive p53 demonstrates wild-type p53 function when cells are incubated at the permissive temperature (32 degrees C) and mutant p53 function at the nonpermissive temperature (38 degrees C). We determined the effect of wt p53 function on FdUrd-mediated radiosensitization and cell cycle progression.

RESULTS

Incubation of ts29G cells at the permissive temperature (32 degrees C) activated p21 and blocked entry of cells into S phase. Whereas FdUrd greatly increased the radiosensitivity of HT29neo cells and ts29G cells incubated at 38 degrees C, FdUrd had no effect on the radiation sensitivity of ts29G cells incubated at the permissive temperature (32 degrees C).

CONCLUSIONS

These findings are consistent with our hypothesis that FdUrd-mediated radiosensitization requires progression into S phase. It is possible that the heterogeneity of clinical responses seen after combined therapy with fluoropyrimidines and radiation is explained, in part, by differences among tumor cells in the control of S phase progression.

Preoperative idoxuridine and radiation for large soft tissue sarcomas: clinical results with five-year follow-up

BACKGROUND

Local control remains an important issue in the management of large soft tissue sarcomas. Radiation is the main adjuvant to surgery for local therapy of sarcomas, but it requires relatively high doses, hitherto considered prohibitive in areas such as the retroperitoneum. We developed a preoperative treatment approach to large soft tissue sarcomas that would deliver a high total dose of radiation administered in conjunction with the halogenated pyrimidine radiosensitizer idoxuridine (IdUrd).

METHODS

Thirty-seven patients with large sarcomas of the head and neck, mediastinum, retroperitoneum, or extremity received three or five cycles of sequential IdUrd infusion (1000-1600 mg/m2/d x 5 d) alternating weekly with twice daily radiation (125-150 cGy per dose) and were then evaluated for resection. The delivered preoperative radiation dose was up to 6250 to 7500 cGy.

RESULTS

Five patients (14%) had a partial response to preoperative therapy, and 28 of 37 patients underwent successful resection. There were no intra- or postoperative deaths. Local control was achieved in 19 of 28 resected patients, but in only 1 of 6 patients who remained unresectable despite therapy. With a median follow-up of 5.8 years, 28% of patients are alive with no evidence of disease, 17% are alive with disease, and 53% have died of their disease.

CONCLUSIONS

Using the dose and schedule we employed, resection of large soft tissue sarcomas was possible after high-dose radiation delivered in conjunction with IdUrd. Although local control was acceptable, the high rate of distant failure represents a limitation of any local approach to the treatment of large soft tissue sarcomas and suggests the need for integration of this approach with an effective systemic therapy.

A phase I trial of intravenous bromodeoxyuridine and radiation therapy for pancreatic cancer

PURPOSE

Improved radiosensitization may lead to improved results of treatment for pancreatic cancer. This Phase I trial was designed to determine the maximum tolerable dose of intravenous bromodeoxyuridine (BrdUrd) when given in an alternating weekly fashion with radiation therapy for patients with pancreatic cancer.

METHODS AND MATERIALS

Patients with resected or locally unresectable pancreatic cancer were eligible if distant metastases were not present. A continuous intravenous infusion of BrdUrd was given on weeks 1, 3, 5, and 7. Twice a day radiation therapy (1.5 Gy per fraction) was given on weeks 2, 4, 6, and 8 to the pancreas/pancreatic bed (total dose 60 Gy) and draining regional lymph nodes (total dose 45 Gy). The starting dose of BrdUrd was 800 mg/m2/day with a planned escalation to 1000 mg/m2/day if at least six out of eight patients were without Grade > or = 3 toxicity. Patients were assessed weekly for toxicity, and were followed every 3 months after treatment for complications and survival.

RESULTS

Fifteen patients with resected (six) or unresectable (nine) pancreatic cancer were enrolled. One patient failed to complete therapy due to tumor progression. One of 11 patients treated with 800 mg/m2/day had a Grade 3 toxicity, while Grade 3 or 4 toxicity was found in all 3 patients receiving 1000 mg/m2/day. The dose-limiting toxicities were hematologic. The acute gastrointestinal toxicity was minimal. Two patients, including one with unresectable disease, were without evidence of disease during exploration for complications (ulcer, small bowel obstruction).

CONCLUSIONS

The recommended dose of BrdUrd for Phase II study is 800 mg/m2/day. The gastrointestinal mucosa did not appear to be sensitized by this method of BrdUrd administration. The presence of a pathologic complete response is encouraging. Further improvements in radiosensitization are possible and may lead to improved local control.

Cell line specific radiosensitizing effect of zalcitabine (2',3'-dideoxycytidine)

The potential of zalcitabine (ddC) to act as an ionizing radiation response modifier was tested on exponentially growing human cancer cells in vitro. Two human cell lines, WiDr (colon) and MCF-7 (breast) were exposed to ddC at 10 microM concentration for various lengths of time (18, 24, 48 and 72 h). On the WiDr cell line the dual effect of concentration and duration of exposure prior to irradiation was investigated. Experimental endpoints were clonogenicity and viability, as measured by colony formation assay (CFA) and MTT assay respectively. The impact on cell-cycle distribution prior to irradiation was assessed by flow cytometry using a double labeling technique (propidium iodide and bromodeoxyuridine pulse label). A significant reduction in surviving fraction and viability was observed for WiDr-cells irradiated after pre-exposure to 10 microM for 18, 48 and 72 h as compared to corresponding irradiated controls. At lower concentrations (1 and 5 microM), the radiosensitizing effect was only significant after a 72-h exposure (assessed by CFA). For MCF-7, ddC induced a significant modification of the dose response only with 24 and 48 h preincubation. However, the overall effect was less pronounced as compared to WiDr. Cell-cycle analysis showed accumulation in S-phase, 48 and 72 h after treatment with 10 microM ddC in the WiDr cells, with a progressive shift to late S-phase as shown by the biparametric analysis. The degree of radiosensitization is cell-line dependent with the most important sensitization observed on the most "radioresistant cell line", i.e., the cell line with the lowest alpha value and highest SF 2 (WiDr). For WiDr, radiosensitization by ddC depends on the duration of exposure and the concentration of the drug.

Survival improvement in anaplastic astrocytoma, combining external radiation with halogenated pyrimidines: final report of RTOG 86-12, Phase I-II study

PURPOSE

This study evaluated the toxicity and tumor efficacy of the halopyrimidine IUdR as a chemical modifier of radiation response in patients with malignant glioma. The preliminary results published in 1993 demonstrated no real advantage in the group of patients with glioblastoma. However, a benefit appeared to be evolving in the group of patients with Anaplastic Astrocytoma (AA). We are now presenting the results on the long-term follow-up of patients with AA.

METHODS AND MATERIALS

Between August 1987 and October 1991, 79 patients were entered in a prospective study with newly diagnosed malignant glioma. Twenty-one of 79 were AA. The study was designed to have a fixed dose of radiation consisting of 60.16 Gy in 32 fractions in 6.5 weeks but varying the dose schedule of IUdR, delivered in a continuous intravenous infusion of long (96 h) or short (48 and 24 h) duration, every week for the 6.5 weeks of radiation treatment.

RESULTS

The last AA patient was entered in March 1991. Ninety-five percent of the AA patients were under 59 years of age and 86% had a Karnofsky score 80. Thirty-eight percent had a tumor diameter of less than 5 cm and 52% had a tumor diameter between 5-10 cm. Seventy-six percent had partial or total tumor resection. The toxicity of this treatment was acceptable and has already been published elsewhere. At the time of this report, 14 out of 21 patients with AA are dead. The median survival, calculated from the Kaplan-Meier, is 3.2 years. Thirty-three percent of the patients have survived 5 years. These results compare favorably with the best results reported in the literature with postoperative external radiation plus chemotherapy, median survival time (MST) of 3 years, and previous Radiation Therapy Oncology Group (RTOG) experience with radiation alone, MST of 2 years.

CONCLUSIONS

Our findings in patients with AA corroborate the improved therapeutic results published recently when combining external radiation with "long" infusion of i.v. BUdR and indicate the need to proceed with randomized Phase III studies utilizing halogenated pyrimidines and radiation. One such study has already been activated, RTOG # 94-04.

PALA enhancement of bromodeoxyuridine incorporation into DNA increases radiation cytotoxicity to human ovarian adenocarcinoma cells

PURPOSE

N-(phosphonacetyl)-L-aspartic acid (PALA) is a transition- state inhibitor of L-aspartate transcarbamylase, which catalyses the biosynthesis of carbamyl-L-aspartate in the de novo pyrimidine biosynthetic pathway. 5-Bromodeoxyuridine (BrdUrd) is known to be a potent radiosensitizer of proliferating cells when it is incorporated into DNA. The experiments described herein were performed to test the hypothesis that depletion of cellular pyrimidine precursors by PALA may increase both the incorporation of BrdUrd into DNA and the sensitivity of these cells to the cytotoxic effect of radiation.

METHODS AND MATERIALS

The effect of PALA concentration and exposure time on the incorporation of BrdUrd into the DNA of exponentially growing BG-1 human ovarian carcinoma cells was determined. BG-1 cells exposed to the most effective PALA + BrdUrd treatment schedule were then irradiated to determine if PALA could enhance the radiosensitization already achieved by pretreatment with BrdUrd alone.

RESULTS

A 72-h exposure to PALA (> or = 25 microM) delayed the growth of human ovarian adenocarcinoma BG-1 cells by 40% compared to that of the untreated control cells. Using a clonogenic assay, the IC50 for a 72-h PALA exposure was approximately 25 microM and the cell killing efficiency was dependent on both the concentration and duration of the exposure. A 72-h exposure to 25 microM PALA produced approximately a 90% decrease in the intracellular uridine-5'-triphosphate (UTP) and cytidine-5'-triphosphate (CTP) levels, but had no effect on the intracellular adenosine-5'-triphosphate (ATP) level. This decrease in the UTP and CTP pools promoted a fivefold increase in the incorporation of [3H]BrdUrd into the DNA of BG-1 cells. The most effective treatment schedule involved a 72-h time course, consisting of a 48-h pretreatment with PALA alone, followed by an additional 24-h treatment with both PALA and BrdUrd. The two agent treatments, PALA (25 microM) + BrdUrd (16 microM), PALA (25 microM) + radiation (6 Gy), and BrdUrd (16 microM) + radiation (6 Gy) produced a 2.1-, 7.4-, and 13.2-fold increase in cytotoxicity, respectively, over that expected if the interaction between the two agents was independent and additive. The most effective three-agent treatment schedule consisting of PALA, BrdUrd, and radiation resulted in a greater than 30-fold increase in cytotoxicity over that expected if the interactions and the three agents were additive (p < 0.05).

CONCLUSIONS

These data indicate that PALA alone enhances radiation cytotoxicity and further enhances the radiosensitization already achieved with the halogenated pyrimidines. These effects could be clinically beneficial.

The treatment of colorectal liver metastases with conformal radiation therapy and regional chemotherapy

PURPOSE

Whole-liver radiation, with or without chemotherapy, has been of modest benefit in the treatment of unresectable hepatic metastases from colorectal cancer. A Phase I/II study combining escalating doses of conformally planned radiation therapy (RT) with intraarterial hepatic (IAH) fluorodeoxyuridine (FdUrd) was performed.

METHODS AND MATERIALS

Twenty-two patients with unresectable hepatic metastases from colorectal cancer, 14 of whom had progressed after previous chemotherapy (2 with prior IAH FdUrd), were treated with concurrent IAH FdUrd (0.2 mg/kg/day) and conformal hepatic radiation therapy (1.5-1.65 Gy/fraction twice a day). The total dose of radiation given to the tumor (48-72.6 Gy) depended on the fraction of normal liver excluded from the high-dose volume. All patients were assessed for response, toxicity, hepatobiliary relapse, and survival. Median potential follow-up was 42 months.

RESULTS

Eleven of 22 patients demonstrated an objective response, with the remainder showing stable disease. Actuarial freedom from hepatic progression was 25% at 1 years. The most common acute toxicity was mild to moderate nausea and transient liver function test abnormalities. There were three patients with gastrointestinal bleeding (none requiring surgical intervention) after the completion of treatment. Overall median survival was 20 months. The presence of extrahepatic disease was associated with decreased survival (p < 0.01).

CONCLUSIONS

Combined conformal radiation therapy and IAH FdUrd can produce an objective response in 50% of patients with hepatic metastases from colorectal cancer. However, response was not durable, and hepatic progression was frequent. Improvements in hepatic tumor control for patients with metastatic colorectal cancer may require higher doses of conformal radiation and/or improved radiosensitization. In an effort to increase radiosensitization, we have recently initiated a clinical trial combining IAH bromode-oxyuridine, a thymidine analog radiosensitizer, with conformal high dose radiation therapy.

Azidothymidine (AZT) as a potential modifier of radiation response in vitro

The potential effect of AZT as a thymidine analogue on radiation response in vitro was investigated. Two human cell lines (WiDr and HeLa) were used. The effect of 10 microM AZT on exponentially growing cells was studied after different exposure times (24, 48 and 72 h). The surviving fraction (clonogenic assay) or metabolic activity (MTT assay) after irradiation of AZT-exposed cells, was compared to unexposed irradiated controls. Flow cytometry was used to assess the cell-cycle effect of pre-exposure of exponentially growing cells to AZT. AZT had a radioprotective effect for all experimental time points as far as WiDr was concerned. For HeLa the effect was significant at 24 h. Cell-cycle analysis showed a significant accumulation in S-phase at 72 h for WiDr. For HeLa there was a significant accumulation in S-phase at 48 h. We conclude that under the reported experimental conditions, AZT as a thymidine analogue seems to reduce the cytotoxic effect of irradiation.

National Cancer Institute (phase II) study of high-grade glioma treated with accelerated hyperfractionated radiation and iododeoxyuridine: results in anaplastic astrocytoma

PURPOSE

We report the outcome of a Phase II study of a cohort of patients with high-grade glioma treated with accelerated hyperfractionated radiation and the radiation sensitizer, iododeoxyuridine (IdUrd).

METHODS AND MATERIALS

Between January 1988 and December 1990, 39 consecutive patients with high-grade glioma were enrolled and treated on a Phase II protocol including hyperfractionated radiation and IdUrd. Thirty-two patients were male and seven were female. Age range was 19 to 71 years with a median age of 38 years. IdUrd (1000 mg/m2 per day) was administered in two separate 14-day courses, the first during the initial radiation field and the second during the final cone-down field. All patients were treated consistently with partial brain technique and received 1.5 Gy/fraction twice daily to a mean total dose of 71.25 Gy (range 66-72 Gy excluding one patient who did not complete treatment). The initial field was treated to 45 Gy followed by a cone-down field covering the tumor volume plus a 1-cm margin to the final dose. Patients were assessed for acute and long-term morbidity and followed for outcome. Two patients had biopsies during the course of treatment. Flow cytometry and high performance liquid chromatography was used to evaluate the labeling index and the percent replacement of IdUrd in the biopsy specimen.

RESULTS

Thirty-eight of 39 patients completed therapy. One patient died on treatment at 48 Gy and is included in the survival analysis. No patient was lost to follow-up. Twenty-one patients had Grade 3 (anaplastic astrocytoma) tumors and 18 patients had Grade 4 (glioblastoma multiforme). Median survival for the entire cohort was 23 months. For the glioblastoma multiforme patients, median survival was 15 months. The median survival of the anaplastic astrocytoma patients has not yet been reached. In the patients assessed, the range of IdUrd tumor cell incorporation was only 0-2.4%.

CONCLUSION

Accelerated hyperfractionated radiation therapy with IdUrd was administered with acceptable acute toxicity. The major acute side effects of mucositis and thrombocytopenia were related to IdUrd infusion and were dose-dependent. There were no unacceptable acute toxicities referable to the radiation as delivered. With a median potential follow-up of 51 months, the actuarial median survival of the glioblastoma multiforme patients is comparable with the best previously published reports. The outcome of patients with anaplastic astrocytoma compares very favorably with even the most aggressive multi-modality approaches in the recent literature with a minimum of acute morbidity.

Bromodeoxyuridine-mediated radiosensitization in human glioma: the effect of concentration, duration, and fluoropyrimidine modulation

PURPOSE

To define the relative influence of duration of exposure, concentration, and modulation by fluorodeoxyuridines (FdUrd) on the incorporation of 5-bromo-2-deoxyuridine (BrdUrd) into DNA of a human malignant glioma line (D-54) in vitro and in vivo.

MATERIALS AND METHODS

IN VITRO STUDIES

an established human malignant glioma line (D-54) was exposed to a clinically achievable concentration of BrdUrd to model intravenous (1 microM BrdUrd) and intraarterial (4 microM BrdUrd) conditions. The influence of modulation was assessed using 1 nM FdUrd. Incorporation of BrdUrd, radiosensitization, and cytotoxicity were determined after 24, 72, and 120 h drug exposures. In Vivo studies: nude mice bearing D-54 xenografts were infused with BrdUrd at 100 mg/kg/day for 7 and 14 days or BrdUrd at 400 mg/kg/day for 5 days. The influence of modulation was assessed by combining 100 mg/kg/day of BrdUrd with 0.1, 0.3, and 1 mg/kg/day FdUrd for 7 days. Incorporation of BrdUrd into the DNA of tumor, gut, and marrow were determined.

RESULTS

In Vitro: thymidine replacement and radiosensitization were a function of concentration, and incorporation began to plateau after 2 to 3 population doublings. Modulation with 1 nM FdUrd significantly increased incorporation. Radiosensitization was a linear function of thymidine replacement under all conditions tested. In Vivo: infusion with 400 mg/kg/day for 5 days resulted in greater tumor incorporation (10.3 +/- 0.4% thymidine replaced) than treatment with 100 mg/kg/day for 14 days (6.0 +/- 0.6% of thymidine replaced). Infusion of FdUrd with BrdUrd increased normal tissue incorporation of BrdUrd, but failed to increase BrdUrd incorporation in tumor cells.

CONCLUSION

These results suggest that relatively short, high dose rate infusions may be preferable to long, low dose rate infusions. The potential benefit of FdUrd modulation demonstrated in vitro may be difficult to realize using continuous systemic infusions.

Kinetics of cell labeling and thymidine replacement after continuous infusion of halogenated pyrimidines in vivo

PURPOSE

Iododeoxyuridine (IdUrd) is a halogenated pyrimidine which has been recognized as a clinical radiosensitizer. It is generally agreed that the extent of radiosensitization correlates with the degree of thymidine substitution in DNA. Controversy exists regarding the optimal administration schedule to achieve maximum radiosensitization. To obtain more information relating to this problem, we present experiments on an in vivo human tumor xenograft continuously exposed to a fixed serum concentration of halogenated pyrimidines so as to study the kinetics of cell labeling and thymidine replacement.

METHODS AND MATERIALS

Human colon tumor (HCT-116) cells were injected subcutaneously into nude mice. After 10 days, most animals (> 90%) developed measurable tumor nodules with a volume doubling time of 5 +/- 1 days. Once the tumors reached a cross-sectional area of 0.25-0.30 cm2, miniosmotic pumps were implanted to deliver a dose of 100 mg/kg/day of IdUrd by continuous infusion. After an IdUrd exposure time of 1-7 days, blood and tumor tissue were collected.

RESULTS

The steady state serum IdUrd concentration was 0.95 +/- 0.1 microM, which is a clinically relevant concentration for a prolonged continuous intravenous infusion. The tumor cell potential doubling time (Tpot) was 25 +/- 2 h. The percent IdUrd thymidine replacement and the fraction of cells labeled, followed exponential saturation kinetics with a halflife of 33 +/- 9 and 27 +/- 2 h, respectively. After 5 days of exposure (congruent to 5 x Tpot), the thymidine replacement in tumor cells was 2.0 +/- 0.2% and the fraction of tumor cells labeled was 94 +/- 1%. Immunohistochemical staining of IdUrd labeled tumor tissues showed an exposure dependent gradient of cellular labeling that was initially highest in regions close to blood vessels. After 4 days of exposure at 100 mg/kg/day, there was an increase in the fraction of cells in G(0) + G1 and a decrease in the S phase population, suggesting a block between G1 and S phase.

CONCLUSION

We conclude that the in vivo kinetics of IdUrd thymidine replacement and fraction of cells labeled after continuous exposure followed exponential saturation kinetics with a halflife of approximately the potential doubling time of the tumor cell population. Simple modeling suggests that some form of prolonged, or briefly interrupted, continuous infusion should be considered for clinical administration because such schedules would leave fewer cells unsensitized than shorter infusions would. Even 10% of unlabeled clonogenic cells could explain the lack of dramatic clinical successes with IdUrd or BrdUrd sensitization.

Sensitization of radiolabeled monoclonal antibody therapy using bromodeoxyuridine

BACKGROUND

Although treatment with radiolabeled monoclonal antibodies (MoAb) against tumor-associated antigens offers the potential for targeted therapy, the efficacy of this approach is limited by the low dose-rate delivered. This could be overcome by increasing tumor sensitivity through the use of radiation sensitizers.

METHODS AND RESULTS

In vitro studies using LS174T human colon cancer cells showed that exposure to 1 microM bromodeoxyuridine (BrdUrd), a thymidine-analog radiation sensitizer, at a plasma concentration easily achievable through systemic administration in both animals and patients, increased the cytotoxicity of continuous low dose-rate irradiation delivered by a cesium-137 irradiator (at 12 cGy/h which resembles the dose-rate delivered by radiolabeled MoAb therapy). It was found that 1 microM BrdUrd produced marked radiosensitization (enhancement ratio of 1.42 +/- 0.03) but did not affect cell cycle distribution. Systemic administration of BrdUrd in athymic nude mice bearing LS174T xenografts was performed using osmotic pumps. Animals tolerated infusions of 200 mg/kg/day BrdUrd for 4 days, which resulted in 8.4 +/- 0.8% of thymidine replacement by BrdUrd in tumors. However, bone marrow incorporation was 15.8 +/- 1.1% under these conditions. To improve the ratio of incorporation in the tumor compared to that in the bone marrow, animals were given an infusion of BrdUrd and the pumps were removed. Incorporation of BrdUrd in the bone marrow and intestine decreased rapidly after the infusion was completed. In contrast, there was relatively little change in the incorporation into the tumor after an initial decrease. Based on these data, experiments were performed comparing the effects of 500 microCi iodine-131-labeled 17-1A MoAb alone to the same dose of iodine-131-labeled 17-1A administered 1 day after discontinuation of the infusion of BrdUrd (200 mg/kg/day for 4 days). BrdUrd tended to increase the delay in tumor growth produced by iodine-131-labeled 17-1A administration.

CONCLUSIONS

Radiosensitization by BrdUrd in vitro appears to be caused not by cell cycle effects but by increased radiation sensitivity. The in vivo data suggests that BrdUrd improved the efficacy (tumor growth inhibition) of radiolabeled MoAb.

Iododeoxyuridine (IUdR) combined with radiation in the treatment of malignant glioma: a comparison of short versus long intravenous dose schedules (RTOG 86-12)

PURPOSE

To evaluate the toxicity and tumor efficacy of the halopyrimidine IUdR (NSC #39661, IND 22475) as a chemical modifier of radiation response when used in a high dose short time infusion versus the acceptable 4 day infusion.

METHODS AND MATERIALS

In August 1987 we initiated a prospective study in patients with newly diagnosed anaplastic astrocytoma and glioblastoma. The study was designed to have a fixed dose of radiation (60.16 Gy = 1.88 Gy in 32 fractions in 6.5 weeks) but varying the dose schedule of IUdR, keeping the total dose between 21 and 24 g/m2. IUdR was delivered in a 96, 48, or 24 hr continuous intravenous infusion per week for 6.5 weeks during radiation treatment.

RESULTS

The study was closed for patient accrual on October 1, 1991. Twenty-two patients were treated on the 96 hrs, 32 on the 48 hr and 25 on the 24 hr schedules. The incidence of glioblastoma ranged between 68 and 75% in the three arms. Seventy percent of the patients had a Karnofsky of 80-90% at the onset of treatment. Over 50% of the patient population were under age 55. Drug tolerance was related to the duration of the IUdR infusion. Toxicities were most pronounced in the 96 hr IUdR infusion schedule where 27.4% of the patients reported a grade 3 drug toxicity. No fatal or grade 4 toxicities were observed. More patients on the 24 and 48 hr schedule received at least 80% of the IUdR dose specified per protocol. We did not observe a trend in acute normal tissue radiation reactions in any of the three arms. The median survivals calculated from the Kaplan-Meier plot are 13.4, 10.5, and 11 months, respectively, for the 96, 48, and 24 hr infusions. The Cox Proportional Hazards model showed that any difference in survival can be attributed to histological grade, type of previous surgery and, to some extent, age of the patient. Dose schedule was not a significant predictor of survival, although statistically nonsignificant trend toward longer survival is seen in those patients with glioblastoma treated in the "long" 4 day schedule.

CONCLUSION

Overall, our treatment combination, particularly for patients with glioblastoma, has not shown convincing evidence of an improvement in survival. Of interest, however, it is the 2 year survival rate of 68% for patients with anaplastic astrocytoma. In our experience, the administration of IUdR is laborious, time consuming and with bothersome acute gastrointestinal and hematological toxicities.

The clinical rationale for S-phase radiosensitization in human tumors

Nonhypoxic cell radiosensitizers, principally the halogenated pyrimidines and hydroxyurea, have been studied in the laboratory and clinical setting for more than 30 years. Early clinical experience in the 1960s and 1970s with the thymidine analogs 5-bromodeoxyuridine (BUdR) and 5-iododeoxyuridine (IUdR) was disappointing because normal tissue toxicity eliminated any potential for therapeutic gain. Inadequate delivery systems for intravenous and intraarterial infusions also contributed to the decline of this strategy. More recently, laboratory investigations have revealed further information regarding the mechanism of IUdR/BUdR radiosensitization. This knowledge provided a rationale for the sequence and timing of drug and radiation exposure, which could be both effective and tolerable. Advancing technology also provided safer infusion devices, and a resurgence in clinical trials combining IUdR or BUdR and radiation resulted. Current laboratory studies are now providing data on tumor cell kinetics, which is being applied to ongoing clinical trials. Fluoropyrimidines, principally 5-fluorouracil (5-FU), were also used in early clinical trials and unlike IUdR/BUdR were found to have significant activity as single agents against a variety of tumor types. The clinical integration of 5-FU and radiation occurred more slowly, but recent trials have demonstrated a therapeutic gain. Improved rates of local control and survival with combined 5-FU and radiation versus radiation alone have now been demonstrated in patients with rectal, esophageal, and anal carcinomas. However, the mechanism of interaction between the fluoropyrimidines and radiation remains uncertain and continues to be investigated with the hope of improved clinical outcome. As the cellular pathways influenced by the halogenated pyrimidines have been defined, the potential for biochemical modulation of these agents has been recognized. Leucovorin, the most commonly applied modulator, has been shown to enhance the activity of 5-FU in patients with metastatic colorectal carcinoma. These studies serve as an example for current trials that use biochemical modulators of IUdR, BUdR, and 5-FU as radiosensitizers. Hydroxyurea, currently used in the treatment of chronic leukemia, has also been considered a radiosensitizer. As with IUdR/BUdR, the clinical trials have often been inconclusive and interest in this radiosensitizer has waned. A poor understanding of the mechanism of action and tumor cell/normal tissue kinetics may be responsible for the lack of overall success with this strategy. Current investigations of cell kinetics in humans and potential mechanisms of hydroxyurea action could provide information critical to future trials of hydroxyurea radiosensitization.

Radiosensitizing and repair-inhibiting properties of dipyridamole

Radioresistance and postirradiation repair of potentially lethal damage (PLD repair) are important factors underlying failure to control local disease in cancer. Dipyridamole (DP) is known as a modifier of the action of cytotoxic drugs. We therefore investigated DP as a potential radiosensitizer and inhibitor of PLD repair in X-irradiated Chinese hamster ovary (CHO) cells in vitro. Exposure to the drug alone resulted in a slight reduction of the clonogenic capacity of the cells. Preincubation for 18 h with 10 and 20 microM DP in cells subcultured at low density, led to a significant radiosensitization. In confluent density-inhibited cultures, preincubation alone as well as pre- and postincubation with 20 microM DP resulted in a significant inhibition of PLD repair. Dipyridamole and related compounds may thus be useful tools for modifying and investigating the response of mammalian cells to radiation.

Modulation of iododeoxyuridine-mediated radiosensitization by 5-fluorouracil in human colon cancer cells

The incorporation of 5-iodo-2'-deoxyuridine (IdUrd), a thymidine analog radiosensitizer, can be increased by the use of modulators such as 5-fluorouracil (FUra). FUra is a particularly attractive potential modulator to use against colorectal cancer, as it is the most active single agent in the treatment of this disease. To begin to define the conditions for the optimal combination of IdUrd and FUra in the treatment of patients with colorectal cancer, a study was conducted of the effect of FUra on IdUrd-mediated radiosensitization in cultured HT29 human colon cancer cells. It was found that when cells were exposed to concentrations of IdUrd typical of those obtained through intravenous exposure (1-3 microM), FUra (1 microM) increased radiosensitization beyond that which would be predicted for the same extent of incorporation produced by incubation with IdUrd alone. This increase appeared to result from a combination of at least two effects: FUra-mediated cell cycle redistribution and increased IdUrd incorporation. When a higher concentration of IdUrd (10 microM) was used with FUra (1 microM), cell cycle distribution returned to nearly normal, and radiosensitization was equal to that predicted by the extent of incorporation of IdUrd. These data demonstrate that the combination of FUra and IdUrd can produce radiosensitization both through increased IdUrd incorporation and cell cycle redistribution. Furthermore, they suggest that, in the presence of a modulator, it may not be necessary to achieve high levels of IdUrd incorporation to produce significant tumor radiosensitization.

Iododeoxyuridine incorporation and radiosensitization in three human tumor cell lines

Iododeoxyuridine is a halogenated pyrimidine and non-hypoxic cell radiosensitizer currently being used in clinical trials. The amount of radiosensitization by IdUrd is related to the amount of incorporation of the drug into a cell's DNA. These experiments were carried out in three human tumor cell lines (lung, glioma, and melanoma) in monolayer culture exposed to concentrations of IdUrd from 0.1-10 microM for one and three cell cycles before irradiation to determine incorporation and sensitization as a function of drug exposure. Except for the lung cell line, which required greater than 1 microM IdUrd, these cells demonstrate radiosensitization when exposed to 0.1 microM or greater of IdUrd. Maximum sensitization occurred at 10 microM IdUrd for all the cell lines at three cell cycles. The percent thymidine replacement by IdUrd increased with increasing concentrations, but was cell line dependent. Maximum percent replacement occurred at 10 microM at three cell cycles for all the cell lines: lung = 22.4%, glioma = 32.0%, and melanoma = 39.1%. The relationships between percent thymidine replacement and sensitization are not identical across these human tumor cell lines. If IdUrd is going to be a successful radiosensitizer in clinical trials, sustained plasma levels of 10 microM or greater for at least three cell cycles should be achieved during irradiation. This may be best accomplished with repeated short exposures to IdUrd (three cell cycles or approximately 4 days in these cell lines) every 1-2 weeks during radiation. Measurements of thymidine replacement in a tumor biopsy should be attempted prior to radiation to develop a predictive assay for radiosensitization.

The effect of fluorodeoxyuridine on sublethal damage repair in human colon cancer cells

Although 5-fluoro-2'-deoxyuridine (FdUrd) has been combined with hyperfractionated radiation therapy in clinical trials, the optimal method of delivering radiation therapy is not yet known. To determine the importance of the time interval between fractions on the survival of tumor cells exposed to FdUrd, we studied the effect of FdUrd on sublethal damage repair in HT29 human colon cancer cells in culture. Cells were exposed to clinically achievable concentrations of FdUrd (10-100 nM) for 14 hr followed by either single dose (8-12 Gy) or split dose (4-6 Gy x 2) external cobalt irradiation. The interval between radiation fractions was varied from 0.5 to 6 hr. FdUrd impaired sublethal damage repair in a dose dependent fashion. FdUrd had no effect on the induction of DNA double strand breaks (DSB's), but significantly reduced the rate of the repair of DNA DSB's. Exposure to 100 nM FdUrd decreased intracellular TTP pools but elevated dATP pools. These findings suggest that FdUrd may decrease sublethal damage repair by perturbing nucleotide triphosphate pools, which leads to a decrease in the ability of the cell to repair DNA DSB's. Furthermore, they suggest that hyperfractionated irradiation will be superior to once daily treatment when combined with regional delivery of FdUrd.