First human treatment of resistant neoplastic meningitis by intrathecal administration of MTX plus (125)IUdR.

PURPOSE: Neoplastic meningitis is often the final outcome of disseminated cancer and is rapidly lethal. Its limited treatment relies on systemic or intrathecal chemotherapy with methotrexate (MTX) or thiotepa. When 5-iodo-2'-deoxyuridine labeled with (125)I ((125)IUdR) is incorporated into the DNA of mitotic tumor cells, the Auger electrons emitted during iodine decay are highly cytotoxic. The radiotherapeutic efficacy of (125)IUdR administered intrathecally has also been established in animals bearing spinal cord tumors, and MTX is known to potentiate the response. This approach has not been tested in the clinic. METHODS: A 44-year-old woman, with locally advanced pancreatic cancer, was treated for three years with complete systemic remission, but then relapsed with cytologically proven neoplastic meningitis. The patient was given four successive intrathecal injections of MTX (10 mg) every 12 h and, with the fourth dose, 1850 MBq (125)IUdR, followed by four additional MTX doses. The response was monitored by cytology and CA19.9 (carbohydrate antigen 19.9) levels in the cerebrospinal fluid (CSF) as well as by clinical status of the patient. RESULTS: The follow-up of cytology and CA19.9 levels in the CSF showed dramatic improvement within 26 days followed by a biological relapse on Day +36. There was no evidence of local central nervous system toxicity. Three months later, neoplastic meningitis recurred and meningeal tumor infiltration was observed on magnetic resonance imaging. Six months after MTX-(125)IUdR treatment, the patient died. CONCLUSION: (125)IUdR treatment proved to be feasible without acute neurological toxicity and seemed to have produced a biological response. This attempt provides the basis for designing prospective clinical trials.

Synthesis and biologic studies of iodinated (125I/127I) ethidium.

An iodinated (125I/127I) ethidium derivative (3,8-diamino-5-[6'-(p-iodobenzoylamino)-4'-azahexyl]-6-phenylphenanthridinium chloride hydrochloride) was synthesized and characterized. The labeling yield of the 125I-labeled derivative was 75% for carrier-free 125I, with a radiochemical purity of 95%. The incubation of iodoethidium with calf thymus DNA resulted in a substantial enhancement of fluorescence yield, indicating the intercalation of this compound into DNA. In the presence of iodoethidium, the nuclei of methanol-treated mammalian cells fluoresced, while those of viable cells did not (since the plasma membrane is impermeable to iodoethidium). When viable cells were incubated with the reduced form of the derivative, 125I/127I-dihydroethidium traversed the plasma membrane, was oxidized in the cytoplasm, and intercalated into nuclear DNA. Finally, we tested the hypothesis that larger malignant solid tumors, containing a relatively greater percentage of degenerating permeable cells, can be targeted with 125I-ethidium. In-vivo studies demonstrated a small but positive correlation (R = 0.72) between tumor volume and the uptake of the derivative. Because of the ubiquitous presence of abnormal permeable cells and necrosis in tumors, our results support the belief that radiolabeled DNA-intercalating or DNA-binding molecules may be of diagnostic and therapeutic value for a variety of solid tumors in humans.

Chemical modification of 5-[125I]iodo-2'-deoxyuridine toxicity in mammalian cells in vitro.

PURPOSE: To address the cytotoxic effects of DNA-incorporated (125)I in Chinese hamster V79 lung fibroblasts under various scavenging conditions. METHODS: The toxic effects of DNA-incorporated 5-[(125)I]iodo-2'-deoxyuridine ((125)IdUrd) were assessed by the colony-forming assay with cells incubated in medium containing serum and/or dimethyl sulphoxide (DMSO). Experiments were carried out at 0.3 or -135 degrees C. RESULTS: When (125)I decays were accumulated at 0.3 degrees C in 10% serum 0, 5 or 10% DMSO, no radioprotection was afforded by 5% DMSO, while the dose modification factor (DMF) for 10% DMSO was 2.0. For cells accumulating decays at 135 degrees C in the presence of 5 or 10% serum, DMSO was radioprotective (DMF= 1.8-1.9). D(0) obtained at each serum concentration correlated strongly (R=0.999) with the scavenging capacity of DMSO. Under these experimental conditions, 10% serum is approximately 3.6 times more protective than 5% serum. CONCLUSIONS: The contribution of indirect mechanisms to the toxicity of (125)I decaying within mammalian cell nuclear DNA can be demonstrated not only with DMSO, but also with the hydroxy radical scavengers present in serum.

Double-strand break yield following 125I decay--effects of DNA conformation.

The decay of iodine-125 (125I) is accompanied by the emission of low-energy electrons that dissipate most of their energy in approximately 10 nm from the decay site. In mammalian cells, the .OH generated by these electrons are also confined to a small volume. Iodine-125 is thus an excellent probe for assessing the radiobiologic effects produced by .OH in close proximity to the site of a decaying atom. We have compared in pUC19 plasmids (naked DNA) and in Chinese hamster V79 lung fibroblasts (chromatin) the modulation by the .OH scavenger dimethyl sulfoxide (DMSO) of 125I-induced DNA double-strand breaks (DSB). The data indicate that DMSO cannot protect plasmid DNA against DSB damage from 125I decaying within a few angstroms from DNA. However, DMSO attenuated DSB production in V79 cells following the decay of DNA-incorporated 125I, thus suggesting that chromatin structure fosters some DSB formation by indirect mechanism(s). DSB production depends on the environment and/or conformation of DNA. Consequently, current biophysical modeling of DNA damage that is based on naked and non-compacted DNA is inadequate for explaining radiobiologic effects at the cellular level.

In vivo therapy of neoplastic meningitis with methotrexate and 5.

We examined whether the administration of methotrexate (MTX) prior to the injection of 5-[125I]iodo-2'-deoxyuridine (125IUdR) in rats with intrathecal (i.t.) TE-671 human rhabdomyosarcoma would enhance 125IUdR uptake by tumor cells and augment the therapeutic efficacy of this Auger-electron-emitting radiopharmaceutical. TE-671 cells were exposed in vitro to medium +/- MTX, and the percentage of cells in various phases of the cell cycle and the uptake of 125IUdR assessed. In addition, nude rats were injected i.t. with TE-671 cells and later infused i.t. with saline or MTX for 24 h prior to 125IUdR injection, and the radioactivity associated with their spinal cords was determined. Exposure of tumor cells in vitro to MTX leads to an increase in the uptake of 125IUdR as a consequence of both a rise in the absolute uptake per cell and an increase in the percentage of S-phase cells. A corresponding increase of radioactivity within the spinal cords of tumor-bearing rats also occurs in the presence of MTX. Tumor-bearing animals were infused/injected with MTX and/or 125IUdR, and the onset of paralysis was determined as a function of time. We find that: (i) MTX infusion leads to a slight increase in time to onset of paralysis (median [M] = 24 vs. 22 days, p = 0.79); (ii) 125IUdR injection results in a statistically significant delay (p < 0.01) in the onset of paralysis (M= 39 days); (iii) MTX administration prior to 122IUdR injection further increases the therapeutic efficacy (M = 45 days).

Morphological transformation of C3H 10T1/2 cells by 99mTc-cardiolite.

UNLABELLED: The induction of in vitro morphological transformation in C3H 10T1/2 cells by 99mTc-Cardiolite (contents of Cardiolite kit [hexakis(2-methoxyisobutylisonitrile) and other components] plus (99m)Tc generator eluate) was examined. METHODS: Cells were grown for 48 h in the presence of 99mTc-Cardiolite or decayed 99mTc-Cardiolite (99mTc-Cardiolite after 1 wk of storage), and cell survival and transformation were assessed by the colony-forming and focus assays, respectively. X-ray was used as a reference for radiation effects, and 20-methylcholanthrene was used as a positive control for focus formation. RESULTS: Exposure of cells to 99mTc-Cardiolite results in a transformation frequency that is not significantly different from that induced by the volume equivalent of decayed 99mTc-Cardiolite. The number of foci per viable cell increases linearly from approximately 0.17 x 10(-4) in the untreated control to 1.7 x 10(-4) at 37 kBq/mL and 30 x 10(-4) at 1100 kBq/mL 99mTc-Cardiolite or its decayed 99mTc-Cardiolite volume equivalent. Furthermore, exposure of cells to low extracellular concentrations of 99mTc-Cardiolite or decayed 99mTc-Cardiolite (cell survival, > or =88%) induces an approximately 20-fold greater number of transformants per viable cell than that observed after 0.5 Gy x-irradiation, a dose that causes the same level of toxicity. CONCLUSION: Radioactive and decayed 99mTc-Cardiolite induce morphological transformation of C3H 10T1/2 cells in vitro. The underlying mechanism does not seem to be related to the radiation effects of decaying 99mTc but to chemical(s) present in the 99mTc-Cardiolite kit.

Strand breaks in plasmid DNA after positional changes of Auger electron-emitting iodine-125: direct compared to indirect effects.

To elucidate the nature and kinetics of DNA strand breaks caused by low-energy Auger electron emitters, we compared the yields of DNA breaks in supercoiled pUC19 DNA in the presence of the (.)OH scavenger dimethyl sulfoxide (DMSO) after the decay of (125)I (1) in proximity to DNA after minor-groove binding ((125)I-iodoHoechst 33342, (125)IH) and (2) at a distance from DNA ((125)I-iodoantipyrine, (125)IAP). DMSO is efficient at protecting supercoiled plasmid DNA from the decay of (125)I free in solution (dose modification factor, DMF = 59 +/- 4) and less effective when the (125)I decays occur close to DNA (DMF = 3.8 +/- 0.3). This difference is due mainly to the inability of DMSO to protect DNA from the double-strand breaks produced by groove-bound (125)I (DMF = 1.0 +/- 0.2). Additionally, the fragmentation of plasmid DNA beyond the production of single-strand and double-strand breaks that is seen after the decay of (125)IH and not (125)IAP (Kassis et al., Radiat. Res. 151, 167-176, 1999) cannot be modified by DMSO. These results demonstrate that the mechanisms underlying double-strand breaks caused by the decay of (125)IH differ in nature from those caused by the decay of (125)IAP.

Comparison of strand breaks in plasmid DNA after positional changes of Auger electron-emitting iodine-125.

To elucidate the kinetics of the induction of DNA strand breaks by low-energy Auger electron emitters, we compared the yields of DNA breaks in supercoiled pUC19 DNA after the decay of 125I (1) in proximity to DNA after minor-groove binding (125I-iodoHoechst 33342, 125IH) and (2) at a distance from DNA (125I-iodoantipyrine, 125IAP). Iodine-125 bound to the minor groove in DNA or free in solution is equally effective per decay in producing single-strand breaks (SSBs), while 125I bound to the minor groove is 6.7-fold more efficient than 125I free in solution in producing double-strand breaks (DSBs) (1.08 +/- 0.13 compared to 0.16 +/- 0.01 DSB/decay). Consequently, SSB to DSB ratios for 125IAP and gamma radiation (20.7 +/- 2.9 and 43.8 +/- 1.5, respectively) are greater than that for 125IH (2.9 +/- 0.4). Finally, the decay of 125IH leads to fragmentation of plasmid DNA beyond SSBs and DSBs.

Cytotoxicity of [125I]iodoHoechst 33342: contribution of scavengeable effects.

PURPOSE: The incubation of the DNA minor-groove binder [125I]iodoHoechst 33342 (125IH) with plasmid DNA leads to the production of one double-strand break (dsb) per decay, both in the presence and absence of dimethylsulfoxide (DMSO). In contrast, when 125I is incorporated into mammalian cell DNA as an iodinated pyrimidine base, DMSO decreases the dsb yield and enhances survival. Because these variations in radioprotective effects may be due either to the location of 125I vis-à-vis the DNA helix or to differences in DNA architecture, the toxicity of 125IH and its modification by DMSO were examined in mammalian cells. METHODS: Uptake and retention of 125IH in V79 cells were measured, and survival was determined after accumulation of 125I decays at 0.3 degrees C +/-10% DMSO. RESULTS: A linear-quadratic survival curve was obtained both in the absence [D37 = 114+/-36 decays/cell, alpha = (5.39 1.17) x10(-3) cell/decay] and presence [D37 = 211+/-65 decays/cell, alpha = (1.27+/-0.52) x10(-3) cell/decay] of DMSO. The dose modification factor for the linear component of the survival curve was 4.25+/-1.97, indicating the predominance of indirect mechanisms. This value is similar to that obtained with DNA-incorporated 125I (4.05+/-1.72) and for the initial slope (alpha) of 137Cs gamma-rays (4.43+/- 1.41). CONCLUSIONS: Cytotoxicity resulting from the decay of the Auger electron emitter 125I in the mammalian cell nucleus is caused mainly by indirect mechanisms.

Fluoresceinated phosphoethanolamine for flow-cytometric measurement of lipid peroxidation.

A new lipophilic fluorescein probe (fluor-DHPE) has been identified that can assay lipid peroxidation in mammalian cells on a cell-by-cell or selected-cell-subpopulation basis by flow cytometry. Application of this approach requires that the fluorescent probe be nonexchangeable among cells. Fluorescein is an appropriate fluorophore, since its fluorescence matches the specifications of common flow cytometers and the compound loses its fluorescence upon reaction with peroxyl radicals. Upon examination of four lipophilic derivatives of fluorescein, fluor-DHPE was found to be the only probe that was nonexchangeable among labeled and unlabeled rat RBC for at least 24 h. The exposure of fluor-DHPE-labeled RBC to benzoyl peroxide followed by mixing the sample with RBC unexposed to peroxide led to a decrease in fluorescence. Furthermore, the flow cytometer could clearly select the subpopulation of cells undergoing lipid peroxidation from those cells that were not. Fluor-DHPE-labeled-RBC obtained from rats and exposed to cumene hydroperoxide also displayed a gradual decrease in fluorescence. This decrease was preventable by either regulation of the vitamin E content in the animal diet or in vitro supplementation of cells with vitamin E. We conclude that fluor-DHPE is a stable and nonexchangeable probe for monitoring lipid peroxidation in cell subpopulations by flow cytometry.

Survival and DNA damage in Chinese hamster V79 cells exposed to alpha particles emitted by DNA-incorporated astatine-211.

Asynchronous Chinese hamster V79 lung fibroblasts were incubated at 37 degrees C for 30 min with the thymidine analog 5-[211At]astato-2'-deoxyuridine (211AtdU, exposure from DNA-incorporated activity) or with [211At]astatide (211At-, exposure from extracellular activity), and DNA-incorporated activity was determined. The 211AtdU content in cellular DNA increased as a function of extracellular concentration. Incorporation of 211At- was less than 1% of that of 211AtdU. After exposure, cells were frozen in the presence of 10% DMSO. One month later, survival was determined by the colony-forming assay, and DNA double-strand breaks (DSBs) were measured by the neutral elution method (pH 9.6). The survival curve for 211AtdU was biphasic (D37 = 2.8 decays per cell), reflecting killing of 211At-DNA-labeled cells and of unlabeled cells irradiated by 211At in neighboring labeled cells. The toxicity of 211At- decaying outside the cell (30-min exposure) was negligible. Analysis of the survival curve produced a D0 of 1.3 decays/cell for 211At-labeled cells. The yield of DSBs from the decay of DNA-incorporated 211At was compared with that from DNA-incorporated 125I. Each decay of 211At produced at least 10 times the number of DSBs as that obtained per 125I decay. The extreme radiotoxicity of DNA-incorporated 211AtdU seems to be associated with considerable damage to the mammalian cell genome.

5-[125I]iodo-2'-deoxyuridine in the radiotherapy of brain tumors in rats.

UNLABELLED: Glial neoplasms of the human central nervous system have defied treatment, in part because of the limited selectivity of available cytotoxic agents. The thymidine analog 5-iodo-2'-deoxyuridine radiolabeled with the Auger electron emitter 125I (125IUdR) is highly toxic to dividing cells when it is deoxyribonucleic acid incorporated, but it is relatively innocuous when located outside the nucleus. Previous studies have shown that 125IUdR has significant antineoplastic potential against mammalian cells in vitro and direct administration of 125IUdR is effective therapy for ovarian ascites tumors in mice and neoplastic meningitis in rats. Studies using external gamma imaging and autoradiography have also shown that direct intratumoral administration of 123IUdR/125IUdR into intracerebral 9L gliosarcomas in rats results in selective uptake of the radionuclide into tumor cells. Based on these encouraging results, we have evaluated the therapeutic potential of 125IUdR in rats bearing intracerebral 9L gliosarcomas. METHODS: Iodine-125-IUdR was infused intracerebrally over a 2-day period into rats bearing 1-day-old 9L tumors and over a 6-day period into animals with 9-day-old 9L tumors; equimolar concentrations of 127IUdR were infused into control animals. Tumor growth was monitored by contrast-enhanced 1H MRI and animal survival was followed over time. RESULTS: Intracerebral tumors (3-7 mm) were readily detected by MRI. Tumor-bearing rats treated with 127IUdR succumbed within 17-24 days, whereas tumor-bearing animals treated with 125IUdR survived significantly longer, and 10%-20% of the animals were cured of tumors. CONCLUSION: These data substantiate the antineoplastic potential of 5-[125I]iodo-2'-deoxyuridine and indicate that it may be a useful agent for the therapy of solid tumors that are accessible to direct radiopharmaceutical administration.

Lysine-directed conjugation of ethidium homodimer to B72.3 antibody: retention of immunoreactivity but altered tumor targeting.

Ethidium homodimer (EHD) was conjugated to B72.3 monoclonal antibody using a method whereby 85-90% of the conjugated EHD remains available for DNA intercalation. Antibody was thiopropionylated by reaction with N-succinimidyl 3-(2-pyridyldithio)propionate and reduction of pyridyldithio groups with dithiothreitol. EHD was maleimido-functionalized with succinimidyl-4-(N-maleimidoethyl)cyclohexane-1-carboxylate and treated with thiopropionylated antibody to obtain a conjugate containing approximately 3.4 EHD per antibody molecule. For biologic studies, 14C-labeled EHD was synthesized by reductive amination and conjugated as above. In vitro the conjugate maintained chemical integrity and immunoreactivity, while in vivo its targeting of LS174T tumors was reduced compared with that of iodinated antibody. A decrease in isoelectric point of the immunoconjugate was also observed.

[125I/127I/131I]Iodorhodamine: synthesis, cellular localization, and biodistribution in athymic mice bearing human tumor xenografts and comparison with [99mTc]hexakis(2-methoxyisobutylisonitrile).

The synthesis of halogenated rhodamine (Rh) derivatives was carried out by controlling the stoichiometry of the halogenating agents, bromine and iodine monochloride. In the no-carrier-added synthesis of radioiodinated rhodamine 123, direct labeling of rhodamine 123 (Rh 123) with Na125I/Na131I required the presence of the oxidant peracetic acid. 125I/131I-Rh 123 was synthesized in modest yields (40-45%). HPLC purification separated Rh 123 from its mono- and diiodo derivatives. Monohalogenation of Rh 123 did not alter the compound's ability to permeate viable cells and localize in mitochondria. 125I/131I-Rh 123 was stable in serum in vitro but rapidly metabolized after intravenous injection into mice. Consequently, scintigraphy and biodistribution data reveal poor targeting of subcutaneously growing human tumor xenografts. The results are compared to those obtained following the administration of [99mTc]hexakis(2-methoxyisobutylisonitrile) which also did not image human tumor xenografts in nude mice.

Indirect mechanisms contribute to biological effects produced by decay of DNA-incorporated iodine-125 in mammalian cells in vitro: double-strand breaks.

We have examined whether nuclear DNA can be protected from double-strand breaks (DSBs) induced by decay of the Auger-electron-emitting radionuclide 125I. Decays were accumulated at 0.3 degrees C in Chinese hamster V79 cells suspended in isotonic buffer containing 0.1 M EDTA in the presence or absence of 10% dimethyl sulfoxide (DMSO). DSBs were measured by the neutral elution method (pH 9.6) and quantified as strand scission factors. DMSO was shown to protect DNA from DSBs caused by the decay of DNA-incorporated 125I. The dose modification factor (DMF) for this radionuclide decreases as a function of 125I decays (389 to 4,100 decays, DMF = 2.5 to 1.3). Extrapolation of the curve for the DMF indicates that at approximately 15,000 decays/cell, a DMF of 1 would be obtained. Experiments using large numbers of 125I decays confirmed these extrapolations. For induction of DSBs by 137Cs gamma rays, the DMF also decreases with dose (50 to 290 Gy, DMF = 2.7 to 1.5). However, extrapolation of the curve for the DMF indicates that protection does not cease at higher doses. The data show that, at the same level of damage, DMSO can protect against gamma-ray-induced DSBs 1.35-fold more efficiently than against DSBs caused by the decay of DNA-incorporated 125I. It appears that when 125I is incorporated into DNA, chromatin structure fosters some DSB formation by an indirect mechanism(s) and that more than one DSB is generated per decaying atom.

Indirect mechanisms contribute to biological effects produced by decay of DNA-incorporated iodine-125 in mammalian cells in vitro: clonogenic survival.

We have examined whether mammalian cells in vitro can be protected against the lethal effects of irradiation by Auger electrons emitted from DNA-incorporated 125I. Chinese hamster V79 lung fibroblasts were cultivated in the presence of 5-[125I]iodo-2'-deoxyuridine (125IdU) for 18 h and resuspended in ice-cold medium in the presence or absence of 10% dimethyl sulfoxide (DMSO). DNA-incorporated 125I activity was measured and the cells were plated for survival. A portion of the cell suspensions were also stored on ice to accumulate 125I decays for 6 to 48 h, after which the cells were plated to determine survival. Storage on ice up to 48 h without radioactivity reduced plating efficiency from 67 +/- 4% (SEM) to 20 +/- 1%. DMSO had a protective effect on colony formation, as the respective cloning efficiencies were 83 +/- 3% and 72 +/- 12% at 0 and 48 h. The survival curves for 125IdU-labeled cells are exponential with D0 = 36 +/- 2 decays per cell in the absence of DMSO and 195 +/- 20 decays per cell in the presence of DMSO. Thus the dose modification factor (DMF) at 37% survival for 10% DMSO is 5.4 +/- 0.6 for DNA-incorporated 125I. In reference experiments, a DMF of 2.5 +/- 0.8 was measured for cells irradiated with 137Cs gamma rays. These results indicate that the radiotoxicity of Auger electrons from 125I decay in mammalian cells is caused mainly by an indirect mechanism(s).

Continuous detection of radiation or metal generated hydroxyl radicals within core chromatin particles.

PURPOSE: The aim of this work was to adapt a recently developed fluorometric method for use in the detection of hydroxyl radical (HO.) generated in the immediate vicinity of chromatin core particles reconstituted from pUC19 plasmid DNA and isolated core histones. MATERIALS AND METHODS: The procedure followed involves labelling nucleosomal histones with SECCA, a non-fluorescent coumarin derivative that generates the fluorescent 7-hydroxy-SECCA after reaction with HO.. Core particles are formed using histones and pUC19 DNA in a salt-dialysis procedure. RESULTS: Electron microscopy and micrococcal nuclease digestion are consistent with successful formation of core particles. No significant differences between core particle formation in the unlabelled and SECCA-labelled samples were detected. Exposure to HO. generated by radiation or copper--ascorbic acid--hydrogen peroxide results in a gradual induction of fluorescence. Studies using dimethyl sulphoxide (DMSO) demonstrate that, unlike HO. produced by radiation, the majority of HO. generated by copper--ascorbic acid--hydrogen peroxide occurs primarily in the immediate vicinity of core particles and DNA and cannot be scavenged. CONCLUSIONS: The present procedure demonstrates the feasibility to quantitate HO. generated by several agents in the immediate vicinity of nucleosomes (chromatin-associated HO.) or associated with specific regions within the genome.

Enhancement of 131I-mediated cytotoxicity by caffeine.

In cell culture, caffeine has been shown to enhance the lethality of DNA-damaging agents including ultraviolet rays, X-irradiation, and alkylating agents. We have previously reported a Phase I clinical trial demonstrating the feasibility of intraperitoneal radioimmunotherapy in patients with refractory ovarian cancer using 131I-labeled monoclonal antibody OC125. We are now exploring the possibility of using caffeine to enhance the toxicity of 131I-irradiation in target cells. As an in vitro model we tested this hypothesis using Chinese hamster ovary (CHO) cells exposed to 131I-labeled human serum albumin at various doses (4 to 70 microCi/ml) for 24 hr followed by 24 hr of incubation with caffeine. Cytotoxicity was measured by clonogenic survival and a nuclear fragmentation assay. The results show that caffeine, at a concentration of 7.7 mM, significantly enhances the cytotoxicity of 131I-irradiation.

Strand breaks after the decay of iodine-125 in proximity to plasmid pBR322 DNA.

To elucidate the kinetics of DNA strand breaks caused by low-energy Auger electron emitters in proximity to DNA molecules, we synthesized (125)I-labeled 2-iodoacridine (2-(125)IA), which intercalates with DNA, and 4-iodoacridine (4-(125)IA), which does not. Supercoiled DNA from pBR322 plasmid, labeled with 3H, was purified and incubated with 2-(125)IA or 4-(125)IA in aqueous solution. Reaction mixtures were stored at 4 degrees C to accumulate radiation dose from the decay of (125)I, and DNA was resolved by gel electrophoresis into supercoiled (DNA-I), nicked-circular (DNA-II) and linear (DNA-III) forms, representing undamaged DNA, single-strand breaks (SSBs) and double-strand breaks (DSBs), respectively. Gamma irradiation from an external (137)Cs source led to an exponential decrease in DNA-I with a D0 value of 10.8 +/- 0.3 Gy. Under identical conditions, the D0 values for 2-(125)IA and 4-(125)IA were 22.4 +/- 0.6 x 10(11) disintegrations and 4.7 +/- 0.4 x 10(11) disintegrations, respectively. External gamma irradiation and 4-(125)IA produced SSB/DSB ratios of 26.5 +/- 2.1 and 15.9 +/- 2, respectively, while that for 2-(125)IA was 0.6. The average number of DSBs from each decay of (125)I was 0.67 for 2-(125)IA and 0.27 for 4-(125)IA. The results indicate that the decay of (125)I bound to a DNA-intercalating compound produces DSBs 2.5-fold more efficiently than (125)I bound to a nonintercalating compound and support the theoretical expectations that predict a DSB yield that is highly dependent on the proximity of the Auger electron emitter to DNA.