OtherBASIC SCIENCE INVESTIGATIONS

Epidermal Growth Factor Receptor Inhibition Modulates the Nuclear Localization and Cytotoxicity of the Auger Electron–Emitting Radiopharmaceutical 111In-DTPA–Human Epidermal Growth Factor

Kristy E. Bailey, Danny L. Costantini, Zhongli Cai, Deborah A. Scollard, Zhuo Chen, Raymond M. Reilly and Katherine A. Vallis
Journal of Nuclear Medicine September 2007, 48 (9) 1562-1570; DOI: https://doi.org/10.2967/jnumed.107.044073

Abstract

111In-DTPA–human epidermal growth factor (111In-DTPA-hEGF [DTPA is diethylenetriaminepentaacetic acid]) is an Auger electron–emitting radiopharmaceutical that targets EGF receptor (EGFR)–positive cancer. The purpose of this study was to determine the effect of EGFR inhibition by gefitinib on the internalization, nuclear translocation, and cytotoxicity of 111In-DTPA-hEGF in EGFR-overexpressing MDA-MB-468 human breast cancer cells. Methods: Western blot analysis was used to determine the optimum concentration of gefitinib to abolish EGFR activation. Internalization and nuclear translocation of fluorescein isothiocyanate–labeled hEGF were evaluated by confocal microscopy in MDA-MB-468 cells (1.3 × 106 EGFRs/cell) in the presence or absence of 1 μM gefitinib. The proportion of radioactivity partitioning into the cytoplasm and nucleus of MDA-MB-468 cells after incubation with 111In-DTPA-hEGF for 24 h at 37°C in the presence or absence of 1 μM gefitinib was measured by cell fractionation. DNA double-strand breaks caused by 111In were quantified using the γ-H2AX assay, and radiation-absorbed doses were estimated. Clonogenic survival assays were used to measure the cytotoxicity of 111In-DTPA-hEGF alone or in combination with gefitinib. Results: Gefitinib (1 μM) completely abolished EGFR phosphorylation in MDA-MB-468 cells. Internalization and nuclear translocation of fluorescein isothiocyanate–labeled EGF were not diminished in gefitinib-treated cells compared with controls. The proportion of internalized 111In that localized in the nucleus was statistically significantly greater when 111In-DTPA-hEGF was combined with gefitinib compared with 111In-DTPA-hEGF alone (mean ± SD: 26.0% ± 5.5% vs. 14.6% ± 4.0%, respectively; P < 0.05). Induction of γ-H2AX foci was greater in MDA-MB-468 cells that were treated with 111In-DTPA-hEGF (250 ng/mL, 1.5 MBq/mL) plus gefitinib (1 μM) compared with those treated with 111In-DTPA-hEGF alone (mean ± SD: 35 ± 4 vs. 24 ± 5 foci per nucleus, respectively). In clonogenic assays, a significant reduction in the surviving fraction was observed when 111In-DTPA-hEGF (5 ng/mL, 6 MBq/μg) was combined with gefitinib (1 μM) compared with 111In-DTPA-hEGF alone (42.9% ± 5.7% vs. 22.9% ± 3.6%, respectively; P < 0.01). Conclusion: The efficacy of 111In-DTPA-hEGF depends on internalization and nuclear uptake of the radionuclide. Nuclear uptake, DNA damage, and cytotoxicity are enhanced when 111In-DTPA-hEGF is combined with gefitinib. These results suggest a potential therapeutic role for peptide receptor radionuclide therapy in combination with tyrosine kinase inhibitors.

The human epidermal growth factor receptor (EGFR) is a transmembrane tyrosine kinase receptor that transmits signals that control cell growth, migration, and proliferation. Ligand-dependent activation of EGFR leads to dimerization and autophosphorylation of the receptor and subsequent activation of downstream molecules involved in mitogenic signaling (1). EGFR overexpression occurs in 30%–60% of breast carcinomas and is associated with hormone resistance and a poor prognosis (2).

Inhibition of the EGFR has been explored extensively as a promising approach to the treatment of cancer in recent years. Several drugs that block EGFR activation or function have been developed and used as single agents or in combination with other treatments (3). An important group of compounds blocks ligand-induced activation of EGFR tyrosine kinase. Gefitinib (Iressa, ZD1839; AstraZeneca), for example, is an orally active tyrosine kinase inhibitor (TKI) that targets the adenosine triphosphate–binding site in the cytoplasmic domain of EGFR (4). Gefitinib has shown efficacy in several solid tumors, including head and neck and non–small cell lung cancer, and clinical trials are being conducted to define the role of gefitinib in the management of these cancers (5). Gefitnib has shown only limited benefit as a single agent in the treatment of metastatic breast cancer (6). However, trials of gefitinib in combination with trastuzumab or chemotherapy are being conducted (7,8). Preclinical studies also suggest synergistic effects of gefitinib in combination with hormonal agents (9).

Ionizing radiation causes EGFR phosphorylation and activation, and an inverse correlation exists between EGFR expression level and radiosensitivity in some tumors (10). In vitro studies have demonstrated synergy between EGFR inhibition and radiation, and this phenomenon is also evident in animal xenograft models (11). These early observations led to the development of clinical protocols that combine radiation with EGFR inhibition. For example, a recently reported phase III trial has confirmed the efficacy of EGFR inhibition in combination with radiation therapy in the treatment of advanced head and neck cancer (12). The molecular mechanisms that are thought to account for radiosensitization by EGFR inhibition include prevention of tumor cell proliferation and repopulation, disruption of normal cell cycle control, and interference with repair of radiation-induced DNA damage (13).

111In-Labeled human epidermal growth factor (111In-DTPA-hEGF [DTPA is diethylenetriaminepentaacetic acid]) is a radiopharmaceutical that binds the EGFR, is rapidly internalized, and translocates to the nucleus (14). 111In emits densely ionizing Auger electrons, 99% of which have a range of <1 μm in tissue and which are highly damaging to DNA (15). On a molar concentration basis, 111In-DTPA-hEGF was found to be 85-, 200-, and 300-fold more effective at inhibiting the growth of the EGFR-positive human breast cancer cell line MDA-MB-468 than paclitaxel, methotrexate, and doxorubicin, respectively (16). 111In-DTPA-hEGF was also found to exert a strong antitumor effect on MDA-MB-468 xenografts in athymic mice (17). Although these results demonstrate a potent anticancer effect of 111In-DTPA-hEGF, it is possible that the radiotoxicity of 111In-DTPA-hEGF might be partially attenuated by activation of the EGFR by the EGF moiety of the drug. To explore this, 111In-DTPA-hEGF was combined with inhibition of the tyrosine kinase function of the receptor using gefitinib. Gefitinib was selected for this purpose because it was shown previously to enhance binding and cellular uptake of astatinated and iodinated EGF in some cell lines (18,19). Strategies that increase the internalization and then nuclear translocation of 111In-DTPA-hEGF are likely to enhance the cytotoxicity of this agent because of the ultrashort range of the Auger electrons.

Our findings indicate that concurrent gefitinib increases the nuclear uptake of 111In-DTPA-hEGF, resulting in a greater level of DNA damage and enhanced cytotoxicity compared with 111In-DTPA-hEGF alone. These observations suggest that combining Auger electron–emitting radiopharmaceuticals that target peptide receptors with small-molecule TKIs may be a useful therapeutic strategy.

MATERIALS AND METHODS

Cell Culture

MDA-MB-468 human breast cancer cells were obtained from the American Type Culture Collection and were cultured in Dulbecco's minimum essential medium ([DMEM] Ontario Cancer Institute) containing penicillin (100 units/mL), streptomycin (100 μg/mL), and l-glutamine (2 mM) and supplemented with 10% fetal calf serum.

Radiopharmaceutical Synthesis

hEGF (Upstate Biotechnology) was derivatized with DTPA and labeled to high specific activity (3–6 MBq/μg) with 111In-acetate as previously described (20). The radiochemical purity of 111In-DTPA-hEGF was 95%–98%, as determined by silica-gel instant thin-layer chromatography in 100 mM sodium citrate, pH 5.

Effect of Gefitinib on EGFR Signaling in MDA-MB-468 Cells

To determine the concentration of gefitinib required to inhibit EGFR phosphorylation, MDA-MB-468 cells were cultured in serum-free DMEM for 3 h. Gefitinib was added in a range of concentrations (0–1 μM) for 3 h, and 20 ng/mL hEGF (Upstate Biotechnology) was added for the last 15 min before cells were harvested. Cells were washed in ice-cold phosphate-buffered saline (PBS) and lysed in NTEN buffer (150 mM sodium chloride, 20 mM Tris, pH 8.0, 1.0 mM EDTA [ethylenediaminetetraacetic acid], 0.5% Nonidet P-40 [NP-40; BDH Chemicals]) containing complete mini-EDTA–free protease inhibitor cocktail tablets (Roche Diagnostics). Lysates were resolved by NaDodSO4–polyacrylamide gel electrophoresis. Proteins were transferred to Immobilon-P transfer membrane (Millipore), blocked with 10% nonfat dried milk in Tris-buffered saline with Tween 20, and probed with anti-EGFR (clone 13, 1:1,000; BD Transduction Laboratories), antiphospho-EGFR (clone 9H2, 1:1,000; Calbiochem), anti-MAPK (1:200; Cell Signaling Technology), antiphospho-MAPK (Thr-202/Tyr-204, 1:100; Cell Signaling Technology), anti-SAPK/JNK (1:200; Cell Signaling Technology), antiphospho-SAPK/JNK (Thr-183/Tyr-185, 1:100; Cell Signaling Technologies), anti-p38 (Cell Signaling Technology), or antiphospho-p38 (Thr-180/Tyr-182, 1:100; Cell Signaling Technology) antibodies at 4°C overnight. After washing 3 times, primary antibodies were detected with horseradish peroxidase–conjugated antimouse or antirabbit secondary antibody as appropriate (Perkin Elmer Life Sciences). Bands were visualized using enhanced chemiluminescence (Super Signal West Pico Chemiluminescent).

Effect of Gefitinib on Internalization and Distribution of EGF and 111In-DTPA-hEGF in MDA-MB-468 Cells

Fluorescein isothiocyanate was conjugated to hEGF as described (21). Fluorescein-hEGF was separated from free fluorescein by size-exclusion chromatography on a P-2 minicolumn (Bio-Rad Laboratories) eluted with 150 mM sterile sodium chloride, pH 7.4. MDA-MB-468 cells (1 × 103) were plated on 8-well chamber slides (Nunclon; Canadian Life Technologies) and cultured for 24 h at 37°C in DMEM. Gefitinib (1 μM) was added for an additional 3 h or cells were left untreated (controls). Cells were washed 3 times with PBS and incubated for 1 h with 100 nM fluorescein-hEGF (with or without 1 μM gefitinib) in 150 mM sterile sodium chloride, pH 7.4. Cells were rinsed with 150 mM sodium chloride, pH 7.4, and fixed with 3.7% paraformaldehyde. After another 3 rinses with 150 mM sodium chloride, pH 7.4, slides were mounted with Vectashield mounting media (Vector Laboratories) containing the nuclear stain DAPI (4′,6′-diamidino-2-phenylindole dihydrochloride) and allowed to dry overnight at 4°C. The intracellular localization of fluorescein-hEGF (λexcit, 470–490 nm) and DAPI (λexcit, 340–380 nm) was evaluated using a Zeiss LSM 510 confocal laser-scanning microscope. Images were obtained of 1-μm slices and processed using Adobe Photoshop 7.0.

To determine the effect of gefitinib on the intracellular distribution of 111In-DTPA-hEGF, MDA-MB-468 cells (3 × 106) were incubated in medium with or without gefitinib (1 μM) in 6-well culture dishes for 3 h, which was followed by the addition of 111In-DTPA-hEGF (5 ng/mL, 3.7 MBq/μg) for 24 h. The 24-h time point was chosen as we have shown previously that nuclear accumulation of 111In-DTPA-hEGF increases for up to 24 h in MDA-MB-468 cells (14). After 24 h, unbound radioactivity was collected by transferring cell suspensions to tubes and recovering the medium by centrifugation at 600g for 10 min. This medium was combined with rinses obtained by resuspending the cells 3 times in PBS and centrifugation. The cells were then resuspended in 1 mL of 200 mM sodium acetate/500 mM sodium chloride, pH 2.5, for 5 min at 4°C to remove cell-surface–bound radioactivity. This acidic wash was recovered by centrifugation. The cells were rinsed 3 times with PBS, and the acidic wash and PBS were combined. Cells were then lysed and separated into cytoplasmic and nuclear fractions. In brief, cells were incubated with 1 mL of lysis buffer (25 mM KCl, 5 mM MgCl2, 10 mM Tris-HCl, pH 8.0, 1 mM dithiothreitol, 0.5% NP-40 plus protease inhibitor cocktail tablets) for 15 min at 4°C. Cell lysates were centrifuged at 1,000g for 5 min, and the supernatant was collected. The pellet was rinsed 3 times in PBS and each rinse was collected by centrifugation. The supernatant and the 3 rinses contained the cytoplasmic radioactivity. The cell pellet was incubated at 4°C for 15 min with nucleus isolation buffer (500 mM sodium chloride, 10 mM Tris-HCl, pH 8.0, 5 mM EDTA, 0.1% NP-40 with protease inhibitors). Vigorous pipeting and vortexing were used to break open the nuclear membrane. Samples were centrifuged at 12,000g for 15 min, and the supernatant was collected. The pellet was rinsed 3 times with PBS and centrifuged for 5 min at 10,000g. The supernatant and the 3 washes contained the nuclear radioactivity. The radioactivity in the 4 fractions (unbound, surface-bound, cytoplasmic, and nuclear) was measured using a γ-counter (Auto Gamma model 5650; Packard Instruments).

DNA Damage in MDA-MB-468 Cells Evaluated Using γ-H2AX Assay

Subconfluent MDA-MB-468 cells were harvested by trypsinization, seeded onto sterilized cover glasses (22 × 22 mm, Fisherfinest; Fisher Scientific), and placed in 6-well tissue culture plates (105 cells/well) in 1 mL of DMEM. After 24 h, the cells were incubated with DTPA-hEGF (250 ng/mL), 111In-acetate (1.5 MBq/mL), gefitinib (1 μM), 111In-DTPA-hEGF (250 ng/mL, 1.5 MBq/mL), or gefitinib plus 111In-DTPA-hEGF in 1 mL of fresh DMEM for 20 h. Cells treated with gefitinib plus 111In-DTPA-hEGF or gefitinib alone had been pretreated with 1 μM gefitinib for 3 h. Control dishes were incubated with medium alone.

After incubation, cells were rinsed with PBS and fixed with 2% paraformaldehyde plus 0.5% Triton X-100/PBS, pH 8.2, for 15 min, permeabilized with PBS containing 0.5% NP-40 for 15 min, and blocked in 2% bovine serum albumin (BSA) plus 1% donkey serum-PBS for at least 1 h. Cells were then incubated with mouse monoclonal anti-γ-H2AX antibody (1:800; Upstate Biotechnology) in 3% BSA-PBS overnight at 4°C and then incubated with Alexa Fluor 488 donkey antimouse secondary antibody (1:500; Invitrogen) for 45 min at room temperature. Finally, cells were stained with DAPI (0.2 μg/mL) for 5 min. After each of these steps, cells were rinsed in PBS twice for 5 min, except that after incubation with antibodies, cells were rinsed 3 times with 0.5% BSA plus 0.175% Tween-20/PBS for 10 min. The cover glasses were removed and mounted on microscope slides (Fisherbrand) using Vectashield Mounting Medium (Vector Laboratories). From the step using the secondary antibody, all procedures were performed in the dark. The cover glasses were wrapped with aluminum foil and stored at 4°C for later image acquisition.

The images of γ-H2AX foci and nuclei were obtained using a Zeiss LSM 510 confocal microscope. Optical sections (1.2 μm) through the thickness of the cells were imaged and combined in a z-projection. The projected images were processed using ImageJ (U.S. National Institutes of Health). At least 30 cells were imaged and analyzed for each data point in each experiment.

Clonogenic Survival Assays

MDA-MB-468 cells were treated with 111In-DTPA-hEGF (5–250 ng/mL; 6 MBq/μg) or unlabeled DTPA-hEGF (250 ng/mL) in medium with or without 1 μM gefitinib. Cells exposed to 1 μM gefitinib plus 111In-DTPA-hEGF were pretreated with 1 μM gefitinib for 3 h. Control dishes were incubated with medium with or without 1 μM gefitinib. All dishes were incubated at 37°C for 24 h. Cells were washed with PBS, trypsinized, and resuspended in medium. Cells were seeded in 6-well dishes for 10 d at 37°C to allow colony formation. Cultures were rinsed with PBS and stained with methylene blue (1% in a 1:1 mixture of ethanol and water). The number of colonies (>50 cells) in each dish was counted under light microscopy. The plating efficiency was determined by dividing the number of colonies formed in the untreated control dishes by the number of cells seeded. The surviving fraction (SF) was calculated by dividing the number of colonies formed by the number of cells seeded multiplied by the plating efficiency.

RESULTS

Effect of Gefitinib on EGFR Signaling in MDA-MB-468 Cells

The treatment of cells with hEGF (20 ng/mL) for 15 min resulted in a marked increase in EGFR phosphorylation (Fig. 1A). The phosphorylation status of EGFR was reduced in cells pretreated for 3 h with gefitinib (0.1–0.5 μM). Gefitinib at a concentration of 1 μM completely blocked EGF-induced phosphorylation of EGFR. Phosphorylation of the downstream targets of EGFR (MAPK, SAPK/JNK, and p38) was also inhibited by 1 μM gefitinib (Fig. 1B). This concentration of gefitinib was selected for use in subsequent nuclear localization and clonogenic survival experiments.

FIGURE 1. 
FIGURE 1. 

EGF-induced EGFR phosphorylation and activation is blocked by 1 μM gefitinib in MDA-MB-468 cells. Western blot analyses of MDA-MB-468 cells without (−) or stimulated with 20 ng/mL EGF (+) for 15 min, following incubation with 0.05–1 μM gefitinib for 3 h at 37°C. (A) Total protein samples were probed with antiphospho-EGFR Tyr-1173 and anti-EGFR antibodies. (B) Anti-MAPK, antiphospho-MAPK (p42/p44), anti-SAPK/JNK, antiphospho-SAPK/JNK, anti-p38, and antiphospho-p38 antibodies were used to determine phosphorylation status of downstream proteins in EGFR signaling cascade. Phosphorylation of EGFR and activation of downstream proteins was completely blocked by 1 μM gefitinib.

Effect of Gefitinib on Internalization and Distribution of EGF and 111In-DTPA-hEGF in MDA-MB-468 Cells

Using confocal microscopy it was found that after 1 h, fluorescein-hEGF was present in both the cytoplasm and the nuclei in the presence and absence of gefitinib, as indicated by the superimposition of green (EGF) on blue (DAPI) staining (Fig. 2). There was no significant difference in the proportion of radioactivity that bound to the cell surface or internalized after 24 h when MDA-MB-468 cells were exposed to 111In-DTPA-hEGF plus gefitinib compared with 111In-DTPA-hEGF alone. However, when cells were treated with 111In-DTPA-hEGF plus 1 μM gefitinib, there was a change in the distribution of radioactivity within the cell, with a significantly greater proportion localized to the nucleus and less in the cytoplasm compared with 111In-DTPA-hEGF alone (Table 1). The combination of gefitinib and 111In-DTPA-hEGF resulted in an almost 2-fold increase in radioactivity in the nuclear fraction compared with 111In-DTPA-hEGF alone (expressed as a percentage of internalized 111In-DTPA-hEGF: 26.0% ± 5.5% vs. 14.6% ± 4.0%, respectively; P < 0.05).

FIGURE 2. 
FIGURE 2. 

Visualization of nuclear fluorescein-labeled-hEGF in presence of gefitinib using confocal microscopy. (A) MDA-MB-468 cells treated with fluorescein-hEGF for 1 h at 37°C. (B) MDA-MB-468 cells pretreated with gefitinib (1 μM) for 3 h followed by fluorescein-hEGF and gefitinib for 1 h at 37°C. Cells were also incubated with DAPI to visualize the cell nucleus. Images represent a 1-μm slice through center of cell. Fluorescein-hEGF was observed in the nucleus in absence and presence of 1 μM gefitinib.

View this table:
TABLE 1

Effect of Gefitinib on Binding, Internalization, and Nuclear Localization of 111In-DTPA-hEGF in MDA-MB-468 Human Breast Cancer Cells

DNA Damage in MDA-MB-468 Cells Evaluated Using γ-H2AX Assay

Cells exposed to gefitinib plus 111In-DTPA-hEGF showed a statistically significant increase in the mean number of γ-H2AX foci per nucleus compared with control untreated cells (35.4 ± 4.2 vs. 6.3 ± 1.3 foci, respectively; P < 0.001) (Fig. 3). There was a 1.5-fold increase in the mean number of γ-H2AX foci in cells exposed to the combination treatment compared with 111In-DTPA-hEGF alone, and this was statistically significant (35.4 ± 4.2 vs. 23.8 ± 5.2 foci, respectively; P < 0.05). These results indicate that the enhancement of nuclear translocation of 111In-DTPA-hEGF in the presence of gefitinib is associated with an increase in DNA double-strand breaks (DNA-dsbs).

FIGURE 3. 
FIGURE 3. 

Effect of gefitinib and 111In-DTPA-hEGF on induction of γ-H2AX foci in MDA-MB-468 cells. γ-H2AX assay was performed after exposure of MDA-MB-468 cells to DTPA-hEGF (250 ng/mL), gefitinib (1 μM), 111In-acetate (1.5 MBq/mL), 111In-DTPA-hEGF (250 ng/mL, 1.5 MBq/mL), gefitinib (1 μM) plus 111In-DTPA-hEGF (250 ng/mL, 1.5 MBq/mL), or DMEM alone (control) for 20 h. Optical sections (1.2 μm) through the cells were obtained using a confocal microscope and were processed using ImageJ (U.S. National Institutes of Health). Error bars represent SEM number of γ-H2AX foci from 3 separate experiments. Thirty cells were counted to generate each data point for each experiment. *P < 0.05 (compared with control).

DTPA-hEGF and gefitinib alone did not cause a statistically significant increase in the number of γ-H2AX foci compared with control untreated cells (P = 0.10 and 0.06, respectively). 111In-Acetate alone caused an increase in the number of γ-H2AX foci compared with control (13.6 ± 2.5 vs. 6.3 ± 1.3 foci, respectively; P < 0.05) (Fig. 3). 111In-DTPA-hEGF, however, resulted in a significantly greater number of foci compared with 111In-acetate alone (23.8 ± 5.2 vs. 13.6 ± 2.5 foci, respectively; P < 0.05).

Clonogenic Survival Assays

The SF of MDA-MB-468 cells was significantly lower when cells were treated with 1 μM gefitinib plus 111In-DTPA-hEGF (5–150 ng/mL, 6 MBq/μg) compared with 111In-DTPA-hEGF alone. For example, the SF of cells treated with 5 ng/mL 111In-DTPA-hEGF was 42.9% ± 5.7%, whereas the SF of cells exposed to 111In-DTPA-hEGF (5 ng/mL) plus gefitinib was 22.9% ± 3.6% (P < 0.01). At the highest concentration of 111In-DTPA-hEGF tested, 250 ng/mL (6 MBq/μg), the SF was reduced to 6%, and the addition of gefitinib was not associated with a further reduction in the SF (Fig. 4). Unlabeled DTPA-hEGF (250 ng/mL) alone resulted in a modest reduction in SF (60.6% ± 9.9%). This is consistent with the reported observation that high concentrations of EGF are inhibitory to the growth of MDA-MB-468 cells (16). The same concentration of 111In-DTPA-hEGF, 250 ng/mL, was approximately 10-fold more cytotoxic than unlabeled DTPA-hEGF (SF, 6.1% ± 1.9% vs. 60.6% ± 9.9%, respectively; P < 0.01). The SF of cells exposed to DTPA-hEGF (250 ng/mL) plus gefitinib (1 μM) did not differ significantly from the SF of cells exposed to 1 μM gefitinib alone (39.9% ± 7.4% vs. 40.3% ± 6.1%, respectively; P = 0.5).

FIGURE 4. 
FIGURE 4. 

Gefitinib enhances 111In-DTPA-hEGF–mediated cytotoxicity. Clonogenic assay was performed using MDA-MB-468 cells treated with increasing concentrations of 111In-DTPA-hEGF (5–250 ng/mL, 6 MBq/μg) for 24 h at 37°C (black bars) or pretreated with gefitinib (1 μM) for 3 h followed by treatment with a range of concentrations of 111In-DTPA-hEGF plus gefitinib (1 μM) for 24 h at 37°C (white bars). At each concentration of 111In-DTPA-hEGF tested, addition of gefitinib resulted in enhanced cytotoxicity. The SF of cells exposed to DTPA-hEGF (250 ng/mL) in the absence or presence of gefitinib (1 μM) was also measured (lined and speckled bars, respectively). 111In-DTPA-hEGF (250 ng/mL) was ∼10-fold more cytotoxic than unlabeled DTPA-hEGF (SF, 6.1% ± 1.9% vs. 60.6% ± 9.9%, respectively; P < 0.01). The SF of cells exposed to DTPA-hEGF (250 ng/mL) plus gefitinib (1 μM) was not statistically significant from the SF of cells exposed to 1 μM gefitinib alone (39.9% ± 7.4% vs. 40.3% ± 6.1%, respectively; P = 0.5). Error bars represent SD of the mean SF calculated from 3 separate experiments. *P < 0.05 (compared with control).

DISCUSSION

111In-DTPA-hEGF is rapidly internalized after binding to the EGFR, and a proportion (5%–10%) translocates to the cell nucleus, where the radionuclide comes in close proximity to chromosomal DNA (14). 111In emits Auger electrons that can cause lethal DNA damage. Microdosimetry models of Auger electron–emitting radioisotopes in mammalian cells predict that the radiation-absorbed dose to the cell nucleus is 20- to 35-fold greater when 111In is localized in the nucleus compared with when it is localized at the cell membrane (22). This prediction is based on the observation that >99% of the Auger electrons emitted by 111In have very low energies (<3 keV) and a range of <1 μm in tissues (15). The lethality of Auger electron–emitting radiopharmaceuticals such as 111In-DTPA-hEGF toward cancer cells is therefore likely to be highly dependent on their ability to translocate to the cell nucleus. Pharmacologic strategies that increase the proportion of 111In-DTPA-hEGF that localizes in the nucleus would thus be expected to amplify its cytotoxicity against EGFR-overexpressing cancer cells. It has been shown that EGFR TKIs can increase the cellular uptake of radiolabeled EGF (18,19), suggesting that combining 111In-DTPA-hEGF with gefitinib might increase its potency. It has previously been reported that 111In-DTPA-hEGF is only cytotoxic to cells that express moderate to high levels of EGFR. The human breast cancer cell line MCF-7, which expresses approximately 1 × 104 receptors per cell, was relatively resistant to cell killing by 111In-DTPA-hEGF, both in vitro and in vivo, compared with the MDA-MB-468 cell line, which expresses approximately 1 × 106 EGFRs per cell (14,17). Therefore, the 111In-DTPA-hEGF–responsive cell line MDA-MB-468 was selected for this initial investigation of the effect of combining 111In-DTPA-hEGF with EGFR inhibition.

Gefitinib would be expected to enhance the cytotoxicity of 111In-DTPA-hEGF only if inhibition of EGFR phosphorylation does not significantly impede the internalization and nuclear translocation of 111In-DTPA-hEGF. Several investigators have reported that EGFR kinase activation is required for ligand-induced internalization of the receptor (23,24). In one study, treatment of NIH3T3 cells expressing EGFR with the TKI PD158780 reduced the internalization rate of the receptor by 90% (24). Conversely, in a recent publication, Wang et al. showed that elimination of EGFR kinase activation by mutation or chemical inhibition did not abolish EGFR internalization. Instead, it was shown that inhibition of EGFR dimerization, by deletion of the dimerization loop in domain II, inhibited EGF-dependent EGFR internalization (25). The apparently contradictory information regarding the need for EGFR phosphorylation in ligand-dependent EGFR internalization could be explained by the existence of redundant pathways of EGFR internalization that may require receptor phosphorylation. It is possible that the dependence of EGFR internalization on EGFR phosphorylation may vary in a cell-type–specific manner (26).

Prevention of EGFR activation through the use of gefitinib did not diminish the internalization and nuclear translocation of fluorescein-labeled hEGF (Fig. 2) or of 111In-DTPA-hEGF in MDA-MB-468 cells (Table 1). Rather, the proportion of radioactivity localizing in the nucleus was 1.8-fold greater in MDA-MB-468 cells treated with the combination of 111In-DTPA-hEGF (5 ng/mL, 3.7 MBq/μg) and gefitinib (1 μM) compared with 111In-DTPA-hEGF alone. Although EGF–EGFR trafficking is incompletely understood, it is apparent that after cellular uptake into clathrin-coated pits, the ligand–receptor complex is internalized into endosomes. From early endosomes, EGFR either is sorted into the late endosome and then sent to the lysosome for degradation or is recycled back to the cell surface. Recent evidence also suggests that ligand-dependent translocation of EGFR to the nucleus can occur (27). The route through which EGFR reaches the nucleus is unclear (28), although a putative nuclear localizing sequence has been identified in the cytoplasmic domain and EGFR has been shown to associate with importins α1/β1 and exportin CRM1 (29). This suggests that nuclear uptake of EGFR is mediated by the nuclear transport receptors and the nuclear pore complex. Sorting of EGFR to the lysosome requires phosphorylation of EGFR at Tyr-1045. Phosphorylation at this site is necessary to recruit c-Cbl, which ubiquitinates EGFR, directing it to the lysosome for degradation (30). Thus, one possible mechanism for increased nuclear uptake of 111In-DTPA-hEGF in the presence of gefitinib is that blocked phosphorylation of EGFR enables the ligand–receptor complex to escape lysosome-mediated degradation. This could allow a proportionately greater amount of the receptor to accumulate in the cell nucleus or to be recycled back to the cell membrane. It is also important to note that the effect of 111In-DTPA-hEGF on EGFR activation and trafficking is likely to be distinct from that of the naked ligand, EGF—because ionizing radiation and other sources of oxyradicals lead to aberrant phosphorylation of the receptor (31). In these circumstances EGFR does not undergo ubiquitination or endocytosis but may traffick to perinuclear and nuclear compartments (32,33).

Overall, the results presented in this article suggest that the enhanced cytotoxicity observed with concomitant 111In-DTPA-hEGF and gefitinib treatment is at least partially due to an increase in the nuclear translocation of the radiopharmaceutical. However, there are likely to be other factors that contribute to the increased lethality of 111In-DTPA-hEGF in the presence of gefitinib compared with 111In-DTPA-hEGF alone. It has been reported that EGFR is involved in the activation and regulation of DNA-dependent kinase (DNA-PK), which participates in the nonhomologous end-joining (NHEJ) DNA repair pathway (33,34). As a result, NHEJ may be less active in EGFR inhibitor–treated cells. Friedmann et al. have shown that exposure of the breast cancer cell line MCF-7 to gefitinib leads to a reduction in nuclear DNA-PK, suppression of DNA-PK activity, and sensitization to DNA damage by cisplatin (35). DNA-PK is known to play a central role in the repair of radiation DNA damage. Therefore, it is possible that the increase in γ-H2AX foci and lower SF that were observed in cells treated with 111In-DTPA-hEGF in the presence of gefitinib compared with 111In-DTPA-hEGF alone may be the result of gefitinib-mediated inhibition of DNA-PK activity.

Small-molecule TKIs have been shown to cause cell cycle arrest in the G0/G1 phase and to reduce the percentage of cells in the radiation-resistant S-phase fraction in malignant cell lines (36). Depending on the duration of cell cycle arrest caused by TKIs and the effect of 111In-DTPA-hEGF itself on cell cycle kinetics, it is possible that partial synchronization of cells by a TKI might render them more susceptible to the cytotoxicity of 111In-DTPA-hEGF, although this has yet to be confirmed experimentally.

The induction of DNA damage by 111In-DTPA-hEGF with and without gefitinib was evaluated by counting the number of γ-H2AX foci formed at the sites of DNA-dsbs. Sedelnikova et al. have reported a positive correlation between the number of γ-H2AX foci and clonogenic survival in cells transfected with 125I-triplex-forming oligonucleotides, suggesting that the γ-H2AX assay is a useful marker of the cytotoxic effects of Auger electron–emitting DNA-targeting agents (37). It was found that the average number of γ-H2AX foci per nucleus increased 1.5-fold when 111In-DTPA-hEGF was combined with gefitinib in comparison with cells incubated with the radiopharmaceutical alone (Fig. 3). Also, 111In-DTPA-hEGF resulted in a significantly greater number of foci than 111In-acetate (23.8 ± 5.2 vs. 13.6 ± 2.5 foci, respectively; P < 0.05). In contrast to Auger electrons, which have a subcellular range, the 171- and 245-keV γ-photons that 111In also emits have a range that is much greater than a cell diameter. Because 111In-acetate is not specifically taken up by the cells, the low level of DNA damage that it causes is likely due to γ-irradiation from extracellular 111In-acetate or to Auger electrons emitted by a limited amount of intracellular 111In. Taken together, these results suggest that it is the Auger electron emissions that cause most of the DNA damage caused by 111In-DTPA-hEGF and that specific binding of the radiopharmaceutical to EGFR is a prerequisite for the significant induction of DNA-dsbs. The increased level of DNA damage observed when 111In-DTPA-hEGF was combined with gefitinib was reflected in lower clonogenic survival of cells exposed to both agents (Fig. 4). Unlabeled DTPA-hEGF at high concentration (250 ng/mL) resulted in a modest reduction in SF, which is consistent with the reported observation that high concentrations of EGF are inhibitory to the growth of MDA-MB-468 cells (16). The same concentration of 111In-DTPA-hEGF was approximately 10-fold more cytotoxic than unlabeled DTPA-hEGF (SF: 6.1% ± 1.9% vs. 60.6% ± 9.9%, respectively; P < 0.01). The SF of cells exposed to DTPA-hEGF plus gefitinib did not differ significantly from the SF of cells exposed to gefitinib alone. These results indicate that the deleterious effect of 111In-DTPA-hEGF plus gefitinib is due to Auger electron radiation and not simply to an increase in nuclear uptake of DTPA-hEGF after gefitinib treatment.

It was calculated that, for the concentration of 111In-DTPA-hEGF used in the cellular fractionation experiments described in this article (5 ng/mL, 3.7 MBq/μg), approximately 12% of EGFs would be occupied. If it is assumed that the cellular distribution is similar under receptor saturation conditions, then—based on the cellular distribution data that were obtained for 111In-DTPA-hEGF alone and in combination with gefitinib (Table 1)—the radiation dose delivered to the entire cell, and specifically to the cell nucleus under both treatment conditions, can be calculated using a microdosimetry model. Goddu et al. tabulated S values (absorbed dose per unit of cumulated activity) for cells and nuclei of various sizes and for several isotopes, including 111In (22). Using these S values and assuming a cell diameter of 10 μm and a nuclear diameter of 6 μm, each MDA-MB-468 cell treated to receptor saturation with 111In-DTPA-hEGF would receive 20.2 Gy to the nucleus. Nuclear 111In-DTPA-hEGF accounts for 16.0 Gy of this total (Table 2). Because radioactivity localized to the nucleus accounts for 80% of the absorbed-radiation dose to the nucleus (22), even a modest increase in nuclear 111In-DTPA-hEGF would result in a significant increase in the absorbed-radiation dose to DNA. It is estimated that the addition of gefitinib to 111In-DTPA-hEGF resulted in a total absorbed dose of 32.3 Gy to the cell nucleus, of which 28.6 Gy were due to 111In-DTPA-hEGF in the nucleus (Table 2). Thus, the 1.8-fold increase in the amount of radioactivity in the nucleus associated with gefitinib treatment results in an additional 12.2-Gy absorbed dose to the nucleus. It is possible that the absorbed-radiation doses shown in Table 2 are overestimated, as the method used to obtain them accounts for radioactive decay but not for biologic clearance of radioactivity from the cell. However, it has been shown previously that nuclear accumulation of 111In-DTPA-hEGF increases for up to 24 h in MDA-MB-468 cells. Given that cells were exposed to 111In-DTPA-hEGF for 24 h in this study, the effect of egress of radioactivity on the absorbed-dose calculation is likely to be small (14).

View this table:
TABLE 2

Effect of Gefitinib on Radiation-Absorbed Dose to Cell Nucleus from 111In-DTPA-hEGF Localized at Cell Membrane, Cytoplasm, or Nucleus of MDA-MB-468 EGFR-Overexpressing Human Breast Cancer Cells*

The current study shows that, in a commonly used model of EGFR-positive human breast cancer, the efficacy of targeted Auger electron radiotherapy can be improved by a pharmacologic intervention that increases nuclear uptake of the agent. The use of combination treatment with a TKI is one way to increase the uptake of 111In-DTPA-hEGF and increase the chance of successful eradication of malignant cells. The concentration of gefitinib used in the experiments described in this report (1 μM) is within the range of human steady-state plasma concentrations (0.4–1.2 μM) resulting from clinically relevant doses of the drug (250–500 mg daily) (38). This suggests that the enhancement of the cytotoxicity of 111In-DTPA-hEGF when combined with gefitinib would be achievable in the clinical setting. Gefitinib is well tolerated at a dose of up to 600 mg/d (38), so it may be possible to further enhance the cytotoxicity of 111In-DTPA-hEGF by increasing the dose of gefitinib with which it is combined, although this has yet to be tested.

It is possible that pharmacologic interventions that alter endosomal trafficking or nuclear transport might further promote nuclear uptake and, thus, efficacy of 111In-DTPA-hEGF. One possible strategy to increase nuclear uptake might involve the addition of a nuclear localization sequence (NLS) to radiopharmaceuticals. A recent study reported a higher internalization rate, prolonged cellular retention, and significantly higher nuclear uptake when the NLS of the simian virus 40 large T-antigen was conjugated to 111In-DOTA-octreotide compared with the unmodified drug, 111In-DOTA-Tyr3-octreotide (39). Similarly, we have recently reported increased nuclear localization and decreased clonogenic survival in leukemia cells exposed to an anti-CD33 monoclonocal antibody, HuM195, radiolabeled with 111In and modified with nuclear-localization-sequence peptides of the simian virus 40 large T-antigen (40).

CONCLUSION

Gefitinib increases the accumulation of 111In-DTPA-hEGF in the nucleus, leading to an increase in DNA double-strand breaks and cell death. Because the localization of 111In-DTPA-hEGF and other Auger electron–emitting radiopharmaceuticals controls lethality, manipulating the nuclear translocation pathway to amplify the amount of short-range radioactivity accumulating in the nucleus is an effective means by which to enhance cytotoxicity. A greater knowledge of the nuclear translocation pathway of 111In-DTPA-hEGF may suggest other mechanisms for enhancing the nuclear uptake of 111In-DTPA-hEGF in the future.

Acknowledgments

This research was supported through grants from the Canadian Breast Cancer Foundation (Ontario Chapter) and Cancer Research-UK. Kristy Bailey, Zhongli Cai, and Danny Costantini were recipients of support from the Canadian Institutes for Health Research (through the Excellence in Radiation Research for the 21st Century Training Program).

Footnotes

  • COPYRIGHT © 2007 by the Society of Nuclear Medicine, Inc.

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  • Received for publication March 14, 2007.
  • Accepted for publication June 14, 2007.
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