Effects of 131I-EGF on cultured human glioma cells

HER2- and EGFR-specific affiprobes: novel recombinant optical probes for cell imaging

The human epidermal growth factor receptors, EGFR and HER2, are members of the EGFR family of cell-surface receptors/tyrosine kinases. EGFR- and HER2-positive cancers represent a more aggressive disease with greater likelihood of recurrence, poorer prognosis, and decreased survival rate, compared to EGFR- or HER2-negative cancers. The details of HER2 proto-oncogenic functions are not deeply understood, partially because of a restricted availability of tools for EGFR and HER2 detection (A. Sorkin and L. K. Goh, Exp. Cell Res. 2009, 315, 683-696). We have created photostable and relatively simple-to-produce imaging probes for in vitro staining of EGFR and HER2. These new reagents, called affiprobes, consist of a targeting moiety, a HER2- or EGFR-specific Affibody molecule, and a fluorescent moiety, mCherry (red) or EGFP (green). Our flow cytometry and confocal microscopy experiments demonstrated high specificity and signal/background ratio of affiprobes. Affiprobes are able to stain both live cells and frozen tumor xenograph sections. This type of optical probe can easily be extended for targeting other cell-surface antigens/ receptors.

Planning for intracavitary anti-EGFR radionuclide therapy of gliomas. Literature review and data on EGFR expression

Targeting with radionuclide labelled substances that bind specifically to the epidermal growth factor receptor, EGFR, is considered for intracavitary therapy of EGFR-positive glioblastoma multiforme, GBM. Relevant literature is reviewed and examples of EGFR expression in GBM are given. The therapeutical efforts made so far using intracavitary anti-tenascin radionuclide therapy of GBM have given limited effects, probably due to low radiation doses to the migrating glioma cells in the brain. Low radiation doses might be due to limited penetration of the targeting agents or heterogeneity in the expression of the target structure. In this article we focus on the possibilities to target EGFR on the tumour cells instead of an extracellular matrix component. There seems to be a lack of knowledge on the degree of intratumoral variation of EGFR expression in GBM, although the expression seemed rather homogeneous over large areas in most of the examples (n=16) presented from our laboratory. The observed homogeneity was surprising considering the genomic instability and heterogeneity that generally characterises highly malignant tumours. However, overexpression of EGFR is, at least in primary GBMs, one of the steps in the development of malignancy, and tumour cells that lose or downregulate EGFR will probably be outgrown in an expanding tumour cell population. Thus, loss of EGFR expression might not be the critical factor for successful intracavitary radionuclide therapy. Instead, it is likely that the penetration properties of the targeting agents are critical, and detailed studies on this are urgent.

Automated, high-resolution cellular retention and uptake studies in vitro

This report describes an automated method for the measurements of cellular retention and uptake of radiolabeled proteins interacting with cell-surface receptors on intact cancer cells. A complete uptake and retention measurement was performed in one cell dish using a rotating radioimmunoassay (RIA) principle. Compared to common manual measurements, rotating RIA saved both labor time and reagents and provided real-time binding traces with superior time-resolution. The rotating RIA retention profiles for different interactions agreed with retention times reported in the literature.

Combined effect of gefitinib ('Iressa', ZD1839) and targeted radiotherapy with 211At-EGF

The EGFR-TKI (epidermal growth factor receptor tyrosine kinase inhibitor) gefitinib ['Iressa' (trademark of the AstraZeneca group of companies), ZD1839] increases the cellular uptake of radiolabelled epidermal growth factor (EGF). We investigated gefitinib treatment combined with astatine-211 EGF targeting in vitro using two cell lines expressing high levels of EGFR: A431 (sensitive to gefitinib) and U343MGaCl2:1 (resistant to gefitinib). In both cell lines, the uptake of 211At-EGF was markedly increased by concomitant treatment with gefitinib. Survival was investigated using both a clonogenic survival assay and a cell growth assay. Combined gefitinib and 211At-EGF treatment reduced the survival of U343 cells 3.5-fold compared with 211At-EGF alone. In A431 cells, 211At-EGF treatment resulted in very low survival, but combined treatment with gefitinib increased the survival by about 20-fold. These results indicate that combined treatment with gefitinib might increase the effect of ligand-mediated radionuclide therapy in gefitinib-resistant tumours and decrease the effect of such therapy in gefitinib-sensitive tumours.

[111In]Bz-DTPA-hEGF: Preparation andIn VitroCharacterization of a Potential Anti-Glioblastoma Targeting Agent

The overexpression of epidermal growth factor receptors, EGFR, in glioblastomas is well documented. Hence, the EGFR can be used as target structure for a specific targeting of glioblastomas. Both radiolabeled anti-EGFR antibodies and the natural ligand EGF are candidate agents for targeting. However, EGF, which has a rather low molecular weight (6 kDa), might have better tissue penetration properties through both normal tissue and tumors in comparison with anti-EGF antibodies and their fragments. The aim of this study was to prepare and evaluate in vitro an EGF-based antiglioma conjugate with residualizing label. Human recombinant EGF (hEGF) was coupled to isothiocyanate-benzyl-DTPA. The conjugate was purified from unreacted chelator using solid-phase extraction and labeled with (111)In. The labeling yield was 87% +/- 7%. The label was reasonably stable; the transchelation of (111)In to serum proteins was about 5% after incubation at 37 degrees C during 24 hours. The obtained [(111)In]benzyl-DTPA-hEGF conjugate was characterized in vitro using the EGFR expressing glioma cell line U343MGaCl2:6. The binding affinity, internalization, and retention of the conjugate were studied. The conjugate had receptor specific binding and the radioactivity was quickly internalized. The intracellular retention of radioactivity after interrupted incubation with conjugate was 71% +/- 1% and 59% +/- 1.5% at 24 and 45 hours, respectively. The dissociation constant was estimated to 2.0 nM. The results indicate that [(111)In]benzyl-DTPA-hEGF is a potential candidate for targeting glioblastoma cells, possibly using locoregional injection.

Cellular processing of (125)I- and (111)in-labeled epidermal growth factor (EGF) bound to cultured A431 tumor cells

Low molecular weight of epidermal growth factor (EGF) enables better intratumoral penetration in comparison with larger targeting proteins, but the cellular retention of EGF-associated radioactivity is poor for directly iodinated EGF. An attempt was made to improve intracellular retention by the use of metal-diethylenetriaminepentaacetic acid or nonphenolic linker (N-succinimidyl-para-iodobenzoate) as labeling agents. The use of nonphenolic linker did not improve retention of the radioactivity in A431 carcinoma cell line. The use of the radiometal label provided an appreciable prolongation of radioactivity residence inside the cell.

Radiolabeling of epidermal growth factor with 99mTc and in vivo localization following intracerebral injection into normal and glioma-bearing rats

High grade gliomas may have amplified expression of the epidermal growth factor receptor (EGFR) gene c-erb-B, which often is associated with increased expression of transmembrane EGFR. The purpose of the present study was to develop a method for labeling EGF with 99mTc and to determine whether the resulting radioligand would localize, following intracerebral injection, in rats bearing EGFR-positive gliomas. EGF has a relatively low molecular mass (approximately 6 kDa) compared to monoclonal antibodies, and this has allowed smaller bioconjugates, which should diffuse more rapidly within the brain and more effectively target disseminated glioma cells, to be constructed. In the present study, EGF has been labeled with either 131I or 99mTc, and in vitro uptake of the resulting radioligand has been investigated using C6EGFR rat glioma cells, which had been transfected with the EGFR gene. Cellular uptake of 131I radioactivity peaked after approximately 30 min of incubation with [131I]EGF, following which time it declined, while 99mTc radioactivity continued to increase over a 6 h incubation with [99mTc]-EGF. To determine if radiolabeled EGF had in vivo tumor-localizing properties, C6EGFR glioma cells were implanted stereotactically into the brains of Fischer rats. Four weeks later, either 99mTc- or 131I-labeled EGF was injected intracerebrally into normal or glioma-bearing animals using the same stereotactic coordinates. External gamma scintigraphy revealed that 131I radioactivity disappeared rapidly from the brain regions of tumor-bearing animals compared to 99mTc, approximately 50% of which remained in the tumor for up to 12 h. In contrast, only approximately 20% remained in the brains of non-tumor-bearing animals after 6 h. These studies are the first to describe a method for radiolabeling EGF with 99mTc and to detect it by external scintigraphy in the brains of tumor-bearing animals.

Development and in vitro studies of epidermal growth factor-dextran conjugates for boron neutron capture therapy

A delivery molecule for directed boron neutron capture therapy against epidermal growth factor (EGF) receptor-rich tumors, such as gliomas, squamous carcinomas, and breast cancers, is presented. EGF and sulfhydryl boron hydride (BSH) were covalently coupled to an allylated 70 kDa dextran chain to form a conjugate. Conjugates with low and high substitution rates of BSH, as well as without BSH, were investigated. The conjugate with a low amount of boron had approximately 6 BSH (72 boron atoms) per dextran, while the conjugates with higher amounts had an average substitution of 55 BSH (660 boron atoms) per dextran. The maximum substitution of boron to dextran in a single experiment was over 800 boron atoms. Binding, retention, and internalization of 125I-labeled conjugates were investigated on cultured human glioma cells. Binding of the conjugates was EGF receptor specific, but the amount of BSH coupled to dextran affected specificity, more than the presence of dextran. The nonspecific binding of the conjugates increased with the amount of attached boron. This was partly due to nonspecific adhesion to the plastic in the culture dishes. [125I]EGF-allyldextran with 6 BSH had a binding maximum after 4 h of continuous incubation and thereafter decreased in binding, while [125I]EGF-allyldextran with the higher substitution rate had a slow increase of binding during 24 h. Over 93% of the radioactivity bound to the cells was internalized, but the retention was quite poor. Only one-third of the cell-bound activity was still associated to the cells 4 h after incubation had ended. In conclusion, it is possible to load the conjugates produced with high amounts of boron, and they retained specificity for the EGF receptor and internalized into cultured cells. Theoretical calculations show that about 10(3) boron atoms per EGF-based conjugate are needed to give a satisfactory therapeutic response. These conjugates are within reach of that level.

In vitro determination of toxicity, binding, retention, subcellular distribution and biological efficacy of the boron neutron capture agent DAC-1

In boron neutron capture therapy (BNCT), 10B is delivered selectively to the tumour cells and the nuclide then forms high-LET radiation (4He2+ and 7Li3+) upon neutron capture. Today much research is focused on development of a variety of boron compounds aimed for BNCT. The compounds must be thoroughly analysed in preclinical tests regarding basic characteristics such as binding and subcellular distribution to enable accurate estimations of dose-modifying factors. DAC-1,2-[2-(3-amino-propyl)-1,2-dicarba-closo-dodecaboran (12)-1-yl-methoxy]- 1,3-propanediol was synthesized at our laboratories and the human colon carcinoma cells LS-174T were used as an in vitro model. The boron compound showed a remarkable intracellular accumulation, 20-100 times higher than the boron content in the culture medium, in cultured cells and was not removed by extensive washes. Approximately half of the boron taken up also remained within the cells for at least 4 days. The DAC-1 compound alone was not toxic at boron concentrations below 2.5 micrograms B/g. The intracellular distribution of the boron compound was investigated by subcellular fractionation experiments and low pH treatments. It is possible that DAC-1 binds to some intracellular molecules or to membranes connected with organelles in the cytoplasm or even to the inside of the outer cell membrane. Another possibility is that the compound, due to the somewhat lipophilic properties, is embedded in the membranes. Thermal neutron irradiations were carried out at the Brookhaven Medical Research Reactor (BMRR). At a survival level of 0.1, DAC-1 + thermal neutrons were about 10.5 times more effective in cell inactivation than the thermal neutrons alone. Monte Carlo calculations gave a mean value of the 10B-dependent specific energy, the dose, of 0.22 Gy. The total physical dose during irradiation of DAC-1-containing cells with a neutron fluence of 0.18 x 10(12) n/cm2 was 0.39 Gy. The dose-modifying factor, at survival level 0.1, when comparing irradiation with thermal neutrons with and without DAC-1 was 3.4, while the dose-modifying factor when comparing neutron irradiations of cells with DAC-1 and irradiation of the cells with 60Co-gamma was 7.3. The results are encouraging and in vivo tests of tissue distributions and tumour uptake should now be carried out.

Boronated epidermal growth factor as a potential targeting agent for boron neutron capture therapy of brain tumors

In order for boron neutron capture therapy (BNCT) to be successful, a large number (approximately 10(9)) of 10B atoms must be delivered to each cancer cell in order to sustain a lethal 10B(n, alpha)7Li reaction. The majority of high grade gliomas express an amplified epidermal growth factor receptor (EGFR) gene, and increased numbers of EGFR are found on the cell surface. If a sufficiently large number of 10B atoms could be attached to EGF, the resulting bioconjugates might be useful for targeting brain tumors. In order to accomplish this, we have boronated a fourth-generation starburst dendrimer (SD) using an isocyanato polyhedral borane, Na(CH3)3NB10H8NCO. For conjugation, reactive thiol groups were introduced into the boronated SD using N-succinimidyl 3-(2-pyridyldithio)propionate (SPDP), and EGF was derivatized with m-maleimidobenzoyl-N-hydroxysulfosuccinimide ester (sMBS). Subsequent reaction of thiol groups of derivatized BSD with maleimide groups of derivatized EGF produced stable BSD-EGF bioconjugates containing approximately 960 atoms of boron per molecule of EGF. As determined by electron spectroscopic imaging, the BSD-EGF initially was bound to the cell surface membrane and then was endocytosed, which resulted in accumulation of boron in lysosomes. The favorable in vitro properties of these bioconjugates suggest that they may be useful for the in vivo targeting of EGFR positive brain tumors.

Strategy for boron neutron capture therapy against tumor cells with over-expression of the epidermal growth factor-receptor

PURPOSE

Gliomas, squamous carcinomas and different adenocarcinomas from breast, colon and prostate might have an increased number of epidermal growth factor (EGF) receptors. The receptors are, in these cases, candidates for binding of receptor specific toxic conjugates that might inactivate cellular proliferation. The purpose of this study was to evaluate whether it is reasonable to try ligand-dextran based conjugates for therapy.

METHODS AND MATERIALS

EGF or TGF alpha were conjugated to dextran and binding, internalization, retention and degradation of eight types of such conjugates were analyzed in EGF-receptor amplified glioma cells. The conjugates were labelled with radioactive nuclides to allow detection and two of the conjugates were carrying boron in the form of carboranyl amino acids or aminoalkyl-carboranes. Comparative binding tests, applying 125I-EGF, were made with cultured breast, colon and prostate adenocarcinoma, glioma and squamous carcinoma cells. Some introductory tests to label with 76Br for positron emission tomography and with 131I for radionuclide therapy were also made.

RESULTS

The dextran part of the conjugates did not prevent receptor specific binding. The amount of receptor specific binding varied between the different types of conjugates and between the tested cell types. The dextran part improved intracellular retention and radioactive nuclides were retained for at least 20-24 h. The therapeutical effect improved when 131I was attached to EGF-dextran instead of native EGF.

CONCLUSION

The improved cellular retention of the ligand-dextran conjugates is an important property since it gives extended exposure time when radionuclides are applied and flexibility in the choice of time for application of neutrons in boron neutron capture therapy (BNCT). It is possible that ligand-dextran mediated BNCT might allow, if the applied neutron fields covers rather wide areas around the primary tumor, locally spread cells that otherwise would escape treatment to be inactivated.

Clonogenic cell survival and rejoining of DNA double-strand breaks: comparisons between three cell lines after photon or He ion irradiation

Three cell lines, human glioma U-343MG, Chinese hamster V79-379A and human colon carcinoma LS-174T, were analysed for clonogenic survival and with pulsed-field gel electrophoresis regarding induction and rejoining of double-strand breaks (dsb) in DNA. The cells were irradiated with either intermediate linear energy transfer (LET approximately 55 keV/microns) He ions or with 60Co photons and showed cell type-specific variations in their survival curves. The induction of dsb, per DNA unit, increased linearly with dose for both radiation qualities, and there were no cell type-dependent differences. However, the dsb induction frequency increased for all three cell lines with an RBE of 1.31 +/- 0.08 (SD) after He ion irradiation. All three cell lines showed biphasic dsb rejoining patterns with cell type-specific differences. The differences between the cell lines were somewhat smaller after irradiation with He ions, although the patterns were very similar to those seen after photon irradiation. The shoulders in the survival curves disappeared after He ion irradiations, however the cells retained the capacity to rejoin dsb. After 8-h rejoining the increase of LET to intermediate values did not give a higher fraction of residual dsb.

Radiation dose heterogeneity in receptor and antigen mediated boron neutron capture therapy

Boron neutron capture therapy, BNCT, might be a valuable tumour therapeutical modality for the treatment of cells that are difficult to handle with conventional methods such as surgery or external radiotherapy. The principle is that tumour associated 10B atoms capture thermal neutrons and thereby forms high-LET helium and lithium ions as reaction products. An interesting development is to conjugate 10B atoms to macromolecules that bind to tumour cells with over-expressed receptors or specific antigens. The targeting macromolecules might be receptor-ligands, antibodies or antibody-fragments containing 10B. The present study deals with the limitations of such an approach. One problem is the background dose from capture of neutrons in physiologically occurring elements, especially nitrogen. We showed, with computer simulations, that the background specific energy (the stochastic analogy of dose) in the cell nuclei, due to captures in nitrogen, had a wide spread and could be rather high, up to 3 Gy in some cells, when relevant neutron fluencies were applied. The maximal amount of 10B that can be delivered to single tumour cells due to receptor-ligand, receptor-antibody or antigen-antibody mediated binding is probably in the range 10(8)-10(10) atoms/cell. Our calculations showed that the tumour cells had to contain about 10(9) 10B/cell to give a therapeutically interesting dose to the nuclei of the targeted cells. The doses were highest when the boron was in the cell nucleus. There was also a wide spread of specific energy absorbed by the nuclei after neutron capture in 10B. When, for example, 10(8) 10(10)B/nucleus were applied the specific energy to the analysed nuclei varied from 0 Gy up to about 7 Gy. These variations were due to the stochastic nature of the capture processes. Some helium or lithium ion tracks passed through the centre of the cell nuclei delivering a lot of energy, some passed through only a smaller part delivering small amounts of energy and sometimes the nuclei escaped without any hits at all. The results were obtained when relevant neutron fluencies (2-5 x 10(12) n/cm2) were applied. Increased neutron fluencies gave higher doses both due to capture in boron and in nitrogen but in order to improve the ratio between the dose to targeted tumour cells and the dose to normal cells, the number of 10B atoms in the targeted cells had to be increased and/or the boron placed in the cell nuclei.

Internalization and excretion of epidermal growth factor-dextran-associated radioactivity in cultured human squamous-carcinoma cells

Certain tumor cells, such as squamous carcinomas and gliomas, can have an increased number of epidermal-growth-factor (EGF) receptors. The EGF receptors can in these cases be targets for toxic conjugates with specific binding. EGF-based toxic conjugates are potential targeting agents. We have analyzed the internalization and excretion of 125I administered in the form of 125I-EGF-dextran in squamous-carcinoma A431 cells. 125I-EGF without dextran was used for comparison. A431 cells have large numbers of EGF receptors and are capable both of recycling and of degradation of internalized receptor-ligand complexes. The binding of 125I-EGF-dextran and 125I-EGF was receptor-specific, since both ligands competed with non-radioactive EGF for binding. The amount of internalized 125I as a function of time increased continuously within 24 hr following administration of radioactivity as 125I-EGF-dextran. The time pattern was quite different when 125I-EGF without dextran was applied. In the latter case, the amount of internalized radioactivity decreased already after a few hours, probably depending on degradation of EGF. Pre-incubation of the cells with 125I-EGF-dextran or 125I-EGF and analysis of retained and released 125I activity at different times after washing showed that the 125I activity was retained for longer periods of time when EGF-dextran was used instead of EGF. About 30% of the internalized 125I activity was retained after 24 hr when EGF-dextran was used, compared with excretion of nearly all the radioactivity within 5 hr when EGF was used. In some experiments a high concentration of non-radioactive EGF, 5 micrograms/ml, was given to the cells after pre-incubation with 125I-EGF-dextran. This changed the retention and excretion patterns, so that a larger amount of 125I was excreted in the macromolecular fraction and a smaller amount of 125I activity was retained in the cells. Gel chromatography of the 125I activity released into the culture medium showed that the variations in molecular weight were larger after administration of a high concentration of non-radioactive EGF, most likely due to partial degradation of EGF-dextran. The results regarding excretion are in conformity with a model of competition between recycling of EGF-dextran-EGF-receptor complexes and "trapping" of EGF-dextran in the lysosomes followed by slow degradation. For targeting purposes, it is worth noting that the radioactivity administered in the form of 125I-EGF-dextran had a longer retention time than when 125I-EGF without dextran was used, and that the retention and excretion rates could be modified by post-treatment with the receptor ligand itself.

Radiation sensitization by estramustine studies on cultured human prostatic cancer cells

In low-stage prostatic carcinoma, local cure can be obtained with radiation therapy alone, while in locally advanced disease the chances for cure are less. In this study, we have addressed the question of whether estramustine (EM), the main cytostatic metabolite of estramustine phosphate (Estracyt), may act as a radiosensitizing agent. This drug accumulates in prostatic cancer and has also been shown to arrest cancer cells at metaphase both in vitro and in vivo. The human prostatic cancer cell line DU 145 was grown as cultures monolayer and incubated with EM in concentrations varying from 1 to 20 micrograms/ml. External beam irradiation was performed with doses ranging from 0 to 8 Gy using gamma rays from a 60Co source. Clonogenic cell survival (CS) was used to analyse the radiation sensitizing effect of EM. The radiation dose modifying factor (DMF) at the survival level 0.1 was found to be 0.77 in the presence of EM (5 micrograms/ml), i.e., 23% sensitization was obtained. When irradiating cells at the standard fraction dose of 2 Gy in the absence of EM, 22% of the cells lost their clonogenic ability. In presence of EM (5 micrograms/ml), 2 Gy caused 40% of the cells to lose their clonogenic ability. Thus a radiation sensitizing effect of EM was established in the CS assay. It was also of interest to determine if the radiosensitizing effect of EM could be confirmed in a rapid assay. The rapid fluorescence assay was used to observe early damage of the cells. Results showed that by 2 days after exposure to irradiation a weak tendency towards sensitization was seen, while a clear sensitization was obtained after 4 days. This indicates that the rapid assay might be developed to a predictive assay for detection of the response of primary prostate tumor cells to the radiation sensitizing effect of EM.

Present status of boron neutron capture therapy

The neutron capture reaction 10B(1n,4He)7Li produces two energetic particles, 4He2+ and 7Li3+ that are strongly cell toxic. Due to the short range of these nuclear fragments (5-9 microns) mainly those cells that have bound or internalized a 10B-containing substance are growth-inactivated. The most critical and difficult step in an efficient boron neutron capture therapy (BNCT) is the tumour targeting. It is today possible to synthesize a large number of boron compounds and conjugate them to tumour-seeking macromolecules, such as monoclonal antibodies or different polypeptides. The boron-containing substances presently considered for therapy are sulfhydryl boron hydride (BSH) and boron-phenylalanine, (BPA) for the treatment of gliomas and malignant melanomas respectively. Other boronated compounds considered are ligands for receptor-amplified tumour cells, antibodies for tumour cells with specific antigens and thioureas for treatment of melanotic melanomas. The required boron concentration is given by the relative dose due to neutron capture in 10B and that of the competing capture reactions in nitrogen and hydrogen. Capture in nitrogen produces protons with a range of about 10-11 microns and this gives a radiation dose to all cells in the neutron activated area. Calculations show that the local concentration of 10B near the critical radiation target, DNA, must be higher than 10 ppm (10 micrograms/g). Increased emphasis will be put on the development of combinations of treatments that fulfil the requirements for attacking the microscopic spread of the tumour.

Strategy for planned radiotherapy of malignant gliomas: postoperative treatment with combinations of high dose proton irradiation and tumor seeking radionuclides

A strategy for improved treatment of malignant gliomas grade III-IV is presented. The strategy can briefly be described as surgical removal of the bulky tumor, high precision external irradiation of small brain volumes over and near the primary tumor area with high doses from proton beams, and thereafter treatment of spread cells with toxic radionuclides. Proton beams suitable for this are under development. The clinical effects of high single doses on malignant gliomas grade III-IV are presently tested with conventional gamma radiation. Targeting of spread glioma cells with toxic radionuclides tagged to epidermal growth factor, EGF, or to EGF-dextran is presently tested in experimental systems and can, in the near future, be tested in combination with local high doses of external proton radiation. The possibilities to combine proton beams with EGF-guided neutron capture therapy will be considered in a longer perspective.

Influence of chloroquine and lidocaine on retention and cytotoxic effects of [131I]EGF: studies on cultured glioma cells

Targeting of toxic substances to the epidermal growth factor, EGF, receptor might be an attractive therapeutic approach because of the increased receptor-expression in some human tumours such as, for example, malignant gliomas and squamous lung carcinomas. Radiation effects of [131I]EGF on human malignant glioma cells growing as monolayers were analysed in this study. The cells were, in all cases, incubated for 25 min with about 350 kBq/ml [131I]EGF, which gave a total binding of 3.2-3.5 kBq/10(5) cells. The rapid release of activity from the cells caused by the normal degradation of EGF was inhibited by incubation with 30 microM chloroquine or 5 mM lidocaine added to the cell culture medium. These substances are, at these concentrations, known to inhibit proteolytic processes in lysosomes. No effects of the inhibitors alone were observed on cell growth and clonogenic survival. Inhibition of EGF degradation by chloroquine or lidocaine resulted in a significantly prolonged association of 131I with the test cells. About 70% of the initially bound radioactivity remained in the cells giving, after 6 h, a binding of 2.1-2.5 kBq/10(5) cells. A 6 h exposure to the radiation from 131I decays, mediated mainly by specifically bound EGF, gave a survival value of about 50%. Such an effect corresponds to a treatment of 2.5 Gy 60Co gamma-radiation. This is promising considering that, when monolayers are applied, only a very small fraction of the released energy from the 131I decays is deposited in the cells. Effects from non-receptor bound [131I]EGF were analysed after presaturation of the receptors with non-radioactive EGF, and gave no or very small changes in survival.