In vivo dosimetry allows for a specified total whole-body radiation dose to be delivered accurately. This schedule of intensification of (131)I-mIBG therapy by dose escalation and radiosensitization with topotecan with a haemopoietic autograft is safe and practicable. This approach should now be tested for efficacy in a phase II clinical trial.

Aims: We investigated whether radiation-induced nontargeted effects are involved in the cytotoxic effects of anticell surface monoclonal antibodies labeled with Auger electron emitters, such as iodine 125 (monoclonal antibodies labeled with 125I [125I-mAbs]). Results: We showed that the cytotoxicity of 125I-mAbs targeting the cell membrane of p53+/+ HCT116 colon cancer cells is mainly due to nontargeted effects. Targeted and nontargeted cytotoxicities were inhibited in vitro following lipid raft disruption with Methyl-β-cyclodextrin (MBCD) or filipin or use of radical oxygen species scavengers. 125I-mAb efficacy was associated with acid sphingomyelinase activation and modulated through activation of the AKT, extracellular signal-related kinase ½ (ERK1/2), p38 kinase, c-Jun N-terminal kinase (JNK) signaling pathways, and also of phospholipase C-γ (PLC-γ), proline-rich tyrosine kinase 2 (PYK-2), and paxillin, involved in Ca2+ fluxes. Moreover, the nontargeted response induced by directing 5-[(125)I]iodo-2′-deoxyuridine to the nucleus was comparable to that of 125I-mAb against cell surface receptors. In vivo, we found that the statistical significance of tumor growth delay induced by 125I-mAb was removed after MBCD treatment and observed oxidative DNA damage beyond the expected Auger electron range. These results suggest the involvement of nontargeted effects in vivo also. Innovation: Low-energy Auger electrons, such as those emitted by 125I, have a short tissue range and are usually targeted to the nucleus to maximize their cytotoxicity. In this study, we show that targeting the cancer cell surface with 125I-mAbs produces a lipid raft-mediated nontargeted response that compensates for the inferior efficacy of non-nuclear targeting. Conclusion: Our findings describe the mechanisms involved in the efficacy of 125I-mAbs targeting the cancer cell surface. Antioxid. Redox Signal. 25, 467–484.

Meta-iodobenzylguanidine conjugated to 131 I-iodine is an methods: (1) survival of clonogens derived from monolayer effective agent for the targeted radiotherapy of tumors of culture; (2) survival of clonogens derived from disaggreneural crest origin which express the noradrenaline transgated multicellular spheroids; and (3) spheroid growth porter (NAT). The therapeutic application of 131 I MIBG is delay. 131 I MIBG was twice as toxic to cells in spheroids presently limited to the treatment of phaeochromocytoma, compared with those in monolayers, consistent with a neuroblastoma, carcinoid and medullary thyroid carcigreater effect of radiation cross-fire (radiological bystander noma. To determine the feasibility of MIBG targeting for a effect) from 131 I ␤-radiation in the three-dimensional tumor wider range of tumor types, we employed plasmidspheroids. The highest concentration of 131 I MIBG tested mediated transfer of the NAT gene into a human glioblas-(1 MBq/ml) was nontoxic to UVW control cells or spheroids toma cell line (UVW) which does not express the NAT transfected with the NAT gene in reverse orientation. gene. This resulted in a 15-fold increase in uptake of MIBG These findings are encouraging for the development of by the host cells. A dose-dependent toxicity of 131 I MIBG NAT gene transfer-mediated 131 I MIBG therapy. to the transfectants was demonstrated using three

Disulfiram has been used for several decades in the treatment of alcoholism. It now shows promise as an anticancer drug and radiosensitizer. Proposed mechanisms of action include the induction of oxidative stress and inhibition of proteasome activity. Our purpose was to determine the potential of disulfiram to enhance the antitumor efficacy of external-beam g-irradiation and 131 I-metaiodobenzylguanidine ( 131 I-MIBG), a radiopharmaceutical used for the therapy of neuroendocrine tumors. Methods: The role of copper in disulfiram-induced toxicity was investigated by clonogenic assay after treatment of human SK-N-BE (2c) neuroblastoma and UVW/noradrenaline transporter (NAT) glioma cells. The synergistic interaction between disulfiram and radiotherapy was evaluated by combination-index analysis. Tumor growth delay was determined in vitro using multicellular tumor spheroids and in vivo using human tumor xenografts in athymic mice. Results: Escalating the disulfiram dosage caused a biphasic reduction in the surviving fraction of clonogens. Clonogenic cell kill after treatment with disulfiram concentrations less than 4 mM was copper-dependent, whereas cytotoxicity at concentrations greater than 10 mM was caused by oxidative stress. The cytotoxic effect of disulfiram was maximal when administered with equimolar copper. Likewise, disulfiram radiosensitization of tumor cells was copper-dependent. Furthermore, disulfiram treatment enhanced the toxicity of 131 I-MIBG to spheroids and xenografts expressing the noradrenaline transporter. Conclusion: The results demonstrate that the cytotoxicity of disulfiram was copper-dependent, the molar excess of disulfiram relative to copper resulted in attenuation of disulfiram-mediated cytotoxicity, copper was required for the radiosensitizing activity of disulfiram, and copper-complexed disulfiram enhanced the efficacy not only of external-beam radiation but also of targeted radionuclide therapy in the form of 131 I-MIBG. Therefore, disulfiram may have anticancer potential in combination with radiotherapy.

The combination of topotecan and [(131)I]MIBG compared with either treatment alone gave rise to greater than additive DNA damage, clonogenic cell kill, and tumor growth delay. These effects were dependent on the scheduling of the two agents.

Radiolabeled benzamides are attractive candidates for targeted radiotherapy of metastatic melanoma as they bind melanin and exhibit high tumor uptake and retention. One such benzamide, N-(2-diethylaminoethyl)-4-(4-fluoro-benzamido)-5-iodo-2-methoxy-benzamide (MIP-1145), was evaluated for its ability to distinguish melanin-expressing from amelanotic human melanoma cells, and to specifically localize to melanin-containing tumor xenografts. The binding of [ 131

No abstract

Summary Radiolabelled meta-iodobenzylguanidine (MIBG) is selectively taken up by tumours of neuroendocrine origin, where its cellular localization is believed to be cytoplasmic. The radiopharmaceutical [1311]MIBG is now widely used in the treatment of neuroblastoma, but other radioconjugates of benzylguanidine have been little studied. We have investigated the cytotoxic efficacy of beta, alpha and Auger electronemitting radioconjugates in treating neuroblastoma cells grown in monolayer or spheroid culture. Using a no-carrier-added synthesis route, we produced 1231-, 1251-, 1311-and 211At-labelled benzylguanidines and compared their in vitro toxicity to the neuroblastoma cell line SK-N-BE(2c) grown in monolayer and spheroid culture. The Auger electron-emitting conjugates ([1231]MIBG and [1251]MIBG) and the alpha-emitting conjugate ([211At]MABG) were highly toxic to monolayers and small spheroids, whereas the beta-emitting conjugate [1311]MIBG was relatively ineffective. The Auger emitters were more effective than expected if the cellular localization of MIBG is cytoplasmic. As dosimetrically predicted however, [211At]MABG was found to be extremely potent in terms of both concentration of radioactivity and number of atoms ml-' administered. In contrast, the Auger electron emitters were ineffective in the treatment of larger spheroids, while the beta emitter showed greater efficacy. These findings suggest that short-range emitters would be well suited to the treatment of circulating tumour cells or small clumps, whereas beta emitters would be superior in the treatment of subclinical metastases or macroscopic tumours. These experimental results provide support for a clinical strategy of combinations ('cocktails') of radioconjugates in targeted radiotherapy.Keywords: meta-iodobenzylguanidine; neuroblastoma; tageted radiotherapy; astatine Meta-iodobenzylguanidine (MIBG) is a structural analogue of the neuroadrenergic blocking drugs bretylium and guanethidine. It is selectively accumulated by an active mechanism in cells of neural crest origin. Radiolabelled MIBG allows the scintigraphic imaging of neural crest tumours (Weiland et al, 1980) and ['311]MIBG is used in the treatment of neuroblastoma and phaeochromocytoma (Hartmann et al, 1987;Mastrangelo, 1987;Schwabe et al, 1987;Voute et al, 1988). Clinical experience since 1984 has demonstrated its potential, with an objective response rate of 35% in patients with progressive heavily pretreated disease (Hoefnagel, 1994). Clinical studies are now evaluating the role of ['311]MIBG at an earlier stage in therapy, either as a first line treatment or in combination with other treatment modalities (DeKraker et al, 1995;Gaze et al, 1995;Mastrangelo et al, 1995).Although many patients show beneficial responses to ['311]MIBG treatment, a significant number subsequently relapse from previously undetected tumour sites (Sisson et al, 1989). This suggests that microtumours below the limit of clinical detectability have survived ['311]MIBG therapy. A possible explanation for the rela...