Auger radiation targeted into DNA: a therapy perspective

BACKGROUND: Auger electron emitters that can be targeted into DNA of tumour cells represent an attractive systemic radiation therapy goal. In the situation of DNA-associated decay, the high linear energy transfer (LET) of Auger electrons gives a high relative biological efficacy similar to that of alpha particles. In contrast to alpha radiation, however, Auger radiation is of low toxicity when decaying outside the cell nucleus, as in cytoplasm or outside cells during blood transport. The challenge for such therapies is the requirement to target a high percentage of all cancer cells. An overview of Auger radiation therapy approaches of the past decade shows several research directions and various targeting vehicles. The latter include hormones, peptides, halogenated nucleotides, oligonucleotides and internalising antibodies. DISCUSSION: Here, we will discuss the basic principles of Auger electron therapy as compared with vector-guided alpha and beta radiation. We also review some radioprotection issues and briefly present the main advantages and disadvantages of the different targeting modalities that are under investigation

Short fluorodeoxyuridine exposure of different human glioblastoma lines induces high-level accumulation of S-phase cells that avidly incorporate 125I-iododeoxyuridine.

PURPOSE: Radio-iododeoxyuridine (IdUrd) is a potential Auger radiation therapy agent incorporated into DNA during the synthesis phase. In this study we sought to optimise S-phase targeting by modulating cellular cycling and radio-IdUrd DNA incorporation using short non-toxic fluorodeoxyuridine (FdUrd) incubations. METHODS: Three human glioblastoma cell lines with different p53 expression were pre-treated with various FdUrd conditions. After different intervals, (125)I-IdUrd DNA incorporation was measured. Fluorescence-activated cell sorter cell cycle analysis was performed after identical intervals post FdUrd pre-treatment. RESULTS: The highest increase in (125)I-IdUrd DNA incorporation was induced by 1-h incubation with 1 muM FdUrd. Increase in radio-IdUrd DNA incorporation was greatest 16-24 h after FdUrd, reaching factors of >or=7.5 over baseline incorporation in the three cell lines. Furthermore, cell synchronisation in S phase was observed with a peak of >or=69.5% in the three cell lines at 16 and 24 h post FdUrd, corresponding to an increase of 2.5-4.1 over baseline. CONCLUSION: FdUrd-induced thymidine synthesis inhibition led to S-phase accumulation that was maximal after an interval of 16-24 h and time-correlated with the highest radio-IdUrd DNA incorporation. These observations might allow the rational design of an Auger radiation therapy targeting a maximal number of S-phase cells in single treatment cycles

Study of the incorporation and the anti-tumour efficacy of radio-iododeoxyuridine according to the cellular cycle and in presence of synthesis inhibitor of thymidine

Résumé La iododeoxyuridine (IdUrd), une fois marqué au 123I ou au 125I, est un agent potentiel pour des thérapies par rayonnements Auger. Cependant, des limitations restreignent son incorporation dans l'ADN. Afin d'augmenter celle-ci, différents groupes ont étudié la fluorodeoxyuridine (FdUrd), qui favorise l'incorporation d'analogue de la thymidine, sans toutefois parvenir à une toxicité associé plus importante. Dans notre approche, 3 lignées cellulaires de glioblastomes humains et une lignée de cancer ovarien ont été utilisées. Nous avons observé, 16 à 24 h après un court pré-traitement à la FdUrd, un fort pourcentage de cellules s'accumulant en phase S. Plus qu'une accumulation, c'était une synchronisation des cellules, celles-ci restant capables d'incorporer la radio-IdIrd et repartant dans le cycle cellulaire. De plus, ces cellules accumulées après un pré-traitement à la FdUrd étaient plus radio-sensibles. Après le même intervalle de 16 à 24 h suivant la FdUrd, les 4 lignées cellulaires ont incorporé des taux plus élevés de radio-IdUrd que sans ce prétraitement. Une corrélation temporelle entre l'accumulation des cellules en phase S et la forte incorporation de radio-IdUrd a ainsi été révélée 16 à 24 h après pré-traitement à la FdUrd. Les expériences de traitement par rayonnements Auger sur les cellules accumulées en phase S ont montré une augmentation significative de l'efficacité thérapeutique de 125I-IdUrd comparé aux cellules non prétraitées à la FdUrd. Une première estimation a permis de déterminer que 100 désintégrations de 125I par cellules étant nécessaires afin d'atteindre l'efficacité thérapeutique. De plus, p53 semble jouer un rôle dans l'induction directe de mort cellulaire après des traitements par rayonnements Auger, comme indiqué par les mesures par FACS d'apoptose et de nécrose 24 et 48 h après le traitement. Concernant les expériences in vivo, nous avons observé une incorporation marquée de la radio-IdUrd dans l'ADN après un pré-traitement à la FdUrd dans un model de carcinomatose ovarienne péritonéale. Une augmentation encore plus importante a été observée après injection intra-tumorale dans des transplants sous-cutanés de glioblastomes sur des souris nues. Ces modèles pourraient être utilisés pour de plus amples études de diffusion de radio-IdUrd et de thérapie par rayonnement Auger. En conclusion, ce travail montre une première application réussie de la FdUrd afin d'accroître l'efficacité de la radio-IdUrd par traitements aux rayonnements Auger. La synchronisation des cellules en phase S combinée avec la forte incorporation de radio-IdUrd dans l'ADN différées après un pré-traitement à la FdUrd ont montré le gain thérapeutique attendu in vitro. De plus, des études in vivo sont tout indiquées après les observations encourageantes d'incorporation de radio-IdUrd dans les models de transplants sous-cutanés de glioblastomes et de tumeurs péritonéales ovariennes. Summary Iododeoxyuridine (IdUrd), labelled with 123I or 125I, could be a potential Auger radiation therapy agent. However, limitations restrict its DNA incorporation in proliferating cells. Therefore, fluorodeoxyuridine (FdUrd), which favours incorporation of thymidine analogues, has been studied by different groups in order to increase radio-IdUrd DNA incorporation, however therapeutic efficacy increase could not be reached. In our approach, 3 human glioblastoma cell lines with different p53 expression and one ovarian cancer line were pre-treated with various FdUrd conditions. We observed a high percentage of cells accumulating in early S phase 16 to 24 h after a short and non-toxic FdUrd pre-treatment. More than an accumulation, this was a synchronization, cells remaining able to incorporate radio-IdUrd and re-entering the cell cycle. Furthermore, the S phase accumulated cells post FdUrd pre-treatment were more radiosensitive. After the same delay of 16 to 24 h post FdUrd pre-treatment, the 4 cell lines were incorporating higher rates of radio-IdUrd compared with untreated cells. A time correlation between S phase accumulation and high radio-IdUrd incorporation was therefore revealed 16 to 24 h post FdUrd pre-treatment. Auger radiation treatment experiments performed on S phase enriched cells showed a significant increase of killing efficacy of 125I-IdUrd compared with cells not pre-treated with FdUrd. A first estimation indicates further that about 100 125I decays were required to reach killing in the targeted cells. Moreover, p53 might play a role on the direct induction of cell death pathways after Auger radiation treatments, as indicated by differential apoptosis and necrosis induction measured by FACS 24 and 48 h after treatment initiation. Concerning in vivo results, we observed a marked DNA incorporation increase of radio-IdUrd after FdUrd pre-treatment in peritoneal carcinomatosis in SCID mice. Even higher incorporation increase was observed after intra-tumoural injection of radio-IdUrd in subcutaneous glioblastoma transplants in nude mice. These tumour models might be further useful for diffusion of radio-IdUrd and Auger radiation therapy studies. In conclusion, these data show a first successful application of thymidine synthesis inhibition able to increase the efficacy of radio-IdUrd Auger radiation treatment. The S phase synchronization combined with a high percentage DNA incorporation of radio-IdUrd delayed post FdUrd pre-treatment provided the expected therapeutic gain in vitro. Further in vivo studies are indicated after the observations of encouraging radio-IdUrd uptake experiments in glioblastoma subcutaneous xenografts and in an ovarian peritoneal carcinomatosis model

Contribution of PaTrin-2 in Radiotherapy with Temozolomide for the Treatment of Glioblastoma Expressing MGMT

Purpose/Objective(s): Current standard treatment of glioblastoma is radiotherapy (RT) concomitant with temozolomide (TMZ), an alkylating agent. O6-methylguanine-DNA methyltransferase (MGMT) expression is a major mechanism of resistance to Proceedings of the alkylating agent chemotherapy, and MGMT gene promoter methylation (present in 30-45 % of tumors) has been shown to be predictive for tumor response to TMZ therapy. MGMT, an exhaustible repair protein can be depleted by specific inhibitors such as O6- benzylguanine or the non-toxic O6-(4-bromothenyl)guanine (PaTrin-2). Here we have studied the efficacy of the combination of TMZ, RT, and PaTrin-2 to improve the treatment outcome in glioblastoma expressing MGMT. Materials/Methods: 3 glioblastoma lines were chosen: LN18 and T98G expressing MGMT and U251 lacking MGMT expression. A shRNA approach was used to selectively and permanently knockdown level of MGMT in LN18 line. Cells were treated with 10 mM PaTrin-2. After 2 h, various concentrations of TMZ were added, cells were incubated for 24 h, and clonogenic assays were performed. After the same PaTrin-2 pretreatment and 100 mM TMZ exposure, cells were plated 4 h before irradiation with increasing RT doses of up to 6 Gy. Clonogenic survival was assessed after 14 days. Results: Western blot analysis confirmed that reduction of MGMT expression was achieved in LN18A1 expressing MGMT-targeting shRNA. The shRNA non-targeting control sequence did not influenceMGMTprotein level (LN18NT). PaTrin-2 showed no toxicity at 10 mMon the 5 cell lines. TMZ induced up to 70 and 97%of cell death on LN18A1 and U251, respectively, but was not toxic up to 50 mMfor T98G, LN18, and LN18NT. Up to 53%increased TMZ toxicity was observed on the 5 cell lines when treated with the 2 drugs. Irradiation of the 5 lines treated or not with PaTrin-2 showed no survival difference at any irradiation dose. When LN18A1 and U251 cells were irradiated post TMZ treatment, an up to 2.5 and 139.4 fold increase in toxicity, respectively, was observed compared to un-pretreated controls. By contrast, TMZ pretreatment did not increase irradiation toxicity on T98G, LN18, and LN18NT. When cells were incubated with PaTrin-2 and TMZ before the irradiation, up to 3.7, 3.9, 5.8, 6.6 and 348.5 fold increase in toxicity was observed compared to controls on LN18, LN18NT, LN18A1, T98G and U251, respectively. Conclusions: We present here results of TMZ and PaTrin-2 combination ± RT on glioblastoma lines. U251 and LN18A1 cells were much more sensitive to TMZ than LN18, LN18NT, and T98G. PaTrin-2 enhanced the toxicity of TMZ on the MGMT expressing glioblastoma lines. RT further increased TMZ and PaTrin-2 efficacy. These results are encouraging for the treatment of patients with glioblastoma expressing MGMT who have the worst prognosis and respond poorly to RT combined with TMZ

Fluorodeoxyuridine mediated cell cycle synchronization in S-phase increases the Auger radiation cell killing with 125I-iododeoxyuridine.

Aim: 125I-iododeoxyuridine is a potential Auger radiation therapy agent. Its incorporation in DNA of proliferating cells is enhanced by fluorodeoxyuridine. Here, we evaluated therapeutic activities of 125I-iododeoxyuridine in an optimized fluorodeoxyuridine pre-treatment inducing S-phase synchronization. Methods: After S-phase synchronization by fluorodeoxyuridine, cells were treated with 125I-iododeoxyuridine. Apoptosis analysis and S-phase synchronization were studied by flow cytometry. Cell survival was determined by colony-forming assay. Based on measured growth parameters, the number of decays per cell that induced killing was extrapolated. Results: Treatment experiments showed that 72 to 91% of synchronized cells were killed after 0.8 and 8 kBq/ml 125I-iododeoxyuridine incubation, respectively. In controls, only 8 to 38% of cells were killed by corresponding 125I-iododeoxyuridine activities alone and even increasing the activity to 80 kBq/ml gave only 42 % killing. Duplicated treatment cycles or repeated fluorodeoxyuridine pre-treatment allowed enhancing cell killing to >95 % at 8 kBq/ml 125I-iododeoxyuridine. About 50 and 160 decays per S-phase cells in controls and S-phase synchronization, respectively, were responsible for the observed cell killing at 0.8 kBq/ml radio-iododeoxyuridine. Conclusion: These data show the successful application of fluorodeoxyuridine that provided increased 125I-iododeoxyuridine Auger radiation cell killing efficacy through S-phase synchronization and high DNA incorporation of radio-iododeoxyuridine

18F-FLT and 125I-IdUrd uptake increase in human tumour cell lines induced by the thymidylate synthase inhibitor FdUrd.

Aim: 5-fluoro-2'-deoxyuridine (FdUrd) depletes the endogenous 5'-deoxythymidine triphosphate (dTTP) pool. We hypothesized whether uptake of exogenous dThd analogues could be favoured through a feedback enhanced salvage pathway and studied the FdUrd effect on cellular uptake of 3'-deoxy-3'-18F-fluorothymidine (18F-FLT) and 5-125I-iodo-2'-deoxyuridine (125I-IdUrd) in different cancer cell lines in parallel. Methods: Cell uptake of 18F-FLT and 125I-IdUrd was studied in 2 human breast, 2 colon cancer and 2 glioblastoma lines. Cells were incubated with/without 1 µmol/l FdUrd for 1 h and, after washing, with 1.2 MBq 18F-FLT or 125I-IdUrd for 0.3 to 2 h. Cell bound 18F-FLT and 125I-IdUrd was counted and expressed in % incubated activity (%IA). Kinetics of 18F-FLT cell uptake and release were studied with/without FdUrd modulation. 2'-3H-methyl-fluorothymidine (2'-3H-FLT) uptake with/without FdUrd pretreatment was tested on U87 spheroids and monolayer cells. Results: Basal uptake at 2 h of 18F-FLT and 125I-IdUrd was in the range of 0.8-1.0 and 0.4-0.6 Bq/cell, respectively. FdUrd pretreatment enhanced 18F-FLT and 125I-IdUrd uptake 1.2-2.1 and 1.7-4.4 fold, respectively, while co-incubation with excess thymidine abrogated all 18F-FLT uptake. FdUrd enhanced 18F-FLT cellular inflow in 2 breast cancer lines by factors of 1.8 and 1.6, respectively, while outflow persisted at a slightly lower rate. 2'-3H-FLT basal uptake was very low while uptake increase after FdUrd was similar in U87 monolayer cells and spheroids. Conclusions: Basal uptake of 18F-FLT was frequently higher than that of 125I-IdUrd but FdUrd induced uptake enhancement was stronger for 125I-IdUrd in five of six cell lines. 18F-FLT outflow from cells might be an explanation for the observed difference with 125I-IdUrd