Sandra Deisenhofer

Is 3'-deoxy-3'- 18 F-fluorothymidine a better marker for tumour response than 18 F-fluorodeoxyglucose?

Abstract3′-Deoxy-3′-18F-fluorothymidine (FLT) was developed in 1998 by Shields and co-workers because monitoring of treatment response would be facilitated by imaging agents able to provide measures of tissue and tumour proliferation. Since then, FLT metabolism has been clarified in more detail in cell culture and experimental animal tumour models and also in clinical studies. Recently, FLT has increasingly been used for the assessment of response to anticancer treatment, mainly in tumour xenograft SCID mouse models; in contrast, clinical data are scarce. In this article we briefly summarise the intermediary metabolism of FLT and its application as an anticancer treatment response probe. The potential value and limitations of FLT as a highly promising proliferation imaging probe and its use for monitoring of treatment response are discussed.

123I-ITdU-mediated nanoirradiation of DNA efficiently induces cell kill in HL60 leukemia cells and in doxorubicin-, beta-, or gamma-radiation-resistant cell lines.

Resistance to radiotherapy or chemotherapy is a common cause of treatment failure in high-risk leukemias. We evaluated whether selective nanoirradiation of DNA with Auger electrons emitted by 5-123I-iodo-4′-thio-2′-deoxyuridine (123I-ITdU) can induce cell kill and break resistance to doxorubicin, β-, and γ-irradiation in leukemia cells. Methods: 4′-thio-2′-deoxyuridine was radiolabeled with 123/131I and purified by high-performance liquid chromatography. Cellular uptake, metabolic stability, DNA incorporation of 123I-ITdU, and the effect of the thymidylate synthase (TS) inhibitor 5-fluoro-2′-deoxyuridine (FdUrd) were determined in HL60 leukemia cells. DNA damage was assessed with the comet assay and quantified by the olive tail moment. Apoptosis induction and irradiation-induced apoptosis inhibition by benzoylcarbonyl-Val-Ala-Asp-fluoromethyl ketone (z-VAD.fmk) were analyzed in leukemia cells using flow cytometry analysis. Results: The radiochemical purity of ITdU was 95%. Specific activities were 900 GBq/μmol for 123I-ITdU and 200 GBq/μmol for 131I-ITdU. An in vitro cell metabolism study of 123I-ITdU with wild-type HL60 cells demonstrated an uptake of 1.5% of the initial activity/106 cells of 123I-ITdU. Ninety percent of absorbed activity from 123I-ITdU in HL60 cells was specifically incorporated into DNA. 123I-ITdU caused extensive DNA damage (olive tail moment > 12) and induced more than 90% apoptosis in wild-type HL60 cells. The broad-spectrum inhibitor of caspases zVAD-fmk reduced 123I-ITdU–induced apoptosis from more than 90% to less than 10%, demonstrating that caspases were central for 123I-ITdU–induced cell death. Inhibition of TS with FdUrd increased DNA uptake of 123I-ITdU 18-fold and the efficiency of cell kill about 20-fold. In addition, 123I-ITdU induced comparable apoptotic cell death (>90%) in sensitive parental leukemia cells and in leukemia cells resistant to β-irradiation, γ-irradiation, or doxorubicin at activities of 1.2, 4.1, 12.4, and 41.3 MBq/mL after 72 h. This finding indicates that 123I-ITdU breaks resistance to β-irradiation, γ-irradiation, and doxorubicin in leukemia cells. Conclusion: 123I-ITdU–mediated nanoirradiation of DNA efficiently induced apoptosis in sensitive and resistant leukemia cells against doxorubicin, β-irradiation, and γ-irradiation and may provide a novel treatment strategy for overcoming resistance to conventional radiotherapy or chemotherapy in leukemia. Cellular uptake and cell kill are highly amplified by inhibiting TS with FdUrd.

Preferential tumor targeting and selective tumor cell cytotoxicity of 5-[131/125I]iodo-4'-thio-2'-deoxyuridine.

Purpose: Auger electron emitting radiopharmaceuticals are attractive for targeted nanoirradiation therapy, provided that DNA of malignant cells is selectively addressed. Here, we examine 5-[123/125/131I]iodo-4′-thio-2′-deoxyuridine (ITdU) for targeting DNA in tumor cells in a HL60 xenograft severe combined immunodeficient mouse model. Experimental Design: Thymidine kinase and phosphorylase assays were done to determine phosphorylation and glycosidic bond cleavage of ITdU, respectively. The biodistribution and DNA incorporation of ITdU were determined in severe combined immunodeficient mice bearing HL60 xenografts receiving pretreatment with 5-fluoro-2′-deoxyuridine (FdUrd). Organ tissues were dissected 0.5, 4, and 24 h after radioinjection and uptake of [131I]ITdU (%ID/g tissue) was determined. Cellular distribution of [125I]ITdU was imaged by microautoradiography. Apoptosis and expression of the proliferation marker Ki-67 were determined by immunohistologic staining using corresponding paraffin tissue sections. Results: ITdU is phosphorylated by thymidine kinase 1 and stable toward thymidylate phosphatase-mediated glycosidic bond cleavage. Thymidylate synthase-mediated deiodination of [123/125/131I]ITdU was inhibited with FdUrd. Pretreatment with FdUrd increased preferentially tumor uptake of ITdU resulting in favorable tumor-to-normal tissue ratios and tumor selectivity. ITdU was exclusively localized within the nucleus and incorporated into DNA. In FdUrd-pretreated animals, we found in more than 90% of tumor cells apoptosis induction 24 h postinjection of ITdU, indicating a highly radiotoxic effect in tumor cells but not in cells of major proliferating tissues. Conclusion: ITdU preferentially targets DNA in proliferating tumor cells and leads to apoptosis provided that the thymidylate synthase is inhibited.