A class of iron chelators with a wide spectrum of potent antitumor activity that overcomes resistance to chemotherapeutics

Abstract

Novel chemotherapeutics with marked and selective antitumor activity are essential to develop, particularly those that can overcome resistance to established therapies. Iron (Fe) is critical for cell-cycle progression and DNA synthesis and potentially represents a novel molecular target for the design of new anticancer agents. The aim of this study was to evaluate the antitumor activity and Fe chelation efficacy of a new class of Fe chelators using human tumors. In this investigation, the ligands showed broad antitumor activity and could overcome resistance to established antitumor agents. The in vivo efficacy of the most effective chelator identified, di-2-pyridylketone-4,4,-dimethyl-3-thiosemicarbazone (Dp44mT), was assessed by using a panel of human xenografts in nude mice. After 7 weeks, net growth of a melanoma xenograft in Dp44mT-treated mice was only 8% of that in mice treated with vehicle. In addition, no differences in these latter animals were found in hematological indices between Dp44mT-treated mice and controls. No marked systemic Fe depletion was observed comparing Dp44mT- and vehicle-treated mice, probably because of the very low doses required to induce anticancer activity. Dp44mT caused up-regulation of the Fe-responsive tumor growth and metastasis suppressor Ndrg1 in the tumor but not in the liver, indicating a potential mechanism of selective anticancer activity. These results indicate that the novel Fe chelators have potent and broad antitumor activity and can overcome resistance to established chemotherapeutics because of their unique mechanism of action.

Fig. 1.

Structures of agents. (A) DFO and Triapine. (B) DpT. (C) PKIH analogs.

Fig. 2.

Dp44mT markedly inhibits clonogenic formation and overcomes resistance to other cytotoxic agents. (A) Dp44mT is more effective than DFO and DOX at preventing clonogenic formation in A2780 human ovarian carcinoma. Cells were treated with chelators or DOX for 48 h, and outgrowth was assessed after 10–15 days in agent-free medium. (B) The MCF-7/VP cell line is resistant to etoposide compared with their sensitive parental counterpart, MCF-7. In contrast, the antiproliferative activity of Dp44mT was equally potent in these cells. (C) KB-V1 cells are resistant to vinblastine compared with their parental counterpart, KB3-1, whereas the antiproliferative activity of Dp44mT is greater in the resistant clone than the sensitive cell. Results are mean ± SD from three experiments.

Fig. 3.

Dp44mT is highly effective at inhibiting growth of a range of human xenografts in nude mice. Dp44mT (0.75 mg/kg per day) is equally or more effective than Triapine (12 mg/kg per day) at inhibiting growth of DMS-53 lung carcinoma (A), SK-N-MC neuroepithelioma (B), and SK-Mel-28 melanoma (C) tumor xenografts in nude mice when administered i.v. once per day, 5 days/week for up to 2 weeks. Each point represents mean ± SEM from 6–12 mice. (D) Dp44mT (0.4 or 0.75 mg/kg per day) inhibit SK-Mel-28 tumor xenografts in nude mice when given i.v. once per day, 5 days/week for up to 7 weeks. Each point represents mean ± SEM from six mice. (E) Photograph of SK-Mel-28 melanoma tumors taken from mice after 7 weeks with the vehicle or Dp44mT (0.4 mg/kg per day) using the regimen in D.

Fig. 4.

Effect of Dp44mT and Triapine on spleen, heart, and liver histology. Histology from nude mice bearing DMS-53 lung carcinoma xenografts and treated i.v. with control (A), Dp44mT (0.75 mg/kg per day) (B), or Triapine (12 mg/kg per day) (C) once per day, 5 days/week for 2 weeks. (i) Hematoxylin and eosin-stained spleen. Asterisks denote increased hematopoiesis. (Magnification: ×40.) (ii) Perls-stained spleen. Asterisks denote increased Fe(III) deposits consistent with hemosiderin. (Magnification: ×200.) (iii) Gomori–Trichrome-stained cardiac tissue. Arrows denote fibrosis. (Magnification: ×20.) (iv) Hematoxylin and eosin-stained liver. Arrows denote hematopoietic cells. (Magnification: ×100.) (v) Perls-stained liver. Arrows denote Fe(III) deposits consistent with hemosiderin in Kupffer cells. (Magnification: ×100.)

Fig. 5.

Administration of Dp44mT and Triapine to mice up-regulates the growth and metastasis suppressor Ndrg-1 in tumor xenografts but not the liver. Expression of the Fe-regulated genes Ndrg1, TfR1, and VEGF1 is down-regulated in liver (A) and up-regulated in tumor (B) after treatment of nude mice bearing DMS-53 xenografts with Dp44mT (0.75 mg/kg per day) or Triapine (12 mg/kg per day) once per day, 5 days/week for 2 weeks. The densitometric results are mean ± SD from three experiments. *, P < 0.05; **, P < 0.01; ***, P < 0.001.