Radiobiological and dosimetric assessment of DNA-intercalated 99mTc-complexes bearing acridine orange derivatives

Abstract

Background Recently, a new family of 99m Tc(I)-tricarbonyl complexes bearing an acridine orange (AO) DNA targeting unit and different linkers between the Auger emitter ( 99m Tc) and the AO moiety was evaluated for Auger therapy. Among them, 99m Tc-C 3 places the corresponding radionuclide at a shortest distance to DNA and produces important double strand breaks (DSB) yields in plasmid DNA providing the first evidence that 99m Tc can efficiently induce DNA damage when well positioned to the double helix. Here in, we have extended the studies to human prostate cancer PC3 cells using the 99m Tc-C 3 and 99m Tc-C 5 complexes, aiming to assess how the distance to DNA influences the radiation-induced biological effects in this tumoral cell line, namely, in which concerns early and late damage effects. Results Our results highlight the limited biological effectiveness of Auger electrons, as short path length radiation, with increasing distances to DNA. The evaluation of the radiation-induced biological effects was complemented with a comparative microdosimetric study based on intracellular dose values. The comparative study, between MIRD and Monte Carlo (MC) methods used to assess the cellular doses, revealed that efforts should be made in order to standardize the bioeffects modeling for DNA-incorporated Auger electron emitters. Conclusions 99m Tc might not be the ideal radionuclide for Auger therapy but can be useful to validate the design of new classes of Auger-electron emitting radioconjugates. In this context, our results highlight the crucial importance of the distance of Auger electron emitters to the target DNA and encourage the development of strategies for the fine tuning of the distance to DNA for other medical radionuclides (e.g., 111 In or 161 Tb) in order to enhance their radiotherapeutic effects within the Auger therapy of cancer.

Validation of the plasmid study to relate DNA damaging effects of radionuclides to those from external beam radiotherapy

“…In this study, kinetic modelling has enabled us to determine the duration of time required to incubate with [ 67 Ga]GaCl 3 or [ 111 In]InCl 3 to induce the same amount of plasmid DNA damage as a specific amount of absorbed radiation dose by EBRT (and vice versa). Traditionally, in vitro or preclinical RBE values are based on clonogenic survival data or studies looking at spermatogenesis in mouse testes [ 49 – 51 ]. These values are then used to inform radiation weighting doses, which in turn are applied to calculate the radiation equivalent dose and finally the effective dose.…”

Synthesis and Biological Evaluation of 99mTc(I) Tricarbonyl Complexes Dual-Targeted at Tumoral Mitochondria

“…Towards this goal, several research groups have studied radiolabeled DNA intercalators to promote accumulation of AE in the cell nucleus in closer proximity with DNA [ 3 ]. For this purpose, we studied 99m Tc(I) tricarbonyl complexes carrying acridine orange (AO) derivatives as DNA intercalating groups [ 5 , 6 , 7 ]. Our results with the 99m Tc(I)-AO complexes have shown that 99m Tc can cause significant radiation-induced DNA damage, both in vitro in plasmid models and in vivo in human prostate cancer PC3 cells [ 6 , 7 ].…”

“…For this purpose, we studied 99m Tc(I) tricarbonyl complexes carrying acridine orange (AO) derivatives as DNA intercalating groups [ 5 , 6 , 7 ]. Our results with the 99m Tc(I)-AO complexes have shown that 99m Tc can cause significant radiation-induced DNA damage, both in vitro in plasmid models and in vivo in human prostate cancer PC3 cells [ 6 , 7 ]. Noticeably, we have proved that there is a marked dependence of the biological effectiveness of 99m Tc Auger electrons on the 99m Tc-DNA distance, even for a relatively small increase of such distance (e.g., 10.80 Å vs. 12.92 Å), as estimated by molecular modelling simulations of the DNA-intercalated complexes [ 6 ].…”

Metal-Based G-Quadruplex Binders for Cancer Theranostics

“…The design of G4 binders using medical radiometals can be considered an attractive approach to explore this type of DNA-targeted molecules in cancer theranostics. For this purpose, a plethora of radiometals suitable for SPECT is available (e.g., 99m Tc "m" stands for metastable, 67 Ga, 111 In) or PET (e.g., 68 Ga, 89 Zr) imaging and for systemic targeted radionuclide therapy (e.g., 90 Y, 177 Lu, 225 Ac). These nuclear medicine modalities have already made an enormous contribution to the diagnosis and treatment of cancer in the clinical onset, based on the use of appropriate radioactive compounds designated as radiopharmaceuticals.…”

“…As mentioned above, Paulo et al performed a detailed and multidisciplinary investigation of the radiation-induced effects of 99m Tc-complexes and structurally related 125 I-labelled derivatives, aiming to assess the influence of the distance to DNA and nature of the radionuclide on the DNA damage. This research study included the spectroscopic characterization of DNA interaction, assessment of DNA damage in vitro and in living cells, cellular and nuclear internalization in tumor cells and computational studies [176,177]. Docking simulations revealed that the compounds 118 (I = 125 I, n = 3, Figure 37A) and 124 (M = 99m Tc, n = 1, Figure 37A) place the corresponding radionuclide at relatively similar distances to the axis of the DNA double helix (10.49 and 10.80 Å, respectively).…”