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

Abstract

For effective Auger therapy of cancer, the Auger-electron emitters must be delivered to the tumor cells in close proximity to a radiosensitive cellular target. Nuclear DNA is considered the most relevant target of Auger electrons to have augmented radiotoxic effects and significant cell death. However, there is a growing body of evidence that other targets, such as the mitochondria, could be relevant subcellular targets in Auger therapy. Thus, we developed dual-targeted 99mTc(I) tricarbonyl complexes containing a triphenylphosphonium (TPP) moiety to promote accumulation of 99mTc in the mitochondria, and a bombesin peptide to provide specificity towards the gastrin releasing peptide receptor (GRPr) overexpressed in prostate cancer cells. The designed dual-targeted complex, 99mTc-TPP-BBN, is efficiently internalized by human prostate cancer PC3 cells through a specific GRPr-mediated mechanism of uptake. Moreover, the radioconjugate provided an augmented accumulation of 99mTc in the mitochondria of the target tumor cells, most probably following its intracellular cleavage by cathepsin B. In addition, 99mTc-TPP-BBN showed an enhanced ability to reduce the survival of PC3 cells, in a dose-dependent manner.

Metal-Based G-Quadruplex Binders for Cancer Theranostics

“…Most relevantly, the presence of a triglycine (Gly-Gly-Gly) linker between the BBN [ 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ] sequence and the pyrazolyl-diamine chelator had a determinant influence on the nuclear internalization of the (radio)conjugates 129 and 131 , which was almost negligible when a Ser-Gly-Ser linker was used instead of Gly-Gly-Gly. Most probably, Gly-Gly-Gly acts as a cathepsin B cleavable linker that leads to the release of an AO-containing Re(I)/ 99m Tc(I) fragment inside the cells with better ability to reach the cell nucleus, as corroborated recently by in vitro enzymatic assays performed by the same authors with Re(I) tricarbonyl complexes [ 184 ]. The success of this dual-targeting strategy encourages the application of similar approaches to deliver classical DNA intercalators or G4-binders to the nucleus of tumor cells, aiming to achieve more selective and efficient anticancer therapies.…”

Searching for a Paradigm Shift in Auger-Electron Cancer Therapy with Tumor-Specific Radiopeptides Targeting the Mitochondria and/or the Cell Nucleus

“…Thus, we considered the same propylenic linker to attach the AO to the dual-targeted chelator AO-Pz-BBN used to obtain the M-AO-BBN (M = Re and 99m Tc) complexes evaluated in this work (Figure 1). As for the BBN peptide, it contains the G3-BBN [7][8][9][10][11][12][13][14] sequence with a triglycine (G3) linker cleavable by cathepsin B, as we recently demonstrated for the Re-BBN complex [27]. The presence of the G3 linker is crucial to obtain an augmented accumulation of 99m Tc or AO chromophore in the nucleus of PC3 cells for dual-targeted M(I) (M = Re and 99m Tc) tricarbonyl complexes similar to M-AO-BBN.…”

“…The obtained ESI(+) mass spectra confirmed the formation of the desired conjugates being observed the expected molecular ion peaks with isotope distributions in agreement with the proposed formulations. The complex 99m Tc-AO-BBN was synthesized by the reaction of the fac-[ 99m Tc(CO)3(H2O)3] + precursor with the dual-targeted chelator AO-Pz-BBN in an aqueous medium (Scheme 2), as we described previously for 99m Tc-BBN and 99m Tc-TPP-BBN [27]. All these 99m Tc complexes were purified by HPLC prior to their evaluation in the several biological studies reported in this work to separate the respective cold chelators.…”

“…Most studies have focused on mitochondria-targeted radiosensitizers for external beam radiation therapy (EBRT), using triphenylphosphonium (TPP) derivatives or small peptides [26]. In contrast, studies with mitotropic therapeutic radioconjugates, namely those carrying AE emitters, are scarce and almost limited to the work that we recently reported for the dual-targeted radioconjugate 99m Tc-TPP-BBN (TPP = triphenylphosphonium; BBN = bombesin derivative) [27].…”

“…Thus, with this in mind, we designed the dual-targeted 99m Tc-TPP-BBN as an AE-emitting mitotropic therapeutic radioconjugate carrying: (i) a BBN sequence to target the gastrin-releasing peptide receptor (GRPR) and promote a selective uptake in GRPR (+) human prostate cancer PC3 cells and (ii) a TPP pharmacophore to provide an increased accumulation in the mitochondria of PC3 cells (Figure 1). Our initial studies showed that 99m Tc-TPP-BBN caused a remarkably high reduction in the survival of human prostate cancer PC3 cells when compared with the singletargeted congener 99m Tc-BBN [27]. This difference was attributed to the enhanced uptake of 99m Tc-TPP-BBN in the mitochondria and the consequent irradiation of this radiosensitive organelle with the AEs emitted by 99m Tc, which present a high-energy deposition and ultra-short trajectories.…”