[en] The gastrin-releasing peptide receptor (GRP-r) is over-expressed in prostate and breast cancer. ^99mTc-Bombesin (^99mTc-Bn) has been reported as a radiopharmaceutical with specific cell GRP-r binding. The HIV Tat(49-57)-derived peptide has been used to deliver a large variety of molecules to cell nuclei. New hybrid radiopharmaceuticals of type ^99mTc-N₂S₂-Tat(49-57)-Lys³-Bn (^99mTc-Tat-Bn) and ¹⁸⁸Re-N₂S₂-Tat(49-57)-Lys³-Bn (¹⁸⁸Re-Tat-Bn), would increase cell uptake and internalized in cancer cell nuclei could act as an effective system of targeted radiotherapy using Auger and internal conversion (I C) electron emissions near DNA. The aim of this research was to prepare and assess in vitro and in vivo uptake kinetics in cancer cells of ^99mTc/¹⁸⁸Re-Tat-Bn and the in vitro nucleus and cytoplasm internalization kinetics in GRP receptor-positive cancer cells as well as to evaluate the subcellular-level radiation absorbed dose associated with the observed effect on cancer cell DNA proliferation. Structures of N₂S₂-Tat-Bn and Tc/Re(O)N₂S₂-Tat-Bn were calculated by an Mm procedure. ^99mTc-Tat-Bn and ¹⁸⁸Re-Tat-Bn were synthesized and stability studies carried out by HPLC and I TLC-Sg analyses in serum and cysteine solutions. In vitro internalization was tested using human prostate cancer Pc 3 cells and breast carcinoma cell lines MDA-Mb 231 and MCF 7. Nuclei from cells were isolated using a nuclear extraction kit. Total disintegrations in each subcellular compartment were calculated by integration of experimental time activity kinetic curves. Nucleus internalization was corroborated by con focal microscopy images using immunofluorescent labelled Tat-Bn. Biodistribution was determined in Pc 3 tumor-bearing nude mice. The Penelope code was used to simulate and calculate the absorbed dose by contribution of β, Auger and I C electrons in the cytoplasm and nucleus using geometric models built from immunofluorescent cell images. A cell proliferation kit was used to evaluate DNA concentration after cancer cell incubation with ^99mTc-Tat-Bn. Results showed a minimum energy of 271 kcal/mol for N₂S₂-Tat-Bn and 300 kcal/mol for Re/Tc(O)N₂S₂-Tat-Bn. ^99mTc/¹⁸⁸Re-Tat-Bn radiochemical purity was >90%. In vitro studies demonstrated stability in serum and cysteine solutions, specific cell receptor binding and internalization in three cell lines was significantly high. The tumor-to muscle radioactivity ratio was 8.5 for ^99mTc-Tat-Bn. Also it showed that 59.7%, 61.2% and 41.5% of total disintegrations per unit of ^99mTc-Tat-Bn activity (1 Bq) bound to the cell occurred in the nucleus of Pc 3, MCF 7 and MDA-Mb 231, respectively and 63.36%, 65.70% and 45.55% of total disintegrations per 1 Bq of ¹⁸⁸Re-Tat-Bn bound to the cell occurred in the nucleus of these three cell lines respectively. The ^99mTc-Tat-Bn absorbed doses delivered to nuclei were 0.142 mGy/Bq·s (Pc 3), 0.434 Gy/Bq·s (MCF 7) and 0.276 mGy/Bq·s (MDA-Mb 231) and from ¹⁸⁸Re-Tat-Bn were 0.156 mGy/Bq·s (Pc 3); 0.368 mGy/Bq·s (MCF 7) and 0.251 mGy/Bq·s (MDA-Mb 231). ^99mTc-Tat-Bn produced a significant decrease in cellular proliferation of Pc 3 (52.98%), MCF 7 (45.71%) and MDA-Mb 231 (35.80%) with respect to untreated cells. These hybrid radiopharmaceuticals are potentially useful in imaging and therapy of breast and prostate cancer. (Author)