Nowadays, radionuclides play an important role in oncology, especially in the developing field of targeted radionuclide therapy (TRT). In this master thesis, the focus is to evaluate different radiolabeled targeting molecules for human epidermal growth factor receptor 2 (HER2) positive breast cancer and metastatic castration-resistant prostate cancer (mCRPC). The chosen targeting molecules are Herceptin® (trastuzumab) and PSMA-617, which will specifically target HER2- and PSMA-receptors, respectively. The HER2- and PSMA-receptors are excellent targets for use in targeted radionuclide therapy and should be explored further to achieve better and more efficient treatments than current treatments and for obtaining more personalized treatment regimens for patients. The β-emitter lutetium-177 (177Lu) has been thoroughly investigated with both trastuzumab and PSMA-617, but the use of the α-emitter actinium-225 (225Ac) is less studied with these targeting molecules. Although the 225Ac-PSMA-617 has been tested in patients, there is still a need for more knowledge about actinium-225, and a deeper understanding of the use of 225Ac in PSMA-617 and trastuzumab with regard to radiolabeling conditions, stability and their effects on cancer cells. This work has resulted in a successful synthesis of 225Ac-DOTA-trastuzumab, 225Ac-PSMA-617, [177Lu]Lu-DOTA-trastuzumab and [177Lu]Lu-PSMA-617 with high radiochemical yields (> 90 %). A new radiolabeling routine was obtained for 225Ac-PSMA-617 giving a yield of 98 % with good reproducibility. Furthermore, the stability of each radiolabeled conjugate has been thoroughly investigated and showed that all radiolabeled conjugates had a shelf-life of five days in the temperature range of 5-37 °C, except for 225Ac-DOTA-trastuzumab which only showed high stability over two days. Moreover, radiolysis effects were not observed for 225Ac-PSMA-617 samples with specific activities up to 216 kBq/µg, showing high stabilities over two weeks with approximately 90 % radiochemical purity. For 225Ac-DOTA-trastuzumab, only the samples with the lowest specific activities (< 1357 kBq/mg) were unaffected by radiolysis. The cell binding assays resulted in high cell-binding values of 94 % and 93 % for 225Ac-DOTA-trastuzumab and [177Lu]Lu-DOTA-trastuzumab on SKBR-3 cells. Furthermore, cell-binding values of 84 % and 85 % were obtained for 225Ac-PSMA-617 and [177Lu]Lu-PSMA-617 on LNCaP cells. Cell viability assays were performed and showed that 225Ac-PSMA-617 (in the range 2.5-600 Bq) is a more efficient therapeutic agent than [177Lu]Lu-PSMA-617 (in the range 50-400 kBq). The cell viability assays also confirmed that 225Ac-DOTA-trastuzumab has the potential of being a more efficient agent for the treatment of HER2-positive breast cancer than non-radioactive Herceptin®. The results of this thesis provide a significant contribution to the field of targeted alpha therapy (TAT) through studying the radiolabeling with 225Ac, the stability of 225Ac-labeled conjugates, their binding affinities and their ability to deplete cancer cells of ATP.