*Article* **<sup>225</sup>Ac-rHDL Nanoparticles: A Potential Agent for Targeted Alpha-Particle Therapy of Tumors Overexpressing SR-BI Proteins**

**Tania Hernández-Jiménez 1,2, Guillermina Ferro-Flores 1,\* , Enrique Morales-Ávila 2,\* , Keila Isaac-Olivé 3 , Blanca Ocampo-García 1 , Liliana Aranda-Lara <sup>3</sup> , Clara Santos-Cuevas <sup>1</sup> , Myrna Luna-Gutiérrez <sup>1</sup> , Laura De Nardo <sup>4</sup> , Antonio Rosato 5,6 and Laura Meléndez-Alafort <sup>6</sup>**


**Citation:** Hernández-Jiménez, T.; Ferro-Flores, G.; Morales-Ávila, E.; Isaac-Olivé, K.; Ocampo-García, B.; Aranda-Lara, L.; Santos-Cuevas, C.; Luna-Gutiérrez, M.; De Nardo, L.; Rosato, A.; et al. <sup>225</sup>Ac-rHDL Nanoparticles: A Potential Agent for Targeted Alpha-Particle Therapy of Tumors Overexpressing SR-BI Proteins. *Molecules* **2022**, *27*, 2156. https://doi.org/10.3390/ molecules27072156

Academic Editors: Alessandra Boschi and Petra Martini

Received: 15 February 2022 Accepted: 23 March 2022 Published: 27 March 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**Abstract:** Actinium-225 and other alpha-particle-emitting radionuclides have shown high potential for cancer treatment. Reconstituted high-density lipoproteins (rHDL) specifically recognize the scavenger receptor B type I (SR-BI) overexpressed in several types of cancer cells. Furthermore, after rHDL-SR-BI recognition, the rHDL content is injected into the cell cytoplasm. This research aimed to prepare a targeted <sup>225</sup>Ac-delivering nanosystem by encapsulating the radionuclide into rHDL nanoparticles. The synthesis of rHDL was performed in two steps using the microfluidic synthesis method for the subsequent encapsulation of <sup>225</sup>Ac, previously complexed to a lipophilic molecule ( <sup>225</sup>Ac-DOTA-benzene-p-SCN, CLog P = 3.42). The nanosystem (13 nm particle size) showed a radiochemical purity higher than 99% and stability in human serum. In vitro studies in HEP-G2 and PC-3 cancer cells (SR-BI positive) demonstrated that <sup>225</sup>Ac was successfully internalized into the cytoplasm of cells, delivering high radiation doses to cell nuclei (107 Gy to PC-3 and 161 Gy to HEP-G2 nuclei at 24 h), resulting in a significant decrease in cell viability down to 3.22 ± 0.72% for the PC-3 and to 1.79 <sup>±</sup> 0.23% for HEP-G2 at 192 h after <sup>225</sup>Ac-rHDL treatment. After intratumoral <sup>225</sup>Ac-rHDL administration in mice bearing HEP-G2 tumors, the biokinetic profile showed significant retention of radioactivity in the tumor masses (90.16 ± 2.52% of the injected activity), which generated ablative radiation doses (649 Gy/MBq). The results demonstrated adequate properties of rHDL as a stable carrier for selective deposition of <sup>225</sup>Ac within cancer cells overexpressing SR-BI. The results obtained in this research justify further preclinical studies, designed to evaluate the therapeutic efficacy of the <sup>225</sup>Ac-rHDL system for targeted alpha-particle therapy of tumors that overexpress the SR-BI receptor.

**Keywords:** scavenger receptor B type I; reconstituted high-density lipoproteins; actinium-225; <sup>225</sup>Ac-rHDL; targeted alpha-particle therapy
