CIGB-300 (P15-Tat) is a cell-permeable cyclic peptide that modulates CK2 (CSNK2) activity by binding to the phosphoacceptor site on CK2 targets, abrogates the CK2 phosphorylation by blocking recombinant substrates in vitro.CIGB-300 (P15-Tat) induced apoptosis as evidenced by rapid caspase activation and cellular cytotoxicity in a variety of tumor cell lines (HSCLC H-82 cell IC50=20 uM).CIGB-300 (P15-Tat) demonstrated substantial regression of the tumor mass C57BL6 mice bearing day 7-established solid tumors.CIGB-300 (P15-Tat) reduced breast cancer cell growth in MDA-MB-231, MCF-7 and F3II cells, exerting a pro-apoptotic action and cell cycle arrest. CIGB-300 decreased cell adhesion, migration and clonogenic capacity of malignant cells.CIGB-300 may interfere with the SARS-CoV-2 life cycle within infected human cells.