<h3>Background</h3> The main limits of current antitumor therapies are chemoresistance, relapses and toxic effects that impair a patient's quality of life. Therefore, the discovery of therapeutic alternatives, such as adjuvants to conventional therapy that modulate the intracellular oxidation state, or the immune response activation remains a challenge.^{1 2} Although the antitumor effects of <i>P. nigrum</i> are well-documented, studies on its potential immunomodulatory effects are limited.^{3 4} This study assessed the antitumor and immunomodulatory activities of an ethanolic extract of <i>P. nigrum</i> in murine models of 4T1 breast cancer and B16-F10 melanoma. <h3>Methods</h3> The <i>P. nigrum</i> extract was prepared by maceration with ethanol and analyzed chromatographically by UPLC-PDA and LC-MS-Q-TOF. The cytotoxic activity of the extract was evaluated against 4T1 and B16-F10 cells and against murine fibroblasts to calculate the selectivity index. Additionally, other biological parameters were measured such as induction of cell death, effect on reactive oxygen species, and modulation of glucose uptake. For in vivo assays, BALB/c and C57BL/6 mice were injected subcutaneously with 4T1 and B16-F10 cells, respectively, and were subsequently treated twice a week with <i>P. nigrum</i> extract or PBS as a control. Subsequently, the tumor and spleen were removed to evaluate different immune cell populations using flow cytometry.⁵ <h3>Results</h3> Chemical characterization showed that the <i>P. nigrum</i> extract has piperines as main components. <i>In vitro</i> evaluations showed that the <i>P. nigrum</i> extract exhibits cytotoxic activity, induces apoptotic cell death, and has a pro-oxidant effect in both cell lines. However, it differentially regulates glucose uptake, decreasing it in 4T1 cells but not in B16-F10 cells. In the in vivo assays, the extract significantly reduced tumor size in both models and also decreased macrometastases in the 4T1 breast cancer model compared to the control group. Analysis of tumor infiltrate by flow cytometry revealed that the extract significantly increases the frequency of dendritic cells and activated CD8+ T cells, while decreasing myeloid-derived suppressor-like cells and regulatory T cells in both models.⁵ <h3>Conclusions</h3> The <i>P. nigrum</i> extract presented a promising antitumor effect, possibly due to the induction of an effector immune response and the modulation of the suppressive immune response.⁵ <h3>Acknowledgements</h3> The authors would like to thank Colombian Environmental Ministry for allowing the use of genetic resources through the Contract of Access to Genetic Resources No. 220 of 2018 for Colombian plant material. Additionally, the authors express their thanks to Pontificia Universidad Javeriana, Ministerio de Ciencia, Tecnología e Innovación, Ministerio de Educación Nacional, Ministerio de Industria, Comercio y Turismo and ICETEX, 2ª Convocatoria Ecosistema Científico - Colombia Científica 792-2017, Program 'Generación de alternativas terapéuticas en cáncer a partir de plantas a través de procesos de investigación y desarrollo traslacional, articulados en sistemas de valor sostenibles ambiental y económicamente' (Contract no. FP44842-221-2018). <h3>References</h3> Gomez-Cadena A, Barreto A, Fioretino S, Jandus C. Immune system activation by natural products and complex fractions: a network pharmacology approach in cancer treatment. <i>Cell Stress</i> 2020;<b>4</b>:154–166. Khan AW, Farooq M, Haseeb M, Choi S. Role of plant-derived active constituents in cancer treatment and their mechanisms of action. <i>Cells</i> 2022;<b>11</b>:1326. Turrini E, Sestili P, Fimognari C. Overview of the Anticancer Potential of the 'King of Spices' Piper nigrum and Its Main Constituent Piperine. <i>Toxins (Basel)</i> 2020;<b>26</b>:12(12):747. Stojanović-Radić Z, Pejčić M, Dimitrijević M, Aleksić A, V Anil Kumar N, Salehi B, C Cho W, Sharifi-Rad J. Piperine-A major principle of black pepper: a review of its bioactivity and studies.<i> Applied Sciences</i> 2019;<b>9</b>(20):4270. Lasso P, Rojas L, Arévalo C, Urueña C, Murillo N, Nossa P, Sandoval T, Chitiva LC, Barreto A, Costa GM, Fiorentino S. Piper nigrum extract suppresses tumor growth and enhances the antitumor immune response in murine models of breast cancer and melanoma. <i>Cancer Immunol Immunother</i> 2023;<b>72</b>(10):3279–3292. <h3>Ethics Approval</h3> This study was approved by Ethics Committee of the Faculty of Sciences, Pontificia Universidad Javeriana (PUJ) and the animal experimentation committee of PUJ; approval number FUA-093-20.