Xymedon Activates the Immune Response in Breast Cancer Xenografts

<b>Background/Objectives:</b> Breast cancer remains a major cause of cancer-related mortality among women worldwide, highlighting the need for new therapeutic strategies. Pyrimidine derivatives have shown promise in oncology due to their ability to modulate immune responses and influence tumor growth pathways. <b>Methods:</b> Cytotoxicity of Xymedon was evaluated using MTT and colony formation assays on cancer MCF-7, NCI-H322M, HCT-15 cells, and primary human foreskin fibroblasts. In vivo efficacy was assessed in an orthotopic MCF-7 xenograft model in female Balb/c nude mice. Xymedon was administered orally at 410 mg/kg daily alone or in combination with intraperitoneal doxorubicin (1 mg/kg weekly). Hematological, histological, and immunohistochemical analyses were performed. <b>Results:</b> In vitro, Xymedon (up to 3 mM) showed no cytotoxicity against cancer cell lines or human skin fibroblasts. In vivo, Xymedon significantly increased tumor necrosis (44.1% vs. 28.5%, <i>p</i> < 0.01) and enhanced intratumoral infiltration of CD3+, CD8+, and CD20+ lymphocytes, with peritumoral counts increasing 2.2-5.3-fold. It mitigated Doxorubicin-induced myelosuppression by improving red blood cell counts, hemoglobin, and hematocrit levels, while platelet recovery remained limited. Combination therapy with Xymedon did not affect tumor volume or weight, but resulted in a non-significant trend toward improved survival (80% vs. 30%, <i>p</i> ≈ 0.11; Hazard Ratio [HR] = 0.268, 95% CI: 0.07082 to 1.012) without affecting fibrous capsule formation. <b>Conclusions:</b> These results suggest that Xymedon is a non-cytotoxic immunomodulator with potential as an adjuvant to enhance antitumor immunity and reduce hematologic toxicity associated with chemotherapy. Further studies are needed to elucidate the molecular pathways and confirm clinical efficacy.