In-vitro Assessment of the Proliferation and Apoptosis of Thyroid Cancer Cells Using Valproic Acid and Retinoic Acid Alone and in Combination with Etoposide and Epirubicin.

Background: Thyroid cancer remains a significant global health concern, necessitating the development of more effective treatment strategies. Methods: This study investigated the therapeutic potential of valproic acid (VA) and retinoic acid (RA), both as single agents and in combination with conventional chemotherapeutics etoposide (Et) and epirubicin (Ep), using in vitro models of thyroid cancer. The research employed two representative cell lines: B-CPAP (poorly differentiated thyroid carcinoma) and SW-1736 (anaplastic thyroid carcinoma). Through comprehensive experimental approaches, including MTT viability assays, flow cytometry-based apoptosis and cell cycle analysis, and scratch wound migration assays, we systematically evaluated the compounds' effects. Results: The results revealed distinct pharmacological profiles: The RA demonstrated superior cytotoxicity with significantly lower IC50 values (3.01 µg/mL for B-CPAP and 1.83 µg/mL for SW) compared to VA (407.29 µg/mL and 584.32 µg/mL, respectively). Importantly, RA exhibited strong synergistic effects when combined with Et/Ep [Combination Index (CI) < 1], while VA showed primarily additive or antagonistic interactions (CI ≥ 1). Mechanistically, RA combined with low-dose Et/Ep (1/5 IC50) significantly enhanced early apoptosis rates (P < 0.05) and induced S-phase cell cycle arrest, effects not observed with VA combinations. In migration assays, RA completely inhibited cancer cell movement (100% inhibition), outperforming VA's partial inhibition (40 - 70%). Conclusions: These findings collectively demonstrate RA's potent anticancer activity and its ability to synergize with conventional chemotherapeutics, highlighting its potential as a promising candidate for combination therapy in thyroid cancer treatment. The study provides compelling preclinical evidence supporting further investigation of RA-based therapeutic strategies through advanced preclinical studies and clinical trials.