Synthesis, In Vitro Antitumor Activity, and In Silico ADMET Evaluation of β-Lapachone-Based Thiosemicarbazones

Background/Objectives: β-Lapachone and triapine are compounds with recognized antitumor potential—the former is an ortho-naphthoquinone, and the latter a thiosemicarbazone inhibitor of ribonucleotide reductase. This study aimed to synthesize and evaluate new β-lapachone-based thiosemicarbazones (TSC1–TSC6) as potential antineoplastic agents. Methods: Lapachol was isolated from Tabebuia sp. and used to obtain ortho-naphthoquinones (2–4), which served as precursors for thiosemicarbazones (TSC1–TSC6). NMR and HRMS spectra were used to characterize the compounds. Their cytotoxic activity was evaluated in vitro against murine melanoma (B16–F10), colon carcinoma (CT26.WT), and breast cancer (4T1) cell lines, as well as normal fibroblasts (L929). Pharmacokinetic parameters were predicted in silico using ADMETLab 3.0. Results: β-Lapachone exhibited strong cytotoxicity toward tumor cells with moderate effects on normal cells, while thiosemicarbazones of β-lapachone, TSC1, and TSC3 demonstrated lower potency but greater selectivity. The β-lapachone-3-sulfonic acid showed high activity against melanoma and breast cancer cells and low toxicity toward normal cells, indicating tumor selectivity. In contrast, their thiosemicarbazones, TSC2, TSC4, and TSC6, showed weak or no antiproliferative activity. The 3-iodo-β-lapachone was cytotoxic to both tumor and normal cells, whereas its derivative TSC5 demonstrated moderate activity with reduced toxicity. β-Lapachone, β-lapachone-3-sulfonic acid, TSC1, and TSC3 exhibited favorable ADME profiles (QED ≈ 0.61–0.66), suggesting good oral bioavailability. Conclusions: The β-lapachone-3-sulfonic acid and the β-lapachone-based thiosemicarbazones TSC1 and TSC3 emerged as promising lead candidates, combining tumor selectivity, favorable pharmacokinetic properties, and structural innovation for the development of safer and more effective antineoplastic agents.