Benzothiazole–Triazole–Indazole Conjugates: Design, Characterization, and Preliminary In Vitro MTT-Based Antiproliferative Evaluation

A new library of benzothiazole–triazole–indazole conjugates (5a–s) was efficiently synthesized via a Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC) strategy. The approach involved the preparation of alkynylated indazoles and benzothiazole–azide derivatives, followed by click-chemistry cycloaddition to provide triazole-linked heterocyclic hybrids in good-to-excellent yields (72–95%), demonstrating the robustness of the modular synthetic design. In silico evaluation using SwissADME indicated that several compounds display physicochemical profiles potentially compatible with early-stage hit exploration, although the relatively high TPSA values suggest that membrane permeability may be limited for some derivatives. The antiproliferative activity of conjugates 5a–s was assessed against A2780 (ovarian), A549 (lung), and MDA-MB-231 (breast) cancer cell lines using the MTT assay. Several conjugates exhibited greater in vitro antiproliferative activity toward A2780 cells, with 5r emerging as the most active derivative of the series (IC50 = 15.7 ± 1.6 μM). In addition, 5b showed notable antiproliferative activity against the MDA-MB-231 cell line (IC50 = 10.8 ± 2.9 μM), highlighting cell-line-dependent antiproliferative profiles. Preliminary structure–activity relationship analysis suggested that electronic effects alone do not fully account for the observed antiproliferative activity, and that substituent position, steric factors, and cell-line-dependent factors may also play important roles in modulating biological response. Given the micromolar activity, the MTT-based nature of the assay, and the absence of non-tumoral cell line data, these compounds are best regarded as chemical starting points for further optimization and mechanistic investigation.