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We describe herein the synthesis of eight new ester-coupled hybrid compounds from thymoquinone and protoflavone building blocks, and their bioactivity testing against multiple cancer cell lines. Among the hybrids, compound 14 showed promising activities in all cell lines studied. The highest activities were recorded against breast cancer cell lines with higher selectivity to MDA-MB-231 as compared to MCF-7. Even though the hybrids were found to be completely hydrolysed in 24 h under cell culture conditions, compound 14 demonstrated a ca. three times stronger activity against U-87 glioblastoma cells than a 1:1 mixture of its fragments. Further, compound 14 showed good tumour selectivity: it acted 4.4-times stronger on U-87 cells than on MRC-5 fibroblasts. This selectivity was much lower, only ca. 1.3-times, when the cells were co-treated with a 1:1 mixture of its non-coupled fragments. Protoflavone-thymoquinone hybrids may therefore serve as potential new antitumor leads particularly against glioblastoma.</div

Calculated IC₅₀ values of the structural combination (hybrid compounds) and experimental combination of the corresponding thymoquinone and protoflavone building blocks on the U-87 cells.

Calculated IC50 values of the structural combination (hybrid compounds) and experimental combination of the corresponding thymoquinone and protoflavone building blocks on the U-87 cells.</p

Antiproliferative effects of thymoquinone-protoflavone hybrids (10–17) and their building blocks (2–4 and 6–9) on cancer cell lines.

Antiproliferative effects of thymoquinone-protoflavone hybrids (10–17) and their building blocks (2–4 and 6–9) on cancer cell lines.</p

Antiproliferative effects of selected compounds on MRC-5 non-cancerous human lung fibroblast cell line and the calculated selectivity indices (SI) of the tested compounds against U-87 glioblastoma cell lines; SI = IC₅₀ (MRC-5)/IC₅₀ (U-87).

Antiproliferative effects of selected compounds on MRC-5 non-cancerous human lung fibroblast cell line and the calculated selectivity indices (SI) of the tested compounds against U-87 glioblastoma cell lines; SI = IC50 (MRC-5)/IC50 (U-87).</p

Scheme 2.

Preparation of the protoflavone fragments (6–9) and the hybrids (10–17). Reaction conditions: a. PIFA/AcN: ROH; 9:1/70°C/1h; b. DDC/DMAP/dry DCM/0°C.</p

Calculated IC₅₀ values of the hybrid; compound 14 and its building blocks (4 and 6) on the tested cancer cell lines.

*Calculated IC50 values of compound 4 on MCF-7 and HeLa cells were above 100 μM.</p

Scheme 1.

Preparation of TQ (2) and its derivatives; compounds 3 and 4. Reaction conditions: a. mCPBA/CHCl3/rt; a’. PIFA/AcN: H2O; 9:1/rt/1h; b. succinic acid/ AgNO3/(NH4)2S2O8/AcN/H2O/100°C.</p