Carboranyl Analogues of Mefenamic Acid and Their Biological Evaluation

Abstract

Mefenamic acid represents a widely used nonsteroidal anti-inflammatory drug (NSAID) to treat the pain of postoperative surgery and heavy menstrual bleeding. Like other NSAIDs, mefenamic acid inhibits the synthesis of prostaglandins by nonselectively blocking cyclooxygenase (COX) isoforms COX-1 and COX-2. For the improved selectivity of the drug and, therefore, reduced related side effects, the carborane analogues of mefenamic acid were evaluated. The ortho-, meta-, and para-carborane derivatives were synthesized in three steps: halogenation of the respective cluster, followed by a Pd-catalyzed B-N coupling and hydrolysis of the nitrile derivatives under acidic conditions. The COX inhibitory activity and cytotoxicity for different cancer cell lines revealed that the carborane analogues have stronger antitumor potential compared to their parent organic compound.

Figure 1

Structures of COX-1 and COX-2. Isoleucine-to-valine substitution opens up the hydrophobic pocket in COX-2 compared to COX-1. Tyr-385 and Ser-530 are essential for COX activity. Adopted from Mengle-Gaw and Schwartz.

Figure 2

Structure of mefenamic acid cocrystallized with human COX-2. Reprinted from Orlando and Malkowski.

Figure 3

Chemical structures of refecoxib, celecoxib, and aspirin.

Scheme 1. Synthesis of Carborane Analogues of Mefenamic Acid

Reaction conditions: (i) halogenation of the cluster: 0.5 equiv of I₂ or Br₂, mixture of HNO₃/H₂SO₄ (1:1, v/v) in glacial acetic acid, 60–80 °C for 1–4.5 h, 81–97%; (ii) B–N coupling of 2a–4a with 2-aminobenzonitrile with Pd(dba)₂–BINAP–KOt-Bu or Sphos-Pd-G3–Sphos–K₃PO₄ as a catalyst in 1,4-dioxane, 70–95 °C for 18.5–46 h, 30–62%; (iii) hydrolysis of 2b–4b using 40 vol % aq H₂SO₄ in glacial acetic acid, 90–120 °C for 7–23 h, 57–91%; (iv) deboronation of 2b using NaF in ethanol/water (3:2, v/v) at 90 °C for 6 h quantitatively gave compound 5.

Scheme 2. Carborane Analogues of NSAIDs

Figure 4

Molecular structures of the meta-carborane analogues of the nitrile precursor (left, 3b) and mefenamic acid (right, 3c). Legend: beige, B; black, C; white, H; yellow, N; blue, O. The ortho and para isomers are presented in the Supporting Information.

Figure 5

Effects of 2b and 3b on the proliferation and apoptotic process. HCT116 and SW480 cells were exposed to an IC₅₀ dose of 2b or 3b and mefenamic acid (MEFA). (A, B) Cell proliferation, (C, D) apoptotic cell fraction, and (E, F) caspase activation were evaluated after 72 h of treatment. Data are representative from three independent experiments. CFSE, carboxyfluorescein succinimidyl ester; Annexin V-FITC, Annexin V conjugated to fluorescein isothiocyanate; PI, propidium iodide; Apostat, FITC-conjugated pan-caspase inhibitor.

Figure 6

HCT116 and SW480 cells were exposed to an IC₅₀ dose of 2b, 3b, and mefenamic acid (MEFA, 1), and ROS/RNS production was evaluated after 72 h of treatment. Treatment with 2b increased ROS/RNS production in both HCT116 and SW480 cells. Representative data from three independent experiments are presented.