Mitochondria‐Targeting Photocytotoxic Ferrocenyl Conjugates of N‐Alkylpyridinium Salts

Abstract

AbstractFerrocenyl (Fc) conjugates (1–3) of alkylpyridinium cations (E)‐N‐alkyl‐4‐[2‐(ferrocenyl)vinyl]pyridinium bromide (alkyl = n‐butyl in 1, N,N,N‐triethylbutan‐1‐aminium bromide in 2, and n‐butyltriphenylphosphonium bromide in 3) were prepared and characterized, and their photocytotoxicities and cellular uptakes in HeLa cancer and 3T3 normal cells were studied. The species with a 4‐methoxyphenyl moiety (4) instead of Fc was used as a control. The triphenylphosphonium‐appended 3 was designed for specific delivery into the mitochondria of the cells. Compounds 1–3 showed metal‐to‐ligand charge‐transfer bands at λ ≈ 550 nm in phosphate buffered saline (PBS). The Fc+/Fc and pyridinium core redox couples were observed at 0.75 and –1.2 V versus a saturated calomel electrode (SCE) in CH2Cl2/0.1 M (nBu4N)ClO4. Conjugate 3 showed a significantly higher photocytotoxicity in HeLa cancer cells [IC50 = (1.3 ± 0.2) μM] than in normal 3T3 cells [IC50 = (27.5 ± 1.5) μM] in visible light (400–700 nm). The positive role of the Fc moiety in 3 was evident from the inactive nature of 4. A JC‐1 dye (5,5,6,6‐tetrachloro‐1,1,3,3‐tetraethylbenzimidazolylcarbocyanine iodide) assay showed that 3 targets the mitochondria and induces apoptosis by the mitochondrial intrinsic pathway caused by reactive oxygen species (ROS). Annexin/propidium iodide studies showed that 3 induces apoptotic cell death in visible light by ROS generation, as evidenced from dichlorofluorescein diacetate assay. Compounds 1–3 exhibit DNA photocleavage activity through the formation of hydroxyl radicals.