Irreversible NADH Cleavage by a Mitochondria-Targeted Metal-Free Photoredox Sensitizer with Activity in Cancer Cells

Intracellular photoredox catalysis has emerged as an alternative to classical photodynamic therapy, yet most systems promote reversible NADH oxidation to NAD⁺. Here we show that the mitochondria-targeted, metal-free organic photosensitizer Cleav-1 induces photoinduced irreversible cleavage of NADH with nicotinamide release. Cleav-1 is accessible in two steps, highly fluorescent, photostable, and efficiently accumulates in mitochondria of MCF-7 breast cancer cells. In an aqueous solution, it catalyzes light-driven NADH consumption, and ¹H NMR detects free nicotinamide, supporting cofactor cleavage rather than redox interconversion. Consistent with a photoredox pathway, its first singlet excited state is efficiently quenched by NADH, whereas no singlet oxygen emission is observed. In cells, photoactivation of Cleav-1 causes marked photocytotoxicity (IC₅₀ ∼ 300 nM), severe depletion of intracellular NAD, mitochondrial dysfunction, and predominantly apoptotic cell death. These results suggest irreversible nicotinamide cofactor damage as a distinct photobiological mechanism and highlight mitochondria-targeted organic photoredox sensitizers for imaging-guided phototherapy.