Plumbagin-Serum Albumin Interaction: Spectral, Electrochemical, Structure-Binding Analysis, Anti-proliferative and Cell Signaling Aspects with Implications for Anticancer Therapy.

: Plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone) is a small molecule with potent anticancer activity. Like other 1,4-naphthoquinones, it exhibits electrophilic reactivity towards biological nucleophiles. We demonstrate that plumbagin and structurally related 1,4-naphthoquinones with at least one unsubstituted quinoid carbon (C2 or C3) bind to albumin, an ubiquitously present nucleophile, with minimum recovery of free drug. Recovery of plumbagin from albumin declined in a one-phase kinetic with a half-live of 9.3min at 10μM. Plumbagin in the presence of albumin exhibited instant changes in UV-VIS absorption bands. Electrochemical analysis using cyclic voltammetry showed a decrease in redox peak currents over time until electro-inactivity, suggesting formation of a supramolecular adduct inaccessible to electron transfer. The adduct inhibited cell growth and caused cell cycle arrest of prostate cancer cells, in part by decreasing levels of cell cycle regulator RBBP. The conjugate displayed similar cellular effects as described for plumbagin such as decreased levels of androgen receptor and protein kinase C-epsilon. The effect of plumbagin-albumin on cancer cells was species-specific, suggesting a receptor-mediated mechanism. It was blocked by cathepsin inhibitor pepstatin A, indicating that lysosomal degradation releases plumbagin to the cell. The spontaneous formation of a plumbagin-albumin adduct is likely to have crucial implications for the pharmacodynamics of plumbagin.