Betulin and betulinic acid have been widely studied for their anticancer activities. However, their further development is limited due to low bioavailability, poor aqueous solubility, and limited intracellular accumulation. In the present study, a triphenylphosphonium cation moiety was linked to betulin and betulinic acid to specifically target them to cancer cell mitochondria. Biological characterization established that uptake of mitochondria-targeted compound 1a in the mitochondria of cancer cells was increased compared to betulin. The mitochondria-targeted derivatives of betulin and betulinic acid showed stronger cytotoxicity than their parent drugs and exhibited more cytotoxic effects in cancer cells than normal cells. The mechanisms may involve the mitochondrial apoptotic pathway, probably caused by the induction of reactive oxygen species production and reducing mitochondrial membrane potential. More importantly, 1a significantly inhibited cancer cell proliferation and migration in an in vivo zebrafish xenograft model. Collectively, these results encourage further study of 1a analogs as anticancer agents.

Mitochondrion is a favorable therapeutic target in cancer, given its regulation of bioenergetics and cell death. Honokiol exhibits antiproliferative effects through mitochondria-mediated death signaling. To enhance its anticancer potential and selectivity, we conjugated honokiol to berberine, a mitochondria-targeting carrier. All designed derivatives displayed 1 order of magnitude increased cytotoxicity compared with the parent compounds, especially with massive cytoplasmic vacuoles. Biological evaluation demonstrated the representative compound 6b localized within the mitochondria, and mitochondrial dilation resulted in vacuolization. 6b induced vacuolation-associated cell death and apoptosis with obvious mitochondrial dysfunction, as demonstrated by booming reactive oxygen species generation, opening mitochondrial permeability transition pore, and reducing mitochondrial membrane potential. The targeting property also conferred 6b with selectivity for tumor cells compared to normal cells. 6b inhibited cancer cell proliferation in the zebrafish xenograft model. These results demonstrate that berberine-linked honokiol derivatives open up a direction for novel mitochondrial-targeting antitumor agents.

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