Ruthenium Complexes are pH-Activated Metallo Prodrugs (pHAMPs) with Light-Triggered Selective Toxicity Toward Cancer Cells

Metallo prodrugs that take advantage of the inherent acidity surrounding cancer cells have yet to be developed. We report a new class of pH-activated metallo prodrugs (pHAMPs) that are activated by light- and pH-triggered ligand dissociation. These ruthenium complexes take advantage of a key characteristic of cancer cells and hypoxic solid tumors (acidity) that can be exploited to lessen the side effects of chemotherapy. Five ruthenium complexes of the type [(N,N)2Ru­(PL)]2+ were synthesized, fully characterized, and tested for cytotoxicity in cell culture (1 A : N,N = 2,2′-bipyridine (bipy) and PL, the photolabile ligand, = 6,6′-dihydroxybipyridine (6,6′-dhbp); 2 A : N,N = 1,10-phenanthroline (phen) and PL = 6,6′-dhbp; 3 A : N,N = 2,3-dihydro-[1,4]­dioxino­[2,3-f]­[1,10]­phenanthroline (dop) and PL = 6,6′-dhbp; 4 A : N,N = bipy and PL = 4,4′-dimethyl-6,6′-dihydroxybipyridine (dmdhbp); 5 A : N,N = 1,10-phenanthroline (phen) and PL = 4,4′-dihydroxybipyridine (4,4′-dhbp). The thermodynamic acidity of these complexes was measured in terms of two pK a values for conversion from the acidic form (X A ) to the basic form (X B ) by removal of two protons. Single-crystal X-ray diffraction data is discussed for 2 A , 2 B , 3 A , 4 B , and 5 A . All complexes except 5 A showed measurable photodissociation with blue light (λ = 450 nm). For complexes 1 A –4 A and their deprotonated analogues (1 B –4 B ), the protonated form (at pH 5) consistently gave faster rates of photodissociation and larger quantum yields for the photoproduct, [(N,N)2Ru­(H2O)2]2+. This shows that low pH can lead to greater rates of photodissociation. Cytotoxicity studies with 1 A –5 A showed that complex 3 A is the most cytotoxic complex of this series with IC50 values as low as 4 μM (with blue light) versus two breast cancer cell lines. Complex 3 A is also selectively cytotoxic, with sevenfold higher toxicity toward cancerous versus normal breast cells. Phototoxicity indices with 3 A were as high as 120, which shows that dark toxicity is avoided. The key difference between complex 3 A and the other complexes tested appears to be higher uptake of the complex as measured by inductively coupled plasma mass spectrometry, and a more hydrophobic complex as compared to 1 A , which may enhance uptake. These complexes demonstrate proof of concept for dual activation by both low pH and blue light, thus establishing that a pHAMP approach can be used for selective targeting of cancer cells.