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)Ru(PL)] were synthesized, fully characterized, and tested for cytotoxicity in cell culture (1: N,N = 2,2'-bipyridine (bipy) and PL, the photolabile ligand, = 6,6'-dihydroxybipyridine (6,6'-dhbp); 2: N,N = 1,10-phenanthroline (phen) and PL = 6,6'-dhbp; 3: N,N = 2,3-dihydro-[1,4]dioxino[2,3-f][1,10]phenanthroline (dop) and PL = 6,6'-dhbp; 4: N,N = bipy and PL = 4,4'-dimethyl-6,6'-dihydroxybipyridine (dmdhbp); 5: 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 values for conversion from the acidic form (X) to the basic form (X) by removal of two protons. Single-crystal X-ray diffraction data is discussed for 2, 2, 3, 4, and 5. All complexes except 5 showed measurable photodissociation with blue light (λ = 450 nm). For complexes 1-4 and their deprotonated analogues (1-4), the protonated form (at pH 5) consistently gave faster rates of photodissociation and larger quantum yields for the photoproduct, [(N,N)Ru(HO)]. This shows that low pH can lead to greater rates of photodissociation. Cytotoxicity studies with 1-5 showed that complex 3 is the most cytotoxic complex of this series with IC values as low as 4 μM (with blue light) versus two breast cancer cell lines. Complex 3 is also selectively cytotoxic, with sevenfold higher toxicity toward cancerous versus normal breast cells. Phototoxicity indices with 3 were as high as 120, which shows that dark toxicity is avoided. The key difference between complex 3 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, 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.

Metastasis is the leading cause of cancer deaths due to the spread of cancer cells through the blood vessels and the subsequent formation of secondary tumors. Metastasizing cancer cells in the human vasculature are called circulating tumor cells (CTCs) and are characterized to express the epithelial cell adhesion molecule (EpCAM). They are further known to survive physiological fluid shear stress (FSS) conditions. However, the effect of FSS on CTC molecular phenotype, such as the epithelial to mesenchymal transition (EMT) and cancer stem cell (CSC) expression, has not been extensively studied. Here, CTCs in FSS are evaluated in an in vitro model system. MCF7 and MDA-MB-231 breast cancer cell lines were grown in adherent and suspension culture media. The cell lines were tested for EMT and CSC genetic and protein markers using qRT-PCR and flow cytometry, respectively. Suspension cells showed a significantly increased EMT signature compared to adherent cells (p<0.05), suggesting that they model cells detaching from primary tumors in vivo. Upon application of FSS, MCF7 and MDA-MB-231 cells did not show a significant change in EMT expression (p>0.05), but there was a statistically significant increase of the CSC population in MCF7 suspension cultures (p<0.05). These results with MCF7 suggest that CTCs can be modeled in vitro as non-adherent cancer cells in FSS and that they show an increased CSC-like signature during circulation, providing new insights to the importance of CSC-targeting strategies when treating metastatic patients.

Ruthenium complexes bearing protic diimine ligands are cytotoxic to certain cancer cells upon irradiation with blue light. Previously reported complexes of the type [(N,N)2Ru­(6,6′-dhbp)]­Cl2 with 6,6′-dhbp = 6,6′-dihydroxybipyridine and N,N = 2,2′-bipyridine (bipy) (1 A ), 1,10-phenanthroline (phen) (2 A ), and 2,3-dihydro-[1,4]­dioxino­[2,3-f]­[1,10]­phenanthroline (dop) (3 A ) show EC50 values as low as 4 μM (for 3 A ) vs breast cancer cells upon blue light irradiation (Inorg. Chem.20177519). Herein, subscript A denotes the acidic form of the complex bearing OH groups, and B denotes the basic form bearing O– groups. This photocytotoxicity was originally attributed to photodissociation, but recent results suggest that singlet oxygen formation is a more plausible cause of photocytotoxicity. In particular, bulky methoxy substituents enhance photodissociation but these complexes are nontoxic (Dalton Trans201815685). Cellular studies are presented herein that show the formation of reactive oxygen species (ROS) and apoptosis indicators upon treatment of cells with complex 3 A and blue light. Singlet oxygen sensor green (SOSG) shows the formation of 1O2 in cell culture for cells treated with 3 A and blue light. At physiological pH, complexes 1 A -3 A are deprotonated to form 1 B -3 B in situ. Quantum yields for 1O2 (ϕΔ) are 0.87 and 0.48 for 2 B and 3 B , respectively, and these are an order of magnitude higher than the quantum yields for 2 A and 3 A . The values for ϕΔ show an increase with 6,6′-dhbp derived substituents as follows: OMe < OH < O–. TD-DFT studies show that the presence of a low lying triplet metal-centered (3MC) state favors photodissociation and disfavors 1O2 formation for 2 A and 3 A (OH groups). However, upon deprotonation (O– groups), the 3MLCT state is accessible and can readily lead to 1O2 formation, but the dissociative 3MC state is energetically inaccessible. The changes to the energy of the 3MLCT state upon deprotonation have been confirmed by steady state luminescence experiments on 1 A -3 A and their basic analogs, 1 B -3 B . This energy landscape favors 1O2 formation for 2 B and 3 B and leads to enhanced toxicity for these complexes under physiological conditions. The ability to convert readily from OH to O– groups allowed us to investigate an electronic change that is not accompanied by steric changes in this fundamental study.

As p38 in the spinal microglia plays a critical role in neuropathic pain, we expect that p38 siRNA NPs could be a promising tool for the treatment of neuropathic pain.

The lipophilic vs. hydrophilic properties of three protic ruthenium compounds were studied as a function of pH. Specifically, we measured log10]phenanthroline (3 A ) and 6,6'-dhbp is the diprotic 6,6'-dihydroxy-2,2'bipyridine ligand) from pH 4.0 to 8.0. This study allowed us to demonstrate that as the ligand is deprotonated at higher pH values the resulting neutral charge on the complex improves its lipophilic properties. Thus, improved uptake by passive diffusion is expected with protic ligands on Ru(II). Furthermore, cellular studies have demonstrated that passive diffusion is the dominant pathway for cellular uptake. However, metabolic inhibition has also shown that energy dependent efflux reduces the amount of the ruthenium complex (as measured by mean fluorescence intensity) in the cells. These compounds have been shown by fluorescence microscopy to accumulate in the nuclei of cancer cells (MCF7, MDA-MB-231, and HeLa). Taken together, this data shows that uptake is required for toxicity but uptake alone is not sufficient. The greatest light activated toxicity appears to occur in breast cancer cell lines with relatively moderate uptake (MCF7 and MDA-MB-231) rather than the cell line with the greatest uptake of complex 3 A (normal breast cell line MCF-10A).