In recent decades, quaternary heteronium salts (QHSs) with its positive charge localized or delocalized on heteroatoms, such as nitrogen, phosphorus or sulfur, have found various applications as reagents, bio-, organo- and metallocatalysts, green solvents, biologically active compounds, and new materials. Among this group of QHS compounds, ionic liquids (ILs) with melting points below 100 °C have become the subject of a great number of studies performed both in academia and industry.1–3 As a consequence of their diverse utility and propagation in the natural environment, various toxicological investigations, including eco-phyto-toxicological, microbiological and cytotoxicological studies, have been carried out.4–9
In medical and pharmaceutical research, QHS derivatives are termed cationic lipophilic salts, delocalized lipophilic cations, or simply lipophilic cations. Due to the difference in trans-mitochondrial negative electric membrane potentials between carcinoma and normal cells, these compounds selectively accumulate in the mitochondria of carcinoma cells, particularly those of human origin.10–13 This in vitro selectivity has been found to range from 10- to 100-fold.14 Therefore, numerous cytotoxicological investigations have been devoted to various lipophilic QHSs, including rhodamine 123, dequalinium chloride, victoria blue BO, safranin O, and various phosphonium salts, as selective, anti-carcinoma chemotherapeutic agents.14, 15
More recently, the interest in biological studies on QHSs has increased, particularly, towards the assessment of the risk of ILs to humans and the environment. The HeLa cell line9 and the two colon carcinoma cell lines, HT-29 and CaCo-2,16 have been used to evaluate toxicity of various nitrogen-containing ILs involving different classes of cations. The CaCo-2 cell line has also been used in evaluation of the toxicity of imidazolium, guanidinium, ammonium, phosphonium, pyridinium, and pyrrolidinium cations,16, 17 and showed that increasing the length of the substituent chain contributed to a significant increase in toxicity. In addition, the cytotoxicity of these compounds was strongly affected by the type of counterion. The effects of different head groups and functionalized side chains on the cytotoxicity of ILs have been described in other investigations.7 Polycationic phosphorus dendrimers bearing various types of protonated amine terminal groups (e.g., pyrrolidine, morpholine, methyl piperazine, or phenyl piperazine) have previously been assayed to determine their cytotoxicity, which was found to be weak against three cell lines, HUVEC, HEK293 and HeLa, the latter two being cancerous.18 Finally, a quantum-chemical-based guide to analyze cytotoxicity of ILs was also introduced in very recent studies.4
The biological activity of phosphonium salts is known, and it is sometimes compared with the activity of analogous ammonium salts.19 However, in a number of such investigations, the activity was very low. Kumar and Malhotra20 investigated the cytotoxicity of phosphonium salts containing 4–14 carbon-atom chains and PF6−, N(CF3SO2)2−, BF4−, or [PF3(C2F5)3]− anions, on a set of human tumor cell lines, not including HeLa cells (Figure 1). They observed that phosphonium salts were more active and less cytotoxic than ammonium salts. Other reports noted that a change from an ammonium to a phosphonium cation elicited no change in inhibitory potential.
Bisphosphonic salts with C12-chains, such as 1,12-bis(tributylphosphonium)dodecyl dibromide 2 (Figure 1), exhibited moderate phospholamban (PLB) inhibition and antifungal activity.21 Delikatny et al. investigated the effect of a triphenyl-containing salt, and tris(4-dimethylaminophenyl) mono- and bisphosphonium salts with short carbon chains on breast cancer cells, DU-4475 and HBL-100, and the triphenyl derivative exhibited selective growth inhibition.15
Among cationic, lipophilic, sulfur-containing salts, the correlation between the number and character of sulfur atoms, and biological activity remains unknown. The cyclic organosulfur compounds explored so far include sulfonium and bivalent sulfur atoms. For example, the thiopyrylium salt AA1 (3)14 strongly inhibited mitochondrial ATPase activity and proved to be 10-times more toxic to colon carcinoma cells (CX-1) than to normal monkey kidney cells (CV-1). Thiacarbocyanine dyes (e.g., 4 and 5, Figure 1) exhibited cytotoxicity in human colon carcinoma cells and additionally inhibited bovine heart mitochondrial NADH-ubiquinone reductase activity.22 Other representatives of the cyanines showed cytotoxicity that in part correlated with the length of alkyl chains.23 MKT-077 (6, Figure 1), formerly known as FJ-776, exhibited a significant antitumor activity in a variety of model systems and was tested in clinical trials.24–26 MKT-077 was found to bind to the hsp70 family member, mortalin (mot-2), and to abrogated its interactions with the tumor suppressor protein p53.27
Initially, we focused on the cytotoxic activity of three phosphonium halides (8–10, Scheme 1), possessing tri-n-butylphosphonium and triphenylphosphonium cations in addition to long and short alkyl chains, on human cervix carcinoma (HeLa) and human chronic myelogenous leukemia (K562) cells. We then synthesized a series of new QHS cations 11–17 (Scheme 1) containing an acyclic, bivalent sulfur atom in a short methylthiomethyl chain. These compounds were screened for their cytotoxic activity towards two cancer cell lines (HeLa and K562) and noncancerous HUVEC cells. We also evaluated their properties as DNA carriers in transfection of mammalian cells.