Recently, the ionic liquid (IL) choline dihydrogen phosphate was demonstrated to improve the thermostability and shelf life of several model proteins, thus exhibiting potential as a stabilizing excipient or solvent for protein therapeutics. Before novel ILs can be used for biomedical applications, comprehensive data is required to establish biocompatibility, including cytotoxicity effects and solution behavior. In this study five phosphate-based anion moieties were analyzed: H2PO_4~- (DHP), dibutyl phosphate (DBP), bis(2-ethylhexyl) phosphate (BEH), bis(2,4,4-trimethylpentyl) phosphinate (TMP), and O,O'-diethyl dithiophosphate (DEP), all paired with the cation choline (C). Toxicity levels for these ILs, and a common sugar and salts, were established using a J774 murine macrophage cell line. The sugar trehalose, and the simple salts sodium chloride and choline chloride yielded EC_(50) values of > 100, 63 and 34 mM, respectively. The EC_(50) values (mM) of CDHP (20), CDBP (9.1), and CDEP (8.2) were lower than, but within the range of simple salts NaCl (62.8) and choline Cl (33.7). The EC_(50) values of CTMP and CBEH were considerably lower, 0.25 and 0.30 mM, respectively. CDHP and CBEH displayed a hormetic response. Osmolality measurements indicated that CDHP, CDBP, and CDEP exhibit nearly complete dissociation in aqueous solution, with osmotic coefficients of 1.0, 0.9, and 0.8, whereas CTMP and CBEH have coefficients of 0.5 and 0.3, and are more molecular in character. A high correlation between the EC_(50) value and the anion mass fraction indicated that anion size and the presence of moderately long and/or branched alkyl chains may affect viability.
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