Ionic liquids for addressing unmet needs in healthcare

Abstract Advances in the field of ionic liquids have opened new applications beyond their traditional use as solvents into other fields especially healthcare. The broad chemical space, rich with structurally diverse ions, and coupled with the flexibility to form complementary ion pairs enables task‐specific optimization at the molecular level to design ionic liquids for envisioned functions. Consequently, ionic liquids now are tailored as innovative solutions to address many problems in medicine. To date, ionic liquids have been designed to promote dissolution of poorly soluble drugs and disrupt physiological barriers to transport drugs to targeted sites. Also, their antimicrobial activity has been demonstrated and could be exploited to prevent and treat infectious diseases. Metal‐containing ionic liquids have also been designed and offer unique features due to incorporation of metals. Here, we review application‐driven investigations of ionic liquids in medicine with respect to current status and future potential.

Application-driven investigations of ILs is a vibrant and rapidly growing research field with emerging frontiers in healthcare, especially drug delivery ( Figure 2). Because many drug candidates exhibit poor solubility profiles, a property that limits their bioavailability, eventually leading to clinical failure, it is expedient to pursue new strategies to solubilize and/or formulate them. An element of this pursuit is the exploration of ILs as drug delivery technologies. In perspective, ILs are liquid salts but with low melting points, typically below 1008C, and in some cases, form an exotic class of room-temperature solvents that can favorably solvate a wide range of compounds. 18 Such extraordinary solvating power enables ILs to dissolve many poorly soluble drugs, and ultimately enhances drug permeation through physiological barriers to boost therapeutic efficacy. 6,[8][9][10]12,56 Also, the innovative concept of active pharmaceutical ingredient ionic liquids (API-ILs) aims at pairing a pharmaceutically active cation and anion to design IL-based drugs that feature better solubility behavior enabling efficient permeation through various barriers to reach target cells. 6,8,10,12 Meanwhile, empirical laboratory-scale results prove the potential of the IL strategy as a promising technology to eliminate several challenges including polymorphism that is associated with some solidstate drugs, and solubility-limited bioavailability of poorly soluble drugs. 6,8,10,12 The ability of ILs to enhance the therapeutic efficacy of drugs is evident from various studies 6,8,10,12 but clinical exploitation of this technology remains limited. Indeed, only a few API-ILs, for example the 1-(2-hydroxyethyl)pyrrolidinium salts of diclofenac sold as FLECTOR® patch in the United States, have entered the market. This review, therefore, aims to draw the attention of stakeholders in the field of drug delivery to the breakthroughs that unequivocally confirm ILs as technologies that improve the therapeutic efficacy of drugs. To encourage research into metal-containing ILs, we review this underexplored subfield and highlight an example of how the synergy between metal-and IL-specific properties enables biosensing application. The antimicrobial activity of ILs is well-demonstrated, and research in this area is continuing. Indeed, these salts are increasingly explored as antimicrobial, antifungal, and antiviral agents. 29 Here, we present several examples of ILs with excellent activity against infection-causing microorganisms. Further, we discuss the central question of toxicity, which must be addressed to translate these innovative materials into marketable healthcare technologies, and recommend future directions to circumvent these problems and expand IL research in healthcare. We are aware of many excellent recent reviews on the biomedical applications of ILs 5-10,12,18 but the prolific research contributions in the field (over 7,000 publications and patents since January 2017) necessitates regular reviews and perspectives.

| Emerging opportunities in drug delivery
The flexibility to fine-tune the physicochemical properties of ILs provides effective solutions to many drug delivery challenges such as low and are now increasingly investigated as chemical enhancers to perturb cell membranes with the goal of improving transcellular and paracellular drug transport. 67 Indeed, computational models, as well as experimental data, prove the feasibility to enhance drug transport using ILs. For instance, molecular dynamics coupled with an empirical force field showed that the cationic head of the amphiphilic 1-octyl-3methylimidazolium-based IL inserts into a model cell membrane, disrupting structural integrity to increase the membrane permeability of small polar molecules such as ammonia. 74 Empirical data confirmed that hydrophilic imidazolium-based ILs fluidize the cell membrane to create pathways for the diffusion of molecules. 75 Although these findings linked the IL-enhanced permeability of biological membranes with cytotoxicity, it is plausible to assume that these mechanisms should enable transport of drugs to target sites. Also, the ability of ILs to extract lipids from physiological structures such as the SC is now recognized. 30  (CAGE), a type 3 deep eutectic solvent (DES), for drug delivery [29][30][31] ( Figure 4). DESs share many general physical properties, such as high viscosity and low vapor pressure, with ILs but are distinct by their facile synthesis and the better-understood toxicity of their precursor. 43 Therefore, for simplicity, this review discusses CAGE and other DESs under ILs. CAGE is a dual-functional technology, acting as a broadspectrum antimicrobial agent and as a chemical enhancer for transdermal delivery. 31 Compared to controls, the drug delivery properties of CAGE is better, being able to enhance the delivery of mannitol, a model hydrophilic drug, and cefadroxil, a model antibiotic, by 5-and 16-fold increases, respectively. 31 We designed other ILs such as choline oleate, tetraalkylphosphonium oleate, tetraalkylphosphonium hexanoate, and tetraalkylphosphonium geranate that act as efficient permeation enhancers, enabling a fivefold enhancement in the transport of cefadroxil into the dermis 31 ( Figure 4). Because the hydrophobic SC poses a barrier to the hydrophilic IL, Goto and coworkers obtained zero drug diffusion through the skin. However, they achieved diffusion by developing an IL-in-oil (IL/oil) microemulsion, in which the IL phase encapsulates acyclovir while the continuous oil phase permeates the hydrophobic barrier to deliver the cargo.
Franz-type diffusion cell experiments showed that the IL/oil microemulsion allows the delivery of acyclovir into Yucatan hairless pig skin. 80 Also, an IL-in-water (IL/water) microemulsion enhances the permeation of poorly water-soluble drugs as demonstrated with the ex vivo topical delivery of etodolac, a nonsteroidal anti-inflammatory drug, to rats. 82 The IL/water microemulsion formulation permits the efficient transport of etodolac into the skin, resulting in a comparatively more effective therapeutic performance compared to formulations without ILs. In a recent development, the Goto group applied a 1-dodecyl-3methylimidazolium-based IL to enhance the skin permeability of a solid-in-oil nanodispersion to deliver ovalbumin in vitro. 83 The potential of IL-assisted drug solubilization and permeation therapeutic impact in a mouse model. 88 The API-IL approach alters the mechanism of action of lidocaine at the cellular level. Lidocaine docusate exerts an effect that is distinct from that of lidocaine on neuritic outgrowth in pheochromocytoma (PC12) cells. 88 Also, the API-IL approach was used to design ranitidine docusate, from ranitidine hydrochloride and sodium docusate. 88 The successful design of liquid ranitidine docusate eliminates polymorphism inherent in ranitidine.
The pioneering work of Rogers and coworkers on molecular engineering of approved pharmaceutically active molecules into API-ILs to access improved pharmacological fingerprints opens new opportunities in pharmaceutics. The ability to facilely obtain and characterize ILs fosters the growing interest in the field as evidenced by the snowballing repository of API-ILs (Table 1) acyclovir. 86 Compared with the parent acyclovir, the API-ILs derived from acyclovir anions exhibit better solubility in water with even the hydrophobic tetrabutylphosphonium counterion giving 200 times improvement in solubility. 86 Also, but to a lesser degree, pairing the acyclovir cation with a chloride or docusate anion gives API-ILs with a better aqueous solubility profile than the neutral acyclovir but no advantage over the sodium salt of acyclovir, whose solubility is superior to that of the docusate anion and comparable to the chloride anion. 86 The excellent solubility of the acyclovir API-IL is evident in simulated gastric and intestinal fluid, where the cholinium-based API-IL, for instance, is 650 times more soluble in the intestinal fluid than the parent drug. 86 Also, by coupling cholinium with pharmaceutically active anions, the Marrucho group advanced the physicochemical and pharmaceutical properties of the anions. 34,90,94 Specifically, API-ILs derived by combining cholinium with anions of nalidixic acid, niflumic acid, 4-aminosalicylic acid, pyrazinoic acid, or picolinic acid exhibit better solubility in water, gastric, and intestinal fluid. 34 Enhanced solubility is presumed to boost membrane permeability, increasing bioavailability and eventually therapeutic efficacy but could lead to over-dosage that is detrimental to critical cellular processes. However, the cholinium, nalidixic, niflumic, and pyrazinoic API-ILs feature similar cytotoxicity toward Caco-2 colon carcinoma cells and HepG2 hepatocellular carcinoma cells as the parent drugs despite the superior solubility of the API-ILs. 34 It may be imperative that a drug penetrates the cell membrane to perform its therapeutic functions. Several parameters, extrinsic and intrinsic to the drug, control the dynamics of cell membrane permeability with the hydrophobic/hydrophilic balance being a key intrinsic factor. 95 A rational design strategy offers the possibility to modulate the balance by an appropriate combination of an ionic drug with a complementary ion. Exemplarily, the Marrucho group tuned the hydrophobic/ hydrophilic balance of L-ampicillin by coupling its anion to ammonium, phosphonium, pyridinium, or imidazolium cations. 94 Compared to the parent drug and its sodium salt, the API-ILs featured high affinity for the cell membrane as revealed by their higher octanol-water partition coefficient, 94 a parameter that describes drugs' lipophilicity. 95 Indeed, lipophilicity is a determinant of how drugs partition into the lipid bilayer, and how they are distributed and metabolized in the body. 95 Marrucho and coworkers manipulated the hydrophobic/hydrophilic balance by molecularly engineering the polarity of the cationic head group to tune hydrophilicity. 94 Counterintuitively, the increased hydrophilicity of the ampicillin-based API-ILs did not compromise the octanol-water partition coefficient. Even the sodium salt of ampicillin with a similar aqueous solubility profile as the cholinium-based API-IL, for instance, is less lipophilic, 94  Neat siRNA is more biocompatible to HEKa than the API-ILs, a finding that concurs with the established positive correlation of hydrophobicity with cytotoxicity. Nonetheless, the API-IL platforms more effectively treat skin diseases than the neat RNA, 93 which is an advantage.

| Incorporating metals in ionic liquids to sense biomolecules
Incorporating a metal center into an organic molecule integrates the unique magnetic, photo-activity, and redox-activity of metals with the inherent stability and processibility of organic molecules. 96 Figure 6).  98 The same year, 1.6 billion people sought treatment for neglected tropical diseases globally,

| Toxicity of ionic liquids
To achieve clinical relevance and secure regulatory approval, the therapeutic benefit of a drug should significantly offset known risks. In contrast to organic solvents, ILs are typically nonvolatile and nonflammable, which led to the concept of "ILs as green chemicals." Obviously, substituting a volatile organic solvent with a nonvolatile IL in a chemical or biochemical process decreases inhalation exposure, flammability hazard, and environmental pollution. Regardless, to categorize ILs, as a class, intrinsically safe and green is premature until a central element of toxicity, biocompatibility, has been established. It is, therefore, pertinent that researchers exploring ILs for biomedical applications establish the biocompatibility profiles.
Compelling empirical data prove the toxicity of many ILs toward diverse life forms, ranging from nucleic acids to multicellular organisms (Table 2). Various in silico 74,75,[113][114][115][116][117] as well as in vivo and in vitro ( Table   2) studies have unraveled the intricacies and molecular origin of this  Our group investigated the biocompatibility of a cholinium-based IL, CAGE, toward a primary adult keratinocyte cell line and found it more benign than the precursors, choline, and geranic acid. 31 Specifically, CAGE causes less skin irritation than its precursors as evidenced by the marginal change in the integrated peak area between 1,650 and As CAGE induces marginal irritation, it is logical to assume that it evades these mechanistic pathways, especially that involving the production of IL-1a. Regardless, CAGE biocompatibility is selective, being toxic to a wide range of microorganisms including fungi, viruses, and bacteria but relatively benign to a human epidermal keratinocytes cell line. The benign behavior of CAGE was attributed to ion-pairing between the cationic choline and the anionic geranate, a phenomenon that shield the ions from interacting with and disrupting keratinocyte membrane to provoke irritation. 29 A recent study from our group confirms that CAGE extracts lipids from the SC as revealed by FTIR data where the integrated peak areas of the CH 2 asymmetric and symmetric stretching of SC lipid at 2,920 and 2,850 cm 21 decrease after incubation with CAGE. 30 The findings suggest that ILs, such as CAGE, are promising biocompatible technologies to enhance transdermal drug delivery.
The concept of a generically benign cholinium-based IL is speculative as toxicity depends, as with other ILs, on the nature of anion, side chain, and biosystem. Indeed, depending on the anion and the biosystem, some cholinium-based ILs could be toxic (Table 3). 31 The IC 50 33 Also, an in vivo study showed that choliniumbased deep eutectic solvents are more toxic than their constituent compounds to ICR mice. 124 This study revealed that the molar ratio of the constituents is central to the toxicity since a 1:2 mixture of choline chloride and urea is less toxic than the 1:3 mixture. 124 Taken together, the toxicity of ILs depends on a complex interplay of structural features, concentration, and the investigated biosystem and so, through rational control of these parameters, it is feasible to design biocompatible ILs. Also, ILs are emerging as technology to combat infectious diseases. It is now established that ILs are potent and broad-spectrum antimicrobial agents with activities that surpass many conventional antimicrobial agents. The excitement generated by this application-driven research has attracted organometallic chemists to incorporate transition metals into ILs with the expectation of accessing exotic properties such as luminescence, redox activity, and magnetism. Although the field of metal-containing ILs is at the early stage of development, it has shown promise in biosensing application as exemplified by the use of a Cocontaining IL to sense glucose. Therefore, ILs should be considered as a promising technology to address many challenges such as drug bioavailability, control of infectious diseases, and biosensing in the biomedical field. We envision that the emerging field of metal-containing ILs could provide new opportunities, for instance in bioimaging given the optical, magnetic, and radioactive properties of metals.
Depending on concentration and structural features, the toxicity profile of ILs may vary, but by rational design, the toxicity can be mitigated. This understanding, albeit encouraging, is over-simplified given that toxicity also depends on the biosystem, which typically co-exists with other life forms in an ecosystem. Hence, it is crucial that susceptibility of the diverse life forms in an ecosystem be considered during the design stage and investigated in vivo using mammalian models.
Toward this, we recommend designing ILs with target specificity, as