Oral administration has advantages over injection and other delivery methods, including ease of use, simple administration, high patient compliance, low production costs of oral formulations, and non-invasiveness ^[1][2][18,97]. However, some common challenges faced in the oral drug delivery route need to be considered, particularly poor solubility and permeability, high levels of P-glycoprotein efflux, pre-systemic metabolism, and high rates of drug molecule degradation ^[2][97]. To overcome these issues, IL-based formulations have been proposed as a unique strategy to solubilize and formulate problematic small and macromolecular drugs/therapeutics for the development of practical and translatable oral drug delivery systems ^[1][3][18,98]. Many biological therapeutics, such as insulin, monoclonal antibodies, and immunoglobulin (IgG), have been successfully delivered via oral administration using ILs. Choline-based ILs (choline/glycolic acid molar ratio of 2:1, 1:1, and 1:2) can be prepared for the oral delivery of insulin and IgG. The IL-based formulation significantly enhances IgG penetration through intestinal mucus and epithelium ^[4][99]. Similarly, improved oral delivery of monoclonal antibodies into the intestinal mucosa can be achieved using choline and glycolate IL ^[5][100]. Choline germinate IL-containing formulations significantly enhance the paracellular transport of insulin by protecting it from enzymatic degradation and interactions with the mucus layer in the gastrointestinal tract ^[6][101]. These studies demonstrate the potential use of ILs for enhancing the oral delivery of macromolecular drugs. IL-based formulations are also considered potential alternatives for the oral delivery of small molecular hydrophobic drugs. For example, the choline oleate IL-based formulation significantly enhances the absorption of PTX delivered orally compared with the marketed chromophore EL-based formulation ^[7][102]. Similarly, the CAGE 1:2-containing formulation has been investigated for the oral delivery of hydrophobic drug sorafenib, resulting in an IL-based formulation with a peak plasma concentration, drug elimination half-life, and mean absorption time 2.2-, 2-, and 1.6-fold higher, respectively, than those of the parent drug suspension (Figure 17A) ^[2][97]. The bioavailabilities of proline ethyl ester-containing methotrexate and alanine ethyl ester-bearing favipiravir delivered orally are 4.6- and 1.9-fold higher than that of the respective parent drugs (Figure 17B) ^[8][9][86,89]. The solubility of lumefantrine docusate IL in lipid-based formulations is 80-fold higher than that of the free drug, resulting in improved plasma exposure (up to 35-fold higher) compared to the control lipid and aqueous suspension formulations of the free drug (Figure 17C) ^[10][103]. Although these IL-based oral delivery systems have successfully delivered both small and large therapeutic molecules, it is yet to be discovered if they are safe within the living body.

The intravenous (IV) route or direct injection at the site of action offers a rapid onset of action and avoids first-pass metabolism. However, several challenges must be overcome in developing injectable formulations of poorly water-soluble drugs due to the high hydrophobicity and potential severe side effects of these drugs. Recently, IL-based formulations have been used to improve the biopharmaceutical properties of drugs delivered via injection. A choline germinate IL-based formulation of a chemotherapeutic drug (doxorubicin) has been developed for percutaneous injection into liver tumors in a rabbit liver tumor model, leading to consistent tumor ablation in the rabbit liver tumor model for prolonged periods ^[11][104]. An IL formulation of doxorubicin exhibits synergistic cytotoxicity against cultured HCC cells with a uniform drug distribution throughout the ablation zone when injected into liver tumors in the rabbit liver tumor model. Similarly, doxorubicin-loaded imidazolium IL ([C₄MIN][PF₆])–polydopamine nanocomposites combined with microwave irradiation have an apparent antitumor efficacy with high inhibition effects ^[12][105]. IL-mediated paclitaxel (PTX) shows excellent antitumor activity with a minor hypersensitivity effect in vitro compared to commercial cremophor EL-mediated paclitaxel (Taxol) ^[13][106]. An IL-based PTX formulation is similar to Taxol in terms of systemic circulation time and antitumor activity. CAGE IL-containing large molecular proteins, such as monoclonal antibodies, significantly enhance monoclonal antibody absorption by ≈200% after subcutaneous injections ^[14][107]. Taken together, IL-mediated injectable formulations open up new possibilities for developing effective and translatable drug delivery strategies for small molecule drugs and biological therapeutics.

Transdermal drug delivery has attracted attention as a non-parenteral administration technique due to its ease of application and termination, noninvasive nature, sustained therapeutic action, and better patient compliance ^[15][16][40,108]. In some circumstances, transdermal drug delivery can circumvent the first-pass metabolism of oral delivery and provide an acceptable therapeutic effect across the skin barrier. An oil-based formulation can facilitate the permeation of the drugs across the skin because it has excellent surfactant properties, and lipophilic oils act as skin penetration enhancers. After being applied to the skin, oils and their components are rapidly metabolized, and the resulting products are quickly excreted without any accumulation in the body, indicating they are potentially useful and safe penetration enhancers ^[17][18][109,110]. Recently, bio-ILs and DESs have been investigated in regard to their ability to increase skin permeability ^[1][18]. ILs have been used in different formulations, including microemulsions, nanoparticles, and (bio)polymer-based drug delivery systems (such as patches and membranes), to deliver poorly water-soluble drugs and macromolecular biological therapeutics via the topical and transdermal routes ^[15][19][1,40]. For topical delivery, ILs are usually used as solubilizing and skin-enhancing agents. Biocompatible choline-containing carboxylic acids, such as lactic acid, oleic acid, formic acid, and propionic acid, are used as the internal nonaqueous phase of IL-in-oil (IL/O) microemulsions (MEs) with IL-based surfactant choline oleate to deliver acyclovir (ACV) topically ^[20][36]. This formulation significantly enhances the topical delivery of ACV, by 9-fold, compared with water-in-oil MEs. Similarly, choline octanoate IL improves the penetration of navitoclax (a BCL-2 inhibitor) across the skin for an extended period ^[21][111]. This formulation has a higher cancer-cell-killing efficacy in topical delivery than in oral delivery. Choline-containing citronellic acid, glutamic acid, caprylic acid, hexenoic acid, glycolic acid, and octanoic acid ILs have been used as solubilizing and skin-enhancing agents of framework nucleic acids (FNAs), resulting in the enhanced penetration of FNAs to the dermis layer with long-term stability ^[22][112]. CAGE IL has been used to deliver many macromolecule therapeutics, such as insulin, siRNA, and dextrans ^[15][40]. A CAGE IL-based formulation significantly improves the transdermal delivery of dextran with various molecular weights up to 150 kDa, due to the potential use of ILs as an effective and noninvasive transdermal drug delivery system for large hydrophilic molecules (Figure 28A) ^[23][113]. CAGE IL-based formulation also enhances the epidermal and dermal penetration of siRNA with suppressed GAPDH expression in mice models compared to the control ^[24][114]. A CAGE-containing thrombin-sensitive nanosensor exhibits significant diffusion into the dermis with sustained release into the blood throughout 72 h ^[25][44]. This formulation releases reporter molecules into the urine by activating the clotting cascade and retains diagnostic power for 24 h. CAGE IL has also been used for the transdermal delivery of insulin, resulting in enhanced delivery into and across porcine skin compared to CPEs (Transcutol) ^[26][115]. This formulation significantly decreased blood glucose levels by 40% within 4 h, with a relatively sustained release of insulin for 12 h, compared to the injection formulation. The transdermal delivery of an antigenic peptide (SIINFEKL) has been formulated using choline fatty acids as biocompatible surfactants. This IL-based system significantly enhanced the skin permeation of the peptide for cancer immunotherapy (Figure 28B) ^[27][116]. A recent study replaced the conventional surfactant, Tween-80, with a surface-active bio-IL to develop a thermodynamically stable IL/O ME ^[28][117]. The developed IL/O ME generates a ME zone that is two times larger than the Tween-80-based IL/O ME, resulting in 4.7- and 5-fold higher loadings of CLX and ACV, respectively. Another IL/O ME was developed for the transdermal delivery of insulin using choline propionate IL as an internal polar phase and [Cho][Ole] as a surfactant and drug-encapsulating agent ^[29][118]. This formulation significantly reduces blood glucose levels, with an improved bioavailability in the systemic circulation and sustained release of insulin for a more extended period, compared to the subcutaneous injection formulation. These results demonstrate that ILs can significantly enhance performance beyond “typical” ME formulations based on traditional surfactants.

Vaccination is a therapeutic approach used to stimulate the body’s immune system by delivering an antigen to antigen-presenting cells in order to initiate an immune response. Vaccines can be administered via different routes, such as oral, intramuscular, and transdermal, in different forms, including suspension, nanoparticle, microparticle, and microemulsion. ILs can be used as penetration enhancers, vaccine stabilizers, or adjuvants in different stages of the vaccine formulation  ^{[30][31][32][33]}[119,120,121,122]. A novel IL-mediated transcutaneous vaccine formulation, developed using a solid-in-oil nano-dispersion technique in which the model antigen ovalbumin is coated with IL[C₁₂MIM][Tf₂N], triggers the production of a higher level of OVA-specific serum IgG compared to both the PBS control and solid-in-oil (S/O) nano-dispersion without IL ^[34][123]. Similarly, a biocompatible choline oleate IL improves the skin permeation of antigenic peptide, by 28-fold, compared to an aqueous vehicle. This IL-based vaccination suppresses tumor growth in vivo compared to 2% wt ethanol containing subcutaneous injection ^[27][116]. Recently, choline lactate IL has been used as a safe adjuvant with OVA, resulting in an enhancement of the immune response against the antigen ^[33][122]. In another study, choline niacinate IL-based O/IL nanoemulsions were formulated for the intranasal vaccine delivery of influenza split-virus antigens, resulting in high levels of mucosal immune responses with secretory IgA titers 25- and 5.8-fold higher than those of naked and commercial MF59-adjuvanted antigens, respectively ^[32][121]. Similarly, humoral immune responses to inactivated foot-and-mouth disease were improved, along with enhanced thermostability and long-term stability compared to the adjuvant of Montanide ISA 206 ^[31][120].