Introduction

Liposomes are small artificial spherical shape vesicles with lipid bilayers which mainly consist of cholesterol and non-toxic phospholipids [1]. Synthesis of the liposome is done by three simple stages which are drying down lipids from an organic solvent, dispersing the lipid in aqueous solution, and purifying the resultant liposome [2]. Generally, small-sized liposomes (size less than 100 nm) are used for the systemic administration of liposome. Small-sized liposomes are made by mechanical dispersion methods such as sonication or extrusion methods. The liposome is amphiphilic, specifically, hydrophobic inside the lipid bilayer and hydrophilic inside the vesicle. Thus, both hydrophobic and hydrophilic drugs can be easily loaded during the dispersion stage. For example, hydrophilic doxorubicin can be loaded within the hydrophilic core of the liposomes for the production of Doxil® [3], while a hydrophobic antifungal agent, amphotericin B, is loaded inside the lipid bilayer of the liposome for production of Ambison® [4]. The liposomal drugs would show reduced toxicity and enhanced efficacy because of the improved biodistribution of the drugs [5]. With these advantages, liposome-based therapeutics were approved for the treatment of various diseases including breast cancer [6, 7], leukemia [8], fungal disease [9], hepatitis [10], and macular degeneration [11]. Also, multiple clinical trials are undergoing currently [12]. The clinically successful liposome-based therapeutics are well summarized in other recent review papers [5, 13]. In this perspective, the conventional advantages of liposome will be overviewed, and newer approaches using liposome for theranostics will be introduced (Fig. 1).

Fig. 1
figure 1

Conventional and newer advantages of liposome for theranostics

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PEGylation: Advantages and Challenges

The first description of liposomes was given in 1965 and liposome has no surface modification [14]. PEGylation enhanced the circulation time and targeting efficiency of liposomes substantially. Many liposome-based therapeutics were based on high enhanced permeability and retention (EPR) effect because of the long circulation time of PEGylated liposome with adequate size [15,16,17]. High hydrophilicity and flexibility of PEG polymers interfere with the hydrophobic interaction between liposomes and serum proteins. Reduced adherence of serum proteins to liposomes reduces the absorption of liposomes by the reticuloendothelial system (RES), such as the liver and spleen, resulting in stealth effects. However, there is still a major challenge for the PEGylated liposome, which is called the accelerated blood clearance (ABC) phenomenon. The ABC phenomenon refers to the faster RES recognition and blood clearance of the nanoparticles (NPs) when the PEGylated NPs are re-injected to the same subject. Formation of anti-PEG IgM antibody is the main reason for the ABC phenomenon. Anti-PEG IgM antibody binds to the re-injected PEGylated NPs. After the recognition by anti-PEG IgM antibody, the NPs are further opsonized by complements and finally cleared by RES [18]. The ABC phenomenon was first noticed in the PEGylated liposomes [19]. The phenomenon was found in mice, rat, rabbits, and guinea pigs [20, 21]. Recently, also in spontaneous canine cancer model, 64Cu-labeled liposomes showed ABC phenomenon by the induction of anti-IgM antibody [21]. Although the immune response to PEGylated liposome is a challenge for the translation of liposomes, the challenge would not be the shortcoming of liposome nanoplatform compared with other types of NPs because the immune response is found in other types of NPs as well. For example, the ABC phenomenon was found in PEGylated reduced-graphene oxide (rGO)-iron oxide hybrid nanoparticles and decreased passive targeting efficiency [18]. Recently, the strategies to overcome ABC have been developed. The strategies include coating the NPs with poly(N-vinyl-2-pyrrolidone) (PVP) [22], hyaluronic acid (HA) [23], or zwitterionic poly(carboxybetaine) (PCB) [24] rather than PEG.

Liposomes as an In Vivo Imaging Platform

Liposomes are utilized as imaging, and theranostic platform since various imaging functionalities can be added easily. For optical imaging, hydrophobic fluorescence dyes are incorporated into the lipid bilayers. A modified fluorescent dye DY-676-C18 was used for in vivo imaging for an edema model of the mouse [25]. Also, fluorescent nanoparticles such as quantum dots can be loaded to the hydrophilic area of the liposomes [26]. Liposome-based magnetic resonance imaging (MRI) contrast agents also developed either loading Gd-DTPA for T1 weighted imaging or iron oxide NPs for T2 weighted imaging [27, 28]. In vivo PET or SPECT imaging can be performed by radiolabeling the liposomes. The liposomes are radiolabeled by loading a chelator in the hydrophilic area of liposome and incubating with radioisotopes [29, 30]. Also, the liposomes can be radiolabeled by the incorporation of the lipid-conjugated bifunctional chelator to the liposome bilayer for further radiolabeling [31]. A glutathione-loaded liposome is used for labeling 99mTc-HMPAO as well [32]. With above-mentioned various methods, therapeutic radiometals also can be labeled to the liposome.

Theranostics Based on Liposome

Recently, there is a growing number of studies using liposome for theranostic applications. Feng et al. reported a 64Cu-labeled and hypoxia-activated prodrug AQ4N and a photosensitizer, hexadecyl amine chlorin e6 (hCe6)-loaded liposome. The liposome can be imaged by PET and has the ability for photodynamic therapy (PDT) under irradiation of 660-nm light emitting diode (LED) [33]. Also, Poly(9,9-dioctylfluorene-2,7-diylco-benzothiadiazole) (PFBT) as fluorescence probe and anticancer drugs (doxorubicin and folate)-loaded liposome showed effective imaging and therapeutic efficacy in tumor-bearing mice [34]. Zhang et al. developed liposome loaded with photosensitizer Ce6, hypoxia-activated prodrug Tirapazamine (TPZ), and gene probe for synergistic photodynamic chemotherapy [35]. Also, Rengan et al. reported the gold-coated liposome NPs for photothermal therapy (PTT) [36]. One report found that 177Lu-labeled liposome showed moderate tumor uptake of 3%ID/g in the tumor-bearing mouse. Thus, 177Lu-labeled liposome could be used for the theranostic platform. While 64Cu-labeled liposome showed similar biodistribution and tumor uptake, thus can be used as a companion imaging agent for 177Lu-labeled liposome [29]. Also, 64Cu liposome (MM-Dx-929) was used for EPR effect surrogate marker of other liposome-based therapeutics. And tumor with a higher uptake of MM-Dx-929 showed better treatment effect of liposome-based therapeutics [30].

Strategies for Controlled Drug Release

The functionality of controlled drug release can be added to liposome [37]. At a certain temperature, the lipid bilayer of the liposome can be transferred from a solid gel phase to a liquid crystalline phase. The temperature is called the melting phase transition temperature, Tm. Encapsulated drugs can be released at Tm. For the synthesis of the thermosensitive liposome, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) is used as a major component, since the Tm of DPPC is 41.4 °C [38]. Tagami et al. reported that Gd-DTPA and DOX-loaded thermosensitive liposome can be utilized as an MRI T1 contrast agent and cancer therapeutics. DOX was released 100% under mild hyperthermia (40–42 °C) [27]. External heating was applied after injection of the theranostic liposome for the treatment of tumor-bearing mouse and showed a superior effect compared with the group without external heating. However, external heating seems not to be applicable in a deeper tumor and bigger animal. The photothermal effect can be used to induce heating for a deeper tumor using near-infrared (NIR) laser coupled with dye or NPs with photothermal effect. Yu et al. developed DOX, indocyanine green (ICG)-loaded liposome, for controlled drug release by 808-nm laser because ICG elicits photothermal effect under the laser irradiation. Cumulative DOX release found to be 20% in vitro by 30 min of NIR laser irradiation, which is a relatively modest amount compared with the local heating method [39]. Meanwhile, the pH-sensitive liposome is used for controlled release of the drugs. Since the extracellular environment of cancer is acidic than of normal tissue or blood, the pH-sensitive liposome is widely utilized for cancer-specific drug release. For pH-sensitive liposome, a neutral cone-shaped lipid dioleoylphosphatidyl-ethanolamine (DOPE) and a weakly acidic amphiphile, such as cholesteryl hemisuccinate (CHEMS), are commonly used [40]. Also, dioleoylphosphatidylcholine (DOPC) [41] or N-succinyl-DOPE [42] is utilized for the pH-sensitive drug release. Zhao et al. reported that tumor-specific pH-responsive peptide (H7K(R2)2)-modified liposome can release the loaded DOX over 80% at a pH of 6.5. The pH-sensitive DOX-loaded liposome showed better tumor control ability than DOX-loaded liposome without pH-sensitive peptide [43]. However, drug release triggered by low pH may not be cancer specific because endosomal and lysosomal lumens of immune cells are also acidic.

Conclusion

Thousands of novel NPs for imaging and therapy other than liposome have been developed after the success of Doxil®. Although these NPs have unique advantages for imaging and therapy, however, in many cases, the NPs suffered from toxicity, low stability in vivo, high RES recognition, and low targeting efficiency [44]. Therefore, liposome is still one of the most viable and promising nanoplatforms because of its biocompatibility, long-circulating ability, and high passive targeting efficiency. Furthermore, liposome has been tested and found to be feasible for adding functionalities including various imaging probes, therapeutics, and controlled release properties. The next goals for the successful theranostics based on liposomes would be to find effective ways to avoid ABC phenomenon on multiple injections and to develop more specific controlled drug release methods.