A noticeable phenomenon: thiol terminal PEG enhances the immunogenicity of PEGylated emulsions injected intravenously or subcutaneously into rats

Engineering aspects of lipid-based delivery systems: In vivo gene delivery, safety criteria, and translation strategies

Defects in the genome cause genetic diseases and can be treated with gene therapy. Due to the limitations encountered in gene delivery, lipid-based supramolecular colloidal materials have emerged as promising gene carrier systems. In their non-functionalized form, lipid nanoparticles often demonstrate lower transgene expression efficiency, leading to suboptimal therapeutic outcomes, specifically through reduced percentages of cells expressing the transgene. Due to chemically active substituents, the engineering of delivery systems for genetic drugs with specific chemical ligands steps forward as an innovative strategy to tackle the drawbacks and enhance their therapeutic efficacy. Despite intense investigations into functionalization strategies, the clinical outcome of such therapies still needs to be improved. Here, we highlight and comprehensively review engineering aspects for functionalizing lipid-based delivery systems and their therapeutic efficacy for developing novel genetic cargoes to provide a full snapshot of the translation from the bench to the clinics. We outline existing challenges in the delivery and internalization processes and narrate recent advances in the functionalization of lipid-based delivery systems for nucleic acids to enhance their therapeutic efficacy and safety. Moreover, we address clinical trials using these vectors to expand their clinical use and principal safety concerns.

Evaluation of the Antitumor Effect and Immune Response of Micelles Modified with a Polysialic Acid-D-α-Tocopheryl Polyethylene Glycol 1000 Succinate Conjugate

D-α-Tocopheryl polyethylene glycol 1000 succinate (TPGS) has shown potential applications in cancer therapy owing to its attractive properties, including reversal of multi-drug resistance and synergistic effects with antitumor drugs. However, its associated shortcomings cannot be underestimated, including activation of the body's immune response and acceleration of blood clearance of polyethylene glycolylated preparations. Polysialic acid (PSA) is a polysaccharide homopolymer, with the dual function of immune camouflage and tumor targeting. PSA and TPGS conjugates (PSA-TPGS) were synthesized to weaken the immune risks of TPGS. We developed PSA-TPGS and TPGS self-assembled mixed micelles and encapsulated the classical antineoplastic, docetaxel. The particle size of docetaxel-loaded mixed micelles was 16.3 ± 2.0 nm, with entrapment efficiency of 99.0 ± 0.9% and drug-loading efficiency of 3.20 ± 0.03%. Antitumor activity studies revealed that the mixed micelles showed better tumor inhibition than Tween 80 and TPGS micelles. Detection of the accelerated blood clearance (ABC) phenomenon demonstrated that insertion of PSA-TPGS into the micelles weakened the ABC phenomenon induced by TPGS. In summary, PSA-TPGS could be a potential nanocarrier to improve antitumor activity and weaken immune responses.

Red blood cell membrane-coated upconversion nanoparticles for pretargeted multimodality imaging of triple-negative breast cancer

Upconversion nanoparticles (UCNPs) have been widely employed for tumor imaging using magnetic resonance imaging (MRI) and upconversion luminescence (UCL) imaging. The short blood clearance time and immunogenicity of UCNPs have limited their further application in vivo. We have designed UCNPs camouflaged with an exterior red blood cell (RBC) membrane coating (RBC-UCNPs) to solve these problems. Moreover, because of some intrinsic disadvantages of MRI and UCL imaging, we investigated the use of pretargeted RBC-UCNPs for positron-emission tomography (PET) imaging to obtain more comprehensive information. Our data showed that RBC-UCNPs retained the immunity feature from the source cells and the superior optical and chemical features from the pristine UCNP cores. The tumor-targeting ability of RBC-UCNPs was enhanced by binding 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[folate(polyethylene glycol)-2000] (DSPE-PEG-FA) molecules onto the cell membranes. PET imaging with short half-life radionuclides to visualize the RBC-UCNPs was successfully realized by a combination of pre-targeting and in vivo click chemistry. Blood chemistry, hematology, and histologic analysis suggested good in vivo biocompatibility of the RBC-UCNPs. Our method provides a new potential biomedical application of biomimetic nanoparticles.

Blood retention and antigenicity of polycarboxybetaine-modified liposomes

Zwitterionic polycarboxybetaines (PCBs) have gained attention as alternative stealth polymers whose liposomal formulation and protein conjugates were reported not to elicit anti-polymer antibodies. Here, we studied the blood retention and antigenicity of liposomes modified with PCBs focusing on their chemical structures and doses. We compared PCBs with either 1 or 3 (PCB1 or PCB3) spacer carbons between the carboxylate and ammonium groups. PCB3-modified liposomes had a short blood retention, whereas PCB1-modified liposomes demonstrated extended blood retention that was somewhat superior to PEGylated liposome. This confirmed the excellent non-fouling nature of PCB1 reported previously. Interestingly, PCB1-liposome as well as PCB3-liposome elicited specific IgMs toward each PCB. The dose-dependent production of specific IgMs to PCB-liposomes was similar to that of PEGylated liposome, i.e., high doses of PCB-liposomes reduced the production of specific IgMs, termed immunological tolerance. These results indicate the importance of investigating the effect of dose to clarify the existence of antigenicity of stealth polymers.

Effects of Uncleavable and Cleavable PEG-Lipids with Different Molecular Weights on Accelerated Blood Clearance of PEGylated Emulsions in Beagle Dogs

To investigate the effect of polyethylene glycol (PEG) molecular weights on circulation time of PEGylated emulsions and the second injection of injected PEGylated emulsions, we studied the effect of molecular weights on the pharmacokinetic behavior of PEG-DSPE (modified emulsions with 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-n-[methoxy (polyethyleneglycol)]) and PEG-CHMC (modified emulsions with poly(ethyleneglycol)-cholesteryl carbonate) emulsions in beagle dogs. The "accelerated blood clearance" (ABC) phenomenon was induced. Through this study, the contribution of PEG molecular weights on the ABC phenomenon was further clarified, and the results provided guidance for lessening or eliminating the ABC phenomenon. We injected different PEG-modified emulsions with 10% PEG-modified density into beagle dogs at 2 μmol phospholipids kg^-1 and the blood samples were drawn after 1 min, 3 min, 5 min, 10 min, 15 min, 30 min, 60 min, 120 min, 240 min, 360 min, 600 min, and 24 h. Then, concentrations of the drug were assayed using high-performance liquid chromatography (HPLC). The results showed that the circulation times of PEG-DSPE-modified emulsions were significantly different because of the difference in molecular weights, whereas those of the PEG-CHMC modified emulsions were not. The spatial conformation of PEG with small molecular weights (PEG400, PEG600, and PEG800) was more likely to induce a strong ABC phenomenon. The results of our work suggest the interaction of circulation time and PEG molecular weights on the ABC phenomenon, implying that the spatial conformation of PEG has advantages that alleviate the ABC phenomenon. Importantly, the results have implications for the choice of molecular weights of PEG for PEGylated formulations.

The Importance of Poly(ethylene glycol) Alternatives for Overcoming PEG Immunogenicity in Drug Delivery and Bioconjugation

Poly(ethylene glycol) (PEG) is widely used as a gold standard in bioconjugation and nanomedicine to prolong blood circulation time and improve drug efficacy. The conjugation of PEG to proteins, peptides, oligonucleotides (DNA, small interfering RNA (siRNA), microRNA (miRNA)) and nanoparticles is a well-established technique known as PEGylation, with PEGylated products have been using in clinics for the last few decades. However, it is increasingly recognized that treating patients with PEGylated drugs can lead to the formation of antibodies that specifically recognize and bind to PEG (i.e., anti-PEG antibodies). Anti-PEG antibodies are also found in patients who have never been treated with PEGylated drugs but have consumed products containing PEG. Consequently, treating patients who have acquired anti-PEG antibodies with PEGylated drugs results in accelerated blood clearance, low drug efficacy, hypersensitivity, and, in some cases, life-threatening side effects. In this succinct review, we collate recent literature to draw the attention of polymer chemists to the issue of PEG immunogenicity in drug delivery and bioconjugation, thereby highlighting the importance of developing alternative polymers to replace PEG. Several promising yet imperfect alternatives to PEG are also discussed. To achieve asatisfactory alternative, further joint efforts of polymer chemists and scientists in related fields are urgently needed to design, synthesize and evaluate new alternatives to PEG.

Main trends of immune effects triggered by nanomedicines in preclinical studies

The application of nanotechnology to emerging medicinal products is a crucial parameter for the implementation of personalized medicine. For example, sophisticated drug delivery systems can target the diseased tissue by recognizing patient-specific biomarkers while carrying pharmacologically active molecules. However, such nanomedicines can be recognized by the immune system as foreign triggering unexpected biological reactions. The anticipation of the immunogenic potential of emerging nanotechnology-based products in the preclinical phase is challenging due to high interspecies variations between the immune systems of laboratory animals and humans. A close monitoring of the scientific literature is required to better understand the relationship between various immune reactions and the diversity of nanomedicines currently in the development pipeline. We have reviewed the most frequent immune reactions induced by the nanomaterials in vivo and have identified the main effects triggered by lipid-based, polymer-based and inorganic nanoparticles, as the main categories of nanomaterials used in medicine. According to our results, almost 50% of the investigated nanomaterials induced effects related to the activation of the immune system. Among them, complement activation-related hypersensitivity reactions and activation of adaptive immune response were the most frequent effects reported for the lipid-based nanoparticles. However, many of these effects are not or are only partially covered by the current regulatory framework applicable for nanomedicines. In addition, we extracted the most relevant nanospecific properties responsible for the observed biological effects. Our analysis led to identification of the most prevalent measurement endpoints relevant for the assessment of the immunotoxic potential of the nanotechnology-based products and will support the smooth and safe translation of the new formulations to clinical applications.

The effect of monosialylganglioside mix modifying the PEGylated liposomal epirubicin on the accelerated blood clearance phenomenon

PEGylated liposomes are potential candidates to improve the pharmacokinetic characteristics of encapsulated drugs, to extend their circulation half-life and facilitate their passive accumulation at tumour sites. However, PEG-modified liposomes can induce accelerated blood clearance (ABC) upon repeated administration, and the extent of ABC phenomenon on the cytotoxic drugs-containing PEGylated liposomes is related to the dose of the cytotoxic drugs. In this study, EPI served as a model cytotoxic drug, a hydrophilic surfactant molecule, monosialylganglioside (GM1) was chosen and modified on the liposomes together with PEG. It was shown that upon mixed modification, when GM1 contents reached 10% or 15% mol, the ABC phenomenon of the PEGylated liposomal EPI significantly reduced. We also found that GM1 played an important role in abrogating the ABC phenomenon in both the induction phase and the effectuation phase. The results suggested that GM1 incorporation unfortunately did not avoid occurrence of ABC phenomenon completely, but GM1 modification on PEGylated liposomes may provide a significant improvement in clinical practice of PEGylated liposomes. Further study must be necessary.

Effects of complement inhibition on the ABC phenomenon in rats

Researchers reported that intravenously injected PEGylated colloidal drug carriers lose their long-circulating characteristic and accumulated extensively in liver when they are administrated twice in the same animal with certain intervals. This phenomenon was referred to as the "accelerated blood clearance (ABC) phenomenon". Some former studies had found that complement-mediated phagocytosis, activated by antigen-antibody complex, was responsible for inducing the phenomenon. According to the theory, we have used cobra venom factor to deplete complement in vivo and to investigate the effect of complement inhibition on the ABC phenomenon. Rats were administered by injection of cobra venom factor solution to build up the model of complement exhaustion/inhibition, and the effect of the inhibition of complement on ABC phenomenon was carried out. It seemed that inhibition of complement didn't affect the pharmacokinetic of the first infection. By contrast, in rats of which complement had been depleted, the second dose of PEGylated nanoemulsions showed enhanced circulation time compared with normal rats in a complement inhibition-independent manner, but the ABC phenomenon was not completely eliminated. It indicated that complement inhibition could certainly weaken the accelerated clearance; meanwhile, there were other factors causing the ABC effect. These findings provide novel insights into the attenuating of ABC phenomenon and lay foundation for further study of immune mechanism.

Accelerated blood clearance phenomenon upon cross-administration of PEGylated nanocarriers in beagle dogs

The cross-administration of nanocarriers modified by poly(ethylene glycol) (PEG), named PEGylated nanocarriers, a type of combination therapy, is becoming an increasingly important method of long-term drug delivery, to decrease side effects, avoid multidrug resistance, and increase therapeutic efficacy. However, repeated injections of PEGylated nanocarriers induces the accelerated blood clearance (ABC) phenomenon, prevents long circulation, and can cause adverse effects owing to alterations in the biodistribution of the drug. Although the nature of the ABC phenomenon that is induced by repeated injections of PEGylated nanocarriers has already been studied in detail, there are few reports on the immune response elicited by the cross-administration of PEGylated nanocarriers. In this study, we investigated the ABC phenomenon induced by the intravenous cross-administration of various PEGylated nanocarriers, including PEGylated liposomes (PL), PEG micelles (PM), PEGylated solid lipid nanoparticles (PSLN), and PEGylated emulsions (PE), in beagle dogs. The results indicated that the magnitude of the immune response elicited by the cross-administration was in the following order (from the strongest to the weakest): PL, PE, PSLN, PM. It is specifically PEG in the brush structure that elicits a significant immune response, in both the induction phase and the effectuation phase. Furthermore, the present study suggests that there is a considerable difference between the effect of repeated injections and cross-administration, depending on the colloidal structure. This work is a preliminary investigation into the cross-administration of PEGylated nanocarriers, and our observations can have serious implications for the design of combination therapies that use PEGylated vectors.

Anti-PEG immunity: emergence, characteristics, and unaddressed questions

The modification of protein and nanoparticle therapeutics with polyethylene glycol (PEG), a flexible, uncharged, and highly hydrophilic polymer, is a widely adopted approach to reduce RES clearance, extend circulation time, and improve drug efficacy. Nevertheless, an emerging body of literature, generated by numerous research groups, demonstrates that the immune system can produce antibodies that specifically bind PEG, which can lead to the 'accelerated blood clearance' of PEGylated therapeutics. In animals, anti-PEG immunity is typically robust but short-lived and consists of a predominantly anti-PEG IgM response. Rodent studies suggest that the induction of anti-PEG antibodies (α-PEG Abs) primarily occurs through a type 2 T-cell independent mechanism. Although anti-PEG immunity is less well-studied in humans, the presence of α-PEG Abs has been correlated with reduced efficacy of PEGylated therapeutics in clinical trials. The prevalence of anti-PEG IgG and reports of memory immune responses, as well as the existence of α-PEG Abs in healthy untreated individuals, suggests that the mechanism(s) and features of human anti-PEG immune responses may differ from those of animal models. Many questions, including the incidence rate of pre-existing α-PEG Abs and immunological mechanism(s) of α-PEG Ab formation in humans, must be answered in order to fully address the potential complications of anti-PEG immunity.

Tolerance-like innate immunity and spleen injury: a novel discovery via the weekly administrations and consecutive injections of PEGylated emulsions

There has been an increasing interest in the study of the innate immune system in recent years. However, few studies have focused on whether innate immunity can acquire tolerance. Therefore, in this study, we investigated tolerance in the innate immune system via the consecutive weekly and daily injections of emulsions modified with polyethylene glycol (PEG), referred to as PEGylated emulsions (PE). The effects of these injections of PE on pharmacokinetics and biodistribution were studied in normal and macrophage-depleted rats. Additionally, we evaluated the antigenic specificity of immunologic tolerance. Immunologic tolerance against PE developed after 21 days of consecutive daily injections or the fourth week of PE administration. Compared with a single administration, it was observed that the tolerant rats had a lower rate of PE clearance from the blood, which was independent of the stress response. In addition, weekly PE injections caused injury to the spleen. Furthermore, the rats tolerant to PEs with the methoxy group (-OCH3) of PEG, failed to respond to the PEs with a different terminal group of PEG or to non-PEG emulsions. Innate immunity tolerance was induced by PE, regardless of the mode of administration. Further study of this mechanism suggested that monocytes play an essential role in the suppression of innate immunity. These findings provide novel insights into the understanding of the innate immune system.