Endosomal escape for cell-targeted proteins. Going out after going in

Protein-based nanocarriers are versatile and biocompatible drug delivery systems. They are of particular interest in nanomedicine as they can recruit multiple functions in a single modular polypeptide. Many cell-targeting peptides or protein domains can promote cell uptake when included in these nanoparticles through receptor-mediated endocytosis. In that way, targeting drugs to specific cell receptors allows a selective intracellular delivery process, avoiding potential side effects of the payload. However, once internalized, the endo-lysosomal route taken by the engulfed material usually results in full degradation, preventing their adequate subcellular localization, bioavailability and subsequent therapeutic effect. Thus, entrapment into endo-lysosomes is a main bottleneck in the efficacy of protein-drug nanomedicines. Promoting endosomal escape and preventing lysosomal degradation would make this therapeutic approach clinically plausible. In this review, we discuss the mechanisms intended to evade lysosomal degradation of proteins, with the most relevant examples and associated strategies, and the methods available to measure that effect. In addition, based on the increasing catalogue of peptide domains tailored to face this challenge as components of protein nanocarriers, we emphasize how their particular mechanisms of action can potentially alter the functionality of accompanying protein materials, especially in terms of targeting and specificity in the delivery process.

Fusogenic peptide delivery of bioactive siRNAs targeting CSNK2A1 for treatment of ovarian cancer

Ovarian cancer has shown little improvement in survival among advanced-stage patients over the past decade. Current treatment strategies have been largely unsuccessful in treating advanced disease, with many patients experiencing systemic toxicity and drug-resistant metastatic cancer. This study evaluates novel fusogenic peptide carriers delivering short interfering RNA (siRNA) targeting casein kinase II, CSNK2A1, for reducing the aggressiveness of ovarian cancer. The peptides were designed to address two significant barriers to siRNA delivery: insufficient cellular uptake and endosomal entrapment. The three peptide variants developed, DIVA3, DIV3H, and DIV3W, were able to form monodisperse nanoparticle complexes with siRNA and protect siRNAs from serum and RNase degradation. Furthermore, DIV3W demonstrated optimal delivery of bioactive siRNAs into ovarian cancer cells with high cellular uptake efficiency and mediated up to 94% knockdown of CSNK2A1 mRNA compared with non-targeting siRNAs, resulting in decreased cell migration and recolonization in vitro. Intratumoral delivery of DIV3W-siCSNK2A1 complexes to subcutaneous ovarian tumors resulted in reduced CSNK2A1 mRNA and CK2α protein expression after 48 h and reduced tumor growth and migration in a 2-week multi-dosing regimen. These results demonstrate the potential of the DIV3W peptide to deliver bioactive siRNAs and confirms the role of CSNK2A1 in cell-cell communication and proliferation in ovarian cancer.

Peptide-based delivery of therapeutics in cancer treatment

Current delivery strategies for cancer therapeutics commonly cause significant systemic side effects due to required high doses of therapeutic, inefficient cellular uptake of drug, and poor cell selectivity. Peptide-based delivery systems have shown the ability to alleviate these issues and can significantly enhance therapeutic loading, delivery, and cancer targetability. Peptide systems can be tailor-made for specific cancer applications. This review describes three peptide classes, targeting, cell penetrating, and fusogenic peptides, as stand-alone nanoparticle systems, conjugations to nanoparticle systems, or as the therapeutic modality. Peptide nanoparticle design, characteristics, and applications are discussed as well as peptide applications in the clinical space.

Identification and optimization of tunable endosomal escape parameters for enhanced efficacy in peptide-targeted prodrug-loaded nanoparticles

Endosomal escape of nanoparticles (NPs) is a weighty consideration for engineering successful nanomedicines. Although it is well-established that incorporation of histidine (His) in particle design improves endosomal escape for NPs, our understanding of its effects for ligand-targeted nanoparticles (TNPs) remains incomplete. Here, we systematically evaluated the cooperativity between targeting ligands and endosomolytic elements using liposomal TNPs with precise stoichiometric control over functional moieties (>90% loading efficiency). We synthesized endosomolytic lipid conjugates consisting of 1 to 10 consecutive His residues presented at the end of linkers between 2 to 45 repeating units of ethylene glycol (His_n-EG_m). His_n-EG_m had minimal effect on NP size (∼115 nm) and had no significant effect on the receptor specificity of TNPs (>90% inhibition by competing peptide). We evaluated various formulations with 8 different targeting ligands relevant to two disease models. Incorporation of His₁-EG₈ resulted in up to ∼170- and ∼12.9-fold enhancement in intracellular accumulation relative to non-endosomolytic NP and TNP, respectively. These observations were time-dependent, targeted receptor-dependent, and showed different trends for NPs and TNPs. Further evaluation demonstrated short linkers (EG_2-4) significantly enhanced nanoparticle internalization compared to EG₈ or longer by up to ∼2.5-fold. Finally, rationally optimized formulation, His₁-EG₂-TNP, improved in vitro toxicity of a DM1 prodrug to SK-BR-3 cells by ∼4.2-fold, with IC₅₀ ∼8.5 nM compared to ∼36 nM for no-His TNP, and >100 nM for non-targeted/no-His NP. This study uncovers an intricate relationship between endosomal escape and ligand-targeted drug delivery, as well as tunable parameters. Furthermore, our findings highlight the value of rational design and systematic analysis for optimization of multifunctional NPs.

Incorporation of Endosomolytic Peptides with Varying Disruption Mechanisms into EGFR-Targeted Protein Conjugates: The Effect on Intracellular Protein Delivery and EGFR Specificity in Breast Cancer Cells

Intracellular delivery of protein therapeutics remains a significant challenge limiting the majority of clinically available protein drugs to extracellular targets. Strategies to deliver proteins to subcellular compartments have traditionally relied on cell-penetrating peptides, which can drive enhanced internalization but exhibit unreliable activity and are rarely able to target specific cells, leading to off-target effects. Moreover, few design rules exist regarding the relative efficacy of various endosomal escape strategies in proteins. Accordingly, we developed a simple fusion modification approach to incorporate endosomolytic peptides onto epidermal growth factor receptor (EGFR)-targeted protein conjugates and performed a systematic comparison of the endosomal escape efficacy, mechanism of action, and capacity to maintain EGFR-targeting specificity of conjugates modified with four different endosomolytic sequences of varying modes of action (Aurein 1.2, GALA, HA2, and L17E). Use of the recently developed Gal8-YFP assay indicated that the fusion of each endosomolytic peptide led to enhanced endosomal disruption. Additionally, the incorporation of each endosomolytic peptide increased the half-life of the internalized protein and lowered lysosomal colocalization, further supporting the membrane-disruptive capacity. Despite this, only EGFR-targeted conjugates modified with Aurein 1.2 or GALA maintained EGFR specificity. These results thus demonstrated that the choice of endosomal escape moiety can substantially affect targeting capability, cytotoxicity, and bioactivity and provided important new insights into endosomolytic peptide selection for the design of targeted protein delivery systems.

Collaborative membrane activity and receptor-dependent tumor cell targeting for precise nanoparticle delivery in CXCR4+ colorectal cancer

By the appropriate selection of functional peptides and proper accommodation sites, we have generated a set of multifunctional proteins that combine selectivity for CXCR4⁺ cell binding and relevant endosomal escape capabilities linked to the viral peptide HA2. In particular, the construct T22-GFP-HA2-H6 forms nanoparticles that upon administration in mouse models of human, CXCR4⁺ colorectal cancer, accumulates in primary tumor at levels significantly higher than the parental T22-GFP-H6 HA2-lacking version. The in vivo application of a CXCR4 antagonist has confirmed the prevalence of the CXCR4⁺ tumor tissue selectivity over unspecific cell penetration, upon systemic administration of the material. Such specificity is combined with improved endosomal escape, what overall results in a precise and highly efficient tumor biodistribution. These data strongly support the functional recruitment as a convenient approach to generate protein materials for clinical applications. More precisely, they also support the unexpected concept that enhancing the unspecific membrane activity of a protein material does not necessarily compromise, but it can even improve, the selective cell targeting offered by an accompanying functional module. STATEMENT OF SIGNIFICANCE: We have shown here that the combination of cell-penetrating and tumor cell-targeting peptides dramatically enhances precise tumor accumulation of protein-only nanoparticles intended for selective drug delivery, in mouse models of human colorectal cancer. This fact is a step forward for the rational design of multifunctional protein nanomaterials for improved cancer therapies.

Recruiting potent membrane penetrability in tumor cell-targeted protein-only nanoparticles

The membrane pore-forming activities of the antimicrobial peptide GWH1 have been evaluated in combination with the CXCR4-binding properties of the peptide T22, in self-assembling protein nanoparticles with high clinical potential. The resulting materials, of 25 nm in size and with regular morphologies, show a dramatically improved cell penetrability into CXCR4⁺ cells (more than 10-fold) and enhanced endosomal escape (the lysosomal degradation dropping from 90% to 50%), when compared with equivalent protein nanoparticles lacking GWH1. These data reveal that GWH1 retains its potent membrane activity in form of nanostructured protein complexes. On the other hand, the specificity of T22 in the CXCR4 receptor binding is subsequently minimized but, unexpectedly, not abolished by the presence of the antimicrobial peptide. The functional combination T22-GWH1 results in 30% of the nanoparticles entering cells via CXCR4 while also exploiting pore-based uptake. Such functional materials are capable to selectively deliver highly potent cytotoxic drugs upon chemical conjugation, promoting CXCR4-dependent cell death. These data support the further development of GWH1-empowered cell-targeted proteins as nanoscale drug carriers for precision medicines. This is a very promising approach to overcome lysosomal degradation of protein nanostructured materials with therapeutic value.

Protein-Based Therapeutic Killing for Cancer Therapies

The treatment of some high-incidence human diseases is based on therapeutic cell killing. In cancer this is mainly achieved by chemical drugs that are systemically administered to reach effective toxic doses. As an innovative alternative, cytotoxic proteins identified in nature can be adapted as precise therapeutic agents. For example, individual toxins and venom components, proapoptotic factors, and antimicrobial peptides from bacteria, animals, plants, and humans have been engineered as highly potent drugs. In addition to the intrinsic cytotoxic activities of these constructs, their biological fabrication by DNA recombination allows the recruitment, in single pharmacological entities, of diverse functions of clinical interest such as specific cell-surface receptor binding, self-activation, and self-assembling as nanoparticulate materials, with wide applicability in cell-targeted oncotherapy and theragnosis.