Peptide-Based Inorganic Nanoparticle – A Promising Intracellular Drug Delivery System

Abstract

Peptide-based inorganic nanoparticles (PINPs) have been a preferred system for drug delivery system. This is due to their functional and unique properties. Basically, PINPs are hybrid in nature – biomolecule and inorganic metal (like Fe, Ag) – thus they exhibit high surface area, changeable optical/magnetic properties. This leads to a better biocompatibility, responsive and more targeted. The PIPNs are used in imaging/theragnostic, tailoring targeted delivery of nanomedicines, enhancing stability of drug and improve cellular uptake. Though various advantages present there are still some drawbacks it contains such as in vivo stability, evaluation techniques and immunogenicity. In this review, formulation methods, types and characterization methods have been briefly discussed. We hope, in future PINPs will be significantly used for drug delivery and diagnostics.

Keywords

Introduction

Table 1: Structure based peptides classification

(a) Based on Function

Table 2: Function based peptides classification

Synthesis Methods Of PINPS

Figure 1: Schematic representation of Peptide-gold NPs synthesis

(a) Conjugation Methods

In this first the MNPs are formed and then modified using functional peptides. Because of this, we can control the orientation of peptide, bioactivity and density. This method is preferred for synthesising stimuli responsive, targeted and multifunctional PINPs. First step involves, the preparation of naked or ligand stabilizes MNPs (e.g.: AuNPs (gold), FeO, AgNPs) cores. These have reactive surface groups such as amine, carboxyl and hydroxyl and these are conjugated with desired peptides through click chemistry, EDH coupling etc. ^{[21, 22]}

(b) One-Pot Green Synthesis

Avoiding synthetic and harsh reducing agents such as sodium borohydride, one-pot green method employs naturally obtained molecules. In this method, the complete process occurs in a single pot under optimal conditions (ambient temperature, aqueous solvent, neutral pH). The modification PINPs can be done by simply changing the conditions such as pH, temperature, concentration of metal ions and peptide. For example, increasing the concentration of peptide results in smaller sized NPs due to rapid surface passivation. Peptide mediated synthesis of PINPs can be reliable and sustainable one which is simple, green and eco-friendly. ^{[23, 24]}

Cellular Uptake Mechanisms of Peptide-INPS (PINPS)

PINPs enter the cytosol by various mechanism which can be classified into in-direct and direct way of cellular uptake. Clathrin mediated endocytosis, phagocytosis, caveolae dependent are comes under in-direct entry. Electroporation, passive diffusion, direct microinjection are come under direct entry mechanism. ^{[25, 26]} The hydrophobic and the cations plays a major role in cellular uptake mechanism. Lipid bilayer of the cell membrane is disturbed by insertion of hydrophobic molecules and loosen the lipid packing thus membrane permeation is increased. Simultaneously, the cations (e.g. arginine, lysine) makes electrostatic interaction with anionic phospholipids and boost the membrane permeation. This combined action of the two paves way for pore formation or full membrane disruption, helps the NPs to translocate into the cytosol. ^[27-29]

Application Of PINPs in Drug Delivery

Targeted Delivery

Many delivery agents or carriers have been developed for targeted drug delivery but PINPs has their own benefits to get into the top. Easy modification of the physicochemical properties, high biocompatibility compared to others and dual action of peptide-metal ion NPs. For example, doxorubicin-loaded AuNPs functionalized with RGD peptides have great tumour accumulation and decreased toxicity compared to free drug formulations. AuNPs has photothermal conversion efficiency and high drug loading capacity thus it acts as a great tool for chemo-photothermal therapy. ^{[30, 31]} PINPs provide us a reliable and robust platform for targeted drug delivery by combining the benefits of peptides and metal nanocores. Thes hybrid systems helps to reduce side effects, increase therapeutic precision, to develop personalized and image-guided medicine.

Stimuli-Responsive

The diseased site is always differ from the normal condition site in a body, it’s because of biological chemical released at the site, when a stimuli-resposive drug delivery system comes into contact with the diseased site, in response to the change (trigger) the drug gets released. Two types of stimuli are present internal (pH, enzymes) and external (light, temperature). ^[32]

(a) pH-responsive systems

The microenvironment condition of tumour has acidic pH (~ 6.5). Peptides, which are sensitive to acidic environment undergo conformational changes then the drug is released. Histidine combined AuNPs undergone conformational changes and releases drug at the site. ^[33]

(b) Redox-responsive systems

High concentration of glutathione (GSH) is present intracellular in cancer cells. This is aa reductive condition thus disulphide-bridged peptides will break in this area which results in intracellular drug release. Paclitaxel loaded AgNPs modified with disulphide bonds showed drug release within tumour cells. ^{[34, 35]}

(c) Enzyme-responsive systems

Proteolytic enzymes such as cathepsins, matric metalloproteinases (MMPs) are present highly in the cancer cells. Simply, linking the MNPs with these proteolytic enzyme cleavable molecules will result in site specific drug release upon exposure to these enzymes. ^[36]

(d) Light and Heat responsive systems

Silver and gold NPs have high surface plasmon resonance (SPR), converts light into heat when exposed to near-IR irradiation. Through photothermal disruption, AuNPs or AgNPs modified using this peptides and drug can release the drug at targeted site. ^[37]

Limitation of PINPs

Stability

Degradation, opsonisation and aggregation are the common problems arises immediately after introduction of PINPs in the plasma. Even though peptides are biocompatible, they are vulnerable to enzymatic action. Metal core (Au, Ag, Fe) may get oxidised or undergo surface changes, especially on salt (Na⁺, K⁺) and physiological pH conditions. These interaction results in reduced circulation time, premature drug release, decreased target action and therapeutic efficacy. ^[38]

Immunogenicity and Toxicity

Peptide alone, biodegradable and less immunogenic but when combined with MNPs they obtain immunogenic properties. Silver and gadolinium metals may release toxic ions or reactive oxygen species, results in cytotoxicity and inflammatory responses. Of all PINPs, AuNPs is the most inert. ^[39]

Scalability

Industry-scale production of peptides is cost and time consuming. A slight changes in the conditions or quality of material used, results in peptide functionalization capacity reduction. Nanoparticle formation also tedious process, maintaining optimal temperature, pH and reactant ratios is crucial one, to confirm the uniform size, drug loading capacity are achieved. More pilot scale-up study should be done to get a standard process, quality control test and to reduce variations and to conform product reproducibility. ^[40]

CONCLUSION

PINPs, a fast-growing delivery system in nanomedicine, which is a great fusion of biological peptides and inorganic metals. Peptides help us to increase the biocompatibility, target specific area and also sometimes carry a biological therapeutic action. While, metals such gold, iron oxide, silver act a carrier, a physicochemical property modifier and helps us to diagnose. Addressing the problems such as stability in in-vivo, immunogenicity, toxicity of metals and scalability will make PINPs a promising delivery system for cancer treatment. PINPs taking place in theragnostic integration, peptide engineering and AI driven design which results in patient-tailored nanomedicines.

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