1. Introduction
The advancements made in nanotechnology in recent years has led to an unprecedented interest in developing targeted therapies for cancer based on nanoparticles (NPs). NPs are defined as nano-sized particles with diameters ranging from 1 to 100 nm [
1,
2,
3]. Although small, NPs have a large surface area and can be used as carriers for a wide range of peptides [
4], antibodies [
5], drugs [
6], or contrast agents [
7]. NPs are widely used as a platform for delivering drugs due to their stable high carrier capacity and their ability to accumulate in tumors through the enhanced permeation and retention effect (EPR) [
8,
9]. Because of the accelerated angiogenesis, tumors are supplied by immature blood vessels with a defective architecture with wide endothelial gaps through which molecules smaller than 700 nm can penetrate [
10,
11,
12]. This characteristic represents the core which led to NPs becoming an important platform for research into cancer theranostics. Inversely, many tumors are heterogenous and possess a dense extracellular matrix which increases interstitial pressure by blocking the passive transport of NPs from the peritumoral vessels [
9], which explains why NPs mostly accumulate in the peritumoral region but fail to penetrate the deep tumoral tissue in experimental applications.
Studies have described techniques to improve the penetration of NPs by using the tumor microenvironment as a targeting site for NPs. One of the constant distinct features of the tumoral microenvironment is the acidic pH, between 0.3 to 0.7 units lower than the pH of normal tissue [
13]. Based on this trait, several studies have designed functionalized NPs, making them responsive to pH changes. Once the pH-functionalized NPs (pH-NPs) penetrate through the endothelium via the EPR effect, they respond to the acidic pH and may either disintegrate and release drugs or change their size and shape, thus enhancing their capacity to diffuse towards the tumors’ core. In recent years, several studies have described various and heterogenous methods to sensitize NPs to pH changes; thus, in this current scoping review, we aimed to map current protocols for pH functionalization and analyze the antitumoral outcomes of drug-loaded pH-NPs.
4. Discussion
Our results show that NPs may be used as pH-responsive platforms with excellent results in tumor penetration and tumor regression rates. pH-NPs, regardless of being metallic or polymeric, were shown to have good tumor penetration in most experimental malignant cell lines in vivo.
Polymers were the most common nanomaterials used in the synthesis of pH-NPs. Besides being used for surface coating to increase the colloidal stability of NPs, polymers (e.g., PEG, PLGA, PHA) were used in the core structure of NPs, making polymeric NPs a widely used platform due to their key advantages: biocompatibility, high stability, non-toxicity, easy synthesis, and versatility. Chemotherapeutics can be linked onto or within the polymers via electrostatic interactions. Once assembled, polymeric NPs have high stability in blood circulation and can maintain the EPR effect, which allows them to escape in the tumoral microenvironment, where drugs are released in a controlled fashion [
140]. Mesoporous silica nanoparticles (MSN NPs) were also commonly used to design pH-responsive nanocarriers. The main advantage of MSN NPs is their large surface area and large porous structure, in which a high volume of drugs can be encapsulated. Their surface can be also chemically modified to attach various linkers which react to pH changes [
141]. Lipid NPs are usually spherical in shape and formed by a bilayer lipid membrane and an aqueous core. They are highly biocompatible and can transport hydrophilic, hydrophobic, and lipophilic drugs; however, lipid NPs can be cleared by the reticuloendothelial system. For this reason, their surface is usually coated with polymers (e.g., PEGylation) to increase their biostability [
142]. Gold NPs can be pH-functionalized using surface pH-responsive linkers. Gold NPs have unique optical characteristics, making them suitable for cancer theranostics and photothermal therapy [
143].
The tumor specificity of pH-NPs was further enhanced using tumor-targeting peptides linked to the surface of NPs which can target specific receptors commonly expressed by cancers. The folate receptor is known to be overexpressed in various tumors [
144] and was used as a target for NPs coated with folic acid, which facilitates the receptor-mediated endocytosis of NPs, where drug cargo can be released in the acidic intracellular environment. Other studies used Fe ions attached to the surface of NPs, as many tumors use Fe for cellular proliferation [
145]. Increased expression of transferrin on tumors promotes NPs attachment and internalization [
146]. Xie et al. [
120] used methotrexate as an antitumor agent and also as a tumor-targeting agent due to its structural similarity to folic acid and capacity to bind to folate expressed by tumors. Gong et al. [
49] used arginine–glycine–aspartate triad (RGD peptide) which is a low-toxicity, highly stable peptide with increased affinity to integrins, which in turn are overexpressed by tumoral neo-vessels.
Doxorubicin is the most used chemotherapeutic in current experiments. Doxorubicin is an anthracycline with potent antimitotic and cytotoxic activity. Its mechanism of action involves intercalation between base pairs where it inhibits DNA synthesis and, in addition, inhibits topoisomerase II activity, thus reducing DNA replication [
147,
148]. Despite having excellent antitumor activity, its use is limited by important side effects, such as cardiotoxicity and myelosuppression [
148]. In a conjugated form, incorporated in the hydrophobic core of nanocarriers, doxorubicin can be administered in higher doses, and can be released at the tumor site where nanoparticles accumulate through enhanced permeability release or by active tumor targeting through pH-dependent conversion, as demonstrated in the included studies.
Drugs are usually loaded into NPs either through core encapsulation or surface bounding. Core encapsulation refers to the organization of NPs around drugs, usually due to their amphipathic property, and the hydrophobic end safeguards the drugs in the center, while the hydrophilic end forms a protective shell, enabling a safe transport of cargo to the tumor. Another way is to attach drugs to the surface of NPs, especially when PEGylation is used to coat the surface. PEG is a stable carrier and binder, and various linkers can be used to attach drugs or tumor-targeting receptors to its surface.
Acid-labile Schiff base linkages were the core from which nanoparticles, regardless of type, were designed to respond to pH changes. Imine Schiff bases undergo hydrolyzation under acidic conditions and such are used as linkers when nanoparticles are assembled. Once the peritumoral acidic pH is sensed, the linkers break, causing disruption of the nanocarriers and release of drugs. In other scenarios, the nanocarriers were coated with tumor-targeting peptides (e.g., folic acid, AS1411 aptamer) which interacted with cancer cells and allowed for the nanocarriers to reach the intracellular environment, via endocytic pathways, where the drugs were released. Another pH sensitization method is the use of electrostatic interactions. pH-NPs were coated with a negative-charged surface which reverted to a positive charge in the acidic environment, leading to the release of positively charged peptides, which were linked to drugs [
42].
Functionalized NPs may become a cornerstone in cancer treatment as they can overcome the barrier of systemic toxicity produced by non-targeted chemotherapeutics and can increase the amount of drug delivered to the tumor. Designing NPs responsive to acidic pH has proven to be a solid option. However, we must consider that, in most studies, the maximal effects of pH-NPs were at a pH lower than 6.5. To ensure similar outcomes in clinical studies, pH-NPs need to be ultra-sensitized to release similar amounts of drugs at pH values of 6.8–7.2, which is the usual pH value in the tumor microenvironment.