Research Status of Carrier-free Nano Antitumor Drugs:

The Mechanism of Action and Future Trends of Four Carrier-free

Nanomedicines

Dongjun Zuo

Northwest University, Xi’an, Shanxi, China

Keywords: Anticancer Drugs, Nano Anticancer Drugs, Self-Assembled Systems.

Abstract: At present, there are four mainstream carrier-free nano anti-tumor drugs. The prodrug self-delivery system

uses self-assembled targeted drugs constructed with small groups and anti-cancer drugs. A pure drug

delivery system that uses two or more pure drugs to construct self-assembled drug-drug conjugates. Based

on the self-delivery system of therapeutic carriers, a self-assembled anticancer drug is constructed using

carriers with auxiliary therapeutic effects. Based on the self-delivery system of non-toxic agents, non-toxic

groups are used to assist anticancer drugs to function. Carrier-free nano anticancer drugs solve the side

effects of traditional nano anticancer drugs that nanocarriers cannot be metabolized by the body, and have

broad research prospects.

1 INTRODUCTION

Cancer is the second most harmful disease to human

health. The annual death toll from cancer is second

only to cardiovascular and cerebrovascular diseases.

There are more than 14 million new cancer cases

worldwide each year. At present, humans have made

very impressive research results in cancer. In fact,

all cancers can be cured if they can be detected in

time and treated correctly. The current mainstream

treatment methods include: (1) surgical resection;

(2) use of chemotherapy or other cancer-specific

drugs; (3) use of radiotherapy; (4) immunotherapy;

(5) gene therapy; (6) small molecule targets to

medication (Roy 2016). Chemotherapy is currently a

widely used treatment method. However,

chemotherapy drugs act on the cells of the whole

body and cause serious side effects. Therefore,

precise and efficient drug delivery systems must be

developed. The drug delivery system must solve the

following problems: (1) PK parameters, especially

half-life, biodistribution and maximum drug

concentration; (2) toxic and side effects; (3) target

fixation at the location of the lesion (Vargason

2021).

https://orcid.org/0000-0002-6094-8851

2 THE DEVELOPMENT OF

NANO ANTI-TUMOR DRUGS

With the development of nanotechnology, huge

innovations have taken place in drug delivery

systems. Researchers have developed numerous

nano-carrier drug delivery systems based on

nanotechnology; liposomes; nanopolymers;

dendrimers; micelles, etc. They have different

molecular targets, sizes and surface properties.

Nanomaterials as carriers have many advantages: (1)

increase water solubility and increase the

concentration of drugs in the blood; (2) accurately

target organs, tissues or cells to prevent drug toxicity

from accumulating in other organs such as the liver;

(3) can Combine imaging technology to monitor

drug effects in real time. The use of nanomaterials as

drug carriers is undoubtedly an effective solution to

the serious side effects of traditional

chemotherapeutic macromolecular drugs (Li 2017).

434

Zuo, D.

Research Status of Carrier-free Nano Antitumor Drugs: The Mechanism of Action and Future Trends of Four Carr ier-free Nanomedicines.

DOI: 10.5220/0011214100003443

In Proceedings of the 4th International Conference on Biomedical Engineering and Bioinformatics (ICBEB 2022), pages 434-439

ISBN: 978-989-758-595-1

3 ADVANTAGES AND

CONSTRUCTION OF

CARRIER-FREE

NANO-ANTITUMOR DRUGS

However, the shortcomings of nano-carrier drugs are

also obvious and inevitable. Their drug loading is

low, and most of the carrier systems cannot be

metabolized by the body, prone to inflammation at

the lesion site, complex synthesis operations, and

high cost. Therefore, some researchers have

proposed the concept of carrier-free nano anti-tumor

drugs. "Carrier-free nanomedicine" mainly refers to

a system that does not use additional carriers during

the administration process. Some researchers also

named the carrier-free nanomedicine "carrier-free

drug delivery system". According to the different

construction methods, carrier-free nanomedicine is

divided into several types: prodrug self-delivery,

pure drug self-delivery, self-delivery based on

therapeutic carrier, self-delivery system based on

non-toxic agent (Zhang 2018). Carrier-free pure

nanospheres (PND) composed of pure medicinal

active molecules are currently considered to be the

field with the most research potential. Nanoparticles

are composed of two or more drugs, and the

treatment efficiency can be doubled (Zhao 2015).

This article summarizes, analyzes and summarizes

several most significant carrier-free nano anti-tumor

drugs in the research field of carrier-free nano anti-

tumor drugs.

3.1 Prodrug Self-delivery System

The prodrug self-delivery system, that is, the active

drug and the small group are connected through a

cleavable bond that is responsive to the internal

environment of the tumor, and self-assembled to

form a nanostructure to achieve a self-delivery

modality. Based on the assembly principle of

carrier-free nanomedicine, that is, the use of

amphiphilic hydrophobic drugs to assemble into

drug-drug conjugates in water (Wang 2012). The

construction principle of this drug-drug conjugate is

generally to use the hydrophobicity or hydrophilicity

of the precursor for synthesis, and the formed

prodrug self-delivery system can protect the drug

from rapid clearance and inhibit premature burst

release. However, the conditions of prodrug design

and self-assembly, such as pH, concentration, ionic

strength, and kinetics, also affect the eventual

spontaneous delivery. Small molecule activity

modification drugs can be used to form amphiphilic

self-assembled drugs and complete drug self-

delivery. Because traditional nano-carrier drugs use

high-molecular-weight nano-carriers, nano-carrier

drugs usually have very low drug loading levels

(Chen 2015). In the prodrug delivery system, the

drug is added in a quantitative manner, so it has a

higher drug loading. A key feature of nanomedicine

is to release active drugs from the self-delivery

system and induce apoptosis of cancer cells after

intracellular delivery. Since cancer cells have

physical and chemical environments different from

normal cells, such as pH, redox potential, and

special enzymes and proteins, building sensitive

links such as acid, enzyme and redox sensitive bonds

can effectively improve the success rate of drug

release. Since many anti-cancer drugs are

hydrophobic and do not have the amphipathic

characteristics for self-assembly (Sun 2014).

Therefore, the hydrophilic group is indispensable for

the construction of amphiphilic prodrugs. Due to its

excellent biodegradability, biological activity,

adjustable amphiphilicity and sophisticated synthetic

methods, short peptides have become ideal

candidates for the preparation of amphiphilic

prodrugs. It is worth noting that prodrugs composed

of short peptides and active drugs have unique

advantages in terms of self-assembly potential and

drug-carrying ability. At present, some researchers

have designed a method to bind β-sheet peptide to

anti-cancer camptothecin (CPT), which is to self-

assemble this amphiphilic prodrug into a

nanostructure with a drug loading capacity of up to

38%. The formation of nanostructures can

simultaneously protect the hydrolyzable CPT and

the biodegradable linker from the external

environment. After reaching the tumor-related sites,

the nanomedicine breaks the reducible dithiobutyrate

linkage by hydrolysis to release CPT and induce

cancer cell apoptosis. Therefore, in vitro toxicity

studies have found that reduction-sensitive

nanomedicine exhibits greater cytotoxicity than

insensitive maleimide-linked nanomedicine

(Cheetham 2013).

3.2 Pure Drug Self-delivery System

The pure drug self-delivery system is composed of

pure drug self-polymerizing nano-medicine and

carries out intracellular transportation. The self-

delivery system is established based on a single drug

or multiple drugs, and is a true carrier-free system,

which is finally prepared into a nano-level drug in an

aqueous solution. According to the molecular self-

assembly/co-precipitation process, drugs can

Research Status of Carrier-free Nano Antitumor Drugs: The Mechanism of Action and Future Trends of Four Carrier-free Nanomedicines

435

become nano-objects with specific sizes and shapes

through self-aggregation. If all the inert excipients

are removed, the pure drug can form a nanostructure

through self-aggregation, which can reach the

optimal drug loading of 100%, which enhances the

anti-cancer activity of the drug and avoids the

relative toxicity and immunogenicity of the carrier.

In order to achieve this goal, a variety of free anti-

cancer drugs have been used to construct pure nano-

drugs, and great progress has been made in this field.

Some researchers have connected water-soluble

irinotecan (Ir) and water-insoluble chlorambucil

(Cb) through ester bonds that are easily hydrolyzed

and broken under acidic conditions, and designed an

amphiphilic drug-drug Conjugate (Huang 2014).

The study found that, compared with the free drug

alone, the Ir-Cb nanoparticles formed by the self-

assembly method of the Ir-Cb copolymer showed a

longer blood circulation half-life and higher tumor

accumulation. After self-delivery in cells, the ester

bonds of Ir-Cb nanoparticles will be hydrolyzed and

broken in the acidic environment of tumor cells, free

Ir and Cb drugs are easily released from Ir-Cb

nanoparticles, thus exerting a synergistic cell

toxicity.

Figure 1: Based on amphiphilic drug-drug conjugated IR-CB nanoparticles and their antitumor activity (Huang 2014).

3.3 Self-delivery System based on

Therapeutic Carrier

The self-delivery system based on the therapeutic

carrier is that both the carrier and the loaded drug

can be used as the therapeutic agent of the

combination therapy. The use of carrier-free

anticancer drugs is to obtain better therapeutic

effects. The idea of the above-mentioned drugs is to

improve the efficacy by reducing the ratio of the

carrier to the drug. Or another way of thinking can

be used, using a therapeutic vector. Such as the self-

assembled micellar nanocomposite of

epigallocatechin gallate (EGCG) derivatives and

ICBEB 2022 - The International Conference on Biomedical Engineering and Bioinformatics

436

protein drugs (Chung 2014). Among them, EGCG

derivative carriers can also show anti-cancer effects.

EGCG derivatives and anti-cancer proteins produce

stable micellar nanocomplexes through sequential

self-assembly, which show better anti-cancer effects

than free protein drugs in in vitro and in vivo

experiments, realizing the development of EGCG

derivative carriers and drugs Combination therapy.

DOX and EGCG can reach the tumor site at the

same time for treatment of resistance to liver cancer

(Liang 2010). Non-toxic doses of EGCG can

increase the sensitivity of chemotherapy-resistant

liver cancer cells to DOX by inhibiting the activity

of the P-glycoprotein (P-gp) efflux pump, thereby

enhancing the DOX-induced killing effect of liver

cancer cells. In the past few decades, some

compounds containing trace elements have been

proposed as therapeutic agents and therapeutic

nanocarriers, because they have a significant ability

to enhance the immune response of cancer cells and

produce anti-cancer metabolites, which can

effectively Interfere with cell metabolism and induce

cell apoptosis (Liu 2015). An example of this

compound in cancer treatment is a selenium-

containing reagent that regulates ROS in vivo to

induce apoptosis. Self-assembled selenium-

containing nanostructures can be used as self-

delivery therapeutics (Liu 2013). A research team

designed and synthesized an amphiphilic

hyperbranched selenium-containing polymer that

self-assembles into nanomicelles and is

automatically delivered to tumor tissues through the

EPR effect. After triggering the exclusive oxidizing

microenvironment in cancer cells, the nanomicelles

decompose, and the released selenium compounds

can effectively induce cancer cell apoptosis. They

further prepared a hyperbranched selenide

macromolecular anticancer drug, which can not only

achieve self-delivery based on its self-assembled

nanomicelles, but also can be used as a carrier

encapsulating hydrophobic DOX for combination

therapy. In order to reduce the cytotoxicity of

selenium-containing polymer anticancer drugs to

normal cells, the researchers introduced PEGylated

polymers to stabilize macromolecular anticancer

drugs and prevent them from being attacked by

proteins in the blood (Li 2015).

3.4 Self-delivery System based on

Non-toxic Agent

There are also some multifunctional local delivery

systems based on the participation of non-cytotoxic

drugs, which can achieve controlled aggregation

around tumors to induce cell apoptosis. This system

is called a non-toxic agent-based self-delivery

system. The anti-cancer effect of the self-delivery

system based on non-toxic agents is dependent on

non-toxic agents rather than conventional

chemotherapeutic drugs. Conventional

chemotherapeutic drugs, such as DOX and CPT,

inhibit the growth of cancer cells by embedding in

cell DNA and inducing cell death. The role of the

non-toxic unit is to show anti-cancer active cells

through self-aggregation and cause cytotoxicity.

Recently, a research team reported a matrix

metalloproteinase-7 (MMP-7) response precursor,

which transforms into a gel before being taken up by

cancer cells, and further enters the cells to form a

hydrogel (Tanaka 2016). The formation of the

hydrogel causes the pressure inside the cell to

increase, thus initiating cell death. Some researchers

have found that due to the dephosphorylation of D-

peptide derivatives by alkaline phosphatase, the

hydrogel/nanomesh will gather in the gaps of cancer

cells to form a self-assembled structure, and block

communication and mass exchange between cells

(Zhou 2016). Way to remove cancer cells. In

addition to low molecular weight precursors as

therapeutic agents, the use of non-cytotoxic

macromolecules has been extended to another

paradigm for cancer treatment. The mechanism of

macromolecular therapeutics in inducing cell

apoptosis is the special biological recognition

between cell surface receptors and natural or

synthetic binding motifs. Based on this theoretical

basis, a research team has developed a new

therapeutic platform mediated by extracellular

hybridization of two complementary nanoconjugates

to induce apoptosis in B-cell lymphoma cells, which

is further cross-linked B-cell lymphoma

overexpresses the CD20 antigen (Chu 2014). In

summary, these self-delivery systems show great

advantages for cancer treatment, but have less toxic

and side effects on healthy cells/tissues. In addition,

since the efflux effect of the efflux pump in drug-

resistant cells is one of the main mechanisms for the

emergence of MDR, and self-assembled

nanomedicine can bypass the efflux pump of cancer

cells due to its size effect, so nanomedicine is the

same for drug-resistant cancer cells. Can produce

higher curative effect, Due to the unprecedented

drug loading capacity, minimization of systemic

toxicity, flexible preparation strategy and the

nanometer size of passive targeted therapy, the

scientific research of carrier-free nanomedicine has

made great progress in recent years (Kunjachan

2013). However, we still know very little about

Research Status of Carrier-free Nano Antitumor Drugs: The Mechanism of Action and Future Trends of Four Carrier-free Nanomedicines

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carrier-free nanomedicine. At present, many reports

point out that the physical and chemical properties

of nanomaterials, such as size, shape, surface

properties, are crucial in regulating their cellular

uptake and transport behavior in the body, and may

affect the overall cancer treatment effect (Williford

2015).

4 PREPARATION OF DOX

NANOPARTICLES

First dissolve DOX in DMSO, then add 1 mL of

triethylamine to 10 mL of 1 mg/mL

DOX.HCl/DMSO solution under moderate agitation

at 25°C, and react for about 4 hours After stopping,

the hydrophilic DOX.HCI is converted into

hydrophobic DOX. Three preparation methods of

DOX nanoparticles: (1) DOX 50 (particle size is

about 50 nm): Drop a 3mg/mL DOX/DMSO

solution into 5ml petroleum ether and stir for 5

minutes. (2) DOX100 (particle size about 100 nm):

Drop the DOX/DMSO solution with a concentration

of 3 mg/mL into 5 ml ultrapure water and stir for 5

minutes. (3) Dox 180 (particle size is about 180 nm):

Drop 1 mg/mL DOX/DMSO solution into 5ml

ultrapure water, and stir for five minutes.

5 CONCLUSION

Carrier-free nanomedicine solves the toxicity

problem of nano-carrier drug carrier system.

According to its construction principle, that is, to

complete the self-assembly process through

hydrophobic and hydrophilic groups, various

construction ideas can be developed. It is worth

mentioning that carrier-free nanomedicine provides

different application ideas for some poorly water-

soluble anticancer drugs. By constructing a carrier-

free nanomedicine system, its water solubility can be

greatly improved. The current frontier research on

carrier-free nano-oncology drugs believes that pure

drug carrier-free nano-oncology drugs have the most

research potential and have very broad prospects.

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