The global health system faced significant challenges in 2018 because of malignant tumors which resulted in 18.1 million new cases and 9.6 million deaths. Surgery and chemotherapy together form the primary treatment method for these tumors. A significant problem with many small-molecule anticancer drugs is their lack of specificity which causes them to harm healthy cells and produce major side effects. The development of tumor-targeted drugs in precision medicine has emerged to minimize side effects and increase treatment effectiveness. Research has progressed tumor-targeted drug delivery by developing nanocarriers and employing prodrug strategies. Prodrugs are parent drugs attached to chemical groups that become active through enzymatic or chemical cleavage inside the body. Small-molecule tumor-targeted prodrugs connected to tumor biomarkers represent another development created by researchers. This method outperforms both small-molecule drugs and nanocarriers in simplicity and efficiency while demonstrating reduced resistance potential and finds use in antibody-drug conjugates (ADC), peptide-drug conjugates (PDC), aptamer-drug conjugates (ApDC), and polymer-drug conjugates.
Antibody-Drug Conjugate (ADC)
As early as the beginning of the 20th century, scientists, led by Paul Ehrlich, the "father of chemotherapy," proposed the "magic bullet" theory, aiming to find a compound that could selectively target diseased areas without causing other side effects. In the 1970s, immunotherapy based on monoclonal antibodies (mAbs) began to emerge. mAbs can reduce nonspecific toxicity by specifically binding to antigens on tumor cells, acting on particular signaling pathways to achieve therapeutic effects, or directly generating an immune response against the tumor cells. To date, approximately 30 mAbs have been approved by the FDA for tumor treatment. To enhance the therapeutic effect, researchers have covalently linked mAbs with various anticancer molecules (such as cytotoxic drugs, radioactive isotopes, immunotoxins, etc.), creating targeted therapies and immunotherapies based on mAbs, with antibody-drug conjugates (ADCs) receiving widespread attention.
ADCs are formed by covalently linking recombinant mAbs with cytotoxic drugs via linkers, with mAbs targeting and delivering the drugs to the target cells for therapeutic action. This immunoconjugate, combined with cytotoxic small molecules (molecular weight ranging from 300 to 1,000, with nanomolar IC50 values), is highly effective in inhibiting tumor cell growth. At the same time, mAbs provide high selectivity, stability, and favorable pharmacokinetic properties, with advantages such as long circulation time, strong therapeutic efficacy due to high specificity, low toxicity to normal tissues, resistance to drug tolerance, and low immunogenicity.
Basic Composition of ADCs
mAbs
mAbs consist of two antigen-binding fragments (Fabs), which mediate antigen recognition, and one constant fragment (Fc), which interacts with immune effector cells. The Fc fragment contains a binding domain for the neonatal Fc receptor (FcRn), which regulates the antibody's half-life in the bloodstream. In ADCs, mAbs typically need to meet the following criteria: (1) low immunogenicity, usually achieved by using humanized or fully human antibodies. (2) target specificity, ensuring sufficient antigen specificity and affinity, along with high internalization efficiency. and (3) a long circulation half-life.
A key challenge in ADC development is the identification and validation of the antigen target for the mAb. Several factors must be considered when selecting an antigen. First, the target antigen should be expressed on the cell surface to facilitate ADC-target antigen complex internalization and intracellular drug release. Second, an ideal antigen should be uniformly expressed on target cells but have low expression in healthy tissues. Lastly, antigen shedding should be minimized to prevent free antigens in circulation from binding to mAbs, which could lead to ADC inactivation.
Currently, a variety of ADC targets are in clinical research globally, primarily classified into hematologic malignancy targets and solid tumor targets. Approved targets for leukemia include CD22, CD30, and CD33, while solid tumor targets include human epidermal growth factor receptor 2 (HER2), nectin-4, prostate-specific membrane antigen (PSMA), and epidermal growth factor receptor (EGFR).
Cytotoxic Drugs
Cytotoxic drugs are the ultimate effector components of ADCs. To be suitable for ADC preparation, these drugs typically need to meet the following criteria: (1) a well-defined mechanism of action. (2) extremely potent cytotoxicity (IC50 in the sub-nanomolar range). (3) the ability to be directly modified or structurally adapted to introduce conjugation sites without affecting biological activity. and (4) stability and sufficient solubility in mAb solutions.
Currently, the most commonly used cytotoxic drugs in clinical applications can be classified into three major categories based on their mechanisms of action:
DNA-damaging agents: This category includes calicheamicins (CLM), doxorubicin (DOX), duocarmycins, and pyrrolobenzodiazepines (PBDs). These drugs bind to the minor groove of the DNA double helix, causing DNA strand breaks and inducing apoptosis.
Microtubule inhibitors: Examples include maytansines and auristatins, which bind to microtubules, preventing their polymerization, disrupting the cell cycle, and ultimately triggering apoptosis.
Topoisomerase inhibitors: Camptothecin (CPT) and its derivatives belong to this class. They interfere with topoisomerases required for DNA replication, leading to DNA strand breaks and cell death.
Linkers
The chemical linker serves to conjugate cytotoxic drugs to mAbs while maintaining the stability of ADCs in the systemic circulation. The chemical properties of the linker and the conjugation site are crucial factors influencing ADC stability, pharmacokinetics, pharmacodynamics, and therapeutic window. An ideal linker should possess the following characteristics: (1) sufficient stability to ensure that ADCs circulate in the bloodstream and reach the target site without premature cleavage, preventing nonspecific toxicity. and (2) the ability to rapidly cleave during internalization to release the cytotoxic drug.
Based on the drug release mechanism, linkers can be classified into cleavable and non-cleavable types. Cleavable linkers are unstable in acidic environments such as early endosomes (pH 5.0–6.0) and lysosomes (pH 4.0–5.0). They enable efficient drug release from ADCs through the following mechanisms:
Acid-sensitive hydrolysis: Hydrolysis of acid-labile bonds, such as hydrazone linkages.
Protease cleavage: Recognition and cleavage of specific dipeptide sequences, such as valine-citrulline (Val-Cit, VC), by lysosomal protease B, which is highly expressed in tumor cells.
Reduction reaction: Reduction of disulfide bonds in the linker by the high intracellular concentration of glutathione (GSH), leading to drug release.
In contrast, non-cleavable linkers form irreversible chemical bonds between the drug and specific amino acid residues on mAbs, making them more stable in circulation. Drug release from ADCs with non-cleavable linkers, such as the thioether linker in ado-trastuzumab emtansine (T-DM1), relies on lysosomal degradation of the antibody. Therefore, effective internalization and optimal lysosomal trafficking pathways must be carefully designed.
Applications of ADCs
The concept of antibody-drug conjugates (ADCs) was introduced in 1958, but the first clinical trial did not occur until 1983, involving a murine anti-carcinoembryonic antigen (CEA) antibody conjugated with vinblastine for treating advanced malignant tumors. The first true ADC, gemtuzumab ozogamicin (Mylotarg), was approved by the FDA in 2000 for treating acute myeloid leukemia (AML). However, it was withdrawn in 2010 due to its failure to improve survival and severe adverse effects. The instability of its acid-sensitive linker and non-site-specific conjugation to lysine residues contributed to its failure. Despite this, Mylotarg paved the way for ADC advancements.
Following Mylotarg's failure, brentuximab vedotin (Adcetris) was approved in 2011 for Hodgkin lymphoma and systemic anaplastic large-cell lymphoma, and ado-trastuzumab emtansine (Kadcyla) was approved in 2013 for HER2-positive metastatic breast cancer. To date, seven ADCs have been approved, with over 100 candidates in clinical trials.
Despite significant progress, several challenges remain in ADC development:
Drug-to-antibody ratio (DAR): ADCs are heterogeneous, with DAR values ranging from 0 to 8. An optimal DAR (3–4) is essential, as higher values reduce tolerability and efficacy.
Lack of site-specific conjugation: Many ADCs have undefined conjugation sites, complicating drug dosage and pharmacokinetic studies.
Instability of cleavable linkers: Premature cleavage in the bloodstream releases non-selective cytotoxic drugs, causing severe toxicity.
Limited tissue penetration: ADCs struggle to penetrate solid tumors, necessitating antibody miniaturization strategies to improve accessibility.
Peptide-Drug Conjugate (PDC)
The design principle of peptide-drug conjugates (PDCs) is similar to that of antibody-drug conjugates (ADCs), primarily serving for drug delivery and tumor targeting. The key difference is that in PDCs, the antibody component of ADCs is replaced by peptide molecules that act as targeting ligands.
Compared to ADCs, PDCs offer several advantages: they are generally easier to synthesize, more cost-effective, and largely non-immunogenic, reducing the risk of autoimmune reactions. The synthesis, storage, and quality control of PDCs are relatively straightforward, and they exhibit greater stability both in vitro and in vivo. Additionally, due to their lower molecular weight, PDCs have improved vascular, tissue, and cellular permeability, allowing them to penetrate deeper into tumors.
However, it is important to note that PDCs are cleared from the bloodstream more rapidly than ADCs due to glomerular filtration. This faster clearance reduces the circulation time of PDCs at the target site, which may impact their targeting efficiency.
Basic Composition of PDCs
Peptide
In recent years, with the rapid advancement of proteomics, phage display, and solid-phase peptide synthesis technologies, numerous novel peptides have been discovered or rationally designed, significantly driving the development of peptide-drug conjugates (PDCs). The targeting peptide molecules in PDCs must exhibit nanomolar-level affinity for their corresponding targets (binding constant < 10-9 mol/L) to ensure high selectivity and minimize nonspecific distribution and toxicity during systemic administration.
Peptide molecules used in PDCs are generally classified into cell-penetrating peptides (CPPs) and cell-targeting peptides (CTPs). CPPs facilitate the transport of drugs across the cell membrane, while CTPs specifically bind to receptors on target cells. Since peptide segments in PDCs are susceptible to degradation by digestive enzymes, they are typically administered via non-gastrointestinal routes. Once in circulation, PDCs pass through capillary walls to reach target cells.
Table.1 Common cell-penetrating peptide products at Creative Peptides.
For CPP-drug conjugates, transmembrane transport is an energy-independent process, allowing them to directly cross the lipid bilayer. Additionally, studies have reported that CPP-drug conjugates can enter cells through transporter-mediated or receptor-mediated, energy-independent, non-endocytic pathways. Commonly used CPPs include trans-activator of transcription (TAT), transportan, penetratin, and their derivatives or other peptides with membrane-penetrating abilities.
In contrast, the transmembrane transport of CTP-drug conjugates relies on receptor-mediated endocytosis. During this process, the conjugate is internalized via early and late endosomes before ultimately entering the lysosome, while the receptor undergoes intracellular recycling back to the cell membrane. Common CTPs include the arginine-glycine-aspartic acid (RGD) peptide series, luteinizing hormone-releasing hormone (LHRH) analogs, and novel tumor-targeting peptides identified through phage display technology.
Table.2 Arginine-glycine-aspartic acid (RGD) related products at Creative Peptides.
Table.3 Luteinizing hormone-releasing hormone (LHRH) analogs at Creative Peptides.
Cytotoxic Drugs
The cytotoxic drugs used for PDC conjugation are usually classic chemotherapy drugs, such as paclitaxel (PTX), DOX, CTP, etc., which exert anti-tumor effects by interfering with or blocking the process of cell proliferation, but due to low selectivity and poor tumor targeting ability, they can easily cause damage to normal cells and tissues. The formation of PDCs can improve the targeting of these drugs to tumor tissues and reduce their distribution in normal tissues, thereby reducing adverse reactions and inhibiting multi-drug resistance.
Linkers
The linker is an effective bridge between the peptide and the drug, a good linker will not affect the function of the peptide or drug, low molecular, appropriate length, suitable stability and polarity are the key factors of the ideal linker. Similar to ADCs, PDC linkers are divided into non-breakable and breakable types, and by employing different linkers, the release of drugs can be adjusted to reduce the risk of potential adverse effects.
Applications of PDCs
In January 2018, the U.S. FDA approved Lutathera (lutetium Lu-177 dotatate), a peptide-drug conjugate (PDC) that targets somatostatin receptors on tumor cells. This drug consists of a radiotherapeutic agent, ¹⁷⁷Lu, chelated to octreotide via the DOTA binding agent. It is effective in treating somatostatin receptor-positive gastroenteropancreatic neuroendocrine tumors (GEP-NETs), as the emitted β-radiation induces damage to both target and neighboring cells. This marked the first use of a radiotherapeutic agent in PDC-based therapy.
Paclitaxel (PTX), a widely used cytotoxic drug, faces limitations in solubility and multidrug resistance. ANG1005, developed by Angiochem, is a novel taxane derivative where three PTX molecules are linked to the angiopep-2 peptide, enabling it to cross the blood-brain barrier (BBB) and target tumors. Phase II trials show ANG1005's effectiveness in treating breast cancer brain metastases, with clinical benefit rates of 77% intracranially and 86% extracranially. It significantly extends survival in patients with leptomeningeal carcinomatosis, particularly in HER2-positive breast cancer patients.
Meanwhile, Chen et al. identified the M1-RGD peptide, which crosses both the BBB and blood-brain tumor barrier (BBTB). When conjugated to PTX, it improved survival in glioblastoma models and showed enhanced effects when combined with temozolomide (TMZ).
Despite these advancements, no chemotherapy PDCs have been approved for market use due to challenges such as rapid clearance, peptide stability, and limited drug-carrying capacity and barrier permeability. Further studies are needed to address these issues and improve PDC efficacy.
Polymer-Drug Conjugate
Polymer-drug conjugates are pharmacologically active macromolecular structures formed by covalently binding to polymers of one or more drugs, which can be small molecule drugs, peptides, proteins, or aptamers. The combination of drugs and polymers has a variety of benefits, including increased drug solubility, controlled drug release rate, increased drug efficacy, and improved pharmacokinetic behavior. In recent years, with the maturity of polymer conjugation technology, more and more polymer-drug conjugates have entered clinical research and shown good development prospects.
Basic Composition of Polymer-Drug Conjugates
The concept of polymer-drug covalent conjugates was first proposed by Professor Ringsdorf in 1975, who referred to them as synthetic polymer drugs or pharmacologically active polymers. It is formed by the connection of a drug to the polymer backbone through an unstable bond and consists of one polymer backbone and three different units: the first is a hydrophilic region that makes the whole macromolecule soluble and non-toxic. the second is the region where the drug is connected to the polymer chain, with the drug typically attached via linkers (such as hydrazone, azo, peptide, disulfide bonds, etc.), which break under specific conditions to release the drug from the polymer carrier. The type of linker used affects the proportion of the drug conjugated to the carrier, the drug stability, and the drug release mechanism. the third is the region for achieving targeted delivery, which functions to deliver the entire polymer system to the target cells or to exert pharmacological effects, similar to the concept of targeted ligands widely applied in current research. Different functions can be achieved by designing different regions of the macromolecule polymer chain.
Currently, the polymers available for drug delivery include: Amino acid derivatives, such as poly-L-lysine (PLL), poly-L-glutamic acid (PGA), gelatin, poly [N-(2-hydroxyethyl-L-glutamine)] (PHEG), and polyaspartic acid (PASP). Polyacids, such as poly-α-malic acid (PAMA) and poly-β-malic acid (PBMA). Polysaccharides, such as dextran, pullulan, hyaluronic acid, and chitosan.
Others, such as N-(2-hydroxypropyl) methacrylamide (HPMA) copolymers and PEG. Some studies focus on conjugating small-molecule chemotherapy drugs, such as anthracyclines, platinum compounds, and taxanes, to polymers. Functional groups that are commonly used for direct drug conjugation or conjugation to the polymer chain via linkers include amino, carboxyl, hydroxyl, and thiol groups. Polymer-drug conjugates can be classified into polymer-macromolecule conjugates, polymer-small molecule conjugates, dendritic macromolecules, and polymer nanoparticles.
Applications of Polymer-Drug Conjugates
The first polymer-drug conjugate used for anti-tumor therapy, which was covalently linked by PGA, p-phenylenediamine nitrogen mustard (PDM), and immunoglobulin, was reported in 1975. This structure included the aforementioned three units. Matsumura and colleagues reported that a polymer conjugate modified with an anti-tumor protein preferentially accumulated in tumor tissue after intravenous injection, and proposed the enhanced permeability and retention (EPR) effect. Polymer-drug conjugates for anti-tumor drugs have demonstrated higher safety and efficacy in preclinical animal models, and many have entered clinical trials, with some even approved for market use.
PK1 is the first water-soluble polymer-small molecule drug conjugate to enter clinical research. It links DOX with HPMA copolymer via a lysosomal-cleavable tetrapeptide (GPLG). PK2 is similar to PK1 but contains galactosamine, which can bind to the asialoglycoprotein (ASGP) receptor on liver cancer cells. Phase I clinical trials showed that in patients with refractory or drug-resistant tumors, the elimination half-life of PK1 was 93 hours, significantly longer than that of free DOX, with reduced cardiac toxicity. Phase II clinical trials of PK1 showed that in patients with breast cancer (7/62), non-small cell lung cancer (16/62), and colorectal cancer (29/62), only 6 patients had partial responses when given a dose of 280 mg/m² every three weeks. However, only a small number of patients exhibited tumor accumulation of PK1. Despite extending the circulation time of DOX and improving in vivo safety, the efficacy was still limited. Unfortunately, in Phase II clinical trials, two patients who showed tumor accumulation effects from PK1 did not respond to treatment, while patients who responded to treatment did not show tumor accumulation. These data suggest that PK1's penetration into tumors is uneven, and it cannot rely on tumor accumulation to achieve therapeutic effects. Considering that tumor accumulation does not necessarily result in therapeutic response, even in tumors with good permeability, and due to the lack of significant increase in PK1's half-life, tumor accumulation was also not ideal, leading to the failure of its development.
PGA-PTX (Opaxio, formerly Xyotax) has been widely studied due to its significant anti-tumor effects. After a single intravenous injection, it completely eliminates mouse breast cancer, and compared to unlinked PTX, the conjugate's maximum tolerated dose increases by 2 times, and tumor accumulation increases by 12 times. Multiple injections of the conjugate had similar efficacy to a single injection, suggesting that long-circulating drug conjugates (such as PGA-PTX) are more effective when administered as a single high-dose injection compared to multiple injections. However, in Phase II clinical trials in ovarian cancer patients, the response rate of this conjugate was only 10% (10/99), with a median survival of 2 months. In a Phase III clinical study of non-small cell lung cancer patients, the survival rate in the PGA-PTX group was similar to the control group, and compared to the first-line treatment regimen of PTX/carboplatin for advanced non-small cell lung cancer, PGA-PTX/carboplatin did not provide better survival. Although PGA-PTX improved PTX's solubility and safety, improvements in clinical anti-tumor efficacy were still limited.
The four-arm PEG conjugate of the cytotoxic drug irinotecan (Onzeald) has entered Phase III clinical trials. Onzeald uses cleavable ester bonds to link one irinotecan molecule to each PEG arm, with a molecular weight of 20,000. In vivo, the ester bonds slowly hydrolyze to release irinotecan, which is then metabolized into the active anti-tumor component SN-38. In animal models, compared to conventional irinotecan, Onzeald exhibited extended circulation half-life, stable concentrations in plasma and tumors, and up to 400 times higher plasma exposure (AUC). Compared to irinotecan, the Cmax in tumors increased 10 times after treatment with Onzeald, while the Cmax in plasma decreased, suggesting a more favorable therapeutic index. Therefore, treatment with Onzeald resulted in sustained tumor inhibition and tumor regression for several weeks. Phase I clinical trials confirmed similar pharmacokinetic characteristics, with SN-38 showing a 50-day elimination half-life, while irinotecan had a half-life of only 12-47 hours. Onzeald has completed Phase III clinical trials and is awaiting approval for market use in the treatment of breast cancer with brain metastasis.
Approved polymer-drug conjugate products are mostly PEG-protein conjugates used to treat conditions such as hepatitis C, acute lymphoblastic leukemia, and rheumatoid arthritis. In contrast, clinical applications of polymer-small molecule drug conjugates have been limited, with only the PEG-naloxone conjugate (brand name Movantik) successfully entering the market for the treatment of opioid-induced constipation in chronic pain patients. Although some polymer conjugates have demonstrated the ability to extend half-life and reduce toxicity, improvements in anti-tumor efficacy remain limited. The preclinical efficacy of anti-tumor polymer therapies is largely attributed to tumor accumulation mediated by the EPR effect, but passive accumulation in human tumors is currently a controversial topic. For example, in a study of 37 patients treated with PK1 and evaluated in Phase I and II clinical trials, only 8 patients showed tumor uptake via radioisotope imaging. The discrepancy between preclinical studies and patient data may be due to mouse models not accurately reflecting human tumor characteristics. The rapid growth of mouse tumors leads to irregular blood vessel formation and leakage, but not all human tumors have leaking blood vessels. A recent study indicated that in tumor drug delivery trials mediated by the EPR effect, only 0.7% of the intravenously injected dose reached the tumor, suggesting that even for drug carriers showing passive accumulation effects in preclinical models, the amount of drug delivered to tumor sites is insufficient. However, researchers did not analyze tumor localization of small molecule drugs, which has confounded the accumulation effects of drug carriers and small molecule drugs at tumor sites. Therefore, future studies need to carefully select patients to confirm which patients can benefit from the intended therapy or develop methods to increase polymer carrier accumulation in tumors (such as introducing targeting ligands). Additionally, comprehensive preclinical evaluations of pharmacokinetics and biodistribution must be conducted to ensure that the selected linker has sufficient stability in vivo to guarantee effective drug delivery.
Aptamer-Drug Conjugate (ApDC)
Nucleic acid aptamers are ligands selected from oligonucleotide libraries using SELEX technology, offering high specificity and affinity for targets, making them ideal for aptamer-drug conjugates (ApDCs) in targeted therapy. Aptamers bind to targets through interactions like van der Waals forces and hydrogen bonding, resembling antibody-antigen interactions. They are smaller, cheaper, more stable, and less immunogenic than antibodies, with high tissue penetration. Aptamers can target small molecules, proteins, and cells. Recent developments like cell-SELEX enable aptamer selection for natural molecular targets. ApDCs, like ADCs, consist of an aptamer, linker, and drug, with potential for various therapies, including chemotherapy and gene therapy.
Non-Covalently Linked ApDC
DNA is a biomolecule that can self-assemble into a double helix structure through base pairing, stabilized by hydrogen bonds, π-π stacking, and hydrophobic interactions, allowing for the simple introduction of drugs through hybridization and intercalation. The core principle of cell-SELEX is to select nucleic acid aptamers capable of recognizing tumor cells, even when the target molecule's characteristics are unknown, enabling direct use in tumor diagnosis and treatment.
Using this characteristic, a nucleic acid aptamer, P19, targeting pancreatic ductal adenocarcinoma, was selected to demonstrate its ability to target and deliver gemcitabine, fluorouracil (FU), and the Mertansine derivative DM1. These chemotherapy drugs first bind to short oligonucleotides and then hybridize with the nucleic acid aptamer, generating an ApDC that specifically delivers the cytotoxic drugs to tumor cells, thereby reducing nonspecific side effects.
DOX, a commonly used broad-spectrum anticancer drug with a flat four-ring structure, can intercalate into DNA base sequences, particularly CG or GC base pairs, inhibiting DNA replication. Based on the ability of DOX to intercalate into nucleic acid structures, it can be incorporated into nucleic acid aptamers rich in CG or GC sequences for targeted DOX delivery.
The nucleic acid aptamer A10, a single-stranded RNA composed of 71 nucleotides, can specifically bind to the PSMA protein overexpressed on the surface of human prostate cancer cells. ADOX was incorporated into the 3D structure of A10 via non-covalent interactions to form an A10-DOX conjugate, which specifically targets and delivers DOX to PSMA-overexpressing human prostate cancer cells. Since A10 and DOX are non-covalently bound, both maintain high biological activity, ensuring that their individual efficacies are not compromised.
To improve drug delivery efficiency, the 5' end of the nucleic acid aptamer molecule was modified with a long double-stranded DNA composed almost entirely of drug insertion sites. This approach functions similarly to guiding a nucleic acid aptamer as the "locomotive" of a nano-train, driving the "train cars" as efficient drug carriers to continuously deliver drugs to target cells, exerting selective cytotoxic effects. This method demonstrates ideal anticancer efficacy, alleviates side effects, and holds strong potential for practical application.
Covalently Attached ApDC
Although the preparation of ApDC through non-covalent binding is relatively simple, many drugs cannot be effectively embedded into nucleic acid aptamers, and the incorporation of drugs may alter the structure of the nucleic acid aptamer, thus affecting its specific binding to the target. Therefore, in addition to non-covalent binding, covalent binding is also widely used to develop more stable ApDC with potential for site-specific modifications. During the process of nucleic acid aptamer-drug conjugation, different linkers can be designed to release the drug in specific tissues or subcellular organelles. For example, by linking the chemotherapy drug DOX to the DNA aptamer sgc8 (which specifically binds to PTK7 overexpressed on the surface of T lymphocytes from acute lymphoblastic leukemia), an ApDC is formed, using the acid-labile hydrazone bond as a linker to release DOX in the acidic tumor environment, acidic endosomes, or lysosomes, thereby inhibiting tumor growth.
A nucleoprotein aptamer (NucA)-PTX conjugate sensitive to cathepsin B has been developed, which selectively delivers PTX to tumor sites, significantly enhancing its anticancer activity and reducing nonspecific toxicity. Using a cathepsin B-sensitive VC dipeptide bond, NucA was linked to the 2' active hydroxyl group of PTX, and the resulting inactive NucA-PTX conjugate remains stable in circulation. The NucA on this conjugate promotes its accumulation in tumor tissue. Once inside tumor cells, the dipeptide linker of the NucA-PTX conjugate is cleaved by enzymes, releasing PTX to exert its effects.
Recently, a nucleolar-specific nucleic acid aptamer AS1411 was conjugated with tripterygium glycoside, resulting in a conjugate that specifically recognizes tumor cells. Upon activation by GSH inside the tumor cells, it triggers a series of bioorthogonal reactions, producing carbon-centered free radicals in situ and in a self-cycling manner. Additionally, the activation of this conjugate significantly reduces the GSH content within tumor cells while increasing the free divalent iron content, generating a synergistic chemical kinetic therapy effect. This conjugate shows strong specificity and high cytotoxicity to human breast cancer cells (MDA-MB-231) and exhibits good in vivo efficacy against triple-negative breast cancer, with minimal adverse reactions to healthy tissues. This approach provides new insights into the design of tumor-targeted drug delivery systems and the study of radical-related molecular mechanisms.
Despite the many excellent properties of nucleic acid aptamers and the proven potential of ApDC in tumor treatment, the development of nucleic acid aptamers or ApDC for targeted therapy is still relatively slow. To date, only one nucleic acid aptamer-based drug, pegaptanib (brand name Macugen, a PEGylated anti-VEGF nucleic acid aptamer used to treat age-related macular degeneration), has been approved by the FDA. The nucleic acid aptamer AS1411 used for tumor treatment is currently in phase II clinical trials. ApDC still faces several challenges in transitioning from the laboratory to clinical use, such as the lengthy aptamer screening process with a low success rate, poor stability of nucleic acid aptamers and ApDC, susceptibility to degradation by ubiquitous nucleases in vivo, a short half-life in the body, and the need for chemical modifications for clinical use. Additionally, immune system recognition of nucleic acids may trigger immune responses.
Summary
This article primarily introduces the applications of antibody, peptide, nucleic acid aptamer, and polymer-drug conjugates in mediating targeted drug delivery. According to current research and clinical trial results, they all have promising applications. Some products have already achieved good therapeutic effects in clinical use, bringing significant benefits to patients. However, there are still some challenging issues, such as adverse reactions, drug resistance, high costs, and the lack of better tumor biomarkers.
Firstly, while targeted therapy can increase the accumulation of drugs at tumor sites, distribution in normal tissues remains inevitable, leading to adverse reactions. For example, although Kadcyla has significantly reduced adverse reactions compared to chemotherapy, it still causes nausea, vomiting, thrombocytopenia, musculoskeletal pain, and hepatotoxicity, leading to poor patient compliance. Secondly, like traditional chemotherapy drugs, targeted therapy also faces drug resistance. Drug efflux proteins highly expressed on the surface of tumor cells can promote drug efflux, and complex mutations may occur during tumor progression, promoting tumor growth and rendering targeted drugs ineffective. Thirdly, the cost of tumor-targeted therapy is high, especially for antibody-based drugs, and additional treatment-related expenses further increase the economic burden on governments and patients. Finally, due to the complexity and heterogeneity of tumors, the efficacy of treatment varies among different patients, requiring further search for potential tumor biomarkers to predict patient efficacy and identify those most suitable for targeted therapy, thus achieving precision treatment. It is believed that with advancements in biomedicine and the improvement of targeted drug delivery theory, more reasonable and effective clinical translation of targeted drugs will be promoted, bringing hope for the treatment of a larger number of cancer patients.
Reference
- Chen, Long, et al., Blood–brain barrier-and blood–brain tumor barrier-penetrating peptide-derived targeted therapeutics for glioma and malignant tumor brain metastases. ACS applied materials & interfaces 11.45 (2019): 41889-41897.
