In the microscopic cellular world, there exists a sophisticated "feeding" mechanism that not only maintains the cell's own life activities but also serves as the core biological basis for the design of cutting-edge modern anticancer drugs, which is endocytosis.
Endocytosis: The Cell's Active "Gateway"
Endocytosis refers to the process by which cells invaginate their cell membranes to form vesicles, encapsulating macromolecules or particulate matter from the outside world and importing them into the cell. It is an important way for cells to actively take up substances, transmit signals, and maintain homeostasis.
Fig 1. Schematic diagram of cellular endocytosis (Image source: rendered using BioRender)
Core process: When an extracellular target substance (ligand) approaches, the cell membrane invaginates locally to encapsulate it. The invaginated portion then breaks off, forming an independent vesicle that enters the cell. Based on the size, state, and mechanism of the ingested substance, endocytosis is mainly divided into three types:
Phagocytosis: Primarily targets larger solid particles (such as bacteria and cell debris), commonly seen in immune cells such as macrophages.
Pinocytosis: Non-specifically ingests extracellular fluid and its soluble substances; almost all eukaryotic cells can perform this.
Receptor-mediated endocytosis: Specific extracellular molecules (such as hormones and transferrin) precisely bind to corresponding receptors on the cell membrane, triggering endocytosis. Like a key (ligand) unlocking a lock (receptor), it initiates the cell membrane's "express delivery" service, representing the most specific and efficient pathway.
Four Precise Approaches to Internalization
Within the broad category of receptor-mediated endocytosis, it can be further subdivided into four main pathways based on its specific molecular mechanisms, which are like four different "channels" for entering the cell:
Fig 2. Schematic diagram of receptor-mediated endocytosis pathway (Image source: Burge, R. (2025). Endocytic Pathway with Macropinocytosis and Phagocytos.)
1. Clathrin-mediated endocytosis: This is the most classic and primary pathway. Clathrin assembles into cage-like structures under the cell membrane, acting like scaffolding to pull the membrane invaginates and form vesicles. Cells take up cholesterol carrier LDL through this pathway.
2. Caveolin-mediated endocytosis: This is mediated by specific microregions on the cell membrane rich in cholesterol-"cavitaries". Caveolins embed themselves in the membrane, forming special invaginations. Some hormones and viruses use this pathway to enter the cell.
3. Macropinocytosis: The cell membrane forms large folds, non-selectively "gulping" large amounts of extracellular fluid to form larger vesicles. This is related to cellular nutrient acquisition and certain immune processes.
4. Non-clathrin / cavitary-dependent endocytosis: This is a general term for other endocytic pathways that do not depend on the two classic proteins mentioned above. The mechanisms are diverse and this is a cutting-edge area of current research.
Using endocytosis as a sharp weapon: The rise and blueprint of ADC drugs
1. ADC: Precise "biological missile"
ADC drugs consist of three parts: antibody (which recognizes tumor cell surface antigens), linker, and toxic drug (warhead). After the antibody binds to the tumor target, the entire ADC molecule is actively "swallowed" into the cell, and the vesicles formed eventually fuse with lysosomes. Under the acidic environment and enzymatic action of lysosomes, the "linker" of the ADC is precisely cleaved, releasing a potent cytotoxic drug, completing a fatal blow to the tumor cells.
Fig 3. Schematic diagram of ADC drug mechanism (Data source: Dumontet C, et al. Nat Rev Drug Discov. 2023)
2. The booming ADC ecosystem
As of the end of 2025, the number of ADC drugs approved for marketing worldwide has increased to 21. From the classic HER2 and TROP2 to the rapidly emerging Claudin 18.2 and B7-H3, and then to the bispecific antibody ADC representing the forefront of technology, it constitutes the current panorama of the ADC field. According to the maturity of the target and the progress of research and development, they can be classified as follows:
Table 1. Overview of the R&D landscape for core ADC targets
The ADC field has become one of the hottest tracks in the global oncology treatment market. As of June 2025, there were more than 1,600 ADC drugs in the research and development stage worldwide, of which more than 90% were in the early clinical stage, showing great potential. Chinese pharmaceutical companies contributed about 34% of the global R&D pipeline. Industry analysis predicts that the global ADC market size is expected to exceed US$60 billion by 2030.
Fig 4. Global ADC Drug Market Size Historical and Forecast (2017-2030E) (Note: Data for 2025 and beyond are forecasts based on CAGR; Data source: Compiled from public reports by Frost & Sullivan et al.)
The receptor-mediated endocytosis of ADC drugs into target cells is a crucial step and the foundation for their subsequent efficacy. However, successful drug penetration into cells is only the first step. How the drug precisely destroys tumor cells, and what factors limit the efficiency of this killing process, are the core questions we will explore in our next installment. We will comprehensively dissect the killing mechanism of ADC drugs and analyze the key variables affecting their efficacy. Stay tuned.
About AlpVHHs
AlpVHHs is an industry-leading nanobody company specializing in VHH antibodies, recombinant antibodies, and recombinant monoclonal antibody development, providing discovery, engineering, expression, and large-scale production services for nanobodies and antibody fragments, supporting both research and diagnostic product development.
AlpVHHs offers a new generation of nano-secondary antibody-drug conjugates linked to MMAE, MMAF, DM1, or Duocarmycin, etc for antibody internalization detection. Using Nano-secondary antibody-drug conjugates (nano-2°ADC) in a cell-based cytotoxic assay is a quick and economical alternative to pre-screening monoclonal antibodies as ADC candidates against tumor cells. Nano-2°ADCs are highly specific with minimally toxicity to cells in the absence of primary antibodies. No obvious change of the primary antibody activity in the presence of the unconjugated secondary antibody.
Code | Description | Application |
023-101-014 | Internalization Test | |
023-101-102 | Internalization Test | |
023-101-103 | Internalization Test | |
023-101-020 | Internalization Test | |
001-101-012 | Internalization Test |
For more information, please visit our website www.alpvhhs.com
