Research Progress of PROTAC in the Treatment of Malignant Tumors

Malignant tumors are a major global public health problem, the leading cause of death in developing countries and the second leading cause of death in developed countries. Traditional treatment methods such as surgical resection, radiotherapy and chemotherapy have serious side effects on the body. Tumor immunotherapies such as small molecule inhibitors and monoclonal antibodies act on the body’s immune system to remove tumor cells and reduce the immunosuppressive signals caused by tumor cells, and achieve good clinical results. However, tumor cells are prone to gene mutations to escape immune surveillance, and small molecule inhibitors are prone to drug resistance and off-target effects. Monoclonal antibodies have large molecular weights, are difficult to penetrate cell membranes, and have high drug costs. Therefore, there is an urgent need for a treatment method that can target tumor-related proteins, enter cells autonomously, and is not prone to drug resistance. Proteolytic targeted chimera (PROTAC) as a targeted protein degradation technology has attracted attention in recent years. It consists of three parts: the ligand that binds to the target protein (POI ligand), the E3 ubiquitin ligase recruitment element and the linker. The ligands at both ends are recruited to the corresponding E3 ubiquitin ligase and target protein (POI), respectively. The target protein hijacks the activity of E3 ubiquitin ligase and initiates the intracellular ubiquitin hydrolysis process, which is degraded by the proteasome system. Then PROTAC was dissociated to participate in a new round of degradation. At present, two PROTAC drugs have passed Phase I clinical trials. Based on the development of PROTAC in recent years, this paper briefly introduces the research progress of the first-generation peptide-based, second-generation small molecule-based and third-generation light-controlled PROTAC targeted therapy for malignant tumors.

Peptidyl PROTAC

In 2001, SAKAMOTO et al. deduced that ubiquitination-dependent degradation of proteins may be an effective means of regulating protein molecules in normal and diseased cells, and proposed the concept of PROTAC for the first time, selecting the ubiquitin ligase SCFβ-TRCP (SKP1-CUL1-F -box) as part of PROTAC, which was subsequently used as a basis for the targeted degradation of breast cancer-associated estrogen receptor (ER) and prostate cancer-associated androgen receptor (AR),  confirming that estradiol-based The PROTAC can promote the ubiquitination and degradation of ER and the efficient degradation of AR by PROTAC based on dihydroxytestosterone, suggesting that PROTAC can be used to target and degrade tumor-related proteins to treat malignant tumors.  However, the early PROTAC has a large molecular weight and poor cell membrane permeability, which greatly limits its application. In 2004, the first PROTAC that could autonomously penetrate the cell membrane was born, named Fu-SMPI, which is a peptidyl PROTAC that recruits E3 by replacing non-peptide small molecules with hypoxia-inducible factor (HIF)-1α-octapeptide Ubiquitin ligases target the degradation of the ER in living cells. Since then, peptidyl PROTACs have been continuously optimized to act on malignant tumor-related proteins, such as peptidyl PROTAC targeting ERα for the treatment of breast cancer, and peptidyl PROTAC containing polyarginine targeting hepatitis B virus X protein for the treatment of hepatocytes Cancer;  peptidyl PROTAC targeting CREPT protein inhibits the proliferation and motility of pancreatic cancer cells, and also inhibits the growth of human pancreatic cancer cell (Panc-1) xenografts in mice. These explorations suggest the possibility of applying PROTACs in the treatment of malignant tumors, and motivate researchers to improve the structure and function of PROTACs from more and broader perspectives to play a better role.

Small molecule-based PROTAC

The early peptidyl PROTACs have low activity, large molecular weights, and are easily recognized by the immune system to produce antibodies, and the cell membrane permeability is not ideal. The continuous improvement of peptidyl PROTAC has promoted the development of small molecule-based PROTAC.  Through research, two types of small-molecule-based PROTACs have been developed: targeting hormone receptors, targeting bromodomain and superterminal domain (BET) protein families, and targeting protein kinases.

Among them, nuclear hormone receptors play an important role in human reproduction, development and physiological processes. The abnormal expression and function of such receptors can lead to major human diseases, so nuclear hormone receptors are also used as therapeutic targets. A targeted hormone receptor PROTAC was designed; BET protein family It plays a very important role in the regulation of gene transcription. Its abnormal expression and function are related to the occurrence and development of various malignant tumors, and it is one of the targets of tumor therapy. Therefore, a PROTAC targeting the BET protein family was designed; Protein kinase is an enzyme that catalyzes the process of protein phosphorylation and is an important link in various signal transduction pathways, and its abnormal activity is associated with various malignant tumors. At present, small molecule inhibitors and monoclonal antibodies are the main clinical anti-cancer drugs against protein kinases, but the therapeutic effect is not ideal. The emergence of PROTACs targeting protein kinases provides a new idea for inhibiting the activity of abnormal protein kinases and treating malignant tumors.

Light-controlled PROTAC

The main drawbacks of PROTACs are non-tumor specificity and relatively high toxicity. In order to solve this problem, a research team has recently developed a light-controlled PROTAC, including two forms of optical cages and optical switches. The photocaged PROTAC (PC-PROTAC) can only be activated under specific wavelength illumination, while the photoswitched PROTAC (photoPROTAC) can achieve reversible activation and inactivation under the appropriate light wavelength illumination. This third-generation light-controlled PROTAC solves the problem of non-tumor specificity, reduces certain toxicity, and can be applied to the treatment of local diseases in the future.