PROTAC uses the natural mechanism of protein degradation in cells, that is, the ubiquitin-protease system, to induce targeted protein degradation. Compared with traditional small molecular inhibitors used in cancer therapy, PROTAC has several potential advantages: it is event-driven (rather than space-occupying driving-directly inhibiting the functional activity of the target protein), so a very low dose can have a good efficacy; PROTAC induces protein degradation and is separated from the complex and enters the next catalytic cycle. Each PROTAC can degrade many protein molecules and has the potential to break through the targets of non-proprietary medicine. Since the first clinical trial of PROTAC in 2019, 18 protein degradants have conducted phase I or I/II clinical trials in multiple tumor types, and the first phase III clinical trial has been launched in 2022. Initial data on the safety, efficacy, and pharmacokinetics of PROTAC are being released, although more evidence is needed to support PROTAC clinical trial data showing initial anti-tumor activity.
The traditional treatments for cancer include chemotherapy, radiotherapy, and surgery. Although these three modes are still the cornerstone of cancer treatment, radiotherapy and chemotherapy have limitations such as toxicity and persistent adverse reactions. Small molecule inhibitors have ushered in a new era of precision medicine. Imatinib is the first tyrosine kinase small molecule inhibitor in the world. It was approved by the FDA in 2001 for BCR-ABL1 gene fusion in chronic myeloid leukemia. Since then, the FDA has approved dozens of small molecular inhibitors, which are widely used in the treatment of solid tumors and hematological diseases.
Although small molecular inhibitors have broad clinical prospects, they still have their limitations in cancer treatment. A major problem is that target proteins are required to have binding pockets, making it difficult for 85% of the targets to become drugs. In addition, small molecular drugs need a high enough concentration to occupy the active site of the target and a long enough half-life to continuously inhibit the target protein. Long-term, high-dose drug exposure not only increases the risk of adverse reactions, but also leads to cumulative toxicity. Continuous treatment with small molecular inhibitors causes cells to synthesize more target proteins, resulting in drug resistance mutations or compensatory activation.
The structure and mechanism of PROTAC
The ubiquitin-proteasome system is the main pathway for intracellular protein degradation. In short, proteins that do not need or are misfolded will be labeled by ubiquitin, and the ubiquitinated proteins can be recognized and degraded by the 26S proteasome. The process of ubiquitination usually requires the synergism of three ubiquitin enzymes: E1 ubiquitin activating enzyme, E2 ubiquitin binding enzyme, and E3 ubiquitin ligase. Among them, E3 ligase is involved in the recognition of protein substrates to be degraded.
PROTAC is a bifunctional molecule that consists of three parts—a target protein ligand, an E3 ubiquitin ligase ligand, and a linker in the middle. PROTAC molecules specifically recognize and bind to the target through the target protein ligand at one end and the E3 ubiquitin ligase ligand at the other end. The target protein-PROTAC-E3 ligase ternary complex is formed, in which the target protein is ubiquitinated by E3 ligase and the ubiquitinated target protein is recognized and degraded by the proteasome, thus inhibiting the function of the target protein.
PROTAC has different pharmacological characteristics from small molecular inhibitors
Pharmacokinetics is irreversible: the formation of the target protein-PROTAC-E3 ligase ternary complex is irreversible, which can promote the degradation of multiple target protein molecules at low concentrations.
Hook effect: according to the protein interaction between target protein and E3 ligase, when the concentration is high, PROTAC molecules will form binary complexes with target protein or E3 enzyme, respectively, resulting in the Hook effect. The emergence of the Hook effect means the independent binding of POI and E3 ligands and prevents the formation of ternary complexes, which may lead to serious side effects. When targeting pathogenic proteins for degradation, our goal is to obtain a higher maximum degradation level (Dmax) (> 90%) and a low half maximum degradation concentration (DC50) (nM–pM).