Latest Research Express

On September 15, 2022, the Harvard Medical School and the School of Pharmaceutical Sciences of Tsinghua University jointly published a research article entitled “A general chemical principle for creating closure-stabilizing integrin inhibitors” in Cell, introducing the challenges in the research and development of small molecule drugs targeting integrins, and proposing the corresponding solutions.

01 Status quo and challenges | Research and development of small molecule drugs targeting integrins

Drug targets are the baton of modern drug research and development. Hot targets such as GPCR and kinase have spawned nearly 600 drugs in multiple categories, including anti-tumor drugs, hypotensive drugs, anti-allergy drugs, analgesics, and schizophrenia drugs, contributing to the rapid development of drug research and development over the past 30 years. In addition to GPCR and kinase, drug targets also include a protein family called “integrin”.

Integrin is a class of transmembrane proteins containing an α subunit and a β subunit, which form heterodimers through non-covalent bonds. So far, it has been found that 18 α subunits and 8 β subunits together form 24 integrin proteins. These proteins are widely distributed on the surface of human cells, responsible for cell adhesion and signal transmission, and closely related to cell growth, differentiation and migration. Integrin is involved in many kinds of diseases, such as platelet disorders, atherosclerosis, cancer, osteoporosis, fibrosis, renal diabetic neuropathy, macular degeneration and other diseases. So it’s no surprise that integrins were once considered one of the most important targets for drug development.

As a matter of fact, since the discovery of integrins in the 1980s by Timothy Springer and others at Harvard Medical School, the following boom in biological research once pushed forward the development of related drugs, with six injectable integrin drugs approved for the treatment of multiple sclerosis, ulcerative colitis, Crohn’s disease, psoriasis, acute coronary syndrome, and complications during percutaneous coronary intervention, fully demonstrating the potential of using integrin as drug target for new drug development. The success of these drugs drove more manufacturers to enter the field, trying to turn integrin drugs that were originally available only as injectable therapies into small-molecule oral drugs. In case of multiple chronic diseases, the convenience of pills would undoubtedly replace injections and bring huge profits to pharmaceutical companies. However, these attempts encountered major setbacks in the development of drugs targeting αIIbβ3, rapidly diminishing the pharmaceutical companies’ enthusiasm for developing small molecule “blockbuster” drugs that can be taken orally.

As an important member of the integrin family, αIIbβ3 is mainly distributed on platelets. It binds to extracellular matrix fibrinogen to regulate the normal coagulation function of human body, and is an important target for the treatment of acute coronary syndrome during surgery. Abciximab was approved to market as early as 1994. Then, in 1998, the small-molecule drug Tirofiban was approved to act as an antithrombotic agent by preventing αIIbβ3 from binding to fibrinogen. However, Tirofiban and several small molecules that entered the second and third clinical phases showed severe side effects of thrombosis aggravation and platelet reduction, which limited their use and subsequent development. These target-related mechanisms may involve almost the entire integrin family, which sharply increased the pharmaceutical community’s concerns about the development of all small molecule drugs targeting integrins, leading to the withdrawal of most companies from the field.

02 Water molecule mediation | A breakthrough in safe conformation

As early as 2014, Professor Timothy Springer from the Harvard Medical School and Professor Zhang Yonghui from the School of Pharmaceutical Sciences of Tsinghua University conducted collaborative research, trying to clarify the molecular mechanism of the toxicity of small molecule drugs targeting integrins in clinical practice through systematic structural biology and chemical biology approaches.

The research shows that these clinically unsafe small molecule drugs can induce specific conformational changes in αIIbβ3. Integrins are usually in a “bent-closed” (BC) resting state, and in the presence of biological ligands, will be transitioned to an “extended-open” (EO) active state. Antibody drugs work by blocking the binding of biological ligands to integrins. Small molecule “antagonists”, including tirofiban, often cause the integrin αIIbβ3 conformation to become partially activated due to its “inappropriate” binding, which enhances the binding ability of biological ligands and aggravates the formation of thrombus. Besides, some “inappropriate” binding will induce conformational changes, exposing integrins to some ligand-induced binding sites (LIBS), which will be identified as allogeneic by the body to produce corresponding antibodies. These are the main causes of clinical side effects of the drugs.

Therefore, how to lock the integrin in a safe “closed” state while blocking the binding of biological ligands is the key to the success of developing small molecule drugs targeting integrins. The team found that the metal-dependent adhesion sites of integrins are important for the transition from “closed” to “open” states. After systematically studying the factors that influence the conformational change of integrins in small molecules, the team found that the introduction of “basic” nitrogen atoms in the design of small molecules could stabilize metal-dependent adhesion sites through hydrogen bonding mediated by water molecules, thus “closing” the entire integrin. The discovery of the chemical principle of water-mediated small molecule design that keeps integrins in a stable “closed” state undoubtedly makes feasible the development of oral small molecule drugs targeting the integrin family.

03 Conclusion

Nearly 40 years have passed since integrin was discovered, and its biological significance has been widely recognized. However, the development of small-molecule oral drugs has gone through twists and turns. This multi-year collaborative research has finally revealed the principle of water-mediated small-molecule drug design that keeps integrin in a safe “closed” state, which undoubtedly has great significance for the development of integrin drugs in the future, just as the Da Vinci quote at the beginning of this paper says: “In time and with water, everything changes.”

Professor Timothy A. Springer and Professor Zhu Jieqing from the Harvard University and Research Fellow Zhang Yonghui from Tsinghua University are co-corresponding authors of the paper. Postdoctoral Fellow Lin Fuyang and Postdoctoral Fellow Li Jing from Harvard University and Postdoctoral Fellow Yonghua Xie from Tsinghua University are co-first authors.