By: Yeonah Suk, PharmD Candidate c/o 2020
Despite the $107 billion a year cancer drug industry there is no substantiated cure for cancer. Cancer can be treated by targeting a variety of cellular mechanisms. One innovative approach incorporates the use of proteasome inhibitors in the regulation of programmed cell death. Damaged or inessential proteins must be degraded in order to reconstruct new proteins which are involved in gene expression, cellular division, and detoxification of reactive oxygen species. Ubiquitin binds to the old or damaged proteins which induces proteasome recognition and degradation. When this ubiquitin-proteasome degradation pathway fails, protein aggregation occurs. This may impede the aforementioned cellular activities, leading to cellular degeneration which can be detrimental to healthy cells. However, this very mechanism can also be manipulated to work against cancer cell proliferation. Current technology has developed a new drug model, proteolysis targeting chimeras (PROTAC), that employs this pathway to combat breast cancer.
Estrogen receptor alpha (ERα) is overexpressed in breast cancer cells and promotes estrogen dependent proliferation making it a good drug target. The conventional approach to combating this malignancy is the inhibition of ERα transcription through modulation of receptor conformation by synthetic ligands. The limitation of this method is that there is a high susceptibility of drug resistance. For example, tamoxifen (Soltamox ®) is currently used as a treatment for breast cancer because it works as an estrogen receptor antagonist. However, over time, it begins to possess agonist function which results in regrowth of cancerous cells. PROTAC offers a solution because it degrades the target protein directly.
PROTAC consists of three components including a stabilized peptide which binds to ERα, a linker, and a hydroxyproline-containing pentapeptide which is recognized by E3 ubiquitin ligase. When a complex is formed, the E3 ubiquitin ligase that is recruited binds ubiquitin and targets the protein ERα, leading to its degradation through the previously described proteasome pathway. PROTAC’s advantage is its ability to target a broad range of proteins and thus, has been applied to create more peptide based PROTACs that selectively degrade multiple protein targets including ERα and estrogen-related receptor alpha. This potential treatment pathway was pursued because peptide modulators show greater potential than PROTACs in certain respects, such as expediting the necessary drug-to-target protein interactions which is challenging for small molecules, like unmodified PROTACs, to achieve on their own.
Currently, PROTAC conjugation with peptidomimetic estrogen receptor modulator (TD-PERM) is being tested to determine whether it can increase PROTAC’s effects. This combined development is named TD-PROTAC and it executes three characteristic actions. TD-PROTAC selectively activates the ubiquitination and subsequent degradation of ERα by a proteasome dependent pathway, reduces the transcription of ERα related genes which provide anti-proliferative properties, and signals apoptosis of ERα positive cancer cells with almost no cell toxicity toward cells that do not contain ERα. To evaluate this drug’s ability to carry out protein degradation, a study conducted at Shenzhen Graduate School of Peking University in Shenzhen, China by Dr. YanHong Jiang, analyzed ERα levels via immunoblotting after treatment with various groups, a control, and TD-PROTAC. Results showed that the control peptides did not exhibit an ability to degrade ERα. This verifies that both the ERα and ubiquitin binding groups are essential to adequately degrade ERα, both of which are moieties present in TD-PROTAC.4
TD-PROTAC also prevents signaling of certain receptors. In the same study conducted by Dr. Jiang, polymerase chain reaction analysis was used to examine the mRNA levels of pS2 which is a gene that ERα regulates via transcription. Results showed that TD-PROTAC treated cells displayed significant down regulation of ps2 gene expression revealing the multi-faceted approach TD-PROTAC has in combating breast cancer. Additionally, the effect TD-PROTAC has on other receptors must be assessed due to the risk of harming non cancerous cells, as well as producing severe side effects. Consequently, the effects TD-PROTAC has on vitamin D receptors, embryonic kidney cells and cells that do not contain ERα were also tested. Results demonstrated clinically insignificant degradation of the aforementioned groups, indicating that TD-PROTAC selectively targets ERα positive breast cancer cells.4
Breast cancer is a highly prevalent disease and is the fourth leading cause of cancer related death in the United States5. Sixty-six percent of all breast tumors express ERα and of these, seventy percent respond to hormone therapy which indicates that PROTAC’s ability to selectively and effectively degrade estrogen receptors as well as impede their transcription is promising in the future of breast cancer therapy.
- Neklesa, T. K., Winkler, J. D, Crews, CM. Targeted Protein Degradation by PROTACs. Pharmacol Ther. 2017;174:138-144. doi:10.1016/j.pharmthera.2017.02.027
- Rodriguez-Gonzalez, A, Cyrus, K, Salcius M, Kim K, Crews, CM, Deshaies, RJ, Sakamoto, Km. Targeting Steroid Hormone Receptors for Ubiquitination and Degradation in Breast and Prostate Cancer. Oncogene. 2008;27(57): 7201–11. doi: 10.1038/onc.2008.320
- Cyrus K, Wehenkel M, Choi EY, Lee H, Swanson H, Kim KB. Josting for Position: Optimizing Linker Location in the Design of Estrogen Receptor-Targeting PROTACs. ChemMedChem. 2010:5(7):979-85. doi. 10.1002/cmdc.201000146.
- Jiang Y, Deng Q, Zhao H, et al. Development of Stabilized Peptide-Based PROTACs against Estrogen Receptor α. ACS Chem Biol. 2018;13(3):628-635. doi: 10.1021/acschembio.7b00985.
- Iacopetta D, Rechoum Y, Furqua SA. The Role of Androgen Receptor in Breast Cancer. Drug Discov Today Dis Mech. 2012;9(1-2):e19-e27.