Molecule of the Month: Glucocorticoid Receptor and Dexamethasone

Glucocorticoid drugs have been widely prescribed for the treatment of inflammatory and autoimmune diseases and have recently been used to treat critically ill COVID-19 patients. COVID-19 progresses swiftly from symptoms like fever and shortness of breath to severe complications like multiple organ failure. Critically ill patients experience a “cytokine storm,” where the body is no longer able to limit the inflammatory response to the coronavirus, and the aberrant production of cytokines (molecular messengers of inflammation) triggers further complications. Clinical trials have shown that the administration of low dose dexamethasone, a potent anti-inflammatory drug that binds to the glucocorticoid receptor, has been effective in reducing mortality rates in hospitalized COVID-19 patients.

Along with the estrogen receptor, the glucocorticoid receptor belongs to the nuclear receptor family. It consists of three parts: a ligand-binding domain, a DNA-binding domain, and a transactivation domain. The most abundant ligand for this receptor in humans is cortisol, a stress hormone. When the receptor binds to cortisol, the receptor changes conformation and migrates from the cytoplasm to the nucleus. In the nucleus, it can bind to target DNA sequences to influence gene expression. Glucocorticoid receptor can also interact with coactivators to further regulate the gene expression machinery. Since the receptor is composed of several domains connected by flexible linkers, structures of the domains have been determined separately. The ligand-binding domain bound to dexamethasone is shown from PDB ID 1m2z and the DNA-binding domain bound to DNA is shown from PDB ID 1glu. The transactivation domain is not shown here. All these domains work together to relay the initial message triggered by the binding of cortisol.

The structure of the drug dexamethasone is very similar to that of natural cortisol. This allows dexamethasone to bind snugly within the glucocorticoid receptor and similarly trigger changes in gene expression that resolve inflammation in the body. This activity makes dexamethasone particularly effective in treating COVID-19, as the damage caused by the coronavirus is not only due to the virus itself, but also from uncontrolled inflammation. However, the anti-inflammatory effects of dexamethasone can be harmful if the drug is administered in the wrong context or at the wrong time. In the early stages of COVID-19, the body must be capable of mobilizing the immune system to fight off the virus, and treatment of early, non-severe COVID-19 with dexamethasone may inadvertently lead to a worse outcome for the patient.

Like many other drugs and hormones, dexamethasone is transported throughout the body by serum albumin, the most plentiful protein found in blood plasma. However, factors related to this protein can complicate the safe and effective administration of dexamethasone when treating COVID-19-associated inflammation. In diabetic patients, for example, a critical amino acid in the protein is frequently bonded to a sugar molecule through a process known as glycation, which can prevent the drug from binding to the protein. Common pain relievers like ibuprofen, and other drugs, can also compete for the same binding site on serum albumin if taken at the same time, hindering dexamethasone transport. Additionally, COVID-19 risk factors such as liver disease, malnutrition, advanced age, or even the virus itself can lower a patient’s serum albumin levels. These complications make it hard for physicians to estimate the relative levels of free and bound dexamethasone in the blood, and can potentially lead to increased drug toxicity, side effects, or loss of drug efficacy.