Corticosteroid-binding globulin and Albumin (CBG; ALB)

Corticosteroid-binding globulin (CBG), also known as transcortin, and albumin are the two primary plasma proteins that govern the pharmacokinetics and bioavailability of glucocorticoids. CBG is a specialized transport protein with high affinity for steroids like cortisol and prednisolone, typically binding the majority of circulating hormone under physiological conditions [1, 4]. Albumin, while having lower affinity, provides a high-capacity reservoir for binding due to its high concentration in the blood [2]. The 'free fraction'—the portion of the drug not bound to these proteins—is the only part capable of crossing cell membranes to interact with the glucocorticoid receptor and exert biological effects [5]. Competitive binding between exogenous drugs, such as prednisolone, and endogenous cortisol for these protein sites can significantly alter the free fraction and thus the potency and clearance of the steroids [3]. Consequently, clinical conditions that alter the levels of these proteins, such as liver cirrhosis, nephrotic syndrome, or systemic inflammation, can lead to unpredictable therapeutic outcomes and increased risk of side effects [4, 5].

Other names

TranscortinSERPINA6Human Serum AlbuminHSAPlasma steroid-binding proteins

These proteins act as carrier molecules that regulate the distribution and free fraction of glucocorticoids. CBG binds cortisol and prednisolone with high affinity, while albumin provides high-capacity, low-affinity binding. Drugs like prednisolone compete for CBG binding sites, displacing cortisol and increasing its biologically active free concentration.