In order to appreciate the mechanism of action of medications used in the treatment of Cushings disease, it is important to first understand basic physiology of the hypothalamic pituitary adrenal axis. Have you ever wondered why certain medications provide a more potent therapeutic effect in Cushings disease compared to others? We will cover this shortly.

Physiology of the HPA Axis

Fig 1. Control of the HPA axis (Cortisol regulation in normal physiology)

The hypothalamic-pituitary-adrenal (HPA) axis controls cortisol production through a complex system of stimulatory and inhibitory mechanisms. Corticotropin-releasing hormone (also known as CRH), produced in the paraventricular nucleus (PVN) of the hypothalamus, travels via the hypophyseal portal system to reach corticotroph cells in the anterior pituitary gland. There, it binds to CRH receptor type 1, triggering the release of adrenocorticotropic hormone (ACTH) from these cells.

Did you also know that arginine vasopressin (AVP) or anti-diuretic hormone, a hormone released from the paraventricular nucleus, binds to specific V1b receptors on corticotrophs (these are cortisol producing anterior pituitary cells). This stimulatory pathway enhances CRH’s effects in the anterior pitutary gland, further stimulating ACTH production.

Conversely, dopamine released from the hypothalamus inhibits ACTH release by activating D2 receptors on corticotroph cells.

The release of CRH is influenced by several factors. Catecholamines, angiotensin II, serotonin, stress, and cytokines stimulate CRH release, while GABA inhibits it. CRH not only triggers the immediate release of ACTH but also increases the expression of the POMC gene, leading to sustained ACTH production.

Now that we know the basic stimulatory and inhibitory factors at the level of the pituitary gland, lets proceed to the adrenal cortex.

Adrenal Steroidogenesis.

So at the level of the adrenal gland, ACTH binds to melanocortin-2 receptors present on cells in the zona fasciculata of the adrenal cortex. This stimulates the conversion of cholesterol into cortisol. Cortisol, in turn, provides negative feedback by suppressing the release of CRH and AVP in the hypothalamus, as well as ACTH in the pituitary gland. This feedback loop helps maintain hormonal balance.

Fig 2.0 Site of Action of various steroidogenesis inhibitors in Cushing’s Disease

Role of Somatostatin Analogs in Cushing’s Disease

Have you ever wondered the relevance of somatostatin analogs in Cushings disease?

Both normal and abnormal corticotroph cells in the pituitary gland have two types of somatostatin receptors: SSR2 and SSR5. Somatostatin, a hormone produced in the hypothalamus, works to reduce ACTH production, which ultimately lowers cortisol levels. However, SSR2 receptors are more sensitive to cortisol’s negative feedback signals and can be easily suppressed, while SSR5 receptors are less sensitive.

Because of this difference, treatments targeting SSR2 receptors, like octreotide, are less effective in managing conditions such as Cushing’s disease. Instead, medications that target SSR5 receptors, such as pasireotide, are often more successful in controlling the disease.

Pasireotide is a medication that mimics the effects of somatostatin and targets four out of the five somatostatin receptor types: SSR1, SSR2, SSR3, and SSR5. It binds most strongly to the SSR5 receptor, which explains its effectiveness in treating Cushing’s disease. Tumors in the anterior pituitary gland, which cause the overproduction of cortisol in Cushing’s disease, tend to have more SSR5 receptors compared to other somatostatin receptor types.

Cortisol’s natural feedback mechanism suppresses the expression of somatostatin receptors, with SSR2 being more affected than SSR5. Because pasireotide specifically targets SSR5 receptors, it is particularly well-suited as a treatment option for managing Cushing’s disease.

As you may recall, dopamine released from the hypothalamus, inhibits the release of ACTH by corticotrophs in the anterior pitutary gland. More importantly, about 80% of corticotroph adenomas have D2 receptors, but these receptors are present in low numbers, which makes dopamine agonists less effective as a treatment option. Medications like bromocriptine and cabergoline work by binding to D2 receptors on corticotroph cells to reduce cortisol production. However, they are not as effective as somatostatin analogs in reducing the high levels cortisol seen in Cushings disease.

Now, let’s discuss the role of retinoic acid in the treatment of Cushing’s disease.

Retinoic Acid in Cushings Disease

In normal corticotroph cells, the POMC promoter gene plays a key role in producing POMC, which eventually leads to the secretion of ACTH. Under normal conditions, transcription factors like activator protein 1 and nuclear receptor 77 activate the POMC promoter gene. Retinoic acid, when bound to its nuclear receptors, suppresses the expression of AP-1 and nuclear receptor 77, effectively blocking the activation of the POMC promoter gene.

Fig 3.0. Role of Retinoic Acid in ACTH regulation

However, a protein called chicken ovoalbumin upstream promoter transcription factor 1 (COUP-TF1) protects AP-1 and nuclear receptor 77 from being inactivated by RA. In some corticotroph tumors, COUP-TF1 expression is reduced, leaving these cells more vulnerable to RA’s inhibitory effects. This makes retinoic acid a potentially effective treatment option for managing Cushing’s disease.

Retinoic acid (RA) lowers cortisol levels in individuals with Cushing’s disease through multiple mechanisms. It reduces the production of ACTH and POMC in corticotroph tumors, thereby decreasing the signals that drive cortisol production. Additionally, RA has direct tumor-killing effects, targeting and damaging corticotroph tumor cells. It also inhibits the growth of overactive adrenal cells, helping to control excessive cortisol release. Furthermore, RA downregulates the expression of melanocortin-2 receptors on adrenal cells, further suppressing cortisol synthesis. These combined actions make RA a promising therapeutic option for managing Cushing’s disease.

How about medications specifically regulating steroidogenesis?

Steroidogenesis Inhibitors

The steroidogenic acute regulatory protein (StAR) plays a key role in adrenal hormone production by transporting cholesterol from the outer to the inner mitochondrial membrane. Once inside, cholesterol is converted into pregnenolone by the cytochrome P450 side-chain cleavage enzyme . This step, which occurs in the inner mitochondrial membrane, is the rate-limiting process in adrenal steroid hormone production. Both ACTH and luteinizing hormone (LH) stimulate StAR’s activity and its downstream effects.

In the adrenal cortex, pregnenolone follows different pathways depending on the specific zone:

1. Zona Glomerulosa: Pregnenolone is converted into progesterone by the enzyme 3β-hydroxysteroid dehydrogenase. Progesterone then undergoes enzymatic changes involving 21-hydroxylase and aldosterone synthase to produce aldosterone, a hormone that regulates salt and water balance.
2. Zona Fasciculata: Here, pregnenolone is first hydroxylated by the enzyme (17-alpha-hydroxylase) into 17-hydroxypregnenolone. This compound is then processed by 3β-hydroxysteroid dehydrogenase, 21 hydroxlyase, and 11β-hydroxylase (CYP11B1) to produce cortisol, a hormone involved in stress response and metabolism.
3. Zona Reticularis: In this zone, 17 alpha hydroxylase and 3β-hydroxysteroid dehydrogenase facilitate the production of androgen precursors such as DHEA (dehydroepiandrosterone) and androstenedione, which are important for sex hormone synthesis.

The mechanisms of action for metyrapone, mitotane, ketoconazole, and the newly approved steroidogenesis inhibitor, osilodrostat, are as follows:

Metyrapone

Metyrapone contains a pyridine group that interferes with the enzyme 11β-hydroxylase, which is crucial for the final step in cortisol synthesis. Additionally, metyrapone inhibits other steroidogenic enzymes, including 17α-hydroxylase and the steroidogenic chain cleavage complex. These effects impact hormone production in both the adrenal cortex and the gonads.

Osilodrostat

Similar to metyrapone, osilodrostat blocks the activity of 11β-hydroxylase in the adrenal cortex, but it is more potent and has a shorter half-life. This allows it to be taken less frequently, typically twice a day instead of four times daily. Osilodrostat also has minor inhibitory effects on enzymes like 17α-hydroxylase and 18-hydroxylase, although these effects are less significant.

Mitotane

Mitotane has both “adrenolytic” (causing adrenal cell death) and “adrenostatic” (inhibiting enzyme activity) effects. It is a chemotherapeutic agent with a diphenylmethane structure that induces mitochondrial damage, leading to cell lysis and necrosis. Its adrenostatic action stems from its ability to inhibit enzymes such as the side-chain cleavage enzyme, 11β-hydroxylase, and 3β-HSD, disrupting cortisol production.

Ketoconazole

Ketoconazole, primarily known for its antifungal properties due to its imidazole group, also inhibits several key enzymes involved in steroidogenesis. Notably, it blocks the side-chain cleavage enzyme, which is essential for cortisol synthesis, as well as other enzymes in the steroid production pathway.

These drugs target various stages of cortisol production, offering therapeutic options for conditions like Cushing’s syndrome that involve excess cortisol levels.