Isturisa (osilodrostat) is an FDA-approved oral medication for adults with Cushing's disease — a form of Cushing's syndrome caused by a pituitary adenoma (tumor) that overproduces ACTH. It belongs to a class of drugs called steroidogenesis inhibitors, which block cortisol production in the adrenal glands.
🧠 Cushing's Disease Specialist Care: Cushing's syndrome is rare and complex to diagnose and manage. Our endocrinologists specialize in the full workup, treatment coordination, and long-term monitoring of Cushing's disease. Call 832-968-7003 to schedule a consultation.
What Is Cushing's Disease?
Cushing's syndrome refers to the clinical effects of chronically elevated cortisol. When the cause is a pituitary tumor secreting excess ACTH (which drives the adrenal glands to overproduce cortisol), it is specifically called Cushing's disease — the most common endogenous cause of Cushing's syndrome.
Symptoms of Cushing's disease include:
- Central (abdominal) weight gain with thin arms and legs
- Round "moon" face and "buffalo hump" (fat pad at upper back)
- Wide purple stretch marks (striae) on the abdomen, thighs, or breasts
- Easy bruising, thin fragile skin
- High blood pressure and blood sugar (often diabetes)
- Muscle weakness, bone loss (osteoporosis)
- Depression, anxiety, cognitive difficulties
- Irregular or absent menstrual periods, low libido
First-Line Treatment: Transsphenoidal Surgery
The preferred first-line treatment for Cushing's disease is transsphenoidal pituitary surgery — removal of the ACTH-secreting adenoma by a neurosurgeon through the nasal passage. When performed by an experienced surgeon at a center of excellence, remission rates are 65–90%. Isturisa is primarily used when:
- Surgery fails to achieve remission
- Cushing's disease recurs after initial surgical remission
- The patient is not a surgical candidate
- As a bridge therapy before or after radiation
How Isturisa Works
Isturisa blocks 11β-hydroxylase, the enzyme responsible for the final step in cortisol synthesis in the adrenal glands. By inhibiting this enzyme, Isturisa reduces cortisol production regardless of how much ACTH is coming from the pituitary. This makes it effective even when the pituitary tumor is still present or recurrent.
Clinical Evidence — LINC 3 Trial
In the pivotal LINC 3 Phase 3 trial:
- 53% of patients achieved normalized urinary free cortisol (UFC) at 48 weeks
- Rapid cortisol normalization — many patients responded within the first 2 months
- Improvements in blood pressure, blood sugar, body weight, and quality of life
- Sustained efficacy with continued treatment
Dosing & Administration
- Route: Oral tablet, taken twice daily
- Starting dose: 2 mg twice daily
- Titration: Adjusted every 2 weeks based on 24-hour urinary free cortisol (UFC) and serum cortisol levels, up to a maximum of 30 mg twice daily
- Target: Normalize UFC without causing adrenal insufficiency
Key Side Effects & Monitoring
| Side Effect | Why It Happens | What to Watch For |
|---|---|---|
| Adrenal insufficiency | Cortisol drops too low from over-suppression | Fatigue, nausea, dizziness, low blood pressure — requires dose reduction or temporary steroid replacement |
| QT prolongation | Cardiac electrical effect | EKG monitoring required; avoid other QT-prolonging drugs |
| Elevated androgens / testosterone | 11β-hydroxylase also involved in androgen pathway | Acne, oily skin, hirsutism (excess hair growth) in women; testosterone monitoring required |
| Nausea, headache, fatigue | Common side effects | Usually manageable; resolve in most patients |
| Hypokalemia | Mineralocorticoid excess as cortisol drops | Potassium monitoring required |
⚠️ Adrenal Insufficiency Risk: The most important monitoring task with Isturisa is avoiding over-suppression of cortisol. Symptoms of adrenal insufficiency — extreme fatigue, nausea, dizziness, low blood pressure — require immediate medical attention. Never stop Isturisa abruptly without medical guidance.
Monitoring Schedule
Patients on Isturisa require frequent lab monitoring, especially during dose titration:
- 24-hour urinary free cortisol (UFC) — every 2 weeks during titration, then every 1–3 months
- Morning serum cortisol and ACTH
- Potassium levels
- EKG (QT interval)
- Testosterone / DHEAS (especially in women)
- Liver function tests
Isturisa vs. Other Medical Therapies for Cushing's
| Medication | Mechanism | Key Distinction |
|---|---|---|
| Isturisa (osilodrostat) | 11β-hydroxylase inhibitor | Oral, twice daily, rapid onset, high efficacy |
| Korlym / Relacorilant (mifepristone) | Glucocorticoid receptor blocker | Blocks cortisol action rather than production; cannot monitor UFC on therapy |
| Metopirone (metyrapone) | 11β-hydroxylase inhibitor | Similar mechanism to Isturisa; older drug; limited US availability |
| Signifor (pasireotide) | Somatostatin analogue (pituitary) | Works upstream at pituitary; causes hyperglycemia in majority of patients |
| Ketoconazole | Steroidogenesis inhibitor | Off-label; liver toxicity concern |
Key Takeaways
- Isturisa is an oral, twice-daily cortisol-lowering medication for Cushing's disease
- Works by blocking 11β-hydroxylase — the final step in cortisol synthesis
- Used after failed pituitary surgery or when surgery is not an option
- ~53% of patients achieve cortisol normalization in clinical trials
- Adrenal insufficiency and androgen excess are key side effects requiring close monitoring
- Requires frequent lab work — UFC, cortisol, potassium, EKG, testosterone
Our Team Treats Adrenal Disorders
All five of our providers evaluate and manage adrenal and endocrine conditions. Book with any member of our team:
Book an Appointment → or call 832-968-7003