Immune Checkpoint Inhibitors and Hormonal Oncology

irAE severity is graded using NCI CTCAE (National Cancer Institute Common Terminology Criteria for Adverse Events) criteria. The general management algorithm follows severity: grade 1 (mild, no functional limitation) — continue ICI with close monitoring; grade 2 (moderate, limiting instrumental activities of daily living) — hold ICI and initiate oral prednisone; grade 3 (severe, limiting self-care) — hold or permanently discontinue ICI and initiate intravenous methylprednisolone; grade 4 (life-threatening) — permanently discontinue ICI, initiate high-dose intravenous methylprednisolone, and escalate to steroid-sparing immunosuppression if no improvement within several days. Steroid tapering must be slow — over at least four to six weeks after symptom resolution — because premature taper reliably causes irAE flares. ICIs must not be rechallenged after grade 4 irAEs, or after grade 3 irAEs involving the heart (myocarditis), nervous system, or eye with vision loss.

Diarrhea and colitis are the most common gastrointestinal irAEs, occurring most frequently with CTLA-4 inhibitors and less frequently with PD-1/PD-L1 inhibitors. Evaluation includes stool culture and Clostridium difficile testing to exclude infectious colitis; colonoscopy for severe or refractory cases shows diffuse mucosal edema and ulceration histologically resembling inflammatory bowel disease. Management follows the general grading algorithm, with one critical escalation agent: infliximab (an anti-TNF [tumor necrosis factor] monoclonal antibody) is the preferred steroid-sparing agent for steroid-refractory grade 3 and 4 colitis, with high efficacy and rapid onset that does not appear to impair ICI antitumor activity.

ICI-associated hepatitis presents as asymptomatic liver enzyme elevation discovered on routine monitoring before each infusion. Liver biopsy shows a pan-lobular hepatitis pattern with cytotoxic CD8-positive T-cell infiltration, distinguishing it from other causes of drug-induced liver injury. One critical distinction from colitis management: infliximab is contraindicated for immune-related hepatitis because it is itself hepatotoxic. Steroid-refractory hepatitis is managed with mycophenolate mofetil (an inhibitor of inosine monophosphate dehydrogenase that blocks lymphocyte proliferation), not infliximab.

ICI-associated pneumonitis is more common in patients with NSCLC and with combination nivolumab-ipilimumab than with PD-1 monotherapy. It presents from asymptomatic bilateral ground-glass opacities on CT (computed tomography) imaging to severe dyspnea and respiratory failure. Bronchoscopy with bronchoalveolar lavage is performed to exclude infectious pneumonia before attributing the finding to immune-related pneumonitis. Pneumonitis is the most common cause of ICI-related death; grade 3 or 4 pneumonitis requires permanent ICI discontinuation. When corticosteroid doses exceed 20 mg prednisone per day for more than four weeks, Pneumocystis jirovecii pneumonia (PJP) prophylaxis is recommended.

Endocrine irAEs are unique in an important way: unlike colitis, hepatitis, and pneumonitis, most endocrine irAEs are permanent because the affected gland is destroyed and cannot regenerate. Corticosteroids may be used for the acute inflammatory phase but do not restore endocrine function. The main endocrinopathies are thyroid disorders (hypothyroidism most common, occurring in approximately 8 to 10 percent of patients on PD-1 monotherapy; managed with levothyroxine; ICI continued in most cases), hypophysitis (pituitary inflammation, more common with ipilimumab than PD-1/PD-L1 agents; presents with headache and fatigue; causes central deficiencies in thyroid, adrenal, and gonadal hormone axes; requires lifelong hormone replacement), primary adrenal insufficiency (fatigue, hypotension, hyponatremia, hyperkalemia; treated with lifelong hydrocortisone replacement), and type 1 diabetes mellitus (rare but dramatic; fulminant insulin-dependent diabetes with diabetic ketoacidosis presentation; requires lifelong insulin).

Adrenal crisis from ICI-related adrenal insufficiency is a medical emergency requiring immediate intravenous hydrocortisone before any diagnostic workup. When the presentation is subacute, cortisol and ACTH (adrenocorticotropic hormone) levels should be drawn before starting steroids to preserve diagnostic utility.

Medical castration is the foundation of androgen deprivation therapy (ADT) for advanced prostate cancer. GnRH agonists (LHRH [luteinizing hormone-releasing hormone] analogs; leuprolide, goserelin, triptorelin) initially cause a testosterone surge due to receptor stimulation before inducing receptor downregulation and castrate levels after two to four weeks. This initial testosterone flare can exacerbate bone pain, urinary obstruction, or spinal cord compression in patients with bulky metastatic disease and is prevented by concurrent antiandrogen administration (bicalutamide for the first four weeks). GnRH antagonists (degarelix; relugolix, the only oral GnRH antagonist) directly block pituitary GnRH receptors without initial stimulation, achieving rapid castration without testosterone flare — a pharmacologically important advantage in patients with high tumor burden or skeletal disease. ADT class toxicities include hot flashes, osteoporosis, muscle loss, and metabolic syndrome.

Enzalutamide is an oral, non-steroidal androgen receptor (AR) inhibitor that provides more complete AR pathway suppression than older antiandrogens: it blocks testosterone and dihydrotestosterone (DHT) binding to AR, inhibits AR nuclear translocation, and prevents AR from binding DNA or recruiting co-activators. It is approved across the spectrum of prostate cancer from castration-sensitive to castration-resistant disease.

Enzalutamide is a strong inducer of cytochrome P450 3A4 (CYP3A4), cytochrome P450 2C9 (CYP2C9), and cytochrome P450 2C19 (CYP2C19) at therapeutic doses. This induction substantially reduces plasma concentrations of co-administered drugs metabolized by these enzymes — including warfarin (INR monitoring essential), direct oral anticoagulants, and multiple oncology agents. The full medication list must be reviewed before starting enzalutamide and after any dose change. CNS (central nervous system) toxicities include fatigue, dizziness, and rare but serious seizures attributed to GABA-A (gamma-aminobutyric acid type A) receptor antagonism; enzalutamide is avoided in patients with prior seizure history.

Darolutamide has a structurally distinct AR-binding domain that reduces CNS penetration, does not cause seizures at therapeutic doses, and has a more favorable cytochrome P450 induction profile compared to enzalutamide, making it preferred in patients with seizure history or extensive polypharmacy. Apalutamide has a similar mechanism and induction profile to enzalutamide with additional risks of skin rash and hypothyroidism.

Abiraterone acetate is an oral prodrug converted to abiraterone, a potent irreversible inhibitor of CYP17A1 (cytochrome P450 17A1; 17-alpha-hydroxylase/17,20-lyase) — the enzyme responsible for androgen synthesis in the adrenal glands and intratumoral tissues. By blocking CYP17A1, abiraterone suppresses androgen production from all sources, achieving more complete androgen deprivation than castration alone.

A mandatory co-administration requirement defines abiraterone prescribing: prednisone 5 mg twice daily throughout therapy. CYP17A1 inhibition in the adrenal cortex blocks cortisol synthesis, causing compensatory ACTH elevation that drives accumulation of mineralocorticoid precursors upstream of the blocked enzyme. These mineralocorticoids cause hypertension, hypokalemia, and fluid retention. Prednisone suppresses ACTH and replaces cortisol. Abiraterone must be taken on an empty stomach because food increases exposure substantially and unpredictably. Blood pressure and serum potassium monitoring is required.

Tamoxifen is a SERM (selective estrogen receptor modulator) that acts as a competitive antagonist at the estrogen receptor in breast tissue while functioning as a partial agonist in endometrium and bone. It is used as adjuvant therapy for estrogen receptor-positive early breast cancer and in premenopausal metastatic disease. Tamoxifen is a prodrug requiring cytochrome P450 2D6 (CYP2D6)-mediated conversion to its active metabolite endoxifen; patients with CYP2D6 poor metabolizer genotype generate substantially less endoxifen and may derive reduced benefit from tamoxifen.

Strong CYP2D6 inhibitors — most notably paroxetine and fluoxetine, commonly prescribed for hot flashes and depression in breast cancer patients — markedly reduce endoxifen levels by approximately 60 to 70 percent and should be avoided. When hot flash management is required in a tamoxifen-treated patient, venlafaxine (a serotonin-norepinephrine reuptake inhibitor with minimal CYP2D6 inhibition), gabapentin, or clonidine are appropriate alternatives. Tamoxifen additionally increases endometrial cancer risk approximately twofold and venous thromboembolism risk; annual gynecological evaluation is recommended for patients on long-term tamoxifen.

Aromatase inhibitors (AIs; letrozole, anastrozole, exemestane) suppress extragonadal estrogen synthesis by blocking aromatase (CYP19A1), which converts androgens to estrogens in adipose tissue, adrenal glands, and breast tissue. AIs are effective only in postmenopausal women (or premenopausal women on ovarian suppression) where extragonadal estrogen is the dominant source. Primary toxicities are musculoskeletal symptoms and accelerated bone loss requiring monitoring and bone-protective therapy.

Fulvestrant is a pure ER (estrogen receptor) antagonist and degrader (SERD; selective estrogen receptor degrader) with no agonist activity, administered as a monthly intramuscular injection. Elacestrant is an oral SERD approved for ESR1 (estrogen receptor 1 gene)-mutant metastatic breast cancer — ESR1 mutations are a common mechanism of acquired AI resistance and are detected by liquid biopsy (circulating tumor DNA). The combination of aromatase inhibitors or fulvestrant with CDK4/6 (cyclin-dependent kinase 4 and 6) inhibitors (palbociclib, ribociclib, abemaciclib) is now the standard approach for most patients with estrogen receptor-positive metastatic breast cancer, as covered in Module 2.

The most common irAE management errors are insufficient steroid dosing and premature taper. Both cause irAE relapse. Steroids must be tapered slowly over at least four to six weeks after symptom resolution, not stopped when symptoms improve. The second critical error is agent selection for steroid-refractory organ toxicity: infliximab for colitis, mycophenolate for hepatitis — never reversed. Giving infliximab for immune hepatitis risks additional hepatotoxicity on top of an already inflamed liver.

All endocrine irAEs require lifelong hormone replacement even after ICI discontinuation because gland damage is permanent. Adrenal crisis from ICI-related hypophysitis or primary adrenal insufficiency is a medical emergency: intravenous hydrocortisone must be given immediately when suspected, without waiting for laboratory confirmation. ICI-induced thyroiditis presents as a transient hyperthyroid phase (released thyroid hormone from destroyed colloid) followed by hypothyroidism; the hyperthyroid phase is managed with beta-blockers alone and typically resolves within four to eight weeks. ICI-induced type 1 diabetes presents with fulminant diabetic ketoacidosis (DKA) and requires immediate insulin therapy; it is not treated with corticosteroids.

Four hormonal therapy drug interactions are highest-yield. First, enzalutamide is a strong cytochrome P450 3A4 inducer — it dramatically reduces plasma levels of warfarin, direct oral anticoagulants, docetaxel, and most other oncology agents metabolized by this enzyme; the full medication list requires review before starting and after any dose change. Second, tamoxifen and CYP2D6 inhibitors — paroxetine and fluoxetine reduce endoxifen levels substantially; substitute venlafaxine or gabapentin for hot flash management. Third, abiraterone requires concurrent prednisone to suppress ACTH-driven mineralocorticoid accumulation and must be taken on an empty stomach to avoid large unpredictable increases in exposure. Fourth, GnRH agonist initiation in patients with high-burden skeletal metastases requires antiandrogen co-prescription (bicalutamide for four weeks) to prevent the testosterone flare from precipitating spinal cord compression or urinary obstruction.

Active autoimmune disease requiring systemic immunosuppression is a relative contraindication to ICI therapy; patients can receive ICI with careful monitoring but require proactive irAE management. Solid organ transplant recipients face a high risk of allograft rejection from PD-1/PD-L1 blockade. Patients on chronic corticosteroids at prednisone-equivalent doses above 10 mg per day have attenuated ICI response in retrospective data; steroid tapering before ICI initiation is recommended when feasible. Live vaccines must not be administered during ICI therapy; inactivated vaccines are safe but immunogenicity may be reduced.