10.4 Trop-2 Antibody-Drug Conjugates, PI3K/AKT/mTOR Inhibitors & New Oral SERDs

Why These Agents Matter

The CBCN outline lists systemic therapy broadly (endocrine therapy, chemotherapy, bispecific antibodies). Metastatic HR-positive and triple-negative breast cancer now move through a sequence that includes Trop-2 ADCs and PI3K-pathway inhibitors, each with toxicities that nurses must anticipate. Many of these drugs are oral, so patient education and adherence support are central.

Trop-2 Antibody-Drug Conjugates

Trop-2 (trophoblast cell-surface antigen 2) is expressed on most breast cancers regardless of subtype, so testing for Trop-2 is not required.

Sacituzumab Govitecan

• Structure: anti-Trop-2 antibody linked to SN-38, the active metabolite of irinotecan (a topoisomerase I inhibitor), with a hydrolyzable linker that allows a bystander effect.
• Evidence: In ASCENT, for metastatic TNBC after at least two prior therapies (one for metastatic disease), median overall survival improved from 6.7 to 12.1 months versus chemotherapy. In TROPiCS-02, it improved progression-free and overall survival in heavily pretreated HR-positive, HER2-negative metastatic disease.
• Dosing: 10 mg/kg IV on days 1 and 8 of a 21-day cycle.
• Boxed warnings: severe neutropenia and diarrhea. Hold for an absolute neutrophil count below 1,500 on day 1 (below 1,000 on day 8) or neutropenic fever; use G-CSF as secondary prophylaxis.
• UGT1A1*28: patients homozygous for this allele clear SN-38 more slowly and have more neutropenia; monitor closely.
• Diarrhea: early cholinergic diarrhea with cramping and sweating can be treated with atropine; later diarrhea is treated with loperamide after infection is excluded.
• Other: hypersensitivity (premedicate with antipyretics and H1/H2 blockers), nausea (two- or three-drug antiemetic prophylaxis), alopecia, and fatigue.

Datopotamab Deruxtecan

• Structure: anti-Trop-2 antibody with the same deruxtecan (topoisomerase I inhibitor) payload used in trastuzumab deruxtecan.
• Evidence and approval: In TROPION-Breast01, it improved progression-free survival versus chemotherapy in HR-positive, HER2-negative metastatic disease after endocrine therapy and one or two lines of chemotherapy. The FDA approved it on January 17, 2025.
• Dosing: 6 mg/kg IV every 3 weeks.
• Signature toxicities: stomatitis/oral mucositis (prophylactic steroid-containing mouthwash, such as dexamethasone swish-and-spit, plus oral care), ocular surface toxicity such as keratitis and dry eye (daily preservative-free lubricating drops, avoid contact lenses, ophthalmic exams), interstitial lung disease (same vigilance as trastuzumab deruxtecan), and nausea.

The PI3K/AKT/mTOR Pathway

The PI3K/AKT/mTOR pathway drives cell survival and growth and interacts with estrogen receptor signaling. About 40% of HR-positive, HER2-negative advanced breast cancers carry PIK3CA mutations; AKT1 mutations and PTEN loss add more. Testing uses next-generation sequencing of tumor tissue or circulating tumor DNA.

Nursing Management

• Hyperglycemia: check fasting glucose and HbA1c at baseline and optimize glucose control before starting; monitor glucose frequently in the first weeks (alpelisib hyperglycemia often begins within about 2 weeks). Metformin is the usual first-line agent; teach symptoms of hyperglycemia and ketoacidosis.
• Rash: prophylactic non-sedating antihistamines reduce alpelisib rash; report blistering or mucosal involvement at once.
• Stomatitis: the SWISH trial showed that alcohol-free dexamethasone mouthwash (swish 2 minutes, spit, four times daily) markedly reduced everolimus stomatitis; the same principle is used with other mTOR/PI3K agents.
• Diarrhea: hydration, loperamide, and early reporting.
• Pneumonitis: new cough or dyspnea on everolimus needs evaluation and possible dose hold.

New Oral SERDs and ESR1 Testing

ESR1 mutations arise under aromatase inhibitor pressure and make the estrogen receptor active without estrogen. Because they are acquired, retest by liquid biopsy at progression. Oral selective estrogen receptor degraders active against ESR1-mutated disease include elacestrant (EMERALD) and imlunestrant, approved by the FDA on September 25, 2025, for ER-positive, HER2-negative, ESR1-mutated advanced breast cancer after at least one endocrine therapy. Imlunestrant is taken as 400 mg once daily on an empty stomach, unlike elacestrant, which is taken with food. Teaching the correct food instruction for each oral agent is a practical nursing task.

Oral Anticancer Therapy Safety

Oral agents shift administration to the home. Nurses verify dose and schedule (including intermittent schedules such as capivasertib's 4 days on and 3 days off), review drug interactions (strong CYP3A4 inhibitors or inducers, grapefruit), teach safe handling and disposal, provide written toxicity action plans, and follow up by phone in the first weeks.

Putting the Agents in Sequence (HR-Positive, HER2-Negative Metastatic Disease)

A common pathway, individualized to biology and prior therapy:

1. First line: an aromatase inhibitor or fulvestrant plus a CDK4/6 inhibitor.
2. At progression: test tumor tissue or circulating tumor DNA for ESR1, PIK3CA, AKT1, and PTEN alterations, and confirm germline BRCA1/2 status.
3. Second-line endocrine-based options: an oral SERD (elacestrant or imlunestrant) for ESR1 mutations; capivasertib or alpelisib with fulvestrant for pathway alterations; inavolisib-based therapy for early relapse with a PIK3CA mutation; everolimus with exemestane; or a PARP inhibitor for germline BRCA carriers.
4. After endocrine resistance: antibody-drug conjugates (trastuzumab deruxtecan for HER2-low or HER2-ultralow disease, datopotamab deruxtecan, or sacituzumab govitecan) and single-agent chemotherapy.

Key Teaching Points for Patients

• Bring a current medication list, including supplements, to every visit; many of these drugs interact through CYP3A4.
• Report mouth sores early, before they limit eating.
• Know your glucose targets and when to call if readings are high.
• Keep a diarrhea log and start loperamide as instructed.
• Report new cough, shortness of breath, or eye pain or blurred vision immediately.