Selpercatinib: A Selective RET Inhibitor for RET-Altered Cancers
Selpercatinib (RETEVMO) is a highly selective RET kinase inhibitor approved for treating RET fusion-positive thyroid cancers, RET-mutant medullary thyroid cancer, RET fusion-positive non-small cell lung cancer, and other RET fusion-positive solid tumors, with demonstrated superior efficacy compared to multi-kinase inhibitors and a generally manageable safety profile. 1, 2
Indications
Selpercatinib is FDA-approved for:
- RET fusion-positive differentiated thyroid cancer (DTC): Adults and pediatric patients ≥2 years with advanced/metastatic disease requiring systemic therapy who are radioactive iodine-refractory (if RAI is appropriate) 2
- RET-mutant medullary thyroid cancer (MTC): Adults and pediatric patients ≥2 years with advanced/metastatic disease requiring systemic therapy 2
- RET fusion-positive NSCLC: Adults with locally advanced or metastatic disease 2
- Other RET fusion-positive solid tumors: Adults and pediatric patients ≥2 years with locally advanced/metastatic tumors that have progressed on prior systemic treatment or have no satisfactory alternatives (accelerated approval) 2
Note the regulatory differences: The FDA approved selpercatinib for patients ≥12 years who are RAI-refractory or ineligible, while the EMA restricted approval to adults only after previous treatment with approved multi-kinase inhibitors (sorafenib, lenvatinib, or both) 1.
Dosing
Adults and Adolescents ≥12 Years 2
Weight-based dosing:
- <50 kg: 120 mg orally twice daily
- ≥50 kg: 160 mg orally twice daily
Pediatric Patients 2 to <12 Years 2
Body surface area-based dosing:
- 0.33-0.65 m²: 40 mg orally three times daily
- 0.66-1.08 m²: 80 mg orally twice daily
- 1.09-1.52 m²: 120 mg orally twice daily
- ≥1.53 m²: 160 mg orally twice daily
Dose Modifications
Severe hepatic impairment: Reduce dose 2
No dose adjustment needed for:
Efficacy Data
RET Fusion-Positive Thyroid Cancer
The LIBRETTO-001 trial evaluated 27 patients with RET fusion-positive thyroid cancers (papillary, poorly differentiated, anaplastic, Hürthle cell) 1:
In previously treated patients (n=19):
- Objective response rate (ORR): 79% (95% CI 54-94%)
- Duration of response ≥6 months: 87% of responders
In treatment-naïve patients (n=8):
- ORR: 100% (95% CI 63-100%)
- Duration of response ≥6 months: 75% of responders
Selpercatinib demonstrated activity across all histological types and regardless of RET fusion partner (CCDC6, CCDC186, ERC1, KTN1, RUFY3) 1.
RET-Mutant Medullary Thyroid Cancer
A phase 3 trial (LIBRETTO-531) compared selpercatinib to cabozantinib or vandetanib as first-line therapy 3:
- Median progression-free survival: Not reached with selpercatinib vs. 16.8 months with control (HR 0.28; 95% CI 0.16-0.48; P<0.001)
- PFS at 12 months: 86.8% vs. 65.7%
- ORR: 69.4% vs. 38.8%
- Treatment failure-free survival: Not reached vs. 13.9 months (HR 0.25; P<0.001)
Network meta-analysis confirmed selpercatinib's superiority over multi-kinase inhibitors (HR 0.10 vs. placebo; P-score 0.999), outperforming cabozantinib (HR 0.28) and vandetanib (HR 0.46) 4.
RET Fusion-Positive NSCLC
The LIBRETTO-431 trial compared first-line selpercatinib to platinum-based chemotherapy ± pembrolizumab 5:
- Median PFS: 24.8 months vs. 11.2 months (HR 0.46; P<0.001)
- ORR: 84% vs. 65%
- CNS progression hazard ratio: 0.28 (95% CI 0.12-0.68)
In early-stage NSCLC (LIBRETTO-432), adjuvant selpercatinib demonstrated remarkable efficacy 6:
- 2-year event-free survival (stage II/IIIA): 92% vs. 61% with placebo (HR 0.17; P<0.001)
- 2-year event-free survival (stage IB-IIIA): 94% vs. 70% (HR 0.16; P<0.001)
Safety Profile
Safety data from 702 patients across all RET-altered solid tumors in LIBRETTO-001 1:
Most Common Grade 3+ Adverse Events:
- Hypertension: 18%
- Elevated ALT: 9%
- Elevated AST: 8%
Common Treatment-Related Adverse Events (≥25%):
- Elevated AST/ALT
- Hyperglycemia
- Decreased leukocytes, albumin, calcium
- Dry mouth
- Diarrhea
- Elevated creatinine, alkaline phosphatase
- Hypertension
- Fatigue
- Edema
- Decreased platelets
- Elevated total cholesterol
- Rash
- Hyponatremia
- Constipation
Treatment Modifications:
- Permanent discontinuation due to adverse events: 2%
- Dose interruptions: 42% (most commonly for elevated ALT/AST)
- Dose reductions: 38.9% (vs. 77.3% with cabozantinib/vandetanib) 3
Critical Safety Considerations
QTc Prolongation 2, 7
Monitor ECG and electrolytes:
- Baseline, after 1 week, 1 month, then every 3 months
- More frequently if QTc >500 msec or if using concomitant QT-prolonging medications
QTc prolongation occurred in ~16% of patients (Grade 3+ in ~4%) 7. Two case reports demonstrated clinically significant QTc prolongation requiring dose reduction or interruption 7.
Hepatotoxicity 2
Monitor ALT/AST:
- Prior to initiation
- Every 2 weeks during first 3 months
- Monthly thereafter
Withhold, reduce dose, or permanently discontinue based on severity.
Renal Tubular Damage 8
A case report documented severe renal tubular damage with symptomatic hyponatremia (112 mmol/L), hypomagnesemia, hypocalcemia, and polyuria occurring 4 weeks after starting selpercatinib 8.
Monitor electrolytes closely, especially:
- Sodium
- Magnesium
- Calcium
- Urine output
Therapeutic drug monitoring may guide dose adjustments after renal toxicity 8.
Other Monitoring Requirements 2
- Hemorrhagic events: Monitor for bleeding
- Impaired wound healing: Withhold for ≥7 days before elective surgery
- Hypertension: Monitor and treat as indicated
- Hypersensitivity reactions: Permanently discontinue for severe reactions
- Interstitial lung disease/pneumonitis: Monitor for pulmonary symptoms
Pediatric-Specific Concerns 2
Growth plate abnormalities: Monitor growth plates in pediatric patients with open growth plates. Juvenile animal studies showed irreversible physeal thickening, decreased bone mineral density, and impaired bone modeling. Consider interrupting or discontinuing therapy based on severity.
Fertility concerns: May impair fertility in males and females. Juvenile rat studies showed irreversible testicular germinal epithelium degeneration and delayed sexual maturity in females 2.
Drug Interactions 2
Selpercatinib is primarily metabolized by CYP3A4 9:
- Strong CYP3A4 inhibitors: May increase selpercatinib exposure
- Strong CYP3A4 inducers: May decrease selpercatinib exposure
- Proton pump inhibitors: Avoid concomitant use (selpercatinib requires acidic pH for absorption)
Comparative Advantages Over Multi-Kinase Inhibitors
Selpercatinib offers several advantages over cabozantinib and vandetanib 1, 3, 4:
- Superior efficacy: Higher ORR and longer PFS
- Better tolerability: Lower rates of dose reductions (38.9% vs. 77.3%) and discontinuations (4.7% vs. 26.8%)
- Fewer off-target toxicities: Avoids palmar-plantar erythrodysesthesia, severe diarrhea, and some cardiovascular toxicities typical of MKIs
- CNS penetration: Effective for brain metastases 5
However, selpercatinib has unique toxicities requiring specific monitoring: QTc prolongation, hepatotoxicity, and potential renal tubular damage 2, 8, 7.
Clinical Practice Recommendations
For RET fusion-positive thyroid cancer: Selpercatinib is the preferred first-line targeted therapy based on NCCN and ESMO guidelines 10. The NCCN designates it as Category 1, preferred regimen for RET-positive medullary thyroid cancer 10.
For RET fusion-positive NSCLC: ASCO guidelines recommend selpercatinib or pralsetinib as first-line therapy (moderate evidence quality, strong recommendation) 11.
Monitoring algorithm:
- Baseline: ECG, electrolytes, liver function tests, blood pressure
- Week 1: ECG, electrolytes
- Weeks 2-12: Liver function tests every 2 weeks
- Month 1: ECG, electrolytes
- Every 3 months: ECG, electrolytes, liver function tests
- Ongoing: Blood pressure monitoring, clinical assessment for bleeding, pulmonary symptoms, electrolyte disturbances
Dose modifications should be guided by severity of adverse events, with consideration for therapeutic drug monitoring in cases of severe toxicity 8.