What is the recommended regimen for using relugolix (oral GnRH antagonist) together with definitive external-beam radiotherapy in patients with advanced prostate cancer?

Relugolix Combined with Radiotherapy for Advanced Prostate Cancer

For patients with advanced prostate cancer receiving definitive external-beam radiotherapy, relugolix should be administered as a 360 mg oral loading dose on day 1, followed by 120 mg once daily, initiated 2-3 months before radiotherapy and continued for a total duration of 24-36 months depending on risk stratification 1.

Risk-Stratified Treatment Regimens

High-Risk and Very High-Risk Disease

For high-risk patients (Gleason 8-10, PSA >20, or ≥T3) and very high-risk patients (N1 disease or ≥2 risk factors: T3-T4, PSA >40, Gleason 8-10):

• Initiate relugolix 2-3 months before starting radiotherapy (neoadjuvant phase) 2, 3
• Continue relugolix concurrently during radiotherapy
• Maintain relugolix for 24-36 months total duration (adjuvant phase) 4, 5
• The 24-36 month duration is based on Level I evidence showing improved overall survival with long-term ADT in this population 4

For very high-risk M0 disease specifically, consider adding abiraterone + prednisone to the regimen for 24 months based on STAMPEDE trial data showing improved metastases-free survival (HR 0.54) and overall survival (HR 0.63) 5.

Intermediate-Risk Disease

For intermediate-risk patients (Gleason 7, PSA 10-20, or T2b-T2c):

• Initiate relugolix 2-3 months before radiotherapy
• Continue for 4-6 months total duration (neoadjuvant + concurrent) 4
• This shorter duration is supported by trials showing significant improvement in overall mortality (HR 0.63) with 6 months of hormonal therapy 4

Relugolix-Specific Dosing and Administration

Loading and Maintenance Dosing:

• Day 1: 360 mg oral loading dose (three 120 mg tablets)
• Day 2 onward: 120 mg once daily at approximately the same time each day 1
• Can be taken with or without food 1
• Swallow tablets whole; do not crush or chew 1

Timing Relative to Radiotherapy:

• Clinical trial data support initiating relugolix at least 2 months (median 3.6 months) before starting radiotherapy 2, 3
• This allows achievement of castrate testosterone levels (<50 ng/dL) before radiation begins
• 56% of patients achieve castration by day 4, and 97% maintain castration through 48 weeks 1

Evidence Supporting Relugolix with Radiotherapy

The combination approach is supported by:

Phase 2 trial data showing relugolix achieved 95% castration rates (testosterone <50 ng/dL) when combined with 24 weeks of ADT and radiotherapy for intermediate-risk disease 2. Median time to castration was only 4 days, significantly faster than injectable alternatives.

Post-hoc analysis of 260 patients receiving radiotherapy demonstrated castration rates of 95% (short-term ADT) and 97% (longer-term ADT) with relugolix 3. Importantly, 52% of patients achieved testosterone recovery to baseline or >280 ng/dL within 12 weeks after discontinuing short-term treatment, compared to only 16% with degarelix 2.

Real-world data with SBRT showed 92.3% of patients achieved profound castration (testosterone ≤20 ng/dL), with 87.2% reaching PSA ≤0.5 ng/mL by 4 months post-treatment 6.

Critical Advantages Over Injectable GnRH Agonists

Relugolix offers distinct clinical benefits as an oral GnRH antagonist:

1. No testosterone flare: Unlike leuprolide and other GnRH agonists, relugolix directly suppresses LH/FSH without initial testosterone surge 1, 7, eliminating risk of symptom exacerbation or spinal cord compression in patients with metastatic disease 8

2. Rapid testosterone suppression: Achieves castrate levels within 4 days versus weeks with depot injections 1, 2

3. Cardiovascular safety: 54% reduction in major adverse cardiovascular events compared to leuprolide 7, particularly relevant given the high cardiovascular risk in this patient population

4. Rapid testosterone recovery: After treatment discontinuation, testosterone recovery is significantly faster than with depot formulations 2, 3, important for quality of life and sexual function recovery

Drug Interactions and Monitoring

Critical drug interactions to avoid:

• P-gp inhibitors (e.g., erythromycin): Increase relugolix exposure 3.5-fold. If unavoidable, take relugolix first, separate dosing by ≥6 hours, and monitor more frequently 1
• Combined P-gp and strong CYP3A inducers (e.g., rifampin): Decrease relugolix exposure by 55%. If unavoidable, increase relugolix dose to 240 mg once daily 1

Monitoring requirements:

• Verify castrate testosterone levels (<50 ng/dL) at week 4 and periodically thereafter
• Monitor PSA levels per standard protocols
• Assess for QT prolongation in patients with cardiac risk factors 1
• Screen for hypersensitivity reactions, though rare 1

Common Pitfalls and Caveats

Adherence considerations: While relugolix is oral and requires daily dosing, studies show 96% testosterone suppression rates without daily reminders 6. However, poor adherence was documented in 2 patients (3.8%) in one series, emphasizing the need for patient education about the importance of consistent daily dosing 6.

Contraindications: Known severe hypersensitivity to relugolix is an absolute contraindication 1. Use caution in patients with pre-existing QT prolongation.

Fertility and contraception: Relugolix causes fetal harm. Males with female partners of reproductive potential must use effective contraception during treatment and for 2 weeks after the final dose 1.

Cost and access: As a newer oral agent, relugolix may face insurance coverage barriers compared to established injectable depot formulations. Address this proactively with prior authorization if needed.

The guideline-based framework remains unchanged from established ADT + radiotherapy protocols 4, 5, 4, but relugolix provides a mechanistically superior oral alternative to achieve the required androgen suppression with potential advantages in cardiovascular safety, testosterone recovery, and patient convenience 2, 3, 7.