Sequencing, Retreatment & Combinations in RPT¹⁷⁷Lu · ²²⁵Ac
Where radioligand therapy sits among the lines, and when to re-treat, combine, or switch emitter
Quick Answer
Choosing whether to give radioligand therapy is only half the decision; where in the treatment sequence, whether to re-treat, and what to combine it with are what separate expert programs. For prostate cancer, the pivotal evidence places ¹⁷⁷Lu-PSMA-617 after an ARPI and a taxane (VISION), with pre-taxane use after ARPI supported by PSMAfore (rPFS benefit, OS confounded by crossover) and reflected in the 2025 pre-taxane label expansion; combination with an ARPI is being defined (ENZA-p). For neuroendocrine tumors, PRRT was second-line after somatostatin-analog progression (NETTER-1) and is now first-line for grade 2–3 GEP-NETs (NETTER-2). Retreatment/rechallenge — a further course after an initial response — has encouraging but less mature evidence for both Lu-PSMA and PRRT. Sequencing must respect marrow and renal reserve, prior myelosuppressive therapy, and the option of escalating to an alpha emitter (²²⁵Ac-PSMA) in β-refractory disease.
Prostate: where Lu-PSMA sits
The mCRPC landscape by evidence:
| Setting | Evidence | Position |
|---|---|---|
| Post-ARPI, post-taxane | VISION (OS + rPFS benefit) | Established indication |
| Post-ARPI, pre-taxane | PSMAfore (rPFS; OS crossover-confounded) | Supported by the 2025 label expansion |
| + ARPI (combination) | ENZA-p (improved PSA-PFS) | Emerging combination strategy |
| β-refractory / progressing on Lu-PSMA | ²²⁵Ac-PSMA series | Alpha escalation (investigational) |
Eligibility always turns on PSMA-PET target expression, and discordant PSMA-negative/FDG-positive disease predicts poorer benefit — dual imaging refines selection.
NET: PRRT moves earlier
PRRT's position has shifted earlier in the NET pathway. NETTER-1 established ¹⁷⁷Lu-DOTATATE for midgut NETs progressing on somatostatin analogs; NETTER-2 showed first-line ¹⁷⁷Lu-DOTATATE (with long-acting octreotide) more than doubled PFS in newly diagnosed grade 2–3 SSTR-positive GEP-NETs, supporting PRRT as an early-line option rather than a last resort. Sequencing considerations include SSTR-expression confirmation, tumor grade/FDG status, and preserving future options.
Retreatment & rechallenge
A patient who responded to an initial course and later progresses may be a candidate for retreatment (rechallenge). Reported experience — additional Lu-PSMA cycles after an initial response, and salvage/retreatment PRRT — suggests meaningful further disease control with acceptable toxicity in selected patients, but the evidence is observational, not randomized. Selection hinges on duration of the first response, retained target expression on repeat companion PET, and adequate marrow/renal reserve.
Combinations & emitter switching
Beyond monotherapy, active strategies include RPT + ARPI (ENZA-p), RPT + immunotherapy or DNA-damage-response inhibitors (trials ongoing), and switching emitter class — escalating from a β-emitter to an α-emitter (²²⁵Ac-PSMA) when disease becomes β-refractory, exploiting the higher linear energy transfer and shorter range of alphas. Each combination raises the stakes on overlapping toxicity (marrow, salivary, renal), so sequencing decisions weigh cumulative absorbed dose to organs at risk, not just the next line's efficacy.
High-Yield Pearls
- VISION = post-ARPI/post-taxane; PSMAfore + the 2025 label = pre-taxane (post-ARPI) Lu-PSMA.
- NETTER-1 = 2nd-line PRRT (post-SSA); NETTER-2 = first-line PRRT for grade 2–3 GEP-NETs.
- Retreatment works in selected responders but rests on observational data — require retained target expression and organ reserve.
- Alpha escalation (²²⁵Ac-PSMA) is an option for β-refractory PSMA-positive disease.
- Sequencing respects cumulative marrow/renal/salivary dose, not just next-line efficacy.
Common Pitfalls
- Reserving PRRT for "last resort" in NETs when first-line use is now evidence-based (grade 2–3 GEP-NET).
- Re-treating without confirming retained target expression on a repeat companion PET.
- Ignoring discordant FDG-positive/PSMA-negative disease when selecting for Lu-PSMA.
- Stacking combinations without accounting for overlapping organ toxicity.
Related Pages
- Agents: ¹⁷⁷Lu-PSMA-617, ¹⁷⁷Lu-DOTATATE; pearl: Discordant PSMA/FDG; trials: VISION, PSMAfore, NETTER-1/2, ENZA-p.
Self-Check
Q1. Per pivotal trials, where does Lu-PSMA-617 sit relative to taxane chemotherapy, and what changed in 2025?
Answer: VISION established it post-taxane (after an ARPI); PSMAfore supported pre-taxane (post-ARPI) use, reflected in the 2025 pre-taxane label expansion.
Q2. How did NETTER-2 change PRRT's position in neuroendocrine tumors?
Answer: It supported first-line ¹⁷⁷Lu-DOTATATE for newly diagnosed grade 2–3 SSTR-positive GEP-NETs (more than doubling PFS), moving PRRT earlier than the post-SSA second-line position from NETTER-1.
Q3. What three factors most influence whether to re-treat a prior RPT responder?
Answer: Duration of the first response, retained target expression on repeat companion PET, and adequate marrow/renal reserve.
Q4. What is the rationale for switching from a β- to an α-emitter in refractory PSMA-positive disease?
Answer: Alpha emitters (e.g. ²²⁵Ac-PSMA) deliver high linear energy transfer over a short range, potentially effective in β-refractory disease — at the cost of distinct toxicity (e.g. xerostomia).
Key References
- VISION, PSMAfore, ENZA-p (prostate) and NETTER-1/NETTER-2 (NET) — see Landmark Trials.
- Reviews of Lu-PSMA rechallenge and salvage/retreatment PRRT; ²²⁵Ac-PSMA series in β-refractory mCRPC.
Evidence & sources
https://nuc.med/theranostics-sequencing-retreatment/ Report an issue