A radiolabeled molecule hunts cancer cells and destroys them from within — the molecular sniper
Radioligand therapy (RLT) is a precision oncology strategy that combines the molecular targeting of antibodies or small molecules with the lethal payload of radiation — a concept sometimes called "theranostics." For prostate cancer, the discovery of PSMA (Prostate-Specific Membrane Antigen) as a near-universal cancer marker created a transformative therapeutic opportunity.
PSMA (also called FOLH1, Glutamate Carboxypeptidase II) is a type II transmembrane metalloprotease (zinc-dependent) with enzymatic activity toward folate polyglutamate substrates (folyl-poly-γ-glutamate carboxypeptidase). It is normally expressed at low levels in prostate epithelium, small intestine, proximal renal tubules, and salivary glands.
In prostate cancer: • Expression is upregulated 10–1,000-fold — one of the highest differential expressions of any tumor marker • Expression correlates with Gleason grade, metastatic status, and castration resistance • Crucially, PSMA upregulation is PRESERVED or increased in castration-resistant prostate cancer (mCRPC) — the disease stage where most patients die • PSMA is a cell-surface protein exposed to the extracellular space, making it accessible to circulating ligands without need for cellular internalization for initial binding
Additional expression contexts: • Tumor-associated neovasculature: PSMA is expressed on endothelial cells of tumor blood vessels in many cancers (lung, renal cell, gastric, colorectal), providing a secondary targeting mechanism even in non-prostate tumors • This neovascular expression pattern makes RLT theoretically applicable beyond prostate cancer
The discovery of a PSMA-targeting PET tracer (⁶⁸Ga-PSMA-11, FDA approved 2020) enabled the simultaneous development of diagnostic imaging AND therapy using the same molecule — the "theranostic" principle: "see it and treat it." The same PSMA-617 ligand can carry either ⁶⁸Ga (PET imaging, positron emitter) or ¹⁷⁷Lu (therapy, β emitter) — diagnostic and therapeutic isotopes on the same molecular chassis.
Prostate cancer is the most common non-skin cancer in men (~1.4 million new cases/year globally). The treatment pathway runs:
1. Localized disease: surgery (prostatectomy) or radiation — curative in ~85% 2. Biochemical recurrence: PSA rise after curative treatment → salvage radiation, hormone therapy (ADT) 3. Metastatic hormone-sensitive prostate cancer (mHSPC): ADT + intensification (docetaxel, enzalutamide, apalutamide) 4. Metastatic castration-resistant prostate cancer (mCRPC): cancer progresses despite castrate testosterone levels — the lethal stage
At mCRPC, therapeutic options are limited: • Enzalutamide/apalutamide (androgen receptor inhibitors) • Docetaxel/cabazitaxel (taxane chemotherapy) • Radium-223 (α-emitter for bone metastases only) • PARP inhibitors (for BRCA1/2-mutated disease) • Sipuleucel-T (cancer vaccine, modest survival benefit)
Median overall survival at mCRPC: ~16–24 months even with best available therapy. This is the clinical setting where ¹⁷⁷Lu-PSMA-617 (Pluvicto) demonstrated unprecedented benefit.
Before RLT, patient selection requires demonstrating that the tumor expresses PSMA. ⁶⁸Ga-PSMA PET/CT imaging provides whole-body visualization of PSMA expression:
• ⁶⁸Ga (Gallium-68): positron emitter, t½ = 68 min, produced from a germanium-gallium generator ("cow") without needing a cyclotron • The same PSMA-617 ligand carries ⁶⁸Ga via DOTA chelation for diagnostic imaging • PET detects the two 511 keV annihilation photons produced when a positron annihilates with an electron • Spatial resolution: ~4–5 mm, far superior to conventional bone scan or CT for detecting small lymph node and bone metastases
Clinical impact of ⁶⁸Ga-PSMA PET: • Detects lesions 3–6× more sensitively than conventional imaging at PSA < 1 ng/mL • Upstages ~16% of high-risk localized prostate cancer patients (finds occult metastases) • Changes management in ~50% of mCRPC patients imaged after conventional workup • Enables pure theranostic workflow: SCREEN patients with ⁶⁸Ga-PSMA → TREAT PSMA+ patients with ¹⁷⁷Lu-PSMA → RE-IMAGE for response assessment
The radioligand itself is a masterpiece of medicinal chemistry: a precisely engineered small molecule (~1,400 Da) that couples the cancer-targeting precision of a biomarker-binding ligand to the lethal energy of a radioactive isotope. Every atom is chosen deliberately — the chelator must hold the metal absolutely, the linker must optimize pharmacokinetics, and the targeting group must bind with nanomolar affinity.
PSMA-617 (the therapeutic version: ¹⁷⁷Lu-PSMA-617 / Vipivotide Tetraxetan) consists of four molecular modules:
1. Targeting ligand (PSMA-617 urea pharmacophore): A Glu-urea-Lys motif that mimics the natural PSMA substrate transition state. The two glutamate groups form bidentate coordination with two zinc ions at the PSMA active site. The lysine side chain projects outward for conjugation. Binding constant Kd ~1–3 nM — comparable to high-affinity antibodies but in a molecule 100× smaller.
2. Linker (4-aminobenzoyl-L-Thr spacer): Connects the pharmacophore to the chelator without disrupting PSMA binding. The linker length and chemical nature profoundly affect pharmacokinetics — too hydrophilic → rapid renal clearance before tumor accumulation; too lipophilic → liver/gallbladder sequestration.
3. DOTA chelator (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid): The 12-membered macrocyclic tetraaza ring forms an extraordinarily stable octadentate coordinate complex with ¹⁷⁷Lu³⁺. Thermodynamic stability constant log K ~25 (vs ~10 for EDTA). Kinetic inertness in vivo is critical — even slow ¹⁷⁷Lu³⁺ release would deposit radiation in bone (where free lutetium accumulates).
4. ¹⁷⁷Lutetium (radioactive payload): Produced in nuclear reactors by neutron activation of ¹⁷⁶Lu or ¹⁷⁶Yb. Decay: ¹⁷⁷Lu → ¹⁷⁷Hf + β⁻ + ν̄ₑ + γ. The β⁻ particle (Emax 498 keV) provides therapy; the 208 keV γ photons enable SPECT imaging for dosimetry and response monitoring — another theranostic feature.
DOTA forms such a thermodynamically stable complex with ¹⁷⁷Lu that the radiolabeling must be done at high temperature (95°C, pH 4–5) for 15–30 minutes — conditions that force the metal into the macrocyclic cage. Once formed, the complex is kinetically inert at physiological conditions — essentially irreversible without extreme conditions.
The choice of ¹⁷⁷Lu as the therapeutic radionuclide is a balance of multiple physical and practical considerations:
Physical properties: • β⁻ particle Emax: 498 keV, mean: 133 keV — deposits most energy within 1–2 mm radius of the decay location • Tissue penetration range: 0.5–2 mm — matches the scale of small metastatic lesions and provides "crossfire" irradiation of PSMA-negative cells adjacent to PSMA+ cells • Half-life: 6.647 days — long enough for tumor accumulation and sustained irradiation over several days; short enough for patient radiation safety and logistical handling • γ emission: 208 keV (11%) and 113 keV (6%) — sufficient for SPECT imaging without being high-energy enough to require heavy shielding • Decay daughter ¹⁷⁷Hf: stable, non-radioactive, biologically inert
Comparison with alternatives: • ¹⁸⁸Re (t½ 17h): too short for tumor accumulation; massive production required • ⁹⁰Y (t½ 64h, Emax 2.3 MeV): pure β⁻ emitter (no imaging capability), longer range → more irradiation of normal tissue around tumor • ²²⁵Ac (α-emitter, t½ 9.9 days): extremely potent but produces radioactive daughters (²¹Fr, ²¹³Bi) that redistribute in body; under active investigation for "PSMA-resistant" RLT
Production: ¹⁷⁷Lu is produced in nuclear research reactors by neutron bombardment. Specific activity matters: "no-carrier-added" (n.c.a.) ¹⁷⁷Lu produced via the indirect ¹⁷⁶Yb(n,γ)¹⁷⁷Yb→¹⁷⁷Lu route achieves higher specific activity than direct irradiation of ¹⁷⁶Lu.
RLT is not limited to PSMA-617. Multiple platforms are advancing through clinical development:
Fibroblast Activation Protein (FAP) targeting: • FAP is overexpressed on cancer-associated fibroblasts (CAFs) in >90% of solid tumors • FAPI (FAP Inhibitor) radioligands (⁶⁸Ga-FAPI for imaging, ¹⁷⁷Lu-FAPI for therapy) are the fastest-growing theranostic platform • Unlike PSMA (tumor cells), FAP targets the tumor stroma — potentially applicable to many cancer types simultaneously
Somatostatin Receptor (SSTR) targeting: • The original theranostic: ¹⁷⁷Lu-DOTATATE (Lutathera®) —FDA approved 2018 for GEP-NETs • DOTATATE: octreotide analog with picomolar affinity for SSTR2 • NETTER-1 trial: 65% progression-free survival benefit vs high-dose octreotide
SSTR2 → PSMA → FAPI represents the chronological expansion of RLT to increasingly common cancers.
HER2-targeting RLT: • ¹⁷⁷Lu-DOTA-trastuzumab: antibody-based; slower kinetics, higher radiation to bone marrow from long circulation • Small-molecule HER2 binders under development for faster pharmacokinetics
| Product | Indication | Trial Design | Key Result |
|---|---|---|---|
| ¹⁷⁷Lu-DOTATATE (Lutathera) | GEP-NETs (SSTR2+) | Octreotide analog + DOTA-¹⁷⁷Lu | FDA approved 2018; 65.2% PFS |
| ¹⁷⁷Lu-PSMA-617 (Pluvicto) | mCRPC (PSMA+) | Urea pharmacophore + DOTA-¹⁷⁷Lu | FDA approved 2022; +7.7 mo rPFS |
| ²²⁵Ac-PSMA-617 | mCRPC (RLT-resistant) | Same PSMA-617 + DOTA-²²⁵Ac | α-emitter; responses in ¹⁷⁷Lu failures |
| ¹⁷⁷Lu-FAPI-46 | FAP+ solid tumors (multiple) | Quinoline FAP inhibitor + DOTA-¹⁷⁷Lu | Phase II/III; broad applicability |
After intravenous infusion, ¹⁷⁷Lu-PSMA-617 must navigate a complex pharmacokinetic journey: distributed by blood flow, extracted by tissues expressing PSMA, and cleared by kidneys. Understanding this journey is essential for dosimetry — calculating the radiation dose delivered to tumor vs. organs at risk.
The biodistribution of ¹⁷⁷Lu-PSMA-617 follows a multi-compartment model:
Phase 1 — Distribution (0–2 h post-injection): • Rapid plasma clearance: ~50% of activity leaves circulation within 30 minutes • High initial PSMA-tissue uptake: salivary glands (~3–5% ID/g), lacrimal glands, proximal renal tubule (PSMA-expressing): all PSMA+ normal tissues • High renal tubule uptake: kidneys receive the highest non-target radiation dose
Phase 2 — Tumor accumulation (2–48 h): • Tumor uptake is slower than normal PSMA+ tissues because: (a) tumor vascularity may be heterogeneous, (b) intratumoral pressure impedes perfusion of large metastases • Peak tumor PSMA-617 uptake: 24–48 h post-injection • Tumor-to-blood ratio: >20:1 at 24 h • Internalization traps 80% of bound ligand inside tumor cells via endocytosis
Phase 3 — Tumor retention + clearance (48 h – 2 weeks): • Internalized radioligand retained in lysosomes for days → slow dissociation constant from tumor • Normal tissue clearance proceeds faster than tumor (favorable tumor-to-normal ratio improves over time) • Typical ¹⁷⁷Lu-PSMA dosimetry: tumor mean dose 10–100 Gy per cycle; kidneys 1–3 Gy; salivary glands 3–10 Gy; bone marrow 0.1–0.5 Gy
Key pharmacokinetic parameters (standard dose: 7.4 GBq): • 24 h whole-body retention: ~40% of injected activity • 72 h whole-body retention: ~10% • Cumulative urinary excretion: ~90% over 7 days
The kidneys are the dose-limiting organ for PSMA-targeted RLT — not because PSMA is highly expressed in kidney, but because ¹⁷⁷Lu-PSMA-617 is predominantly renally cleared. Pre-infusion amino acid infusion (lysine + arginine) competitively inhibits proximal tubule PSMA-mediated reabsorption, reducing kidney dose by 10–30% — a critical nephroprotective strategy.
Unlike external beam radiation therapy (EBRT) where dose can be precisely planned with CT-based treatment planning, internal radiation from RLT distributes based on the patient's individual tumor biology and pharmacokinetics. Dosimetry quantifies the absorbed radiation dose:
DosiMetric approach: 1. SPECT/CT imaging at multiple timepoints (e.g., 4h, 24h, 96h, 168h post-injection) 2. Quantitative SPECT reconstruction of ¹⁷⁷Lu activity in each organ/tumor over time 3. Time-activity curve fitting → calculate cumulative activity (area under curve) 4. Absorbed dose calculation: D = Ã × S-value (MIRD formalism) where à = cumulated activity, S-value = dose distribution kernel for the anatomy 5. Organ dose estimates: tumor, kidneys, salivary glands, bone marrow
Clinical dosimetry findings in PSMA RLT: • Median tumor absorbed dose: 22–46 Gy per cycle (range 5–300+ Gy in individual lesions) • Kidneys: 0.8–3.0 Gy/cycle (cumulative limit: ~23 Gy over full treatment) • Salivary glands: 2–12 Gy/cycle — clinically significant xerostomia in ~15% of patients • Red marrow: 0.05–0.3 Gy/cycle — hematological toxicity in ~5–10% of patients
Practical challenge: significant inter-patient and inter-lesion variability in uptake means that the same external dose produces very different tumor doses across patients. Personalized dosimetry-guided dosing (increasing doses for patients with low kidney exposure) is an active research priority.
¹⁷⁷Lu is a β⁻ emitter with low-energy γ; its radiation hazard requires specific but manageable precautions:
Patient radiation safety: • Patients retain significant radioactivity for the first week after infusion • ¹⁷⁷Lu β⁻ particles deposit energy within ~2 mm — not a significant external dose to others (mean free path in air: ~1 m) • The 208 keV γ photons do contribute external dose: dose rate at 1 meter from patient ~10 μSv/h at 24h post-infusion • Recommended precautions (1–2 weeks): limit close contact with pregnant women/children (<1 m for prolonged contact), separate toilet use, careful hand hygiene after uranium excretion in urine
Hospitalization requirements: • Most countries: outpatient or brief observation (2–4 hours), then discharge with written radiation safety instructions • Some jurisdictions: 1–2 day inpatient admission for first cycle • Comparison to ¹³¹I therapy (thyroid cancer): ¹³¹I is a far stronger γ emitter (364 keV, 81% abundance) → requires multi-day isolation ward admission. ¹⁷⁷Lu is much more ambulatory-friendly.
Staff dosimetry: • Pharmacy preparation (GMP): lead-lined laminar flow hood, syringe shields, lead aprons • Administration: typical nursing dose per infusion: ~10 μSv (within annual background variation) • Cumulative staff doses well below occupational limits (20 mSv/year)
The therapeutic efficacy of radioligand therapy depends critically on what happens after the radioligand reaches the tumor: binding must be tight enough to resist washout, and internalization must trap the radioactivity inside the cancer cell — preventing redistribution to healthy tissues. PSMA's rapid receptor-mediated endocytosis makes it an ideal trapping mechanism.
PSMA (Glutamate Carboxypeptidase II) is a zinc-dependent metalloprotease with a catalytic mechanism that PSMA-617 exploits with extraordinary precision:
PSMA active site structure (from crystal structure): • Two Zn²⁺ ions separated by ~3.2 Å, bridged by a water molecule that acts as the nucleophile in catalysis • Zn1 coordinated by His377, Asp387, His553 • Zn2 coordinated by Asp387, His553, His557 • The catalytic water (Zn1-bridging) is attacked by the transition-state of peptide hydrolysis • An arginine-rich "arene-binding" patch (Arg463, Arg534, Arg536) interacts with carboxylate groups
PSMA-617 binding mechanism (Glu-urea-Lys pharmacophore): • The urea carbonyl oxygen mimics the transition-state: coordinates both Zn²⁺ ions simultaneously (bidentate coordination) • The glutamate moiety makes ionic contacts with the zinc-binding histidines and the Arg patch • The lysine side chain exits through the entrance funnel without steric clash • Combined, this creates a ground-state transition-state analog that binds with Kd ~1–5 nM — 10,000× more potently than the natural substrate's Km
Key design insight: the glutamate-urea-lysine motif was discovered by screening phosphonate and phosphinate transition-state analogs against PSMA — the glutamate is essential, and the urea dipeptide isostere proved most metabolically stable while retaining potency.
PSMA undergoes constitutive clathrin-mediated endocytosis — making it a self-recycling "delivery trap" for radioligands:
Endocytosis cycle: 1. PSMA-ligand complex clusters in coated pits (clathrin-assembled lattices at the plasma membrane) 2. Dynamin GTPase pinches off the pit → forms a clathrin-coated vesicle containing PSMA + bound radioligand 3. Clathrin dissociates; vesicle acidifies → early endosome (pH 6.5) → late endosome (pH 5.5) → lysosome (pH 4.5–5.0) 4. At lysosomal pH, PSMA-617 dissociates from PSMA (protons compete with zinc coordination) 5. Free ¹⁷⁷Lu-DOTA fragment is trapped in lysosomes (too hydrophilic to permeate the lysosomal membrane at neutral pH) 6. PSMA is recycled back to the plasma membrane (via ERC) — ready to bind more ligand 7. The radioligand is retained intracellularly until physical decay releases the lethal β particles
T½ of internalization: ~3–5 minutes per bound ligand; steady-state 60% internalized at 1h.
Therapeutic implication: internalization converts PSMA from a surface receptor into a concentrating pump. Tumor cells with high PSMA expression accumulate radioactivity far above extracellular levels, creating a steep radiation dose gradient. Non-expressing cells in the tumor that happen to be close to PSMA+ cells also receive radiation (crossfire effect).
The lysosomal trapping mechanism solves a major problem in radioligand design: without efficient internalization, radioligands that dissociate from surface PSMA would redistribute to normal tissues. Internalization effectively "locks" the radioactivity inside tumor cells for the 6.7-day ¹⁷⁷Lu half-life, concentrating radiation dose where it is needed.
Despite the elegant targeting mechanism, not all PSMA+ tumors respond equally, and tumors can develop resistance:
Sources of heterogeneity: • Intra-tumoral PSMA heterogeneity: individual lesions in the same patient may have 50-fold variation in PSMA expression • Neuroendocrine differentiation: 15–20% of mCRPC tumors undergo neuroendocrine trans-differentiation (NEPC) with drastically reduced PSMA expression and androgen receptor — "double-negative" disease • Post-AR-inhibitor PSMA paradox: newer androgen receptor inhibitors (enzalutamide) can paradoxically increase PSMA expression via androgenic suppression → sequential RLT after enzalutamide may have higher PSMA expression
Resistance mechanisms: 1. PSMA loss: rare — PSMA is not usually driver of survival; cells can lose it under selective pressure from RLT 2. AR-PSMA decoupling: PSMA transcription is AR-driven; events that uncouple AR from PSMA promoter (CDK7 mutations, alternative promoter usage) reduce expression 3. DNA repair upregulation: surviving cells after sub-lethal irradiation upregulate homologous recombination and NHEJ repair — especially relevant for BRCA-wild-type tumors
Strategic responses: • Combination with PARP inhibitors (olaparib, rucaparib): inhibit DNA repair → synergize with radiation-induced DSBs • Combination with ²²⁵Ac-PSMA: α-particles create complex DSBs less repairable than β-mediated damage • Theranostics-guided retreatment: re-image with ⁶⁸Ga-PSMA after RLT; lesions losing PSMA can be identified and treated with alternative strategies
The final act of radioligand therapy is physics meeting biology: a radioactive decay event inside a tumor cell sends a high-energy electron particle through the nuclear DNA, creating irreparable double-strand breaks. This is radiation biology at the cellular scale — and the reason radioligands achieve responses in tumors that have failed every other treatment.
¹⁷⁷Lu undergoes β⁻ decay (negatron emission) as a neutron converts to a proton:
¹⁷⁷Lu₇₁ → ¹⁷⁷Hf₇₂ + β⁻ + anti-neutrino
The emitted β⁻ particle (high-energy electron) has: • Continuous energy spectrum from 0 to Emax = 498 keV (mean energy 133 keV) • LET (Linear Energy Transfer): ~0.2 keV/μm at mean energy — "low LET" radiation • Range in tissue: 0.2–2.2 mm depending on energy • RBE (Relative Biological Effectiveness): ~1.0 (similar to X-rays)
Energy deposition pattern: As β⁻ particles travel through tissue, they lose energy primarily through inelastic Coulomb interactions with orbital electrons — causing ionization and excitation of molecules along their path (the "track"). The particle deposits its entire kinetic energy within the tissue range (~1 mm mean).
Crossfire effect: Beta particles from ¹⁷⁷Lu travel up to 2 mm — far enough to irradiate neighboring tumor cells even if they don't express PSMA. This is critical for: • Heterogeneous tumors where not every cell has PSMA • Small metastases where tumor cells are adjacent to stroma • Micrometastases too small to "see" on imaging but within range of PSMA+ anchor cells
γ emission (SPECT imaging photons): • 208 keV (10.4% abundance) and 113 keV (6.4%) • These escape the patient and enable SPECT/CT dosimetry imaging — a unique dual-purpose emission
The 0.5–2 mm β-particle range matches the scale of small tumor deposits remarkably well. It is long enough to achieve "crossfire" killing of neighboring tumor cells but short enough that adjacent normal structures (bowel, bladder wall) are largely spared if the radioligand concentrates in the tumor. This spatial selectivity is what distinguishes β-emitting RLT from systemic chemotherapy.
The primary mechanism of radiation cell killing is DNA double-strand breaks (DSBs):
1. Direct effect (30% of damage): β⁻ particle directly ionizes DNA atoms → deoxyribose radical → strand scission. DSBs occur when two breaks occur within ~10 bp on opposite strands.
2. Indirect effect (70% of damage): β⁻ ionizes water molecules → ·OH (hydroxyl) radicals → react with DNA bases and sugars. ·OH diffuses ~4 nm from formation site — hitting DNA with high probability near the decay event.
DNA damage types per cell (¹⁷⁷Lu at therapeutic dose, ~10 Gy): • Single-strand breaks (SSBs): ~1,000 per Gy per cell — efficiently repaired within hours • Double-strand breaks (DSBs): ~20–40 per Gy per cell — lethal if unrepaired or misrepaired • Oxidized bases, DNA-protein crosslinks: numerous but not immediately lethal
Cell killing mechanisms: • Apoptosis: TP53-activation → p21/Bax → caspase cascade (hours to days) • Mitotic catastrophe: cells with unrepaired DSBs attempt mitosis → chromosomal segregation failure → nuclear fragmentation — the dominant mechanism in radioresistant cells with TP53 mutations • Senescence: cells permanently exit the cell cycle (especially in p53-competent cells at sub-lethal doses) • Necrosis: at very high doses — rapid membrane disruption
Dose-response: cell survival follows the Linear-Quadratic (LQ) model: S = exp(−αD − βD²) where D = dose, α = direct lethal events, β = lethal interaction between sub-lethal events. For prostate cancer, α/β ≈ 1.5–3 Gy — the low α/β means prostate cancer is especially sensitive to high dose-per-fraction radiation (hypofractionation benefit).
The landmark VISION phase III trial (2021, NEJM) established ¹⁷⁷Lu-PSMA-617 as a new standard of care:
VISION trial design: • 831 patients with PSMA-positive mCRPC; prior taxane + AR-pathway inhibitor • Randomization: ¹⁷⁷Lu-PSMA-617 (7.4 GBq IV q6 weeks × 6 cycles) + SoC vs. SoC alone • Primary endpoints: radiographic progression-free survival (rPFS) + overall survival (OS)
Results: • rPFS: 8.7 vs 3.4 months (HR 0.40, p<0.001) — 60% risk reduction in progression • Overall survival: 15.3 vs 11.3 months (HR 0.62, p<0.001) — unprecedented for this patient population • PSA response (≥50% decline): 46% vs 7% • Toxicities: xerostomia 39% (grade 1–2), fatigue 33%, nausea 35%; grade 3–4 toxicities manageable (thrombocytopenia 8%)
Subsequent developments: • PSMAfore trial (2023): RLT moved to earlier mCRPC (before cabazitaxel); superior rPFS vs abiraterone/enzalutamide switch (41% vs 23% PSA50 response) • PSMA4 trial: exploring RLT in metastatic hormone-sensitive PCa (first-line) • Combination studies: RLT + olaparib (PARP inhibitor, exploiting DNA damage synergy) actively enrolling • Next-generation: ²²⁵Ac-PSMA-617 — α-emitter with higher LET (80 keV/μm vs 0.2 keV/μm for β⁻); early data showing responses in ¹⁷⁷Lu-PSMA failures