Four Molecular Targets Are Reshaping Targeted Radiation Therapy for Cancer

Radionuclide therapy is quietly becoming one of the most consequential stories in modern oncology. A comprehensive review published in Holistic Integrative Oncology by a team at the Medical University of Vienna, led by Honghu Zhang and senior authors Xiang Li and Marcus Hacker, maps the clinical maturity of four molecular targets—prostate-specific membrane antigen (PSMA), the somatostatin receptor (SSTR), the chemokine receptor CXCR4, and the fibroblast activation protein (FAP)—and draws a striking conclusion: the field is no longer defined by a single winning target, but by multiple therapeutic pathways shaped by distinct tumor biologies. What unites them is the theranostic principle, in which the same molecular target guides imaging, patient selection, treatment delivery, dosimetry, and response monitoring within one coherent framework.

The core logic of the approach is deceptively simple. A target-specific ligand is first labeled with a positron-emitting radionuclide such as gallium-68 or fluorine-18 and used in PET imaging to verify that a patient’s lesions actually express the target and that tracer distributes across the whole body. Only then is a therapeutic radionuclide—commonly lutetium-177, actinium-225, or yttrium-90—attached to the same or a closely related ligand and administered, delivering cytotoxic radiation with relative selectivity to tumor tissue. The authors emphasize that imaging in this context is not an adjunct to therapy but the starting point and foundation of the entire treatment pathway. Target stability, interlesional heterogeneity, physiologic uptake in normal tissues, and the feasibility of dosimetric assessment all directly influence benefit and risk, which is why pretreatment molecular imaging has become an indispensable gatekeeping step rather than a diagnostic formality.

PSMA stands out as the most mature system, and its success rests on elegant biology. PSMA is a type II transmembrane glycoprotein with a large extracellular domain and ligand-induced internalization, making it highly accessible to small molecules built on the Glu-urea-Lys scaffold. Expression is markedly upregulated in prostate cancer, particularly in high-risk, metastatic, and castration-resistant disease. The pivotal phase III VISION trial demonstrated that lutetium-177-labeled PSMA-617 plus standard care extended median radiographic progression-free survival from 3.4 to 8.7 months and median overall survival from 11.3 to 15.3 months in patients with PSMA-positive metastatic castration-resistant prostate cancer previously treated with androgen receptor pathway inhibition and taxane chemotherapy. Subsequent real-world studies and meta-analyses have consistently supported both efficacy and an acceptable safety profile, cementing beta-emitter PSMA therapy as established clinical practice rather than an experimental strategy.

Yet the beta emitter is not the end of the story. Beta particles have an intermediate tissue range and low linear energy transfer, producing a cross-fire effect suited to multifocal metastatic disease but potentially inadequate for microscopic residual lesions or tumors with heterogeneous uptake. Actinium-225 offers a different physics: an extremely high linear energy transfer and a micrometer-scale path length that induce clustered, difficult-to-repair DNA damage. In the multicenter retrospective WARMTH Act study of 488 heavily pretreated patients, actinium-225-PSMA therapy yielded a median overall survival of 15.5 months and progression-free survival of 7.9 months, though xerostomia emerged in 68 percent of evaluable patients after the first cycle. The Vienna reviewers position actinium-225 not as a stronger version of lutetium but as a distinct intensification strategy, best reserved for selected refractory scenarios while its dosimetry and sequencing with lutetium remain under active investigation.

Somatosatin receptor targeting, meanwhile, remains the model of stability in neuroendocrine tumors. Among the five SSTR subtypes, SSTR2 predominates in well-differentiated gastroenteropancreatic and midgut neuroendocrine tumors, providing the molecular basis for gallium-68-labeled DOTATATE, DOTATOC, and DOTANOC imaging and for lutetium-177-DOTATATE peptide receptor radionuclide therapy (PRRT), now a standard option for advanced SSTR-positive disease. European practice recommendations designate SSTR PET/CT as the reference-standard imaging modality for candidate selection, with FDG PET/CT complementing it in dedifferentiated or aggressive lesions. Perhaps the most intriguing recent development is the shift from agonists to antagonists. Because antagonists bind a broader range of receptor conformations and more binding sites, they can achieve higher lesion uptake and better tumor-to-background contrast, particularly in liver metastases, as confirmed by a 2025 systematic review and meta-analysis. The antagonist therapeutic lutetium-177-satoreotide tetraxetan has shown high tumor uptake and promising disease control in phase I/II testing, though hematologic toxicity and dosimetric challenges mean it remains an evolving complement rather than a replacement for agonist PRRT.

CXCR4 illustrates a fundamentally different clinical logic. This chemokine receptor drives tumor-cell migration, homing, and marrow invasion, and is associated with aggressiveness and poor prognosis in hematologic malignancies. Imaging with gallium-68-pentixafor PET is especially valuable in B-cell lymphomas and multiple myeloma, sometimes offering higher lesion contrast than FDG PET, and a prospective study in primary central nervous system lymphoma found that a fluorine-18-labeled CXCR4 tracer identified more lesions than FDG PET/CT. Therapy, using lutetium-177 or yttrium-90-labeled pentixather, has deliberately bypassed broad application and instead concentrated on high-intensity bridging or myeloablative treatment before hematopoietic stem-cell transplantation in heavily pretreated lymphoma and myeloma. The marrow toxicity in this setting is not merely an adverse event but part of the therapeutic concept, demanding close coordination between nuclear medicine, hematology, and transplant teams.

FAP targeting extends theranostics beyond tumor cells to the tumor microenvironment itself. FAP is a type II transmembrane serine protease overexpressed on cancer-associated fibroblasts but largely silent in normal adult tissue, making FAPI imaging remarkably effective across stromal-rich solid tumors including pancreatic, biliary, gastrointestinal, and ovarian cancers, sarcomas, and peritoneal metastases. Therapy, however, has lagged imaging for a fundamental pharmacologic reason: early small-molecule FAPI tracers clear rapidly, which produces pristine PET images but limits tumor retention and absorbed dose when labeled with lutetium-177. A 2025 phase II study of lutetium-177-LNC1004 achieved a disease control rate of 46 percent in 28 end-stage metastatic patients with high FAP expression, and a prospective study of the peptide ligand FAP-2286 showed feasibility and safety in FAP-positive gastrointestinal tumors, though benefit was mostly disease stabilization. The competitive frontier now lies in ligand engineering—Evans blue conjugation, albumin-binding modifications, dimerization, and novel scaffolds—all aimed at prolonging intratumoral retention to match the physical half-life of the radionuclide.

Across all four systems, the review identifies recurring translational bottlenecks. Target heterogeneity, both between patients and among lesions within the same patient, can evolve under treatment pressure, as seen when prostate tumors undergo neuroendocrine transdifferentiation and lose PSMA expression. Individualized dosimetry has become clinically decisive rather than academic: patient-specific studies reveal substantial interpatient variation in renal, salivary, lacrimal, and marrow radiation burden even under identical dosing, and the 2025 DUONEN trial showed that per-cycle organ dosimetry can safely guide activity adjustments below cumulative thresholds of 23 Gy for kidneys and 2 Gy for bone marrow. Organ protection is shifting from passive management toward active optimization through ligand design and dosimetry-guided dosing, particularly for the salivary glands that limit alpha-emitter PSMA therapy.

The wider horizon is expanding on two axes simultaneously. New targets including CCK2R, GRPR, and CAIX are entering clinical development, while novel radionuclides such as lead-212, terbium-161, and copper- and lead-based theranostic pairs offer different half-lives, emission profiles, and tissue ranges that change which ligands and lesion types they suit. The first-in-human VIOLET study of terbium-161-PSMA-I&T in metastatic castration-resistant prostate cancer exemplifies this dual expansion within an established target system. The Vienna authors argue that radioligand therapy must ultimately be embedded in multidisciplinary pathways that weigh sequencing against androgen receptor inhibitors, chemotherapy, PARP inhibitors, and immunotherapy, and that integrate supportive care, symptom control, and quality-of-life assessment rather than relying solely on imaging or biochemical endpoints. Their verdict is measured but clear: PSMA and SSTR constitute the proven backbone of nuclear theranostics, while CXCR4 and FAPI point toward a future in which the tumor microenvironment and the marrow niche, not just the tumor cell, become therapeutic targets in their own right.

Subject of Research: Molecular targets in oncologic radionuclide therapy and theranostics

Article Title: Advances in molecular targets for oncologic radionuclide therapy

Article References: Zhang, H., Xue, S., Wen, S., Wang, Z., Li, X., Jia, Y., Hacker, M., & Li, X. (2026). Advances in molecular targets for oncologic radionuclide therapy. Holistic Integrative Oncology, 5(1), Article 77. https://doi.org/10.1007/s44178-026-00293-6

Image Credits: AI Generated

DOI: 10.1007/s44178-026-00293-6

Keywords: radionuclide therapy, theranostics, PSMA, SSTR, CXCR4, FAPI, lutetium-177, actinium-225, dosimetry, prostate cancer, neuroendocrine tumors, radioligand therapy