gallium (68Ga) gozetotide
68Ga-PSMA-11, 68Ga-PSMA-HBED-CC, gallium Ga 68 gozetotide, PSMA-11 (unlabelled ligand), 68Ga-HBED-CC-PSMA
Gallium-68 PSMA-11 is the PET imaging agent that made prostate-specific membrane antigen targeting clinically routine. Like PSMA-617 it is not a peptide but a small-molecule urea-based enzyme inhibitor, in this case attached to an HBED-CC chelator that binds gallium at room temperature. It substantially outperforms conventional CT and bone scanning for staging, and it identifies which patients are suitable for PSMA-targeted radioligand therapy.
Mechanism
PSMA-11 shares the glutamate-urea-lysine pharmacophore of PSMA-617. The urea carbonyl coordinates the two zinc ions in the active site of prostate-specific membrane antigen (glutamate carboxypeptidase II), while the glutamate arm occupies the S1' pocket that normally accepts the glutamate of the enzyme's natural substrates. An aminohexanoic acid spacer links this motif to the chelator, and the chelator's own aromatic rings contribute to binding at the enzyme's arene-binding site, a nice example of a chelator that is not merely a passenger. The complex is internalised on binding and the radiometal is retained intracellularly, giving high tumour-to-background ratios.
The chelator choice defines the practical character of this agent. HBED-CC (N,N'-bis(2-hydroxy-5-(carboxyethyl)benzyl)ethylenediamine-N,N'-diacetic acid) is an acyclic chelator that complexes Ga³⁺ rapidly and quantitatively at room temperature, unlike macrocyclic DOTA which requires heating. That allowed straightforward kit-based or module-based radiolabelling in ordinary hospital radiopharmacies, which is why gallium-68 PSMA-11 spread so quickly through European nuclear medicine in the mid-2010s before any regulatory approval existed. The trade-off is that HBED-CC does not usefully chelate lutetium-177 or actinium-225, so PSMA-11 is an imaging agent only and cannot be converted to a therapeutic, which is exactly why the DOTA-bearing PSMA-617 was developed separately. Gallium-68 decays by positron emission (89% branching ratio, Emax 1.90 MeV) with annihilation photons detected in coincidence by PET.
What the research shows
The prospective single-arm trial reported by Fendler and colleagues in 635 men with biochemically recurrent prostate cancer found that gallium-68 PSMA-11 PET localised recurrence in 475 of 635 (75%) patients, with detection rates rising steeply with PSA (38% at PSA below 0.5 ng/mL up to 97% at PSA of 5 ng/mL or more) and a positive predictive value of 0.84 to 0.92 depending on the validation standard used. The proPSMA trial randomised 302 men with high-risk prostate cancer before curative-intent treatment to PSMA PET/CT or conventional CT plus bone scan; PSMA PET/CT had 92% accuracy for pelvic nodal or distant metastasis against 65% for conventional imaging, produced fewer equivocal results (7% versus 23%), changed management more often (28% versus 15%) and delivered less radiation (8.4 versus 19.2 mSv).
The most honest and clinically useful result comes from the multicentre phase 3 study reported by Hope and colleagues. That trial enrolled and imaged 764 men with intermediate- to high-risk prostate cancer, of whom 277 (36%) went on to prostatectomy with pelvic lymph node dissection and formed the efficacy analysis cohort; 75 of those 277 (27%) proved to have pelvic nodal metastasis. Against that histopathological reference standard, sensitivity for pelvic nodal metastasis was 0.40 (95% CI 0.34-0.46), specificity 0.95 (0.92-0.97), positive predictive value 0.75 (0.70-0.80) and negative predictive value 0.81 (0.76-0.85). That is a much less flattering sensitivity figure than the enthusiasm around PSMA PET might suggest, and it carries a direct clinical message: a positive PSMA PET finding in a pelvic node is highly likely to be real, but a negative scan does not exclude micrometastatic nodal disease, because a PET scanner cannot resolve deposits of a few hundred cells. Interpreting the technology as a rule-in rather than a rule-out test is the correct reading of the evidence. Known false-positive patterns (coeliac and other sympathetic ganglia, healing rib fractures, Paget's disease, fibrous dysplasia, granulomatous disease, and PSMA expression in the neovasculature of other tumour types) have been catalogued and are handled by standardised reporting frameworks.
Evidence assessment
High-quality evidence
Multiple prospective diagnostic accuracy trials including a phase 3 study using histopathology as the reference standard, a prospective randomised comparison against conventional imaging with a clear accuracy advantage, and full FDA and EU marketing authorisation. All three citations verified against PubMed. Note that 'strong' here means the evidence is robust and well characterised, including robustly demonstrating that sensitivity for pelvic nodal disease is poor.
Tiers are applied consistently across the library and re-checked when new trials read out. Read the grading method.
Key studies
Assessment of 68Ga-PSMA-11 PET accuracy in localizing recurrent prostate cancer: a prospective single-arm clinical trial Preclinical only
Recurrence localised in 475 of 635 (75%) patients; detection rate rose from 38% at PSA below 0.5 ng/mL to 97% at PSA of 5 ng/mL or above. Positive predictive value 0.84-0.92 depending on reference standard.
Prostate-specific membrane antigen PET-CT in patients with high-risk prostate cancer before curative-intent surgery or radiotherapy (proPSMA): a prospective, randomised, multicentre study Preclinical only
Accuracy 92% versus 65% for conventional imaging, fewer equivocal findings (7% versus 23%), more frequent management change (28% versus 15%) and lower radiation exposure (8.4 versus 19.2 mSv).
Diagnostic accuracy of 68Ga-PSMA-11 PET for pelvic nodal metastasis detection prior to radical prostatectomy and pelvic lymph node dissection: a multicenter prospective phase 3 imaging trial Preclinical only
Sensitivity for pelvic nodal metastasis 0.40 (95% CI 0.34-0.46), specificity 0.95 (0.92-0.97), positive predictive value 0.75 and negative predictive value 0.81. Establishes PSMA PET as a rule-in rather than a rule-out test for nodal disease.
Safety
Exceptionally well tolerated, as expected for a nanomole-quantity diagnostic tracer. Reported adverse reactions are uncommon and minor: nausea, dizziness, headache, fatigue or injection-site discomfort. There is no hormonal, anabolic or pharmacological activity. The main considerations are radiation exposure of roughly 3-4 mSv per study (lower than the conventional imaging pathway it replaces) and the interpretive pitfalls that come with high sensitivity for a target that is not entirely prostate-specific. Physiological uptake occurs in kidneys, ureters and bladder, salivary and lacrimal glands, liver, spleen, small bowel and coeliac ganglia. Non-prostatic causes of uptake include healing fractures, Paget's disease, fibrous dysplasia, vertebral haemangiomas, sarcoidosis and tuberculosis, and PSMA expression in tumour neovasculature means renal cell carcinoma, hepatocellular carcinoma, glioblastoma and several other malignancies can be avid. Recent androgen deprivation therapy can transiently alter PSMA expression and affect scan interpretation.
Regulatory status
| Jurisdiction | Status |
|---|---|
| United Kingdom | EU marketing authorisation granted 9 December 2022 for gozetotide, radiolabelled with gallium-68, for detection of PSMA-positive lesions by PET in adults with prostate cancer, covering initial staging of high-risk disease before curative treatment, suspected recurrence with rising PSA, and identification of patients with PSMA-positive progressive metastatic castration-resistant prostate cancer suitable for PSMA-targeted therapy (indication verified against the EMA record). Licensed in Great Britain by the MHRA. Widely used in UK nuclear medicine before authorisation under radiopharmacy manufacturing arrangements. |
| United States | FDA approved 1 December 2020 for gallium Ga 68 PSMA-11 injection at the University of California Los Angeles and University of California San Francisco, the first PSMA-targeted PET imaging drug approved in the United States, and an unusual approval granted to two academic institutions. Commercial kit products for the preparation of gallium Ga 68 gozetotide injection followed, in December 2021 and March 2022. Indicated for PSMA-positive lesions in men with prostate cancer with suspected metastasis who are candidates for initial definitive therapy, with suspected recurrence based on elevated PSA, and for selection of patients for PSMA-directed therapy. |
| WADA (sport) | Not on the WADA Prohibited List. It is a diagnostic radiotracer administered in nanomole quantities with no pharmacological activity. |
Questions
No. Its targeting element is a glutamate-urea-lysine small molecule that inhibits the PSMA enzyme's active site. Glutamate and lysine are amino acids, but they are joined by a urea bridge rather than a peptide bond. It is regularly filed alongside peptide radiopharmaceuticals and is a useful thing to have corrected.
No, and this is the most important limitation. The phase 3 trial using surgical histopathology as the reference found sensitivity for pelvic nodal metastasis of only 0.40, against specificity of 0.95. A positive finding is very likely real; a negative scan does not exclude small-volume nodal disease, because deposits below a few millimetres are below the resolution of PET.
HBED-CC binds gallium rapidly at room temperature, whereas DOTA needs heating. That made kit-based preparation practical in ordinary hospital radiopharmacies. The cost is that HBED-CC will not usefully hold lutetium-177 or actinium-225, so PSMA-11 cannot be converted into a therapeutic, which is why the DOTA-bearing PSMA-617 exists as a separate molecule.
Not entirely. PSMA is expressed physiologically in salivary and lacrimal glands, kidneys, small bowel and sympathetic ganglia, and it appears in the neovasculature of several other tumours including renal cell and hepatocellular carcinoma. Benign bone lesions, Paget's disease and granulomatous conditions can also be avid. Standardised reporting frameworks exist to handle these patterns.
Roughly 3 to 4 mSv from the tracer for a typical administered activity. In the proPSMA randomised trial the whole PSMA PET/CT pathway delivered 8.4 mSv compared with 19.2 mSv for conventional CT plus bone scintigraphy, so it replaced the older pathway with less exposure, not more.