Gallium Ga-68 PSMA-11

PSMA-11 uses the same glutamate-urea-lysine warhead as the therapeutic PSMA ligands, coupled to an HBED-CC chelator that binds gallium-68 rapidly at room temperature — which is why it became the first widely produced PSMA tracer. Binding is followed by internalisation, so tumour signal continues to accumulate over the uptake period while blood-pool activity clears renally. Sensitivity for nodal and distant disease at low PSA levels far exceeds conventional imaging: detection rates climb steeply above a PSA of about 0.5 ng/mL. Physiological uptake in kidneys, bladder, salivary and lacrimal glands, liver, spleen and small bowel is expected. PSMA is not perfectly specific to prostate cancer — some renal, thyroid and other tumours and even benign lesions express it, so the scan is read in clinical context.

Targets: Prostate-specific membrane antigen (PSMA / FOLH1)

• · 111 to 259 MBq (3 to 7 mCi). Patients are usually asked to hydrate and void immediately before imaging so bladder activity does not obscure the prostate bed.

Work out your exact syringe units →

• very commonNo clinically meaningful pharmacologic effects— The chemical mass injected is in the microgram range.
• very commonDiagnostic radiation exposure— In the range of a contrast CT.
• uncommonInjection-site reaction
• rareNausea, dysgeusia or fatigue— Reported at low single-digit rates and self-limiting.

• None absolute. The clinically important pitfall is interpretive, not toxic: a false-positive PSMA-avid lesion can send someone down the wrong treatment path.

• synergylu-177-vipivotide-tetraxetan — Locametz was approved specifically as the companion diagnostic for Pluvicto.
• cautionAndrogen deprivation therapy — Recent ADT can transiently increase or decrease PSMA expression, so scan timing relative to hormone therapy affects interpretation.
• redundantpiflufolastat-f-18 — Both are PSMA PET tracers answering the same question; the choice is driven by local isotope supply.

• · No laboratory monitoring required for the tracer.
• · PSA at the time of scanning, because detection rate is strongly PSA-dependent.
• · Correlate PSMA-avid findings with anatomic imaging before acting on them.

FDA and EMA approved. Nuclear medicine only.

• Fendler et al. 2019, prospective multicentre trial of 68Ga-PSMA-11 PET in biochemical recurrence, JAMA Oncology (trial)
• FDA prescribing information for Locametz and Illuccix (label)

Same warhead chemistry as the therapeutic PSMA radioligand — a urea that mimics the tetrahedral transition state of PSMA's carboxypeptidase reaction and coordinates the two active-site zincs. The diagnostic difference is the chelator. HBED-CC forms a stable gallium complex at room temperature in a few minutes, which is why PSMA-11 became the first widely produced PSMA tracer while DOTA-based ligands needed heating. That chemistry choice is also why PSMA-11 cannot simply be swapped to lutetium: HBED-CC does not hold lutetium well, which is the reason the therapeutic molecule is PSMA-617 with DOTA rather than PSMA-11 with a different metal. Clinically, the signal to understand is the PSA dependence of detection. Detection rate climbs steeply as PSA rises: it is modest below 0.5 ng/mL, respectable between 0.5 and 1, and high above 2. That gradient is not a limitation of the scanner, it is a statement about tumour volume — a 3 mm nodal deposit expressing PSMA still emits less signal than the PET can resolve above background. PSA doubling time matters too, because fast-doubling disease has more volume by the time you scan. The interpretive risk is that PSMA is not prostate-specific in any literal sense: renal cell carcinoma, hepatocellular carcinoma, thyroid tumours, and the neovasculature of many solid tumours express it, and so do some benign findings including Paget disease, fibrous dysplasia, healing fractures and coeliac ganglia — the last of which is a classic mimic of a retroperitoneal node.

Coeliac ganglion uptake called nodal disease. Benign bone lesions — Paget disease, fibrous dysplasia, healing rib fractures, haemangiomas — called metastases and used to justify systemic therapy in a man who was curable with local treatment. Renal, hepatic and thyroid tumours that express PSMA read as prostate metastases. And the opposite error, over-reassurance from a negative scan at a PSA of 0.2 ng/mL where the detection rate was never going to be good. The structural problem is the isotope: gallium-68 comes from a generator with limited elutions per day and a finite lifetime, so capacity is constrained and slots are scarce, which is precisely why the fluorine-18 tracers took market share. The most consequential real-world failure is not a misread image at all — it is stage migration. PSMA PET finds disease that CT and bone scan never saw, so men are being reclassified as metastatic on the basis of imaging that no historic trial used, and the treatment evidence for those newly-visible deposits does not exist yet.

• Prostate-specific membrane antigen (PSMA / FOLH1) — Low nanomolar inhibition constant; reported figures for PSMA-11 sit in the low double-digit nanomolar range but vary by assay

  Active-site binding followed by internalisation, so lesion signal accumulates across the uptake period while background clears.

• Physiological PSMA in salivary and lacrimal glands, kidney, liver, spleen and small bowel — Normal tissue expression

  Expected normal uptake. Liver takes about 15 percent of injected dose, kidneys 7 percent, spleen 2 percent, salivary glands 0.5 percent.

• PSMA in coeliac ganglia and sympathetic ganglia — Normal neural expression

  The single most common false positive. Symmetric, band-like ganglion uptake gets called retroperitoneal nodal disease by inexperienced readers.

• Prospective multicentre trial of 68Ga-PSMA-11 PET in biochemical recurrence (UCLA/UCSF) Prospective diagnostic accuracy study · n=635 · 2019

  Positive predictive value of 68Ga-PSMA-11 PET against histopathology and a composite reference standard in men with biochemically recurrent prostate cancer. Positive predictive value was high, and detection rate rose with PSA.

Injection, then a 50 to 100 minute uptake period, then imaging. Patients void immediately before the scan so bladder activity does not swamp the prostate bed. Total visit around two hours. No pharmacological onset — the mass injected is far too small for any effect.

The pairing that defines this tracer is with the therapeutic PSMA radioligand — Locametz was approved specifically as its companion diagnostic, and no one gets radioligand therapy without a scan showing target expression. The interaction worth thinking about is with androgen deprivation. Short-term androgen receptor blockade transiently upregulates PSMA in preclinical and some clinical data, which would make a scan more sensitive, while prolonged suppression can reduce lesion detectability by shrinking disease and dropping PSA. The practical consequence is that scan timing relative to hormone therapy affects the result, and the request form should state the hormonal status. Beyond that, there is no meaningful drug interaction — the injected mass is micrograms and there is no pharmacological effect to interact with. The other tracer relationship is with fluorine-18 alternatives, which answer the same question with a longer-lived isotope.

Against piflufolastat F-18, the two answer the same question and head-to-head data show broadly comparable detection performance. The differences are practical: fluorine-18 has a 110-minute half-life and cyclotron production, so doses ship and centres without a generator can scan, and its shorter positron range gives marginally better spatial resolution; gallium-68 is generator-produced on site, which suits centres that already have one. There are also small differences in benign bone uptake patterns that experienced readers learn to discount. Against conventional imaging — CT plus technetium bone scan — this is not a competition, and the nodal staging accuracy difference before prostatectomy is large. Against fluciclovine F-18, the older amino acid tracer, PSMA imaging outperforms it at low PSA. Against the therapeutic PSMA radioligand, the warhead is the same but the chelator is different and deliberately so.

Not characterised, and not a monthly cost in any case. It is a per-scan facility charge with technical and professional components that varies widely by centre and payer, and in the US it has been subject to separate pass-through payment arrangements that change over time.

• ?PSMA-11 binding affinity is quoted as a range; reported inhibition constants vary by assay and I did not resolve a primary value in this session.
• ?The Fendler 2019 participant count of 635 is as commonly cited for that prospective trial; I resolved the paper record but not a participant-level breakdown from the full text.
• ?Detection-rate-by-PSA thresholds are given as approximate bands. The published figures differ between series and depend on the reference standard used.
• ?Whether androgen receptor blockade should be deliberately timed to boost PSMA expression before scanning is an open question with conflicting evidence.
• ?No volume of distribution or clearance figure is published, so those are left null.
• ?The claim that HBED-CC does not adequately chelate lutetium is standard radiochemistry teaching but was not re-verified against a primary source here.

• Assessment of 68Ga-PSMA-11 PET Accuracy in Localizing Recurrent Prostate Cancer: A Prospective Single-Arm Clinical Trial Fendler WP, Calais J, Eiber M, et al., JAMA Oncology, 2019 · PMID 30920593

  The prospective histopathology-anchored accuracy study that underpinned the December 2020 FDA approval.

• LOCAMETZ (kit for the preparation of gallium Ga 68 gozetotide injection) — US prescribing information Advanced Accelerator Applications / Novartis, FDA prescribing information

  Source of the 33 percent protein binding, the 14 percent urinary excretion at 2 hours and the organ distribution percentages.

• ILLUCCIX (kit for the preparation of gallium Ga 68 gozetotide injection) — US prescribing information Telix Pharmaceuticals, FDA prescribing information

  The competing kit. The pharmacokinetic section reports the same distribution and excretion figures, which is expected since the labelled molecule is identical.