Mechanism 05 of 16

Receptor binding and internalisation

The tracer is a ligand for a receptor the target cell has far too many of. Signal equals receptor density — which is why the same scan that finds the disease also decides who can be treated.

This family is where imaging and therapy became one thing. A Ga-68 DOTATATE scan is not only a staging study; it is the eligibility test for Lu-177 DOTATATE, because the picture and the treatment share a targeting vector and differ only in the isotope bolted to the chelator.

The corollary is the flip-flop. Well-differentiated neuroendocrine tumours express somatostatin receptors and are quiet on FDG. As they dedifferentiate they lose the receptor and gain glucose avidity. A lesion that is DOTATATE-negative and FDG-positive in a patient with known NET is the aggressive clone, and it is the one that will not respond to peptide receptor radionuclide therapy.

ligand approachingreceptor–ligand complexreceptor

Binding, then swallowing. The octreotide analogue docks on SSTR2 and the receptor–ligand complex is drawn inside by clathrin-mediated endocytosis — the chelated metal ends up in an endosome and stays there for hours. That internalisation is also what makes Lu-177 DOTATATE therapeutic. Ioflupane, by contrast, binds the dopamine transporter and stays on the presynaptic membrane: the count is a census of surviving nerve terminals.

The agents

Ga-68 DOTATATE / Cu-64 DOTATATENetspot · Detectnet

PET

Ga-68 t½ 68 min (Ge-68/Ga-68 generator) · Cu-64 t½ 12.7 h (distributable)

Handle: Somatostatin receptor subtype 2 (DOTATATE is highly SSTR2-selective)
Trapping: Octreotate binds SSTR2 with high affinity; the complex is internalised, keeping the chelated metal inside the cell.
Use: Well-differentiated neuroendocrine tumour staging, occult primary, and selection for Lu-177 PRRT.
Pitfall: Physiologic uptake is intense in spleen (highest), adrenals, pituitary, kidneys and the pancreatic uncinate process — the classic false positive. Hold long-acting octreotide about 3–4 weeks; it competes for the receptor. Splenules, accessory spleen, inflammation, meningioma and vertebral haemangioma are avid. High-grade NETs lose SSTR — pair with FDG when behaviour and images disagree.

In-111 pentetreotideOctreoScan

SPECT / planar

In-111 · t½ 2.8 d · 171 and 245 keV γ · image at 24 and 48 h

Handle: Somatostatin receptor 2/5
Trapping: Octreotide–DTPA conjugate; same receptor, lower affinity and far poorer resolution than DOTATATE.
Use: Historical NET imaging; still used where Ga-68 is unavailable.
Pitfall: Bowel activity requires laxative preparation and delayed imaging. Superseded by Ga-68 DOTATATE on sensitivity, resolution, radiation dose and same-day workflow.

Lu-177 DOTATATELutathera

Therapy

Lu-177 · t½ 6.6 d · β⁻ 498 keV max · 113 and 208 keV γ allow post-therapy imaging

Handle: SSTR2
Trapping: Identical targeting vector to the diagnostic agent; internalisation delivers the β⁻ dose inside the cell rather than to its surface.
Use: SSTR-positive gastroenteropancreatic neuroendocrine tumours; four cycles at 8-week intervals.
Pitfall: Amino acid co-infusion (lysine and arginine) blocks proximal tubular reabsorption and is required to protect the kidneys. Myelosuppression, and a small late risk of MDS or leukaemia. Requires a positive companion DOTATATE scan — this is the theranostic pairing in its purest form.

I-123 ioflupaneDaTscan

SPECT / planar

I-123 · t½ 13.2 h · 159 keV γ · image 3–6 h after injection

Handle: Presynaptic dopamine transporter (DAT) on nigrostriatal terminals
Trapping: A cocaine analogue that binds DAT. Uptake is a direct measure of surviving presynaptic terminals — an anatomical count, not a functional one.
Use: Separates degenerative parkinsonism (asymmetric, putamen-first loss producing the "comma to period" shape) from essential tremor, drug-induced, psychogenic and vascular parkinsonism, which are usually normal. Supports a diagnosis of dementia with Lewy bodies.
Pitfall: It does not distinguish Parkinson disease from MSA, PSP or CBD — all are abnormal. Thyroid blockade required. Cocaine, methylphenidate, amphetamines, bupropion and modafinil interfere. Normal ageing causes a symmetric decline of roughly 5–7% per decade.

Tc-99m tilmanoceptLymphoseek

SPECT / planar

Tc-99m · t½ 6.01 h · particle diameter ~7 nm

Handle: CD206 mannose receptor on nodal macrophages and dendritic cells
Trapping: A dextran backbone carrying mannose units binds CD206 and is held in the first draining node. Because the molecule is tiny it clears the injection site fast, yet the receptor keeps it from washing through to second-echelon nodes.
Use: Sentinel lymph node mapping in melanoma, breast cancer and oral cavity squamous cell carcinoma.
Pitfall: The advantage over sulfur colloid is rapid injection-site clearance with less distal node spillover — useful in head and neck, where the sentinel node may sit close to the primary. More expensive.

← 04Chemisorption onto bone mineral 06 →Enzyme-target binding and internalisation IndexAll 16 mechanisms