Theranostic pair
A matched set of two radioactive drugs built on the exact same molecule: one labelled with a radioactive tag good for taking pictures (a PET scan), the other labelled with a different radioactive tag good for treatment, so the scan predicts where the treatment will go.
Overview
A theranostic pair is a diagnostic and a therapeutic radiopharmaceutical that share the same, or a near-identical, targeting molecule (peptide, small molecule or antibody), so that the biological distribution seen on the diagnostic scan is expected to closely mirror where the therapeutic radiopharmaceutical will localise and deliver radiation dose. The classic and historically first example is 123I/131I sodium iodide, where both agents follow the identical physiological pathway (the sodium-iodide symporter) into the thyroid. The current dominant paradigm labels the same peptide or small-molecule vector with two different chemical elements: a positron emitter for PET imaging (commonly gallium-68, sometimes copper-64 or fluorine-18) and a therapeutic emitter for treatment (commonly lutetium-177 for beta therapy, or actinium-225 for alpha therapy), examples being 68Ga/177Lu-DOTATATE for somatostatin-receptor-positive neuroendocrine tumours and 68Ga/177Lu-PSMA-617 (or PSMA-I&T) for prostate cancer.
Because the imaging and therapeutic agents in a "different-element pair" are chemically distinct compounds rather than isotopes of the same element, their pharmacokinetics can diverge somewhat (different positron/beta energies, different in vivo stability of the chelate, sometimes different affinity), so a positive diagnostic scan is a necessary but not perfect predictor of therapeutic biodistribution; this is one rationale for post-therapy SPECT/CT imaging with the therapeutic radionuclide itself, when it emits an imageable gamma (as 177Lu does), to confirm actual dose delivery. Lutetium-177 is unusual in being "theranostic on its own": its beta emission treats while its co-emitted 113/208 keV gamma photons allow SPECT imaging of the same injection, without needing a separate diagnostic radionuclide.
Other reported or developing theranostic pairs include 43/44Sc/47Sc, 64Cu/67Cu, 86Y/90Y, 152Tb/161Tb, 89Zr/177Lu or 225Ac (for CAIX and other antibody-based pairs), and 68Ga/225Ac or 213Bi combinations for alpha therapy planning. The theranostic pair concept underlies the pairing entries in this graph (for example psma-pet-to-rlt and sstr-pet-to-prrt) and is the organising principle behind most modern radioligand development programmes, which typically design a diagnostic and a therapeutic analogue of a new ligand together from the outset.
Similar pages
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