Glossary
The words this site uses, defined.
62 terms from radiopharmaceutical development and machine learning. Each definition follows the authority cited at the foot of the page; links show where the site uses the term.
Last updated September 16, 2026
Terms beginning with A
4 terms
- Actinium-225
An alpha-emitting radionuclide, half-life 9.9 days, whose decay chain to stable bismuth-209 releases four net alpha particles of 5.8 to 8.4 MeV. Their range in tissue is tens of micrometres.
Read moreIsotope half-lives
- Affinity (Kd)
How tightly a ligand binds its target. The equilibrium dissociation constant Kd is the ratio [A][B]/[AB] at equilibrium, equal to koff/kon; a lower Kd, in nM or pM, means tighter binding.
Where Epitorix uses it Science · affinity ranking
Read moreKd, pKd and binding affinity, explainedpKd calculator
- Alpha vs beta emitters
Therapeutic radionuclides sorted by particle. Alpha emitters deposit high energy across tens of micrometres — a few cell diameters. Beta emitters deposit less energy over roughly one to a few millimetres, reaching more surrounding cells.
- Antibody–drug conjugate (ADC)
An antibody, or antibody fragment, joined by a chemical linker to one or more payload molecules — usually a cytotoxic drug released once the antibody has bound and the conjugate is internalised.
Terms beginning with B
2 terms
- Biodistribution
Where an administered compound goes in the body, and how much of it reaches each tissue over time. For a radiopharmaceutical it drives both tumour uptake and the dose absorbed by healthy organs.
Where Epitorix uses it Science · biodistribution and organ risk
- BLI (biolayer interferometry)
A label-free binding assay. Molecules accumulating on a fibre-optic sensor tip change its optical thickness, shifting an interference pattern; the shift over time gives association and dissociation rates, and from them Kd.
Where Epitorix uses it Science · affinity ranking
Terms beginning with C
3 terms
- Cell-type risk
On-target, off-tumour risk: the chance that a drug's target is also expressed by a healthy cell type the drug will reach. Assessed per cell type, from single-cell expression data, rather than from whole-tissue averages that average small vulnerable populations away.
Where Epitorix uses it Science · biodistribution and organ risk
- Chelator (DOTA)
The cage that holds a metal radionuclide onto a targeting molecule. DOTA — 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid — is the macrocyclic standard for gallium-68, lutetium-177 and actinium-225.
- CMC (chemistry, manufacturing and controls)
The part of a regulatory filing describing what a drug is made of, how it is made, and how its identity, strength, quality, purity and stability are controlled from batch to batch.
Terms beginning with D
2 terms
- De novo design
Designing a molecule from first principles rather than from a known binder or a natural template — for proteins, generating new backbones and sequences for a chosen site on a chosen target.
Where Epitorix uses it Science · targeted generation
Read moreAI in radiopharmaceutical design
- Dosimetry
Determining absorbed dose — the radiation energy deposited per unit mass of tissue, in gray. In radiopharmaceutical therapy it sets how much dose a tumour receives, and how much each organ receives.
Terms beginning with F
3 terms
- Fab
The fragment antigen-binding arm of an antibody: one light chain paired with the heavy chain's VH and CH1 domains, about 50 kDa, obtained by papain cleavage of an IgG or expressed directly.
- FEP+
Schrödinger's free-energy perturbation package. Alchemical molecular-dynamics simulations estimate the difference in binding free energy between related ligands, at roughly 1 kcal/mol accuracy and hours of compute per pair.
Where Epitorix uses it Science · affinity ranking
- First-in-human
The first trial in which an investigational product is given to people — usually Phase 1, sized at tens of subjects and designed around dose escalation, safety, pharmacokinetics and pharmacology rather than efficacy.
Terms beginning with G
2 terms
- Gallium-68
A positron emitter for PET, half-life 68 minutes, eluted from a germanium-68 generator rather than made in a cyclotron. Short-lived, so labelling, injection and imaging happen the same morning.
Read moreIsotope half-lives
- Generative model
A model that samples new candidates from a learned distribution instead of only scoring existing ones — new sequences, structures or molecules, conditioned on a target, a site or a set of constraints.
Where Epitorix uses it Xenoclea · ligand ranking
Read moreAI in radiopharmaceutical design
Terms beginning with H
2 terms
- Half-life (physical, biological, effective)
The time for a radionuclide's activity to halve (physical). Biological half-life: the time for the body to clear half of a compound. Effective half-life combines both (1/Teff = 1/Tphys + 1/Tbio) and sets the dose a tissue receives.
Read moreIsotope half-lives
- Hit rate
The fraction of designs tested that meet a stated binding criterion. It means little without three numbers alongside it: how many designs were tested, against what target, and at what threshold.
Where Epitorix uses it Science · our in-house models
Terms beginning with I
5 terms
- IC50
The concentration of an inhibitor or antagonist that reduces enzyme activity or agonist response by half. It shifts with assay conditions, so it is not interchangeable with Kd or Ki.
- IND-enabling studies
The nonclinical package assembled before an Investigational New Drug application: pharmacology, pharmacokinetics and GLP toxicology in vitro and in animals, enough to conclude that first-in-human dosing is reasonably safe (21 CFR 312.23; ICH M3(R2)).
Where Epitorix uses it Science · two paths to a candidate
- IND / IMPD
The dossiers that open a clinical trial: an Investigational New Drug application to the FDA, an Investigational Medicinal Product Dossier in the EU. Both carry manufacturing, non-clinical and clinical data for the candidate.
- Inverse folding
Going from structure to sequence: given a fixed protein backbone, predict amino-acid sequences that fold into it. The complement of structure prediction, and the usual second step in protein design.
- In vitro · in vivo
In vitro: experiments outside a living organism — cells, proteins or assays in the laboratory. In vivo: experiments in a living organism, usually an animal model before humans. Preclinical evidence uses both.
Where Epitorix uses it Pipeline · in vivo validation
Terms beginning with L
1 term
- Lutetium-177
A beta-minus emitter for radiopharmaceutical therapy, half-life 6.6 days, maximum beta energy 498 keV. Its 113 and 208 keV gamma photons also make SPECT imaging and patient dosimetry possible.
Read moreIsotope half-lives
Terms beginning with M
5 terms
- Macrocycle
A molecule built around a ring of twelve or more atoms. Macrocyclic peptides trade some size for rigidity, protease resistance and enough surface to cover flat protein interfaces.
Where Epitorix uses it Xenoclea · ligand ranking
- Maximum-intensity projection (MIP)
A flat rendering of a three-dimensional scan that keeps, for each viewing ray, only the brightest voxel along it. It is the rotating whole-body view a physician reads first on PET.
Where Epitorix uses it Home · our approach
- Mini-protein
A small designed protein, roughly 30 to 100 residues: smaller than an antibody, more rigid and more stable than a peptide, and cheap to express in bacteria.
Where Epitorix uses it Science · our in-house models
- Molecular dynamics (MD)
Simulating atomic motion by integrating Newton's equations with a force field. It shows how a complex moves and whether it holds together, at a cost of hours to days per system.
Where Epitorix uses it Science · affinity ranking
- mRNA display
In vitro selection in which each translated peptide stays covalently tethered to its own mRNA through a puromycin linker, so libraries of 10¹³ members and more can be selected and amplified.
Where Epitorix uses it Science · targeted generation
Terms beginning with N
1 term
- Nanobody
The single variable domain (VHH) of a camelid heavy-chain-only antibody, about 15 kDa: one domain binds the antigen. Nanobody is a registered trademark; VHH and single-domain antibody are the generic terms.
Where Epitorix uses it Science · our in-house models
Terms beginning with O
3 terms
- Off-target
Binding to a protein other than the intended target. Off-target binding in healthy tissue is a common reason a candidate that looked potent in vitro fails, or proves toxic, in vivo.
Where Epitorix uses it Science · off-target prediction
Read moreOff-target binding, explained
- On-target, off-tumour toxicity
Damage to healthy tissue that expresses the drug's intended target — distinct from off-target binding. For radiopharmaceuticals the renal proximal tubule, which retains many agents, is the usual example; see cell-type risk.
Where Epitorix uses it Science · biodistribution and organ risk
- Organ at risk (OAR)
A healthy tissue whose radiation exposure may cause clinically significant toxicity, so its absorbed dose is monitored or constrained. For ¹⁷⁷Lu-labelled ligands: kidneys, bone marrow, salivary and lacrimal glands; the liver usually is not.
Where Epitorix uses it Science · biodistribution and organ risk
Terms beginning with P
13 terms
- PBPK model
A physiologically based pharmacokinetic model: compartments for organs and blood joined by flows, used to predict whole-body distribution and organ uptake of a drug — the established way to predict radiopharmaceutical biodistribution before therapy.
- Pearson R
The linear correlation between predicted and measured values, from −1 to 1. It measures linear agreement, not rank order (Spearman's ρ) and not absolute error. For affinity it is computed on pKd, not on Kd in nM.
Where Epitorix uses it Science · affinity ranking
- Peptide
A short chain of amino acids joined by amide bonds, conventionally up to about fifty residues. Large enough to cover a binding site, small enough to synthesise chemically.
Where Epitorix uses it Science · our in-house models
- PET
Positron emission tomography. A positron from the tracer annihilates with an electron; the two 511 keV photons are detected in coincidence, placing the decay on a line between two detectors.
Where Epitorix uses it Science · biodistribution and organ risk
- PET/CT
One scanner, two acquisitions: PET for where the tracer went, CT for anatomy and for attenuation correction. The fused images are what a reading room actually looks at.
Where Epitorix uses it Science · biodistribution and organ risk
- Phage display
Peptide or antibody libraries displayed on the surface of bacteriophage and enriched by binding to an immobilised target. Introduced in 1985; recognised by the 2018 Nobel Prize in Chemistry.
Where Epitorix uses it Science · targeted generation
- Pharmacodynamics (PD)
What the drug does to the body: the relationship between exposure and effect, on the target and away from it. Read together with pharmacokinetics, it sets dose and schedule.
Where Epitorix uses it Science · biodistribution and organ risk
- Pharmacokinetics (PK)
What the body does to the drug: absorption, distribution, metabolism and excretion over time. For a radioligand, blood clearance decides how much reaches the tumour and how much irradiates healthy tissue.
Where Epitorix uses it Science · biodistribution and organ risk
- pKd
The negative base-10 logarithm of Kd in molar units: Kd of 1 nM is pKd 9. Working in logarithms keeps affinity data comparable, and is what most predictors are trained on.
Where Epitorix uses it Science · affinity ranking
Read morepKd calculatorKd, pKd and binding affinity, explained
- Preclinical (nonclinical)
Studies before any human trial: pharmacology, toxicology, pharmacokinetics and biodistribution in vitro and in animals. FDA prefers “nonclinical” and notes the two terms are used interchangeably before first-in-human studies.
Where Epitorix uses it Science · two paths to a candidate
- Proteome-wide screening
Testing a candidate against every protein a genome encodes rather than a chosen panel — computationally, since no assay covers the proteome. The output is a ranked list of possible off-targets.
Where Epitorix uses it Science · off-target prediction
Read moreOff-target binding, explained
- PRRT (peptide receptor radionuclide therapy)
Systemic treatment with a radiopharmaceutical made of a beta-emitting radionuclide chelated to a peptide that targets a cell-surface receptor — classically somatostatin receptor 2 on neuroendocrine tumours.
- PSMA
Prostate-specific membrane antigen: a type II transmembrane protein highly expressed on prostate cancer cells and also in the renal proximal tubule. The target of the approved ¹⁷⁷Lu-PSMA-617 therapy and of ⁶⁸Ga-PSMA PET imaging.
Where Epitorix uses it Xenoclea · the PSMA structure (PDB 4NGM)
Terms beginning with R
6 terms
- Radioligand
A targeting molecule — peptide, small molecule, antibody or fragment — carrying a radionuclide. The ligand decides where it goes; the radionuclide decides whether it images the target or treats it.
Where Epitorix uses it Products · PharmPhax
- Radioligand therapy (RLT)
Therapy that joins a targeting molecule — antibody, small molecule or peptide — to a therapeutic radioisotope through a chelator-linker, delivering ionising radiation to tumour cells. Two systemic RLTs are FDA-approved, both from Novartis.
- Radiopharmaceutical
A medicinal product that, when ready for use, contains one or more radionuclides included for a medical purpose — the definition in EU Directive 2001/83/EC. Diagnostic and therapeutic agents are both included.
Where Epitorix uses it Products · PharmPhax
- Radiopharmaceutical therapy (RPT)
Cancer treatment that delivers radioactive atoms to tumour-associated targets systemically, using pharmaceuticals that bind preferentially to cancer cells or accumulate by physiological mechanisms — the umbrella term for radioligand therapy, PRRT and targeted alpha therapy.
Where Epitorix uses it Xenoclea · the recorded demo
- Recall (sensitivity)
The fraction of true positives a classifier recovers: true positives divided by all actual positives. It says how much of the real signal was found, not how many predictions were wrong — that is precision.
Where Epitorix uses it Science · off-target prediction
- Retrieval-augmented generation (RAG)
Retrieving the relevant documents or records first, then conditioning a language model's answer on what was retrieved. It grounds an answer in sources that can be cited and checked.
Where Epitorix uses it EPiQ · the RAG system
Terms beginning with S
4 terms
- SPECT
Single-photon emission computed tomography. A gamma camera rotates around the patient, a lead collimator selects the direction photons may arrive from, and the projections are reconstructed into a three-dimensional image.
- SPR (surface plasmon resonance)
A label-free binding assay. Analyte binding to a sensor surface changes the local refractive index; the signal over time gives association and dissociation rates, and from them Kd.
Where Epitorix uses it Science · affinity ranking
- SSTR2 (somatostatin receptor 2)
The G-protein-coupled receptor subtype most often overexpressed on neuroendocrine tumours, and the target of somatostatin-analogue imaging and of peptide receptor radionuclide therapy such as ¹⁷⁷Lu-DOTATATE.
Where Epitorix uses it Science · targeted generation (the PDB 7T11 render)
- SUV (standardized uptake value)
The tracer concentration in a region of a PET image divided by the injected activity per unit of body weight — a dimensionless, semi-quantitative index of uptake. SUVmean averages a region; SUVmax takes its hottest voxel.
Where Epitorix uses it Home · our niche (the uptake scale)
Terms beginning with T
6 terms
- Targeted alpha therapy (TAT)
Radiopharmaceutical therapy with an alpha-emitting radionuclide such as actinium-225: alpha particles deposit 5–9 MeV over 50–100 µm — a few cell diameters — with high linear energy transfer, so tumour cells die with little dose to tissue beyond them.
Read moreWhat is targeted radiopharmaceutical therapy?Isotope half-lives
- Targeting vector
The molecule that carries a radionuclide to its target and binds it selectively — an antibody, antibody fragment, peptide, small molecule or mini-protein. A chelator or linker joins it to the radionuclide; together the three make a radiopharmaceutical.
Where Epitorix uses it Science · our in-house models
- Theranostics
Using the same targeting molecule twice: labelled with an imaging radionuclide to find the disease and confirm the target is there, then with a therapeutic radionuclide to treat it.
- Therapeutic index
The ratio of the exposure that causes a beneficial effect to the exposure that causes the first adverse effect; commonly the toxic dose for 50 % of a population over the minimum effective dose for 50 %.
- Translational de-risking
Testing a candidate for the liabilities that end programs — off-target binding, biodistribution, organ and cell-type exposure, PK and PD — before trials begin, so failures happen on paper instead.
Where Epitorix uses it Products · PharmPhax
- Tumour sink effect
Reduced uptake of a radiopharmaceutical in healthy organs because a large tumour burden sequesters much of the injected activity; with a low tumour burden the same activity exposes salivary glands, kidneys and marrow more.
Sources
- IUPAC Gold Book — Kd, IC50, macrocycle, therapeutic index
- SNMMI — theranostics, dosimetry, PET, MIP
- US FDA (21 CFR 312, guidances) — IND, CMC, first-in-human, ADC
- US FDA, ICH M3(R2) nonclinical safety studies — IND-enabling studies (with 21 CFR 312.23)
- EMA and EU Directive 2001/83/EC — radiopharmaceutical, IMPD
- RCSB PDB — Fab, nanobody, structure terms
- Google Machine Learning Glossary — recall (sensitivity)
- US FDA, Drug Development and Review Definitions — in vitro, in vivo, preclinical (nonclinical)
- PharmaDossier, Radioligand Therapy Clinical Trials 2026 — radioligand therapy (RLT)
- EANM Dosimetry Committee recommendations (PMC9015994) — organ at risk (OAR)
- Sgouros et al., Nat Rev Drug Discov 2020 (PMC7390460) — radiopharmaceutical therapy (RPT), targeting vector
- Kinahan and Fletcher, Semin Ultrasound CT MR 2010 (PMC3026294) — SUV
- Poty et al., J Nucl Med 2018 (PMC6004557) — targeted alpha therapy (TAT)
- StatPearls, Nuclear Medicine Physics (NBK568731) — half-life (physical, biological, effective)
- Joint IAEA/EANM/SNMMI practical guidance on PRRNT — PRRT
- Eur J Nucl Med Mol Imaging 2025, in vitro and in vivo models for targeted radionuclide theranostics — PSMA
- J Nucl Med 2017, somatostatin receptor antagonists; Pharmaceuticals 2023 (PMC10222684) — SSTR2
- J Nucl Med 2026, in-silico microdosimetry of renal toxicity — on-target, off-tumour toxicity
- Tumor sink effect with PSMA-targeted theranostics (PMC10177482) — tumour sink effect
- EJNMMI Research 2023, PBPK model for [68Ga]Ga-(HA-)DOTATATE — PBPK model
Definitions describe general scientific and regulatory usage, not Epitorix performance. Numbers, methods and program status live on Science and Pipeline; the longer explanations are in Learn, and two of the quantities can be worked in the pKd calculator and the isotope half-lives table.