# Fluorothymidine F-18 (FLT)

Source: https://nuclide.cc/drugs/f-18-fluorothymidine/  
Nuclide record `f-18-fluorothymidine` (Agent). Data CC BY-NC 4.0, attribute "Data from Nuclide (nuclide.cc)"; commercial use needs a licence.

## TL;DR

An experimental PET tracer that measures how fast tumour cells are dividing, rather than how much sugar they burn. It has been studied in research settings for over 20 years but has never been approved as a medicine.

## Summary

18F-FLT has been an investigational PET agent since it was first reported in 1998, and remains one so today: it is supplied only under Investigational New Drug (IND) exemptions or Radioactive Drug Research Committee (RDRC)-approved basic science protocols, and no marketing application has been filed anywhere. The US National Cancer Institute's Cancer Imaging Program holds a centralised, multi-source IND that lets academic sites use FLT made by any of several suppliers with a Drug Master File on record, which has supported dozens of single- and multi-centre trials, mostly in breast, lung and brain cancer, evaluating early response to chemotherapy.

FLT's main limitation is that its signal is diluted by TK1 activity in normal proliferating tissue -- bone marrow and liver in particular -- giving high background uptake that can obscure metastases in those organs, and its plasma level is affected by co-administered chemotherapy and some pain medications via hepatic glucuronidation. Studies correlate FLT uptake with the Ki-67 proliferation marker in breast, lung and brain tumours, and FLT-PET has shown early, quantifiable drops in uptake after starting chemotherapy, but larger controlled trials with survival endpoints have not been completed, and no clinical guideline currently recommends it outside a trial setting.

## Fields

- Kind: Agent
- Status: phase-2
- Last checked: 2026-09-22
- Also known as: 18F-FLT; 3'-deoxy-3'-[18F]fluorothymidine
- Tags: proliferation imaging; investigational; TK1
- Modality: PET tracer
- Mechanism: A thymidine analogue that is taken up and phosphorylated by thymidine kinase 1 (TK1) during the S-phase of the cell cycle, then trapped intracellularly because the phosphorylated form cannot cross the cell membrane back out. Because TK1 activity tracks DNA synthesis, FLT uptake is a direct measure of cellular proliferation rather than glucose metabolism (which 18F-FDG measures) -- in principle a closer PET analogue of the Ki-67 proliferation index measured on biopsy.

## Sources

- NCI Investigator's Brochure: [F-18]FLT, an investigational PET radiopharmaceutical: https://dctd.cancer.gov/drug-discovery-development/reagents-materials/imaging-ind-resources/documentation/flt-ib.pdf
- Applications of PET imaging with the proliferation marker 18F-FLT (review, 2015): https://pmc.ncbi.nlm.nih.gov/articles/PMC4415691/
- SNMMI PET Center of Excellence FLT factsheet: https://snmmi.org/common/Uploaded%20files/Web/Centers/PET%20Center%20of%20Excellence/FLT_07-11-12%20Final.pdf

## Connected records

- isotopes: [Fluorine-18](https://nuclide.cc/isotopes/f-18/)
- indications: [Glioma & glioblastoma](https://nuclide.cc/indications/glioblastoma/), [HER2-positive breast cancer](https://nuclide.cc/indications/breast-her2-positive/), [Non-small cell lung cancer](https://nuclide.cc/indications/nsclc/)
- technologies: [PET (positron emission tomography)](https://nuclide.cc/technologies/pet/)

---
JSON: https://nuclide.cc/api/v1/entities/f-18-fluorothymidine.json