Fluorescent probes have the potential to enhance visualization of small tumours and help guide surgical procedures. Unfortunately, existing probes are often limited by high background signal. Activatable probes offer lower background signals, but often require hours to days to activate, making them impractical for routine or real-time clinical use. The new probe – developed by researchers from the University of Tokyo and the NIH's National Cancer Institute (Bethesda, MD) – lights up tumours within minutes and remains fluorescent for at least one hour.

The probe is based on a green dye called γ-glutamyl hydroxymethyl rhodamine green (gGlu-HMRG). The dye is intramolecularly caged so that it is initially non-fluorescent. Upon contact with γ-glutamyltranspeptidase (GGT), an enzyme that's overexpressed in several human tumours, the gGlu part is cleaved and fluorescent HMRG is activated. The authors note that while GGT might not be a specific biomarker for all cancers, similar probes can be developed for activation by enzymes located at the surface of other tumours.

An important feature is that gGlu-HMRG can be sprayed onto tissue surfaces suspected of harbouring tumours. The substantially lower dose that's required for this topical administration, compared to systemic intravenous administration, likely reduces potential toxicity concerns.

The researchers tested the probe in 11 different human ovarian cancer cell lines in vitro. All 11 cell lines showed activation and accumulation of gGlu-HMRG probe by 60 min, with some cell lines strongly fluorescing after 10 minutes. This fast instant activation of the probe suggests its suitability for incorporation into surgical or endoscopic biopsy or resection procedures.

Six cell lines that form peritoneal tumours in athymic mice were then used for an in vivo study. An endoscope was inserted into the abdominal cavity of anaesthetized tumour-bearing mice and diluted gGlu-HMRG sprayed on the peritoneal surface. Four of six peritoneal implants were detected within minutes after spraying the probe, with significantly increasing fluorescence signal over the first 10 minutes.

For one cell line (SHIN3), small nodules were sharply defined in the peritoneum as early as 30 s after spraying the probe. Small tumours (1 mm in diameter) could be easily removed with forceps under fluorescence-guided endoscopy from the living, anesthetized mice.

"The gGlu-HMRG probe could aid surgeons in detecting tiny cancerous nodules for accurate biopsy and tumour resection, delineating the borders of tumours for complete removal and confirming no residual tumour," concluded the researchers.