Systematic Exploration of the Potential of HPQ as the Core Scaffold of Probes for in Vivo Imaging: A Case Study of FAP-Responsive Probes

The
limited tissue penetration depth of optical probes
based on
2-(2′-hydroxyphenyl)-4(3H)-quinazolinone (HPQ) hindered dynamic
monitoring of the actual distribution and metabolism of them in deep
tissues. Herein, using fibroblast activating protein (FAP)–activatable
dual-modal probes as a prototype, we explored the potential of HPQ
as a core scaffold for in vivo imaging probes via stepwise biological
characterization. The tumor-to-muscle uptake ratios (T/M) of LWF-1, LWF-2, and LWF-3 were consistently
above 4.05 during in vivo fluorescence imaging with sustained tumor
visualization exceeding 48 h. All 68Ga-radiolabeled probes
exhibited a maximum tumor uptake of up to 12.83 ± 9.37%ID/g in
model mice, with the T/M of [68Ga]LWF-3 remaining above 4.89 throughout the PET scanning.
The ex vivo biodistribution study revealed that [177Lu]LWF-3 reached the peak uptake
in the tumor at 4 h postinjection (10.41 ± 0.68%ID/g). Accordingly,
structural design and optimization governing their in vivo metabolism
are critical for acquisition of ideal HPQ-based probes.