# Translational Potential of Fluorescent PARP1 Inhibitor as a Molecular Contrast Agent for Diagnosis of Basal Cell Carcinoma

## TL;DR

A fluorescent PARP1 inhibitor shows promise as a noninvasive tool for diagnosing basal cell carcinoma by improving imaging contrast in skin.

## Contribution

PARPi-FL is a novel fluorescent contrast agent that can detect BCC noninvasively with high specificity and safety in preclinical models.

## Key findings

- PARP1 is overexpressed in BCC compared to nonmalignant lesions.
- PARPi-FL penetrates intact skin within 2–5 minutes and labels BCC lesions with strong fluorescence.
- No serious toxicity was observed in preclinical animal models.

## Abstract

Basal cell carcinoma (BCC), the most common skin cancer, typically requires biopsy for definitive diagnosis. Reflectance confocal microscopy is a noninvasive rule-out test, but the lack of nuclear contrast in BCC often leads to missed diagnoses. Our previous research in ex vivo human skin indicated that PARPi-FL, a poly(adenosine diphosphate ribose) polymerase 1 (PARP1) inhibitor–targeted fluorescent contrast agent, penetrated intact skin when applied topically and improved the diagnosis of BCC compared with reflectance confocal microscopy alone. This study refined PARPi-FL’s concentration and application timing to assess its detectability, safety, and feasibility for topical use in clinics and investigated its potential to distinguish BCC from nonmalignant clinical mimickers. Methods: We assessed PARP1 expression in 86 BCCs and 76 nonmalignant mimickers via immunohistochemistry. PARPi-FL (10 μM) permeability on ex vivo human skin was evaluated using a fluorescent confocal microscope (FCM) after 5 min compared with the previously evaluated times of 10–30 min. We evaluated the detectability of PARPi-FL’s fluorescent signal (10 μM; 5 and 30 min) via FCM through intact skin in excised human BCC tumors and compared it with the signal in 4 benign lesions. FCM imaging was conducted in anesthetized tumor-bearing B6 K5-Gli2 mice to evaluate topical application with gauze, simulating in vivo human imaging. Quantitative measurements of FCM signal intensities were taken with varying PARPi-FL concentrations (>10 μM) at 5 min. Systemic and cutaneous toxicity were assessed in SKH1-Hrhr mice and Yorkshire pigs. Results: PARP1 was overexpressed in BCC lesions compared with nonmalignant lesions. PARPi-FL penetrated intact human skin and labeled dermal structures within 2–5 min. A strong fluorescent signal from BCC lesions was detectable through the skin surface to a depth of approximately 100 µm after 5 min, whereas benign lesions showed lower and more variable signals. PARPi-FL application using gauze proved effective, with detectable signals in tumor-bearing mice. Increasing the concentration and reducing application time enhanced the nuclear fluorescent signal. No serious toxicity was observed in preclinical species. Conclusion: PARPi-FL is a fluorescent molecular contrast agent that has shown preclinical safety and translational potential for use in topical application for noninvasive BCC diagnosis. Its diagnostic accuracy and safety in humans require validation in clinical trials.