Combined PARP1-targeted nuclear contrast and reflectance contrast enhances confocal microscopic detection of basal cell carcinoma

Reflectance confocal microscopy (RCM) with endogenous backscattered contrast can noninvasively image basal cell carcinomas (BCCs) in skin. However, BCCs present with high nuclear density and the relatively weak backscattering from nuclei impose a fundamental limit on contrast, detectability, and diagnostic accuracy. We investigated PARPi-FL, an exogenous nuclear poly (ADP-ribose) polymerase (PARP1)-targeted fluorescent contrast agent and fluorescence confocal microscopy (FCM) towards improving BCC diagnosis. Methods: We tested PARP1 expression in 95 BCC tissues using immunohistochemistry, followed by PARPi-FL staining in 32 fresh surgical BCC specimens. Diagnostic accuracy of PARPi-FL contrast was evaluated in 83 surgical specimens. Optimal parameters for trans-epidermal permeability of PARPi-FL through intact skin was tested ex vivo on 5 human skin specimens and in vivo in 3 adult Yorkshire pigs. Results: We found significantly higher PARP1 expression and PARPi-FL binding in BCCs, as compared to normal skin structures. Blinded reading of RCM-and-FCM images by two experts demonstrated a higher diagnostic accuracy for BCCs with combined fluorescence and reflectance contrast, as compared to RCM-alone. Optimal parameters (time and concentration) for PARPi-FL trans-epidermal permeation through intact skin were successfully determined. Conclusion: Combined fluorescence and reflectance contrast may improve noninvasive BCC diagnosis with confocal microscopy.