Methods of tagging cancerous tissue for diagnostic applications, mainly for cancer screening purposes, include oral or luminal administrating of a tag molecule into a patients gastrointestinal lumen either orally or through an enema, and tagging cancerous tissue present within the lumen with the tag molecule. The tag molecule may be a near infrared dye, e.g., IR783 or a derivative thereof, which showed to be of high concentration within cancerous tissue compared to normal tissue. The near infrared IR783 dye is administered, either orally or through an enema, as part of a pharmaceutical composition or formulation either as is or conjugated to a specific polymer. The formulation may comprise an enteric coating in order to keep the formulation intact prior to it reaching the target organ, which is to be tagged by the formulation.
