Fluorophore-drug conjugates as imaging based targeted delivery and therapy systems in combating cancer

Cancer is one of the dreadful diseases having the highest mortality rate throughout the year worldwide. The treatments with conventional chemotherapeutics face the problems of drug resistance, insolubility, non-selectivity, toxicity and biological barriers that weaken the patient to cure effectively. To overcome these barriers, fluorophore-drug conjugates as imaging based targeted delivery and therapy systems have emerged as attractive platform against cancer to maximize their therapeutic efficacies to the specific site/s of interest, reducing the potential side effects. Anticancer drugs and other ligands anchored to fluorescein based chemosensors by various linkers have been investigated to monitor the real time fluorescent imaging in drug delivery followed by stimuli responsive therapies. The drug liberation triggered by bio and chemo hydrolytic environments is visualized based on ‘switch on’ fluorescence of the fluorophore-moiety of fluorescein-drug conjugates for non-invasive cancer imaging. The conjugation of free phenolic hydroxyl or carboxylic groups of fluorescein chemosensor to the hydroxyl group of drugs or specific carriers are employed by biodegradable amino and oxy acrylate linkers for targeted drug delivery applications. This review mainly demonstrates the biomedical applications of small molecule-based non-cleavable and cleavable fluorophore-drug conjugates as delivery systems through fluorescent monitoring of drug liberation, accompanied with chemo, photothermal and photodynamic therapies to avail higher targeted therapeutic efficiencies against cancer.