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"Liposomal Texaphyrin Theranostics for Metastatic Liver Cancer" J. Am. Chem. Soc., 2016, 138 (50), pp 16380–16387
  • Reported here is a new theranostic agent, 1, which consists of a Gd3+-texaphyrin core conjugated to a doxorubicin prodrug via a disulfide bond. Conjugate 1 was designed to undergo cleavage in the presence of glutathione (GSH), a species typically upregulated in cancer cells. As prepared, conjugate 1 displays no appreciable fluorescence. However, when exposed to excess GSH an increase in the fluorescence intensity at 592 nm is observed that is ascribed to release of free doxorubicin. To improve the solubility and enhance the tumor targeting of 1, it was loaded into folate-receptor-targeted liposomes to produce FL-1 (for folate liposome loaded with 1). As inferred from both fluorescence turn on studies and independent HPLC analyses, FL-1 was found to undergo selective uptake and cleavage to release free Dox in the KB and CT26 cell lines, which express folate receptors on the cell surface, relative to the HepG2 and NIH3T3 cell lines, which show low expression of those receptors. FL-1 was found to produce a greater antiproliferative effect in the case of the KB and CT26 cell lines as compared to that in the HepG2 and NIH3T3 cell lines. FL-1 was also found to provide enhanced magnetic resonance imaging in vivo under conditions of T1 contrast in the early stage of metastatic cancer progression. Finally, time-dependent tumor regrowth studies involving both subcutaneous and metastatic liver cancer mouse models revealed that FL-1 is capable of reducing the tumor burden in vivo
"Disulfide-Based Multifunctional Conjugates for Targeted Theranostic Drug Delivery" Acc. Chem. Res., 2015, 48 (11), pp 2935–2946
  • Theranostics, chemical entities designed to combine therapeutic effects and imaging capability within one molecular system, have received considerable attention in recent years. Much of this interest reflects the promise inherent in personalized medicine, including disease-targeted treatments for cancer patients. One important approach to realizing this latter promise involves the development of so-called theranostic conjugates, multicomponent constructs that selectively target cancer cells and deliver cytotoxic agents while producing a readily detectable signal that can be monitored both in vitro and in vivo. This requires the synthesis of relatively complex systems comprising imaging reporters, masked chemotherapeutic drugs, cleavable linkers, and cancer targeting ligands. Ideally, the cleavage process should take place within or near cancer cells and be activated by cellular components that are associated with cancer states or specifically expressed at a higher level in cancer cells. Among the cleavable linkers currently being explored for the construction of such localizing conjugates, disulfide bonds are particularly attractive. This is because disulfide bonds are stable in most blood pools but are efficiently cleaved by cellular thiols, including glutathione (GSH) and thioredoxin (Trx), which are generally found at elevated levels in tumors. When disulfide bonds are linked to fluorophores, changes in emission intensity or shifts in the emission maxima are typically seen upon cleavage as the result of perturbations to internal charge transfer (ICT) processes. In well-designed systems, this allows for facile imaging. In this Account, we summarize our recent studies involving disulfide-based fluorescent drug delivery conjugates, including preliminary tests of their biological utility in vitro and in vivo. To date, a variety of chemotherapeutic agents, such as doxorubicin, gemcitabine, and camptothecin, have been used to create disulfide-based conjugates, as have a number of fluorophores, including naphthalimide, coumarin, BODIPY, rhodol, and Cy7. The resulting theranostic core (drug-disulfide-fluorophore) can be further linked to any of several site-localizing entities, including galactose, folate, biotin, and the RGD (Arg-Gly-Asp) peptide sequence, to create systems with an intrinsic selectivity for cancer cells over normal cells. Site-specific cleavage by endogenous thiols serves to release the cytotoxic drug and produce an easy-to-monitor change in the fluorescence signature of the cell. On the basis of the results summarized in this Account, we propose that disulfide-based cancer-targeting theranostics may have a role to play in advancing drug discovery efforts, as well as improving our understanding of cellular uptake and drug release mechanisms.
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