Molecular umbrellas are a novel class of amphiphiles that can create hydrophobic or hydrophilic exteriors when exposed to hydrophobic or hydrophilic microenvironments, respectively. They are unique in that they can promote the passive transport of lipophilic as well as hydrophilic molecules across lipid bilayers. In addition, molecular umbrellas do not follow the classic size/lipophilicity rule, where small and lipophilic agents are expected to cross lipid bilayers faster than ones that are relatively large and hydrophilic. Based on these unique properties, molecular umbrellas provide an opportunity for promoting the passive transport of biologically-active agents across the BBB in ways that have not previously been possible. In essence, this research will test this hypothesis, rigorously, using a variety of conjugates made from molecular umbrellas and amphotericin B (AmB) in (i) model membrane studies, (ii) cytotoxicity, hemolytic and antifungal measurements, and (iii) in vivo BBB-transport experiments. Several of the molecular umbrella-amphotericin B conjugates that will be synthesized are also expected to exhibit a wide therapeutic index relative to unconjugated AmB. For this reason, they may represent attractive alternatives for systemic use as well. In a broader context, success in promoting the transport of AmB across the BBB would provide a strong incentive for exploring other molecular umbrella-drug conjugates for treating other diseases. At a more fundamental level, this program challenges the current dogma that drug transport is limited by molecular size and lipophilicity. The immediate objectives of this research are four-fold: (1) to synthesize a broad series of molecular umbrella-AmB conjugates that vary in the number of polysulfated or polyhydroxylated walls, the size and flexibility of the umbrella framework, the length of the poly(ethylene glycol) spacer used, and the presence of a cleavable disulfide bond, (2) to determine the ability of the molecular umbrella-AmB conjugates to recognize ergosterol-containing liposomes and fungal cells, and to assess their cytotoxicity, (3) to characterize the membrane-binding and bilayer transport properties of each new molecular umbrella-AmB conjugate, and (4) to determine the efficacy of selected molecular umbrella-AmB conjugates in crossing the BBB.
Promoting the transport of hydrophilic drugs across cell membranes, and the blood-brain barrier in particular, represent major challenges for modern medicinal chemistry. Molecular umbrellas (molecules that can create an oil-loving or a water-loving exterior on demand) offer a fundamentally new approach for this long-standing problem. This program will test the feasibility of creating molecular umbrella-amphotericin B conjugates, which can be used as therapeutic agents for the treatment of fungal infections in the brain.
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