Atomic structure of antifungal drug confirms unusual mechanism, opens
door to less-toxic derivatives

Date:
December 9, 2021
Source:
University of Illinois at Urbana-Champaign, News Bureau
Summary:
Advanced molecular imaging technology has now mapped the
structure of a drug widely used to treat fungal infections but
whose workings have mystified researchers and physicians for
nearly 70 years. Researchers now described in atomistic detail
the structure of the drug amphotericin B, a powerful but toxic
antifungal agent. Seeing the structure provides illumination in
the researchers' quest to formulate less-toxic AmB derivatives.



FULL STORY ========================================================================== Advanced molecular imaging technology has now mapped the structure of
a drug widely used to treat fungal infections but whose workings have
mystified researchers and physicians for nearly 70 years.


==========================================================================
In a new study, researchers at the University of Illinois
Urbana-Champaign, the University of Wisconsin, Madison and the National Institutes of Health described in atomistic detail the structure of the
drug amphotericin B, a powerful but toxic antifungal agent.

Seeing the structure provides illumination in the researchers' quest
to formulate less-toxic AmB derivatives, said Dr. Martin D. Burke, a
professor of chemistry at Illinois and a member of the Carle Illinois
College of Medicine, as well as a medical doctor. Burke co-led the
study with Chad Rienstra, a Wisconsin professor of biochemistry,
and Taras Pogorelov, an Illinois research professor of chemistry. The researchers reported their findings in the journal Nature Structural & Molecular Biology.

"It's like we were driving in the dark at night, and all of a sudden we
were able to put the lights on. With the clarity of this structure, we
can see where we need to go to reach our goal of a less-toxic antifungal
drug," Burke said.

Previously, researchers and physicians thought that AmB killed fungal
cells by forming channels in the cell membrane, the outer envelope that
encases the cell. However, in 2014, while Rienstra was a professor at
Illinois, Burke and Rienstra's group found that amphotericin primarily
kills cells by robbing the membrane of sterol molecules -- cholesterol
in human cells and ergosterol in fungal cells. Individual amphotericin molecules aggregated into a larger structure that absorbed sterol
molecules out of cell membranes like a sponge, causing the cells to die.

"The ion channel is a secondary action to the antifungal activity. That
let us disconnect the ion channel-forming function from the fungicidal
activity of amphotericin," Burke said. His group has applied the channel-forming abilities of AmB as a "molecular prosthetics" approach
to treat cystic fibrosis, yet greater understanding of the fungicidal
sterol sponge remained elusive.



==========================================================================
"We had some images but no details," said Agnieszka Lewandowska, a senior research scientist at Illinois and first author of the new study. "Now we
can really see the part of the structure that we think is responsible for interacting with cholesterol, which we don't want. So then we could modify
that and make sure it only interacts with ergosterol, which we do want."
Since AmB forms a large aggregate, the usual molecular imaging techniques
such as nuclear magnetic resonance are difficult to apply. In the new
study, the researchers developed novel sample preparation protocols and
used an advanced molecular imaging technique called magic-angle spinning solid-state NMR. They also used advanced computational modeling methods
to visualize the structures represented by the NMR data.

The result was a picture in atomistic detail of how small AmB
molecules fit together in a head-to-tail configuration, staggered
into a large lattice, leaving a void shaped and sized just right for
sterol molecules. There was also some flexibility within the aggregate, potentially allowing it to flex a bit to accommodate cholesterol, which
is slightly larger than ergosterol.

"We wanted to know how the AmB sponge fits together to accommodate
ergosterol," Rienstra said. "Just like sponges that absorb water, if it's
dried out and crusty, it doesn't move well and won't do a very good job
of absorbing sterols.

Once it's a little soft, it does a better job of absorbing because then
it's flexible." The detailed structure validates earlier work and also provides a road map for synthesizing derivatives, the researchers say.

"We are already in the process of investigating the structures of the
AmB complexes with both cholesterol and ergosterol. It opens the door
to finally build or find nontoxic derivatives of this important drug
and help a lot of people without the horrible side effects that AmB has
right now," Lewandowska said.

Next, the researchers plan to continue collaborating to synthesize
derivatives and then study their atomistic structures to see how they
aggregate and interact with both cholesterol and ergosterol, as well as
to explore the potential of other small molecules.

"Amphotericin works differently than any other drug we know about. It
doesn't bind to a protein; it self-assembles into this interesting
aggregate," Burke said. "We saw this whole new area of small molecule interactions. This imaging technique is giving us new tools to
understand small molecule interactions and how they can perform higher
order, proteinlike functions. We're finally in a position where we can rationally tap into AmB's huge functional potential, both for antifungal treatment and for molecular prosthetics." The National Institutes of
Health supported this work.

========================================================================== Story Source: Materials provided by University_of_Illinois_at_Urbana-Champaign,_News_Bureau.

Original written by Liz Ahlberg Touchstone. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Agnieszka Lewandowska, Corinne P. Soutar, Alexander I. Greenwood,
Evgeny
Nimerovsky, Ashley M. De Lio, Jordan T. Holler, Grant S. Hisao,
Anuj Khandelwal, Jiabao Zhang, Anna M. SantaMaria, Charles
D. Schwieters, Taras V. Pogorelov, Martin D. Burke, Chad
M. Rienstra. Fungicidal amphotericin B sponges are assemblies
of staggered asymmetric homodimers encasing large void
volumes. Nature Structural & Molecular Biology, 2021; 28 (12):
972 DOI: 10.1038/s41594-021-00685-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211209124210.htm

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