This is a model of amphotericin, the most relied-upon drug for treating fungal infections, despite its toxicity. Image: Martin Burke |
With
one simple experiment, University of Illinois chemists have debunked a
widely held misconception about an often-prescribed drug.
Led
by chemistry professor and Howard Hughes Medical Institute early career
scientist Martin Burke, the researchers demonstrated that the top drug
for treating systemic fungal infections works by simply binding to a
lipid molecule essential to yeast’s physiology, a finding that could
change the direction of drug development endeavors and could lead to
better treatment not only for microbial infections but also for diseases
caused by ion channel deficiencies.
“Dr.
Burke’s elegant approach to synthesizing amphotericin B, which has been
used extensively as an antifungal for more than 50 years, has now
allowed him to expose its elusive mode of action,” said Miles Fabian,
who oversees medicinal chemistry research grants at the National
Institute of General Medical Sciences. The institute is part of the
National Institutes of Health, which supported the work. “This work
opens up avenues for improving upon current antifungals and developing
novel approaches for the discovery of new agents.”
Systemic
fungal infections are a problem worldwide and affect patients whose
immune systems have been compromised, such as the elderly, patients
treated with chemotherapy or dialysis, and those with HIV or other
immune disorders. A drug called amphotericin (pronounced
AM-foe-TARE-uh-sin) has been medicine’s best defense against fungal
infections since its discovery in the 1950s. It effectively kills a
broad spectrum of pathogenic fungi and yeast, and has eluded the
resistance that has dogged other antibiotics despite its long history of
use.
The downside? Amphotericin is highly toxic.
“When
I was in my medical rotations, we called it ‘ampho-terrible,’ because
it’s an awful medicine for patients,” said Burke, who has an M.D. in
addition to a Ph.D. “But its capacity to form ion channels is
fascinating. So my group asked, could we make it a better drug by making
a derivative that’s less toxic but still powerful? And what could it
teach us about avoiding resistance in clinical medicine and possibly
even replacing missing ion channels with small molecules? All of this
depends upon understanding how it works, but up until now, it’s been
very enigmatic.”
While
amphotericin’s efficacy is clear, the reasons for its remarkable
infection-fighting ability remained uncertain. Doctors and researchers
do know that amphotericin creates ion channels that permeate the cell
membrane. Physicians have long assumed that this was the mechanism that
killed the infection, and possibly the patient’s cells as well. This
widely accepted dogma appears in many scientific publications and
textbooks.
However,
several studies have shown that channel formation alone may not be the
killing stroke. In fact, as Burke’s group discovered, the mechanism is
much simpler.
Amphotericin
binds to a lipid molecule called ergosterol, prevalent in fungus and
yeast cells, as the first step in forming the complexes that make ion
channels. But Burke’s group found that, to kill a cell, the drug doesn’t
need to create ion channels at all – it simply needs to bind up the
cell’s ergosterol.
Burke’s
group produced a derivative of amphotericin using a molecule synthesis
method Burke pioneered called iterative cross-coupling (ICC), a way of
building designer molecules using simple chemical “building blocks”
called MIDA boronates joined together by one simple reaction. They
created a derivative that could bind ergosterol but could not form ion
channels, and tested it against the original amphotericin.
If
the widely accepted model was true, and ion channel formation was the
drug’s primary antifungal action, then the derivative would not be able
to wipe out a yeast colony. But the ergosterol-binding,
non-channel-forming derivative was almost equally potent to natural
amphotericin against both of the yeast cell lines the researchers
tested, once of which is highly pathogenic in humans. The researchers
detailed their findings in the journal Proceedings of the National Academy of Sciences.
“The
results are all consistent with the same conclusion: In contrast to
half a century of prior study and the textbook-classic model,
amphotericin kills yeast by simply binding ergosterol,” Burke said.
“The
beauty is, because we now know this is the key mechanism, we can focus
squarely on that goal. Now we can start to think about drug discovery
programs targeting lipid binding.”
The
researchers currently are working to synthesize a derivative that will
bind to ergosterol in yeast cells, but will not bind to cholesterol in
human cells, to see if that could kill an infection without harming the
patient. They also hope to explore other derivatives that would target
lipids in fungi, bacteria and other microbes that are not present in
human cells. Attacking these lipids could be a therapeutic strategy that
may defy resistance.
In
addition to exploiting amphotericin’s lipid-binding properties for
antimicrobial drugs, Burke and his group hope to harness its
channel-creating ability to develop treatments for conditions caused by
ion-channel deficiencies; for example, cystic fibrosis. These new
findings suggest that the ion-channel mechanism could be decoupled from
the cell-killing mechanism, thus enabling development of derivatives
that could serve as “molecular prosthetics,” replacing missing proteins
in cell membranes with small-molecule surrogates.
“Now we have a road map to take ampho-terrible and turn it into ampho-terrific,” Burke said.
The paper, “Amphotericin Primarily Kills Yeast by Simply Binding Ergosterol,” is available from the News Bureau or PNAS.
