Fighting Fungi by Capturing Sugars

For many Yale students, the word “fungus” might call to mind the dining hall’s hybrid mushroom and beef burgers. But fungus also has another unsavory meaning—fungal infections are a major public health concern and can be deadly, particularly to patients with weak immune systems such as those with organ transplants, HIV, or cancer.

Fungal infections are responsible for roughly half of AIDS-related deaths globally Candidemia, a fungal infection common in organ transplant patients, has a 30-40% mortality rate.While antifungal treatments exist, they can have side effects, similar to how chemotherapy can damage a cancer patient’s body while destroying cancer cells. Furthermore, fungi are capable of developing resistance to conventional therapeutics.

Dr. Egor Chirkin and Dr. Viswanathan Muthusamy, researchers in the Spiegel lab of the Yale Chemistry department, collaborated with Merck to generate a novel antifungal compound that would minimize such side effects and prevent the development of resistance. Their recent paper describes how Chirkin and Muthusamy designed and tested a compound that can activate the body’s own immune system, destroying fungal cells without harming the patient’s own cells.

“What the field is moving toward is clean killing of pathogenic cells without a lot of side effects,” said Muthusamy, who headed the biology aspect of the study. “What better to do it with than your own immune cells, which are meant to fight these diseases?”

The scientists were able to harness the power of the immune system by creating a small molecule with two different ends, so that one end of the molecule binds to the fungus, and the other to antibodies already present in human blood.

“On one side, we need something which can always interact with antibodies, the antibody-recruiting terminus. On the other side, we need something which can interact with the pathological cells, some specific target which is expressed only on the fungal cell wall,” said Chirkin, who used his chemistry background to design and synthesize potential molecules.

The researchers began by creating modified versions of a molecule called calcofluor, which selectively binds to chitin, a sugar found in fungal cell walls but not human cells. Because there was not a good test to measure the efficacy of the molecules Chirkin created, Muthusamy devised a novel test. The common human pathogen C. albicans was treated with different antibody-recruiting molecules targeting fungi (ARM-Fs). When the ARM-F was able to recruit antibodies to the fungal cell, the complex could be recognized and “eaten up” by human immune cells. Looking for fluorescently labeled fungal cells, the scientists were able to quantify how efficiently the fungal cells were eaten up by the immune cells.

“It is very unique in biology to target fungal cells using immune effectors; it is not usual in the literature, so we had to develop our own assays to test the efficacy of these compounds,” said Muthusamy. This new methodology can be used by scientists in the future to continue study in the field.

In addition to avoiding side effects, an advantage to the ARM-F approach is that fungi are unlikely to develop drug resistance. Unlike common antibiotics, which quickly become obsolete, an ARM-F drug could likely be used without drug resistant strains developing. Chitin, the target molecule on fungal cell walls, is a sugar polymer. While proteins evolve quickly to evade our immune defense system due to errors in DNA replication, sugars are not coded for by DNA and therefore do not possess the same ability to quickly mutate.

“Chitin is a key element of the fungal cell wall, and it is a polysaccharide. This is a chemical substance, so the fungus really cannot change it. The fungus can make less of it, but that also would reduce its chance of survival,” said Muthusamy.

Moreover, small molecules tend to be more stable than other drug compounds and can be ingested, unlike biological molecules. This means that a drug from an ARM-F could likely be taken orally as a pill—much easier than an injection or other delivery method.

While this study holds promise, the molecule must undergo further testing before it can be used on patients. So far, the ARM-F has only been tested in cells. The molecule must next undergo rigorous testing in mouse models before entering clinical trials. If all goes well, many patients stand to benefit from this work.