Small amounts of carbon monoxide may help protect vision in diabetes

The Medical College of Georgia scientists have early evidence that HBI-002, a low-dose oral compound developed by Hillhurst Biopharmaceuticals and already in early stage trials for sickle cell disease, can safely reduce oxidative stress and inflammation in the retina, both early, major contributors to diabetic retinopathy.

"Inflammation and oxidative stress go hand in hand," says Dr. Pamela Martin, cell biologist and biochemist in the MCG Department of Biochemistry and Molecular Biology and Vision Discovery Institute at Augusta University. "If you impact one, you generally impact the other."

At the right dose, carbon monoxide can impact both.

While we likely think of chirping detectors in our homes, toxic fumes from cars and trucks and even death when we think of carbon monoxide, many of our own cells and tissues are actively and regularly producing small amounts of the colorless, odorless gas to protect themselves from the damage of high and/or chronic inflammation and oxidative stress, says Dr. Ravirajsinh Jadeja, MCG biochemist.

He's talking about the enzyme heme oxygenase 1, a common component of many tissues including oxygen-carrying hemoglobin and immune cells, whose jobs include reducing oxidative stress and inflammation, and one of the ways it does that is by releasing small amounts of carbon monoxide. In fact heme oxygenase 1 is naturally upregulated in our cells in response to increased levels of the destructive states.

"Basically what the enzyme does is produce a very low amount of carbon monoxide, 1,000 times lower than what we inhale outside," Jadeja says. Unlike in our homes when fuel-burning furnaces can cause high levels to accumulate, the small, steady production and the cell machinery that use it also mean the carbon monoxide we make doesn't accumulate in our body.

"When you introduce diseases like diabetes, then these natural mechanisms fail, and we have to think of ways to enhance, or reestablish, those mechanisms that would normally protect us," Martin says.

The scientists have early evidence that HBI-002, a liquid that converts to the familiar gas when it hits our intestines, can help our retinas, which are damaged in nearly half of people with diabetes, do that.

Martin and Jadeja are co-principal investigators on a $300,000 grant (1R41EY033264-01), for the first phase of an R41/R42, Small Business Technology Transfer Grant from the National Institutes of Health that is enabling them to further explore HBI-002's potential.

For the new studies, they are looking at the impact of the compound in both an acute ischemic model, when the retina suddenly is not getting sufficient oxygen because of oxidative stress followed by its normal companion inflammation, as well as a model of more natural disease progression.

The retina, which is considered an extension of the brain, is comprised of intricate layers of nerves that line the back part of the eye and sense light, which the brain transforms into images. Cell types in the retina are among the highest metabolically active cell types, which means they naturally generate a lot of oxidative stress, Martin says.

One of her many long-term interests is identifying noninvasive methods to get help to the retina, like oral antihypertensives help reduce blood pressure.

Their preliminary studies indicate HBI-002 can make the trip. The liquid turns into carbon monoxide's more familiar gaseous state in the intestines, where it naturally binds to hemoglobin— the oxygen-carrying component of blood — then travels up to the eye in the blood, Jadeja says. Carbon monoxide is good at finding hemoglobin, the scientists note. In fact, it's strong affinity for hemoglobin is what makes it deadly since oxygen can no longer bind to hemoglobin when carbon monoxide has made the connection, Jadeja says.

"It's bad, it's deadly. When you first hear carbon monoxide, that is everyone's reaction," says Martin. "That is true in a broad, uncontrolled setting. But this is in a very controlled setting where you are getting minimal concentrations that target specific cellular processes."

Once in the retina, the small dose of carbon monoxide finds the heme oxygenase enzyme in retinal cells and helps induce the steps that normally yield its desired antioxidant, anti-inflammatory action.

To objectively measure its impact, the scientists have functional tests that — much like an electrocardiogram of the heart, but in this case it's an electroretinogram — produce peaks and waves, which indicate how different cells are firing and signaling. They can also look directly at the retina for telltale signs of damage: Diabetes prompts cell death and a thinning of the retina, and can cause retinal detachment. RNA and protein analysis will give more evidence of whether they are getting increased expression of good genes and decreased expression of the problematic ones in response to treatment.

Meanwhile, levels of the complex carboxyhemoglobin, which forms inside red blood cells when hemoglobin is exposed to carbon monoxide and whose levels can be easily monitored with a blood test, are a good indicator of whether carbon monoxide levels remain safe.

The unique liquid to gas transformation of HBI-002 enables more targeted dosing since the amount needed can vary between individuals and, if one day approved for humans, will help ensure dosing accuracy and the ability of patients eventually to easily and safely use the compound at home, the scientists and its developers say.

It also effectively leaves nothing behind, while a pill form previously studied left behind metal complexes that had to be cleared by the kidneys, which also are often compromised in people with diabetes, Jadeja says.

If the evidence of HB1-002's benefit continues to hold in their studies over the next 12 months, it will lead to another grant and more studies honing in on how this carbon monoxide compound is working against the ravages of diabetic retinopathy, exploring optimal doses, how often it should be given and more.

While they and other scientists studying the compound have reported no side effects, the sickle cell trial just getting underway should provide more insight. Martin notes they expect to see minimal impact, if any.

Earlier this year, the U.S. Food and Drug Administration gave Hillhurst approval to move forward with a first-phase trial of their HBI-002 for sickle cell disease. Phase 1 trials are intended to test safety and are done in healthy individuals. The company has preclinical evidence that the compound will prevent the classic pain crises that result when adequate blood flow is disrupted by sickled red blood cells in the arms, legs, bones, joints and other tissues.

While the causes of sickle cell and diabetes may be disparate, they have in common players like oxidative stress and inflammation, Martin notes.

"While we are generating this data, they will be continuing with this trial," Jadeja says. "We are working on something that is very close to clinical usage."

The back of the eye has been a tough target for drugs to reach. For example, using eye drops as a mechanism of delivery typically is not effective because often the drug doesn't reach the retina, and if you give sufficient doses to help it make the journey, it can damage the front of the eye, Martin says. So diabetic retinopathy therapies tend to be invasive, like intravitreal injections of drugs that help reduce the destructive, abnormal blood vessel growth that happens in response to blockage of existing blood vessels but which ultimately instead contributes to vision loss.

Diabetic retinopathy is the most frequent cause of new cases of blindness among those age 20-74, and is strongly related to how long the individual has had diabetes, according to the American Diabetes Association.

Carbon monoxide also has been shown to have important, natural roles in helping blood vessels dilate and repairing cell powerhouses called mitochondria, the scientists say.

Hillhurst Biopharmaceuticals is a clinical stage company that focuses on blood and inflammatory disorders. Preclinical studies of their HBI-002, like the ones Martin and Jadeja are doing in diabetic retinopathy, also are underway in conditions like Parkinson's and acute pain