Glutaminase is an enzyme that converts the amino acid glutamine into glutamate and ammonia. Glutaminase inhibitors are compounds designed to block this enzyme’s activity. These inhibitors are being studied for their potential to influence cellular processes and their implications in various biological contexts.
The Role of Glutamine and Glutaminase
Glutamine is the most abundant free amino acid in the human body. It supports cellular functions like energy production, cell growth, and immune responses. Cells with high proliferative rates, such as those in the immune system and certain disease states, particularly rely on glutamine as a fuel source.
Glutaminase converts glutamine into glutamate. This enzymatic reaction releases glutamate and ammonia. Glutamate can then be processed to enter the tricarboxylic acid (TCA) cycle as alpha-ketoglutarate, a central pathway for generating cellular energy in the form of ATP, NADH, and FADH2.
Glutamine also contributes to the synthesis of other biomolecules, including nucleotides for DNA and RNA, and other amino acids. It plays a role in maintaining redox balance within cells by contributing to the production of glutathione, a molecule that helps protect cells from oxidative stress.
There are two main types of glutaminase enzymes in humans: kidney-type glutaminase (GLS1) and liver-type glutaminase (GLS2). GLS1 is predominantly expressed in the kidneys and regulates acidic amino acid levels. GLS2 is mainly found in the liver and contributes to the urea cycle. Both isoforms are primarily located in the mitochondria.
Mechanism of Glutaminase Inhibitors
Glutaminase inhibitors disrupt the enzyme’s activity, reducing the availability of glutamine-derived metabolites that cells need for energy production and biosynthesis. These inhibitors bind to the glutaminase enzyme, blocking its catalytic activity.
One common strategy involves competitive inhibition, where the inhibitor molecule structurally resembles glutamine and binds directly to the enzyme’s active site. By occupying this site, competitive inhibitors prevent glutamine from binding and undergoing conversion. Examples of competitive inhibitors include 6-diazo-5-oxo-L-norleucine (DON) and its prodrugs, such as JHU-083 and DRP-104.
Another approach involves allosteric inhibition, where the inhibitor binds to a site on the enzyme distinct from the active site. This binding induces a change in the enzyme’s shape, which then alters the active site and prevents or reduces glutamine binding and conversion. Allosteric inhibitors, such as those in the BPTES class (e.g., CB-839), can trap the glutaminase enzyme in an inactive tetrameric state, preventing it from functioning.
The inhibition of glutaminase activity results in a decrease in glutamate production and a subsequent buildup of glutamine within the cell. This metabolic shift can have several downstream consequences, including reduced cell growth and proliferation, and in some cases, the induction of programmed cell death.
Therapeutic Applications
Glutaminase inhibitors are primarily being investigated for their therapeutic potential in cancer treatment. Many cancer cells exhibit a phenomenon known as metabolic reprogramming, altering their metabolic pathways to support rapid growth and survival. A common feature of this reprogramming is an increased reliance on glutamine metabolism, often referred to as “glutamine addiction.” By inhibiting glutaminase, researchers aim to starve cancer cells of the necessary metabolites for proliferation and survival.
Preclinical studies have demonstrated that these inhibitors can reduce tumor growth in various cancer models, including breast, lung, and hematological malignancies such as multiple myeloma. Their use in cancer therapy targets metabolic vulnerabilities, leading to effects like reduced glutathione levels, increased oxidative stress, and ultimately, apoptotic cell death in cancer cells.
Beyond directly impacting cancer cell growth, glutaminase inhibitors are also being explored for their ability to enhance other cancer treatments. They can make cancer cells more susceptible to therapies like chemotherapy and radiation. These inhibitors may also enhance the anti-tumor activity of immune cells, such as T-cells and natural killer cells, making them candidates for combination therapies with immune checkpoint inhibitors or adoptive cell therapies.
While the main focus is on cancer, glutaminase inhibitors are also being explored in other areas. Research suggests potential applications in neurodegenerative diseases like Alzheimer’s and Parkinson’s, where glutamine metabolism has been implicated. Their ability to modulate cellular metabolism suggests broader applications, though these areas are still under active investigation.
Ongoing Research and Future Directions
Glutaminase inhibitor research is ongoing, with efforts to develop new compounds and explore their therapeutic potential. Several glutaminase inhibitors are currently in various phases of clinical trials, particularly for cancer treatment. For instance, CB-839 (Telaglenastat) has been a lead compound in clinical trials for various cancer indications. IACS-6274 has shown promising early results in a Phase I trial for advanced cancers, demonstrating successful target inhibition and some anti-tumor activity. DRP-104 is also undergoing a Phase 2 clinical trial for solid tumors, showing lower toxicity compared to older compounds like DON.
Researchers are also focusing on understanding and overcoming potential resistance mechanisms that cancer cells might develop against glutaminase inhibitors. Resistance can arise from compensatory metabolic pathways that allow tumor cells to bypass their dependence on glutamine. This has led to the exploration of combination therapies, where glutaminase inhibitors are used alongside other anticancer agents or inhibitors targeting these alternative pathways, such as glycolysis or fatty acid oxidation.
New compounds are continuously being developed, with efforts directed at improving potency, selectivity, and pharmacokinetic properties. For example, prodrugs like JHU-083 and JHU-395 are designed to circulate inertly in the bloodstream but release the active inhibitor within target tissues, aiming to reduce systemic toxicity. The development of pan-glutaminase inhibitors, which can inhibit both GLS1 and GLS2 isoforms, is also an area of active investigation, as GLS2 can sometimes compensate for GLS1 inhibition.