Researchers at Purdue University have developed a small-molecule compound that could open a new route toward cancer immunotherapy by targeting a protein that normally restrains immune activity. The compound, known as L-32, inhibits protein tyrosine phosphatase non-receptor type 22, or PTPN22, an immune-regulating enzyme that has attracted growing interest because of its connections to both autoimmune disease and antitumor immunity. In studies reported in the Journal of Medicinal Chemistry, L-32 reduced tumor growth in living models and displayed improvements in potency, selectivity, cellular activity and drug-like properties compared with earlier compounds aimed at the same target. The work was led by Zhong-Yin Zhang, a Distinguished Professor of medicinal chemistry at Purdue University, whose team describes the inhibitor as a promising lead rather than a finished medicine. Its significance lies in the possibility of stimulating the immune system from within tumor-bearing tissues while avoiding the limitations that have slowed the development of earlier PTPN22 inhibitors.
PTPN22 belongs to a broad family of protein tyrosine phosphatases, enzymes that remove phosphate groups from proteins. These phosphate groups act as molecular switches, controlling the activity, location and interactions of signaling proteins. In immune cells, reversible phosphorylation is essential for transmitting signals from receptors that recognize antigens, pathogens or tissue damage. PTPN22 functions as a negative regulator in several of these pathways, helping prevent immune activation from becoming excessive. That protective role is important for maintaining tolerance to the body’s own tissues, but it can also dampen immune responses that might otherwise recognize and attack cancer cells. Genetic studies have linked PTPN22 variants to susceptibility to multiple autoimmune disorders, while experimental deletion of the gene has been associated with stronger antitumor immunity. These observations led Zhang’s group to investigate whether carefully designed inhibitors could temporarily reduce PTPN22 activity and release a controlled immune response against tumors.
The strategy is technically challenging because protein tyrosine phosphatases have highly conserved catalytic regions that bind phosphate-containing substrates. Compounds designed only for the active site can therefore struggle to distinguish one phosphatase from another, increasing the risk of unwanted effects. Many phosphatase inhibitors also have difficulty entering cells, remaining stable in biological environments or reaching sufficient concentrations in tissues. Zhang’s team pursued a different design principle: instead of relying solely on the catalytic pocket, the researchers sought molecules that could engage both the active site and neighboring structural pockets unique to the target enzyme. This approach is intended to improve binding strength and selectivity by exploiting a larger molecular surface. It also reflects a broader shift in drug discovery, in which researchers use adjacent or “allosteric” regions around an enzyme’s catalytic center to create inhibitors with more favorable biological behavior.
The Purdue researchers began with quinolone-3-carboxylic acid, a chemical scaffold identified as a PTPN22 inhibitor from an in-house collection of drug-like small molecules. A chemical scaffold is the core framework of a compound, providing the architecture onto which additional functional groups can be attached. The team used a fragment-based focused library approach, linking carefully selected molecular fragments to different positions on the quinolone core. Such fragments are relatively small chemical units chosen for their ability to establish specific interactions with a protein, including hydrogen bonds, electrostatic attractions and hydrophobic contacts. By systematically modifying the scaffold, the scientists searched for structures that would occupy the catalytic site while extending into nearby pockets. This process produced L-32, a quinolone-based inhibitor with stronger activity and improved selectivity compared with the group’s previous derivatives.
Laboratory testing indicated that L-32 could inhibit PTPN22 and retain activity in cellular systems, an important distinction in medicinal chemistry. A compound may bind effectively to a purified enzyme but fail inside cells because it cannot cross the cell membrane, is rapidly degraded, binds nonspecifically to other proteins or is pumped out by cellular transporters. Cellular efficacy suggests that at least part of the compound’s biochemical activity survives the complex environment of a living cell. The researchers also evaluated characteristics related to pharmacokinetics, the study of how a compound is absorbed, distributed, metabolized and eliminated by the body. According to Zhang, L-32 demonstrated a more favorable pharmacokinetic profile than earlier molecules, including oral bioavailability. An orally bioavailable drug can be absorbed through the gastrointestinal tract, a property that may simplify treatment compared with medicines requiring injection, although further optimization and safety testing would be needed before any clinical use could be considered.
The compound was then examined in syngeneic MC38 tumor models, systems in which cancer cells and immune cells come from genetically compatible animals. These models are particularly useful for immunotherapy research because they preserve interactions between the tumor and an intact immune system, unlike some models that rely on severely immune-deficient animals. In the Purdue study, L-32 was reported to reduce MC38 tumor growth in vivo more effectively than earlier compounds. The researchers attributed this effect to the promotion of antitumor immunity, involving the coordinated activity of innate and adaptive immune defenses. Innate immune cells provide rapid, broad responses to abnormal tissue, while adaptive immune cells, including T lymphocytes, can recognize specific tumor-associated antigens and develop more durable responses. By inhibiting an immune checkpoint within signaling pathways rather than directly poisoning cancer cells, L-32 represents an immunomodulatory approach: its aim is to improve the body’s capacity to attack tumors rather than act solely as a conventional cytotoxic agent.
The findings also illustrate why PTPN22 has remained an intriguing but underdeveloped therapeutic target. The biological rationale for inhibiting the enzyme has been strengthened by genetic and immunological evidence, yet the field has lacked high-quality chemical tools capable of probing the target reliably. Selective inhibitors are essential not only as potential drugs but also as research instruments. They allow scientists to determine which effects result specifically from blocking PTPN22 and which arise from unintended interactions with related phosphatases. L-32’s reported combination of biochemical potency, selectivity and cellular activity could therefore help clarify how PTPN22 influences immune-cell signaling within tumors. At the same time, the relationship between immune stimulation and autoimmunity will require close attention. Because PTPN22 helps regulate immune tolerance, prolonged or excessive inhibition could theoretically increase inflammatory reactions or autoimmune complications. The compound’s therapeutic window, or range between effective and toxic doses, will be a central question in future studies.
Zhang’s team plans to refine L-32 and conduct additional tests of its efficacy in vivo. The researchers are particularly interested in cancers that are difficult to treat with existing approaches, including pancreatic and liver cancers, as well as tumors that have become resistant to current immunotherapies. Resistance can develop when tumors exclude immune cells, suppress antigen presentation, alter inflammatory signaling or create a microenvironment that disables T cells and other immune effectors. A PTPN22 inhibitor could potentially be evaluated as a standalone treatment or in combination with established immunotherapies, although the appropriate combinations and dosing schedules remain unknown. Before such possibilities can be assessed in humans, researchers must establish detailed toxicology profiles, confirm reproducible pharmacokinetics, study how the compound behaves across different tumor types and determine whether its immune effects are sufficiently selective. Animal results, even when encouraging, do not guarantee clinical benefit.
The research has been published in the Journal of Medicinal Chemistry under the title “A Potent and Selective Quinolone-Based PTPN22 Inhibitor with Improved Immunotherapeutic Activity.” Zhang and his collaborators have disclosed the quinolone-based PTPN22 inhibitors, including L-32, to the Purdue Innovates Office of Technology Commercialization, which has applied for patent protection through the U.S. Patent and Trademark Office. The intellectual property is available for potential development or commercialization through Purdue’s licensing program. The study was supported in part by the National Institutes of Health and the Robert C. and Charlotte Anderson Chair Endowment. While L-32 remains an experimental lead compound, its development adds momentum to efforts to drug protein tyrosine phosphatases and suggests that targeting a previously underexplored immune regulator may eventually broaden the range of strategies available against cancer.
Subject of Research: Development of L-32, a quinolone-based small-molecule inhibitor of PTPN22 for cancer immunotherapy.
Article Title: A Potent and Selective Quinolone-Based PTPN22 Inhibitor with Improved Immunotherapeutic Activity
Web References: Purdue University College of Pharmacy; Purdue Institute for Cancer Research; Purdue Institute for Drug Discovery; Purdue Innovates Office of Technology Commercialization; Journal of Medicinal Chemistry article page: https://pubs.acs.org/jmcmar/article/69/14/16401/5172625/A-Potent-and-Selective-Quinolone-Based-PTPN22
References: Journal of Medicinal Chemistry, DOI: 10.1021/acs.jmedchem.5c03467
Image Credits: Purdue University photo/Ashley Jensen
Keywords: PTPN22, L-32, cancer immunotherapy, small-molecule inhibitors, quinolone-based inhibitors, protein tyrosine phosphatases, antitumor immunity, drug discovery, cancer research, pancreatic cancer, liver cancer, MC38 tumor model
Tags: autoimmune disease and cancer linkcancer immunotherapydrug development for cancerimmune system activation within tumorsnovel approaches to cancer treatmentprotein tyrosine phosphatase inhibitorsPTPN22 enzyme inhibitionPurdue University cancer researchsignal transduction in immune cellssmall-molecule anticancer compoundstargeted cancer therapytumor immune response stimulation
