Abstract: Immune checkpoint blockade has revolutionized cancer therapy, yet a significant proportion of patients remain refractory, necessitating the development of novel immunotherapeutic strategies. The protein tyrosine phosphatases PTPN2 (TC-PTP) and PTPN1 (PTP-1B) have emerged as critical negative regulators of inflammation and immune cell activation. Historically considered undruggable due to their highly polar active sites, these phosphatases are now the target of Osunprotafib (ABBV-CLS-484), a first-in-class, orally bioavailable small-molecule active-site inhibitor. This review synthesizes current preclinical evidence demonstrating that ABBV-CLS-484 unleashes potent anti-tumor immunity by acting dually on tumor cells and host immune cells. By enhancing JAK-STAT signaling, the compound sensitizes tumors to interferon-gamma (IFNγ), promotes the activation and effector functions of CD8+ T cells and natural killer (NK) cells, and favorably remodels the tumor microenvironment. With robust efficacy in multiple immunotherapy-resistant solid tumor models and a manageable, reversible safety profile, ABBV-CLS-484 represents a promising new frontier in solid tumor immunotherapy and is currently undergoing clinical evaluation.
1. Introduction
While immune checkpoint blockade (ICB) therapies targeting PD-1 and CTLA-4 have transformed the clinical management of various cancers, many tumors exhibit primary or acquired resistance, often characterized by a lack of T cell infiltration or impaired antigen presentation [2]. To overcome these limitations, researchers have focused on identifying intracellular checkpoints that restrict anti-tumor immunity. The protein tyrosine phosphatases PTPN2 (also known as TC-PTP) and its paralogue PTPN1 (PTP-1B) have been identified as central negative regulators of several cytokine signaling pathways, including the JAK-STAT pathway, and T cell receptor (TCR) signaling [1]. Genetic deletion of PTPN2 or PTPN1 in either tumor cells or immune cells has been shown to profoundly promote anti-tumor immunity [1][2].
Despite their validation as therapeutic targets, phosphatases have long been considered challenging to drug—particularly at their active sites—due to the requirement for highly polar inhibitors that typically exhibit poor cellular permeability and pharmacokinetic properties [1]. Osunprotafib (ABBV-CLS-484, or AC484) represents a major breakthrough as a first-in-class, orally bioavailable, potent active-site inhibitor of both PTPN2 and PTPN1. The discovery of ABBV-CLS-484 paves the way for a new class of small-molecule immunotherapies capable of achieving efficacy comparable to or exceeding that of antibody-based immune checkpoint inhibitors [1].
2. Pharmacological Activity
ABBV-CLS-484 exhibits profound pharmacological activity both in vitro and in vivo by amplifying immune responses and directly sensitizing cancer cells to immune-mediated destruction. In vitro, ABBV-CLS-484 dose-dependently enhances IFNγ-driven growth arrest in tumor cells, phenocopying the genetic deletion of PTPN2/N1 [1]. It increases the sensitivity of a broad range of human cancer cell lines to IFNγ, particularly those exhibiting features of intrinsic inflammation [1].
In vivo, systemic oral administration of ABBV-CLS-484 generates potent anti-tumor immunity across multiple syngeneic mouse models, including B16 melanoma, KPC pancreatic adenocarcinoma, and the PD-1-resistant 4T1 and EMT-6 breast carcinoma models [1]. Treatment induces highly significant tumor regression and increases survival, with efficacy comparable to or greater than anti-PD-1 monotherapy. Furthermore, combining ABBV-CLS-484 with anti-PD-1 in the CT26 colon cancer model demonstrated additive therapeutic effects [1]. Beyond primary tumor control, ABBV-CLS-484 effectively suppresses metastatic dissemination, as evidenced by the complete prevention of lung metastases in the B16 intravenous model and a significant reduction of spontaneous lung metastases in the orthotopic 4T1 model [1]. Related preclinical studies using a similar dual PTPN1/PTPN2 inhibitor (Compound 182) corroborate these findings, showing robust T cell-dependent repression of AT3-OVA mammary tumors and MC38 colon tumors [2].
3. Molecular Mechanism of Action
The mechanism of action of ABBV-CLS-484 is multifaceted, engaging a dual anti-cancer mechanism that acts directly on tumor cells while simultaneously hyperactivating host immune cells [1].
Effects on Tumor Cells: By inhibiting PTPN2 and PTPN1, ABBV-CLS-484 prevents the dephosphorylation of JAK and STAT family members, thereby amplifying type I and type II interferon signaling. In tumor cells, this leads to the upregulation of MHC class I molecules, enhancing antigen presentation and increasing the diversity of the presented peptide repertoire. It also drives the production of T cell chemoattractants such as CXCL9 and CXCL10, which recruit effector immune cells into the tumor microenvironment (TME) [1].
Effects on Immune Cells: ABBV-CLS-484 lowers the threshold for TCR signaling and enhances T cell activation, as marked by increased CD69 expression and elevated production of IFNγ and TNF [1]. In the TME, the inhibitor drives a distinct epigenetic and transcriptional reprogramming of CD8+ T cells. It enhances IL-2–STAT5 signaling, which promotes a robust effector and memory T cell phenotype while significantly reducing the expression of exhaustion-associated markers such as TIM-3 and TOX [1]. Furthermore, ABBV-CLS-484 enhances the cytotoxic function of Natural Killer (NK) cells, which are crucial for controlling tumors that have downregulated MHC class I or lost IFN sensing (e.g., JAK1-deficient tumors) [1].
Remodeling the Tumor Microenvironment: Single-cell transcriptomic profiling reveals that ABBV-CLS-484 inflames the TME by increasing the overall lymphoid-to-myeloid cell ratio. It promotes a shift in the myeloid compartment, significantly increasing the ratio of pro-inflammatory M1 macrophages to immunosuppressive M2 macrophages, and reducing the frequency of myeloid-derived suppressor cells (MDSCs) [1]. It also enhances the activation of dendritic cells, increasing their expression of MHC molecules and co-stimulatory markers like CD86 [1].
4. Structure-Activity Relationship (SAR)
The discovery of ABBV-CLS-484 overcame the historical challenge of drugging the highly polar and homologous active sites of PTPN2 and PTPN1. Structure-based drug design was utilized to optimize interactions within the active site while maintaining drug-like properties, such as a low molecular weight (<500 Daltons) and low clearance [1].
ABBV-CLS-484 features a highly acidic thiadiazolidinone dioxide moiety that makes up to nine critical interactions within the active site Cys216 region, including hydrogen bonds with residues Cys216, Arg222, Asp182, Ser217, and Ile220 [1]. To further enhance binding, an isopentyl-amine group was designed to engage in a hydrogen bond with Asp50 and a hydrophobic interaction with Met256 proximal to the tetralin ring [1].
Crucially, ABBV-CLS-484 is a zwitterionic compound, possessing a thiadiazolidinone dioxide NH with a pKa of 0.9 and an amine NH with a pKa of 10. This specific zwitterionic character is pivotal for its favorable physiochemical properties, granting it low plasma protein binding (86% unbound in mouse plasma and 50% in human plasma) and preventing hepatic clearance. Instead, the compound is cleared via renal and biliary mechanisms. These optimized ligand-target interactions and physiochemical properties result in low nanomolar biochemical potency (IC50 of 1.8 nM for PTPN2 and 2.5 nM for PTPN1) and excellent oral bioavailability [1].
5. Current Limitations
While ABBV-CLS-484 demonstrates a favorable therapeutic window, systemic inhibition of fundamental inflammatory regulators like PTPN2 and PTPN1 carries the risk of immune-related adverse events. In preclinical toxicity studies, high doses of ABBV-CLS-484 (e.g., 100 mg/kg twice daily in mice or 300 mg/kg/day in rats) led to overt systemic immune activation. This was characterized by significant increases in circulating cytotoxic Granzyme B+ CD8+ T cells and systemic chemokines (CXCL9, CXCL10), resulting in dose-dependent inflammatory immune cell infiltrates in the kidneys, joints, and liver [1].
However, a key advantage of the small-molecule approach over antibody-based therapeutics is its relatively short half-life. The observed inflammatory infiltrates were fully reversible and resolved within 28 days following the cessation of treatment, highlighting that toxicity can be managed through dose modulation and treatment holidays [1]. Additionally, the inherent difficulty in targeting highly polar phosphatase active sites means that any future structural modifications must carefully balance enzymatic potency with cellular permeability and oral bioavailability [1].
6. Future Perspectives
ABBV-CLS-484 represents a landmark achievement as the first active-site phosphatase inhibitor to enter clinical evaluation for cancer immunotherapy. It is currently being evaluated in Phase I clinical trials (ClinicalTrials.gov identifier NCT04777994) for patients with advanced solid tumors, both as a monotherapy and in combination with anti-PD-1 blockade [1][2].
The unique mechanism of action of ABBV-CLS-484 offers a promising strategy to overcome common mechanisms of tumor immune evasion. Because it simultaneously enhances the activity of multiple cytotoxic immune subsets (including both CD8+ T cells and NK cells), it has shown efficacy in tumors that are typically resistant to T cell-mediated immunity, such as those with JAK1 mutations (impaired IFN sensing) or B2M mutations (loss of MHC class I expression) [1]. Future clinical and translational research will likely focus on identifying predictive biomarkers of response—such as baseline tumor inflammation or specific genetic mutations—and optimizing dosing regimens to maximize anti-tumor efficacy while mitigating systemic autoimmune toxicities. Broadly, the successful development of ABBV-CLS-484 may pave the way for a new generation of small-molecule therapeutics targeting other challenging intracellular immune checkpoints [1].
