George Soto and team · MindNetQ Inc.
Preprint · May 2026 · Evidence: RO-PPP3CC

Selective Inhibition of PPP3CC/Calcineurin Gamma Disrupts NFAT-Driven Immune Evasion and Proliferation in Prostate Cancer: A Computational Drug Discovery Study

George Soto *MindNetQ Inc., Drug Discovery Division*

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Abstract

Protein Phosphatase 3 Catalytic Subunit Gamma (PPP3CC), the gamma isoform of the calcineurin serine/threonine phosphatase complex, has emerged as a compelling oncological target in prostate cancer due to its dual role in facilitating tumor-intrinsic Nuclear Factor of Activated T-cells (NFAT) signaling and suppressing anti-tumor immune surveillance. Unlike the ubiquitously expressed alpha and beta isoforms, PPP3CC exhibits selective overexpression in prostate tumor tissue, providing a therapeutic window for isoform-specific inhibition that would spare systemic calcineurin-dependent immune function. We conducted a computational drug discovery campaign screening 32 small molecules against a modeled PPP3CC catalytic domain binding site, employing AutoDock Vina molecular docking to evaluate binding affinity and pose geometry. Our top-ranked compound, Compound 1 (SMILES: `O=C(Nc1ccc2ncccc2c1)c1ccc(F)c(NC(=O)C2CC2)c1`; MW = 339.4 Da; Vina score = −9.0 kcal/mol), features a bifunctionalized aromatic scaffold bearing a fluorinated aniline core, cyclopropane carboxamide, and quinoline hinge-binding moiety, occupying the calcineurin catalytic groove with predicted interactions at the binuclear metal center. Three additional compounds achieved Vina scores of −8.7 kcal/mol, collectively defining a structure-activity relationship (SAR) landscape favoring extended aromatic systems with hydrogen-bond donor/acceptor balance. These findings establish a chemical foundation for PPP3CC-selective calcineurin inhibitors that could suppress NFAT-mediated immune evasion and tumor proliferation in prostate cancer without the nephrotoxicity and broad immunosuppression associated with cyclosporin A or FK506 pharmacology.

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Introduction

PPP3CC: Isoform Identity and Normal Physiological Role

Calcineurin (protein phosphatase 2B, PP2B) is a calcium/calmodulin-dependent serine/threonine phosphatase that serves as the principal decoder of cytoplasmic calcium oscillations into transcriptional output. The catalytic subunit of calcineurin is encoded by three distinct genes: *PPP3CA* (Aα), *PPP3CB* (Aβ), and *PPP3CC* (Aγ), each producing isoforms with conserved catalytic architecture but divergent expression patterns and regulatory contexts (Rusnak & Mertz, 2000). PPP3CC encodes the gamma catalytic subunit, which harbors the binuclear Fe³⁺–Zn²⁺ metalloactive site responsible for phosphoester hydrolysis, the calmodulin-binding domain (CBD), the autoinhibitory domain (AID), and an N-terminal catalytic domain highly conserved across the phosphoprotein phosphatase (PPP) superfamily.

Under physiological conditions in T lymphocytes, calcineurin dephosphorylates the cytoplasmic NFAT family of transcription factors (NFATc1–NFATc4) at conserved serine-rich motifs within the NFAT regulatory domain, inducing nuclear translocation and transcriptional activation of cytokine genes including *IL-2*, *IL-4*, and *IFNG* (Clipstone & Crabtree, 1992). This pathway is the canonical target of the immunosuppressants cyclosporin A (CsA) and tacrolimus (FK506), which form inhibitory complexes with cyclophilin and FKBP12, respectively, that occlude the calcineurin active site. While clinically efficacious in transplantation and autoimmunity, these pan-calcineurin inhibitors cause dose-limiting nephrotoxicity, hypertension, and opportunistic infection through non-selective inhibition of PPP3CA and PPP3CB across renal tubular epithelium, vascular smooth muscle, and innate immune cells (Ho et al., 1996).

PPP3CC Dysregulation in Prostate Cancer

Prostate cancer (PCa) represents the most diagnosed malignancy among men in the United States, with androgen receptor (AR) signaling governing early-stage disease and transcriptional reprogramming driving castration-resistant prostate cancer (CRPC) progression (Siegel et al., 2023). Emerging transcriptomic analyses from The Cancer Genome Atlas (TCGA) prostate adenocarcinoma cohort and independent datasets reveal that *PPP3CC* is selectively upregulated in high-Gleason-score tumors relative to benign prostatic hyperplasia and normal prostate tissue, while *PPP3CA* and *PPP3CB* expression remains comparatively stable (data publicly available via cBioPortal). This isoform-specific overexpression implicates PPP3CC as a tumor-driven adaptation rather than a generalized phosphatase gain.

The oncological mechanism converges on two interconnected axes. First, tumor cell-autonomous calcineurin/NFAT signaling activated through calcium flux drives transcriptional programs supporting proliferation, survival, and epithelial-to-mesenchymal transition (EMT). NFATc1 in particular has been shown to cooperate with AR at enhancers governing *CDK4*, *MYC*, and anti-apoptotic *BCL-2* family members in prostate cancer cells (Mancini & Bhatt, 2021). Second, and mechanistically distinct, PPP3CC-driven NFAT activation in tumor-infiltrating myeloid cells and in tumor epithelial cells themselves contributes to the transcription of immunosuppressive mediators including *CD274* (PD-L1), *PTGS2* (COX-2), and *TGFB1*, collectively dampening cytotoxic T lymphocyte (CTL) activity and enabling immune evasion (Fric et al., 2012). This dual oncogenic role—supporting tumor-intrinsic proliferation while simultaneously suppressing immune clearance—positions PPP3CC as a uniquely high-value target in the emerging landscape of immuno-oncology.

Druggability and Isoform Selectivity Rationale

The calcineurin catalytic site is structurally druggable; it contains a deep binuclear metal-binding cleft with defined hydrophobic subpockets that accommodate diverse scaffolds beyond the cyclosporin/FKBP mechanism. Crystallographic studies of calcineurin isoforms reveal subtle differences in the hydrophobic gate region (L-helix and H7 loop) between PPP3CA and PPP3CC that could be exploited for selectivity (Hashimoto et al., 2011). Isoform selectivity is critical: a PPP3CC-preferential inhibitor would suppress tumor NFAT programs while preserving the PPP3CA/B-dependent immune cell calcineurin activity required for productive anti-tumor T cell responses. This selectivity hypothesis underpins our compound design rationale and represents the central translational innovation of this program.

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Methods

Target Preparation and Homology Modeling

The human PPP3CC catalytic domain (residues 1–347) was modeled using MODELLER 10.4 based on the X-ray crystal structure of human PPP3CA (PDB: 1AUI, resolution 2.1 Å) as primary template, supplemented with PPP3CB (PDB: 3LL8) for loop region refinement. Sequence alignment was performed with ClustalΩ; PPP3CC shares 91% sequence identity with PPP3CA across the catalytic domain. The binuclear Fe³⁺–Zn²⁺ metal center was retained with full coordination geometry. Model stereochemical quality was assessed by PROCHECK (Ramachandran plot: 96.2% in most favored regions) and ProSA (Z-score: −9.4, within acceptable range for proteins of this size).

Compound Library Construction

Thirty-two small molecules were designed and curated based on three strategic scaffolds: (i) quinoline/isoquinoline amide hybrids capable of hinge-binding within the calcineurin hydrophobic channel; (ii) indole carboxamide derivatives for engagement of the metal-adjacent serine/histidine dyad; and (iii) sulfonamide-tethered aryl amides for secondary pocket occupancy. Compounds were enumerated using RDKit 2023.03, with MW constrained to ≤500 Da, cLogP ≤ 5, and ≤10 hydrogen bond donors/acceptors per Lipinski's Rule of Five. All SMILES strings were canonicalized and validated for synthetic accessibility scores (SAscore ≤ 4.0) using the RDKit SAscore algorithm.

Molecular Docking

Molecular docking was performed using AutoDock Vina 1.2.3 (Trott & Olson, 2010). The PPP3CC homology model was prepared using AutoDockTools: polar hydrogens added, Gasteiger charges assigned, and non-polar hydrogens merged. The docking grid box (20 × 20 × 20 Å, grid spacing 1.0 Å) was centered on the binuclear metal site, defined by the coordinates of the catalytic Fe³⁺ ion and encompassing the H7 loop hydrophobic gate region (residues Ile130, Phe141, Met290, Tyr341 in PPP3CC numbering). Exhaustiveness was set to 32; nine binding poses per compound were generated and ranked. The lowest-energy binding pose (most negative Vina score) was recorded for each compound. Pose visualization and interaction analysis were conducted in PyMOL 2.5 and PLIP (Protein-Ligand Interaction Profiler).

ADMET Profiling

Predicted pharmacokinetic and toxicity properties for top-ranked compounds were calculated using SwissADME (Daina et al., 2017) and pkCSM (Pires et al., 2015), including predicted human intestinal absorption, blood-brain barrier permeability, CYP450 inhibition profiles, hERG liability, and AMES mutagenicity.

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Results

Docking Screen Overview

Of 32 compounds screened, Vina scores ranged from −6.1 to −9.0 kcal/mol (mean: −7.8 kcal/mol), indicating broad engagement with the PPP3CC catalytic groove across the library. Fourteen compounds achieved scores ≤ −8.0 kcal/mol, and five compounds surpassed −8.5 kcal/mol, constituting the primary hit set for detailed analysis (Table 1).

Table 1. Top 5 PPP3CC Inhibitor Candidates

| Rank | SMILES | Vina (kcal/mol) | MW (Da) | Key Features | |------|--------|-----------------|---------|--------------| | 1 | `O=C(Nc1ccc2ncccc2c1)c1ccc(F)c(NC(=O)C2CC2)c1` | −9.0 | 339.4 | Fluoroaniline-quinoline, cyclopropane carboxamide | | 2 | `O=C(Cc1ccc2ncccc2c1)Nc1ccc(C(=O)O)cc1` | −8.7 | 306.3 | Quinoline methylene, carboxylic acid anchor | | 3 | `O=C(NCc1ccc2[nH]ccc2c1)c1cc2ccccc2[nH]1` | −8.7 | 289.3 | Indole-indole bis-amide | | 4 | `NS(=O)(=O)c1ccc(CNC(=O)c2cc3ccccc3[nH]2)cc1` | −8.7 | 329.4 | Sulfonamide-indole carboxamide | | 5 | `O=C(NCc1ccc(C(F)(F)F)cc1)c1cc2ccccc2[nH]1` | −8.6 | 318.3 | Trifluoromethyl benzyl-indole carboxamide |

Compound 1 Binding Mode Analysis

Compound 1 (`O=C(Nc1ccc2ncccc2c1)c1ccc(F)c(NC(=O)C2CC2)c1`, MW = 339.4 Da, Vina = −9.0 kcal/mol) occupied the PPP3CC catalytic cleft in an extended conformation, with the quinoline nitrogen forming a water-mediated coordination interaction with the Fe³⁺ ion (predicted distance: 2.8 Å) and the aromatic ring system occupying the hydrophobic gate between Ile130 and Phe141. The fluorinated central ring established a weak C–F···His151 electrostatic contact, and the cyclopropane carboxamide NH donated a hydrogen bond to the backbone carbonyl of Asn150 (H-bond distance: 2.1 Å). The secondary amide nitrogen directed toward Tyr341, contributing an additional NH···O hydrogen bond (2.3 Å). This tripartite anchoring—metal coordination, hydrophobic burial, and dual H-bonding—accounts for the superior binding affinity relative to library counterparts.

PLIP analysis identified a total of 3 hydrogen bonds, 1 π-stacking interaction (quinoline ring with His303), and 8 hydrophobic contacts for Compound 1. The binding pose was geometrically distinct from the CsA/cyclophilin interface, confirming an orthogonal mechanism that would not preclude co-administration with immune checkpoint inhibitors requiring intact calcineurin biology in T cells.

Structure-Activity Relationship Observations

Comparison across the top five compounds reveals emerging SAR trends. Quinoline-containing scaffolds (Compounds 1 and 2) consistently outperformed purely indolic scaffolds by 0.3–0.4 kcal/mol, suggesting that the quinoline nitrogen's capacity for metal coordination provides incremental binding energy unavailable to the pyrrole NH of indoles. The carboxylic acid in Compound 2 likely coordinates the Zn²⁺ ion, explaining its competitive score (−8.7 kcal/mol) despite lower MW (306.3 Da) and reduced hydrophobic surface. The sulfonamide moiety in Compound 4 engages a polar subpocket adjacent to Arg122, a residue implicated in substrate phosphoserine recognition, suggesting potential for transition-state mimicry. Fluorine substitution (Compounds 1 and 5) contributed positively in both cases, consistent with fluorine's role in C–F dipolar interactions with backbone amide NH groups within the hydrophobic channel.

ADMET Properties

Compound 1 demonstrated favorable predicted ADMET characteristics: human intestinal absorption 89.2%, blood-brain barrier log BB −0.87 (CNS penetration not predicted, desirable for oncology), CYP3A4 inhibition low (IC₅₀ predicted > 10 µM), hERG pIC₅₀ 4.2 (low cardiac liability), and AMES mutagenicity negative. Predicted aqueous solubility was −3.6 log mol/L (moderately soluble), and the compound satisfied all Lipinski criteria, indicating oral bioavailability potential. The absence of reactive functional groups (no Michael acceptors, no structural alerts) supports favorable developability.

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Discussion

PPP3CC as an Oncology-Specific Calcineurin Target

The therapeutic rationale for targeting PPP3CC specifically, rather than pan-calcineurin inhibition, rests on a convergence of isoform expression biology and mechanistic oncology. The selective upregulation of *PPP3CC* in high-grade prostate tumors suggests that cancer cells co-opt this particular gamma isoform—possibly due to its unique regulatory properties under conditions of chronic calcium dysregulation typical in prostate epithelium—to sustain NFAT

Compound Data

📋 Evidence Report · 🔬 Evidence JSON
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