---

Drug Target Identification: INPP5D / SHIP1

1. Gene and Protein Overview

---

2. Disease Relevance and Genetic Evidence

Alzheimer's Disease (primary indication)

• Two intronic SNPs link INPP5D to late-onset AD (LOAD):
  • rs35349669 (intron 10): minor allele increases AD risk (p = 4.85 × 10⁻⁹; accounts for ~3.8% of AD genetic risk)
  • rs10933431 (intron 2): minor allele is protective (p = 8.92 × 10⁻¹⁰)
• INPP5D expression is elevated overall in AD brain, but active protein levels are reduced, particularly in phagocytic microglia
• TREAT-AD (NIH-funded) ranked INPP5D/SHIP1 as the top priority target among five novel AD targets after evaluating hundreds of candidates [PMID: 41527522]

Other Disease Contexts

• Hematologic malignancies (leukemia, lymphoma) - via PI3K pathway dysregulation
• Autoimmune/inflammatory diseases (COPD, asthma, atopic dermatitis, interstitial cystitis)
• Graft-versus-host disease
• Diabetes, hypertension (emerging evidence)

---

3. Molecular Mechanism and Target Biology

Canonical Phosphatase Function

• PI(3,4,5)P3 → PI(3,4)P2

Domain Architecture

SHIP1 in Microglia and AD

1. TREM2 signaling axis: SHIP1 inhibits TREM2-driven PI3K signaling, dampening microglial phagocytosis of amyloid-beta plaques
2. NLRP3 inflammasome: Reduced INPP5D activity triggers lysosomal lipid accumulation → cathepsin B leakage into cytosol → NLRP3 inflammasome assembly → CASP1 cleavage → IL-1β and IL-18 secretion [PMID: 38016942]
3. NF-κB inhibition: SHIP1 also suppresses NF-κB inflammatory signaling
4. Autophagy regulation: INPP5D disruption impairs autophagic flux in iPSC-derived human microglia
5. Plaque-associated microglia: SHIP1 deficiency promotes recruitment of microglia to plaques and reduces plaque burden in 5XFAD mouse models

---

4. Druggability Assessment

Target Tractability

• High druggability: SHIP1 has a well-defined catalytic phosphatase active site
• Crystal structures available: SGC deposited apo structure and Mg²⁺/phosphate-bound structure; 91 fragment-bound structures of SHIP1 with the phosphatase-C2 construct (PDB entries 5RW2-5RXW series)
• Key selectivity challenge: SHIP1 and SHIP2 (encoded by INPPL1) share 52% sequence identity in the phosphatase-C2 domains with near-identical catalytic sites - developing selective inhibitors is a major medicinal chemistry challenge

Druggable Binding Sites

1. Catalytic active site - substrate-competitive inhibitors (primary site)
2. C2 domain interface - allosteric modulation; fragment hits identified near the phosphatase-C2 interface [Structure, 2024 - PMID: 38309262]
3. SH2 domain - potential for disrupting protein-protein interactions with ITIMs
4. Scaffold interactions - SHIP1 has phosphatase-independent signaling roles that could be targeted

---

5. Pharmacological Tool Compounds and Drug Pipeline

SHIP1 Inhibitors

SHIP1 Agonists (opposite strategy)

Note: The therapeutic strategy for AD favors inhibition (enhance microglia activity). For autoimmune/inflammatory indications outside the CNS, agonism (dampen immune responses) was explored with AQX-1125.

---

6. Target Validation Evidence

Genetic Validation

• GWAS associations (AD risk SNPs)
• Germline SHIP1 haploinsufficiency in 5XFAD mice reduces plaque burden
• iPSC-derived human microglia with heterozygous INPP5D loss-of-function phenocopy AD microglial transcriptional profiles

Functional / Pharmacological Validation

• CETSA (Cellular Thermal Shift Assay) confirms intracellular target engagement by inhibitors
• pAKT levels as on-target pharmacodynamic biomarker
• Phagocytosis of myelin debris and amyloid-beta enhanced by compound 32 in murine microglia [PMID: 42059452]
• Oral administration of compound 32 alters microglial gene expression and reduces IL-1β in amyloidosis mouse model

Postmortem Brain Validation

• Multi-analytic study of hundreds of AD brains: microglia with reduced INPP5D activity show higher ASC speck formation (inflammasome activation) [PMID: 38016942]

---

7. Key Assays for Drug Discovery

---

8. Challenges and Risks

1. SHIP1/SHIP2 selectivity: Near-identical catalytic sites make selective inhibitor design difficult; off-target SHIP2 inhibition could affect peripheral metabolic pathways
2. Isoform complexity: Multiple INPP5D isoforms (exon 1 vs intron 14 transcriptional start sites) may differ in function; some truncated isoforms lack the SH2 domain
3. Dual inhibitor/agonist biology: The therapeutic direction (inhibit vs activate) depends on the disease context and tissue; intermittent dosing of inhibitors may be needed to avoid bone marrow and immune-related side effects
4. BBB penetration: Achieving adequate CNS exposure for AD indication requires optimization of physicochemical properties
5. Scaffold vs enzymatic functions: SHIP1 has phosphatase-independent signaling roles; purely catalytic inhibitors may not fully recapitulate genetic loss-of-function

---

9. Key Literature

---

10. Strategic Summary

1. Medicinal chemistry optimization of compound 32 scaffold for improved selectivity (vs SHIP2), metabolic stability, and CNS penetration
2. Biomarker development - pAKT, IL-1β, lipidomics as PD biomarkers for clinical translation
3. Indication expansion - hematologic malignancies and autoimmune disease represent additional opportunities with distinct pharmacological strategies (inhibition vs agonism)
4. Open science resources available via SGC Target Enabling Package (TEP) and AD Knowledge Portal (TREAT-AD)