Suppression of cns inflammation by phosphodiesterase-1 (pde1) inhibitors: toward new treatments for neurodegenerative diseases

Neuroinflammatory processes are implicated in the cause and progression of neurodegenerative disorders, like Alzheimer's disease. Elevated intracellular cyclic nucleotide levels suppress neuroinflammatory responses by controlling microglial activation. The second messenger cyclic adenosine monophosphate (cAMP) is a negative modulator of inflammatory cell responses, including cytokine secretion and leukocyte recruitment. Similarly, increasing intracellular cyclic guanosine monophosphate (cGMP) attenuates lipopolysaccharide (LPS)-induced responses in microglia. Hence, increasing intracellular cAMP or cGMP using analogs or phosphodiesterase (PDE) inhibitors, antagonizes changes in microglial cell morphology and production of proinflammatory cytokines in the context of inflammation. Intra-Cellular Therapies, Inc. has developed ITI-214, a potent, specific, and clinically-safe inhibitor of PDE1, a dual-specificity (cAMP/cGMP) PDE, and tested its ability to control neuroinflammation. Effects of LPS and/or ITI-214 on mRNA cytokine expression were measured by qPCR using BV2 cells or iPSC-derived astrocytes in vitro and mouse brain tissue. RNAseq was performed (GENEWIZ, LLC) followed by gene ontology using AmiGo 2 software. LPS-induced increases in TNFα, IL-1β, and Ccl2 mRNA expression were reduced by >50% by ITI-214 in BV2 cells in vitro and in mice in vivo in brain regions with high PDE1 expression (striatum, cortex and hippocampus) while brain expression of the anti-inflammatory cytokine IL-10 was increased after ITI-214. Effects of ITI-214 appeared microglial-specific as it did not attenuate induction of IL-6 levels in hiPSC-derived astrocytes after proinflammatory cytokine stimulation. We then employed RNA-Seq to identify a subset of genes whose transcript expression was significantly changed with PDE1 inhibition in LPS-treated BV2 cells. These genes were significantly enriched in cell migration and extravasation pathways as well as inflammatory pathways. Of genes induced by LPS, a subset were attenuated by PDE1 inhibition, all of which were significantly associated with inflammatory pathways. Importantly, inhibition of another PDE isoform, PDE4, attenuated a different subset of LPS-induced genes, demonstrating the unique properties of our target. The data demonstrate a key role for PDE1 inhibition in regulating inflammatory responses in microglia and support PDE1 inhibition as a viable therapeutic approach in disorders in which neuroinflammation and microglia activation are known components of the disease, like Alzheimer's disease.