BPN14770

BPN14770 (zatolmilast) is a first-in-class, subtype-selective, allosteric inhibitor of phosphodiesterase-4D (PDE4D) that was discovered at Tetra Discovery Partners (later Tetra Therapeutics) and is currently in late-stage clinical development under Shionogi, which acquired Tetra in May 2020. The compound is distinguished from the two approved PDE4 inhibitors (roflumilast and apremilast) and from the classical PDE4 research tool rolipram by three principal features: allosteric rather than competitive inhibition of the PDE4D catalytic site, high selectivity for the PDE4D subtype over PDE4A, PDE4B, and PDE4C (approximately 730-fold selectivity over PDE4B), and exploitation of a primate-specific amino acid residue in the PDE4D N-terminal regulatory domain (UCR2 helix) that confers approximately 100-fold greater potency in humanized transgenic mice relative to wild-type mice, while simultaneously producing low potency in the xylazine/ketamine emesis surrogate assay. These properties yield a therapeutic index of 40- to 100-fold between plasma exposures that produce cognitive and neurochemical benefit (10 to 30 ng/mL) and those projected to cause emesis in non-rodent species (approximately 1,300 ng/mL), a margin that has historically been the dose-limiting constraint for PDE4 inhibitors in cognitive indications.

The molecular pharmacology of BPN14770 is centered on the cAMP-PKA-CREB signaling cascade. Inhibition of PDE4D elevates intracellular cyclic adenosine monophosphate (cAMP) in hippocampal and cortical neurons, activating protein kinase A (PKA), which phosphorylates the transcription factor CREB (cAMP response element-binding protein). Phosphorylated CREB drives expression of brain-derived neurotrophic factor (BDNF), synapsin, postsynaptic density protein 95 (PSD-95), and other effectors of synaptic plasticity and memory consolidation. In humanized PDE4D mice, a single acute oral dose of BPN14770 at 0.01 mg/kg elevated hippocampal cAMP nearly threefold, augmented the late phase of long-term potentiation (LTP), reversed scopolamine-induced impairment of short-term memory, and improved long-term memory through a PKA-dependent mechanism confirmed by the PKA inhibitor H-89. Repeated dosing for 14 days at 0.03 mg/kg elevated hippocampal BDNF 2.1-fold and phospho-CREB 2.3-fold. In an amyloid-beta neurotoxicity model, 14-day oral BPN14770 at 0.01 to 0.03 mg/kg protected hippocampal pyramidal neurons from dendritic atrophy, preserved spine density, restored pCREB/CREB and BDNF/VGF ratios, and normalized spatial and working memory. A separate pathway analysis confirmed that BPN14770 engages the cAMP-PKA-SIRT1-Akt-Bcl-2/Bax signaling module, producing neuroprotective and anti-apoptotic effects.

Clinically, BPN14770 has been evaluated in two Phase 1 trials in 109 healthy adults (single doses up to 100 mg, multiple doses of 10 to 40 mg twice daily in elderly volunteers), a Phase 2 PICASSO trial in 255 patients with early Alzheimer's disease (10 or 25 mg twice daily for 12 weeks), and a Phase 2 crossover trial in 30 adult males with Fragile X syndrome (25 mg twice daily for 12 weeks). The Phase 1 trials established linear pharmacokinetics, oral bioavailability of 70 to 80 percent, a plasma half-life of 8 to 10 hours, and a brain-to-plasma ratio of approximately 0.4. The PICASSO Alzheimer's trial missed its primary endpoint (RBANS Delayed Memory Index) but showed a signal on the Clinical Dementia Rating Sum of Boxes (CDR-SB) in a higher-dose subgroup. The Fragile X Phase 2 trial met its primary endpoint of safety and tolerability and demonstrated significant improvement on NIH Toolbox Oral Reading Recognition, Picture Vocabulary, and Cognition Crystallized Composite Score, with clinically significant caregiver-rated improvement in language and daily functioning. Shionogi subsequently initiated the EXPERIENCE Phase 2b/3 program comprising three studies (EXPERIENCE-204 in adolescents, EXPERIENCE-301 in adults, EXPERIENCE-302 open-label extension) for Fragile X syndrome. Topline results from the Phase 3 trials indicated that neither study met its originally specified primary endpoint of cognitive improvement on the NIH Toolbox, though the adult study (EXPERIENCE-301) showed statistically significant improvement on the caregiver-assessed Numeric Rating Scale. The compound holds FDA Fast Track designation, Orphan Drug designation in both the United States and European Union, and Rare Pediatric Disease designation for Jordan's syndrome (Houge-Janssens syndrome 1), for which a Phase 2 trial enrolling 30 participants was initiated in February 2025.

The compound is well tolerated in clinical studies. The most common adverse events are headache, transient nausea, and vomiting, occurring at rates modestly above placebo. No serious adverse events attributable to the compound have been reported. The favorable emetic profile relative to classical PDE4 inhibitors reflects the allosteric mechanism and PDE4D subtype selectivity, which avoid the PDE4B-mediated emesis that limits rolipram and constrains roflumilast dosing.

This monograph reviews the chemistry, structural pharmacology, and primate-specific binding of BPN14770; the cAMP-PKA-CREB-BDNF signaling mechanism in molecular detail; the comprehensive preclinical pharmacology across scopolamine, amyloid-beta, and Fragile X models; human pharmacokinetics; the clinical evidence base in Alzheimer's disease, Fragile X syndrome, and Jordan's syndrome; sourcing and quality verification for research applications; reconstitution and handling; stack-interaction considerations; adverse-event signal; and a structured comparative assessment of five PDE4 inhibitor candidates (roflumilast, apremilast, rolipram, MK-0952, GEBR-7b) against BPN14770 on five competency standards.