CSTI-500: A Serotonin-Preferred, Tunable Triple Monoamine Reuptake Inhibitor in Phase 2 Clinical Development
Overview
CSTI-500 is an investigational, orally administered new chemical entity and serotonin-preferred triple monoamine reuptake inhibitor that modulates serotonin, dopamine, and norepinephrine signaling. It is currently being evaluated in a U.S. Phase 2 clinical study in individuals with Prader-Willi syndrome, focusing on safety, tolerability, pharmacokinetics, and preliminary effects on behavioral dysregulation and hyperphagia.
Mechanism of Action
CSTI-500 is a serotonin-preferred triple monoamine reuptake inhibitor (TRI) that inhibits SERT, DAT, and NET, thereby modulating serotonin, dopamine, and norepinephrine signaling. “Serotonin-preferred” means that CSTI-500 has greater potency at SERT than at DAT and NET, producing broader monoamine modulation as concentrations rise. These systems are involved in behavioral and emotional regulation, reward, attention, motivation, and feeding-related behavior
Multiple lines of evidence implicate dysregulated monoamine signaling in the behavioral and feeding-related symptoms of PWS. Based on this rationale, CSTI-500 is being evaluated as a single-molecule approach to behavioral dysregulation and hyperphagia. CSTI-500 is investigational. Initial safety, tolerability, and pharmacokinetic data have been generated in completed studies involving healthy volunteers and individuals with PWS; longer-term safety and clinical efficacy in PWS remain to be established in clinical trials.
Tunable & Personalized Dosing
CSTI-500 has a serotonin-preferred, concentration-dependent transporter-occupancy profile.
Human PK/PET studies in healthy volunteers established the relationship between plasma exposure and SERT and DAT occupancy: SERT occupancy approaches a plateau at lower exposures, while DAT and predicted NET occupancy increases more gradually as exposure rises.
Illustrative representation of CSTI-500 brain occupancy vs plasma concentration at steady state in healthy humans
What does "tunable" mean?
SERT engagement predominates at lower exposures, while DAT and predicted NET engagement increase as exposure rises.
What is the supporting evidence?
Human PK/PET studies established exposure–occupancy relationships for SERT and DAT.
Why does the profile matter?
At lower exposures, CSTI-500 pharmacology is dominated by SERT engagement, modulating serotonin pathways associated with emotional regulation, behavioral flexibility, and compulsivity. As exposure rises, DAT and predicted NET engagement increase, adding modulation of dopamine pathways associated with food-related reward and motivation and norepinephrine pathways associated with arousal and attention.
This concentration-dependent profile provides the rationale for PK-guided dose titration across behavioral and feeding-related symptom domains in PWS. Whether this mechanistic rationale translates into clinical benefit remains to be established in clinical trials.
How is this applied clinically?
In the ongoing U.S. Phase 2 study in PWS, measured plasma concentrations guide dose titration toward predefined target-exposure ranges. Brain occupancy is not directly measured in study participants.
This quantitative approach supports individualized dose selection while safety, tolerability, pharmacokinetics, and preliminary clinical activity are evaluated.
Clinical Development Status
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Generally safe and well-tolerated in Phase 1 studies in healthy volunteers (n = 87)
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Demonstrated CNS target engagement via Positron Emission Tomography (PET) imaging
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Phase 1 in PWS patients completed (n = 10), confirming PK and tolerability in the target population
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Phase 2 clinical trial currently recruiting (NCT07348601)