Ad Hoc Announcement Pursuant to Art. 53 LR 

Geneva, Switzerland, April 23, 2026 - Addex Therapeutics (SIX: ADXN and Nasdaq: ADXN), a clinical-stage biopharmaceutical company focused on developing a portfolio of novel small molecule allosteric modulators for neurological disorders, today announced that its spin-out company, Neurosterix, is on track to complete its Phase 1 clinical study evaluating NTX-253 in Q2 2026. NTX-253 is an investigational potent, selective, orally available positive allosteric modulator (PAM) of the muscarinic M4 receptor being developed for the treatment of schizophrenia. The study is designed to generate a robust early-stage clinical data package assessing safety, tolerability, and pharmacokinetics (PK) across both healthy adult participants and those with stable schizophrenia.

The clinical study includes a multi-part, ascending-dose design intended to efficiently characterize NTX-253’s clinical profile and support subsequent patient-focused development. The study includes both single ascending dose (SAD) and multiple ascending dose (MAD) components, incorporating key translational elements such as food-effect and cerebrospinal fluid (CSF) assessments.

“This thoughtfully constructed Phase 1 clinical study has been designed to deliver a comprehensive understanding of NTX-253’s safety and pharmacokinetic profile, while incorporating early patient data and CNS exposure metrics,” said Tim Dyer, CEO of Addex. “We believe the study’s integrated, comprehensive design will help to de-risk subsequent clinical studies and could accelerate development timelines as it combines healthy volunteer and patient data within a single Phase 1 program.

Comprehensive Phase 1 Study Design
Part 1: Single Ascending Dose (SAD)
Healthy adult participants will receive a single oral dose of NTX-253 or placebo across sequentially escalating dose cohorts. This phase includes a dedicated food-effect cohort to assess the impact of food on NTX-253 pharmacokinetics. A separate SAD cohort in healthy volunteers will evaluate NTX-253 concentrations in cerebrospinal fluid, providing early insight into central nervous system exposure and brain penetration.

Part 2: Multiple Ascending Dose (MAD)
In the MAD phase, participants will receive once-daily oral dosing of NTX-253 or placebo for 10 consecutive days, with sequential dose escalation. This phase is designed to evaluate safety, tolerability, and steady-state pharmacokinetics following repeated dosing.

Importantly, the MAD phase includes two dedicated cohorts of adults with stable schizophrenia, representing early clinical evaluation in the target patient population. Participants in these cohorts will have their antipsychotic medications withdrawn for up to eight days prior to dosing with NTX-253, enabling assessment of safety and PK in patients while maintaining clinical stability.

Neurosterix was spun-out of Addex in April 2024, raising $65 million in a Series A financing led by funds affiliated with Perceptive Advisors. Addex retains a 20% equity interest in Neurosterix.

About NTX-253 and M4 PAMs

The M4 muscarinic receptor is a validated target for treating schizophrenia and related disorders through indirect modulation of dopamine signaling. NTX-253 is a potent, selective, orally available PAM of M4 that fine-tunes muscarinic signaling with the potential to reduce psychosis symptoms while avoiding the movement disorders and metabolic complications associated with traditional dopamine antagonists. Preclinical studies demonstrate robust antipsychotic-like activity and a favorable safety profile, supporting progression into first-in-human clinical studies. Currently available antipsychotics typically target dopamine receptors, providing some success in ameliorating the positive symptoms of the disorder. However, targeting dopamine also can induce metabolic, cognitive, and motor side effects, limiting their therapeutic utility. Research suggest that M4 PAMs could indirectly modulate dopamine levels and induce antipsychotic activity without peripheral muscarinic side-effects seen with direct agonists. Highly selective M4 PAMs have been found to have robust antipsychotic-like effects in multiple rodent models and reverse multiple in vivo effects of psychomotor stimulants that induce increases in extracellular dopamine.

About Addex Therapeutics

Addex Therapeutics is a clinical-stage biopharmaceutical company focused on developing a portfolio of novel small molecule allosteric modulators for neurological disorders. Addex’s lead drug candidate, dipraglurant (mGlu5 negative allosteric modulator or NAM), is under evaluation for future development in brain injury recovery, including post-stroke and traumatic brain injury recovery. Addex’s partner, Indivior, has selected a GABAB PAM drug candidate for development in substance use disorders and has successfully completed IND enabling studies. Addex is advancing an independent GABAB PAM program for chronic cough. Addex holds a 20% equity interest in a private spin out company, Neurosterix LLC, which is advancing a portfolio of allosteric modulator programs, including M4 PAM for schizophrenia, psychosis and mood-related disorders and mGlu7 NAM for mood disorders. In addition, Addex has invested in Stalicla, a private Swiss company pioneering a precision medicine approach for neurodevelopmental and neuropsychiatric disorders.

Addex shares are listed on the SIX Swiss Exchange and American Depositary Shares representing its shares are listed on the NASDAQ Capital Market, and trade under the ticker symbol “ADXN” on each exchange. For more information, visit www.addextherapeutics.com.

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