Material Event (Form 8-K)

Item 8.01. Other Events.

On May 26, 2026, Gain Therapeutics, Inc. (the "Company") disclosed interim clinical and biomarker data from Part 2 (the nine-month extension) of its Phase 1b open-label clinical study of GT-02287 in patients with Parkinson's disease with or without a GBA1 mutation. The data was released via an oral presentation at the 3rd International GBA1 Meeting 2026, held May 22-23, 2026, in Phoenix, AZ.

The data presented, which includes safety, tolerability, biomarkers, and clinical scores from the Phase 1b nine-month study extension support continued development of GT-02287 for PD. As previously reported, 16 of 19 participants who completed dosing in Part 1 of the Phase 1b chose to continue in the ongoing nine-month extension (Part 2), further supporting the tolerability of GT-02287. A Data Monitoring Committee meeting on March 5, 2026, concluded that the study should continue without any changes.

To date, all 16 participants remain on study and have completed five months of dosing (Day 150).

As previously reported, in all participants with elevated baseline levels of glucosylsphingosine (GluSph) in cerebrospinal fluid (CSF), GluSph decreased by an average of 81% after 90 days of treatment with GT-02287. Elevated GluSph, a result of glucocerebrosidase (GCase) dysfunction, has been shown to increase the aggregation of alpha synuclein and to impair mitochondrial and lysosomal function in neurons. Furthermore, in individuals with high levels of CSF GluSph at baseline, levels of DOPA decarboxylase (DDC) decreased following 90 days of treatment with GT-02287. DDC is responsible for converting levodopa into dopamine in the brain and is elevated in the CSF of people with Parkinson's - likely due to dopaminergic neuron dysfunction

Figure 1: Change in MDS-UPDRS scores from Baseline at Day 150 in all patients for whom CSF GluSph and MDS-UPDRS scores were available, patients with low baseline GluSph, and patients with high baseline GluSph.

Participants with elevated baseline levels of GluSph in CSF continued to benefit more than those with low baseline levels of GluSph in CSF after 150 days of dosing with GT-02287, with a difference of 4.8 points in the sum of MDS-UPDRS Part II and Part III scores between the two groups at Day 150. MDS-UPDRS scores remained stable and durable across overall study population after 150 days of treatment with GT-02287.

Additionally, participants in the Phase 1b study provided unsolicited descriptions of perceived benefit after 90 days of dosing with GT-02287. Perceived benefits most commonly described included four instances of improved sense of smell and taste, four instances of improved balance or gait, and three instances of improved sleep. The planned Phase 2 clinical trial is anticipated to include endpoints to further assess treatment effects beyond benefits perceived and informally reported, including administration of the University of Pennsylvania Smell Identification Test (UPSIT) at baseline and at the end of the study, as well as the use of Opal wearable sensors for in-clinic gait assessments to be conducted at multiple timepoints throughout the study.