The safety and tolerability of multiple oral administrations of GRT9906 at different doses was investigated in this clinical study. The prolonged-release tablets slowly release the active compound in the intestine. In addition, absorption into the body, distribution, metabolization and excretion of GRT9906 was characterized. Pharmacological effects of GRT9906 in healthy participants was assessed using pupillometry (diameter and reactions of the pupil) and a Cold Pressor Test where pain is measured while hands are placed in icy water for two minutes.

The primary objective of the study was to investigate the safety and tolerability of escalating doses of GRT9906 after multiple oral dose administration of prolonged-release (PR) tablets to healthy male and female participants

Secondary objectives were:

• To evaluate the pharmacokinetics (serum and urine) of escalating doses of GRT9906 after multiple oral dose administration of PR tablets and
• To assess the pharmacodynamic effects of escalating doses of GRT9906 by means of pupillometry and Cold Pressor Test after multiple oral dose administration of PR tablets.

The doses of GRT9906 in this study were 80, 120, 160, and 200 milligrams (mg) twice daily in dose groups 1-4.

Participants were screened within 28 days prior to the first dosing. Treatment periods consisted of 5.6 (first dose group) or 6.6 (second and subsequent dose groups) days:

• Day 0 was the day prior to first dosing
• Day T was the titration day (day before Day M1; not applicable for the 80 mg dose group)
• Days M1 to M4 were the days with repeated dosing
• Days F1 and F2 were the follow-up days (24 hours and 48 hours after the last drug administration)
• A final examination was performed within 4 to 14 days after last dosing.

Inclusion Criteria:

• Male or female Caucasian participants aged 45-70 years;
• Body Mass Index (BMI) between 18 and 30 kilograms per square meter (extremes included);
• Participants must be in good health as determined by medical history, physical examination, 12-lead electrocardiogram (ECG), electroencephalogram (EEG), vital signs and clinical laboratory parameters (haematology, sedimentation rate, clotting, clinical chemistry, urinalysis and virus serology). Minor deviations of laboratory values and ECG parameters from the normal range may be accepted, if judged by the investigator to have no clinical relevance and if not considered to interfere with the study objectives;
• Adequate contraception. For females of childbearing potential (i.e., all females except surgically sterilized or at least 2 years postmenopausal) this was defined as follows: any form of hormonal contraception or intra-uterine device had to be used, at least from 4 weeks prior to the first dosing up to 4 weeks after the last dosing and an additional barrier contraception (condom or diaphragm) had to be used from 2 weeks prior to the first dosing up to 4 weeks after the last dosing;
• Participants must give written informed consent to participate within this study;
• Negative human immunodeficiency virus-1/2-antibodies, surface antigen of the hepatitis B virus (HBs-antigen), hepatitis B core (HBc)-antibodies, hepatitis C virus (HCV)-antibodies determined at screening examination;
• Negative drug abuse screening test determined at screening examination and prior to first dose administration in each period (the test will include screening for amphetamines, barbiturates, benzodiazepines, cocaine, cannabinoids and opiates);
• Negative blood beta-human chorionic gonadotropin (beta-HCG) test for females of childbearing potential determined at screening examination and prior to first dose administration in each period (the test is not necessary in females who are in post-menopausal state for at least 2 years or in females with status after surgical sterilisation).

Exclusion Criteria:

• Pulse rate below 50 or above 100 beats per minute. The measurement must be performed in supine position after a resting period of at least 5 minutes
• Systolic blood pressure below 100 or above 160 millimeters Mercury (mmHg), diastolic blood pressure below 50 or above 100 mmHg. The measurement must be performed in supine position after a resting period of at least 5 minutes;
• Participants with history or presence of diseases or functional disorders of the central nervous system, endocrinological system, gastrointestinal tract, connective tissue, hepatobiliary system, renal system, respiratory system or cardiovascular system or other conditions known to interfere with the absorption, distribution, metabolism or excretion of drugs;
• Marked repolarisation abnormality (e.g., suspicious or definite congenital long QT syndrome) or co-medication that is known to influence cardiac repolarisation substantially;
• History of seizures or at risk (i.e. head trauma, epilepsy in family anamnesis, unclear loss of consciousness); abnormal, clinically relevant EEG findings at screening;
• History of orthostatic hypotension;
• Bronchial asthma;
• Definite or suspected history of drug allergy or hypersensitivity;
• History of Raynaud´s disease or phenomenon;
• Malignancy;
• Participation in another clinical study within the last 3 month prior to the start of this study (exception: characterisation of metaboliser status);
• Blood donation (above 100 milliliters [mL]) or comparable blood losses within three month prior to the start of this study;
• Excessive consumption of food or beverages containing caffeine or other xanthines (more than 1000 mL coffee or equivalent per day) within 2 weeks before administration of the investigational products or during the study;
• Drinking of alcohol containing beverages within 48 hours before administration of the investigational products or during the study;
• Evidence of alcohol, medication or drug abuse;
• Smoking of more than 3 cigarettes per day. Smokers who are not able to abstain from smoking for 24 hours prior to the administration of any cold pressor test, and during the period of hospitalisation must be excluded;
• Intake of drugs that are substrates to cytochrome P2D6 (CYP2D6) isoenzyme within the last 4 weeks prior to the start of this study. Intake of more than 1000 mg paracetamol within 3 days prior to administration of the investigational products. Use of any other medication within 4 weeks prior to the start of the study (self-medication or prescription), if not on a stable basis;
• Neurotic personality, psychiatric illness or suicide risk;
• Known or suspected of not being able to comply with the study protocol or of not being able to communicate meaningfully with the investigator and staff;
• Participants who in the opinion of the investigator should not participate in the study;
• Not able to perform the Cold Pressor Test reproducibly, i.e., the difference in the area under the pain-time curves (AUC) is above 20 percent during the 2 tests performed to evaluate the reproducibility at screening;
• Not able to perform the Vienna Test System (Determination Test, Visual Pursuit Test, Tachistoscopic Traffic Test Mannheim for Screen or equivalent subtests);
• Pregnant or breastfeeding women.