This study is designed to characterize the regional distribution of [11C] ABP688 in brain by positron emission tomography (PET).

Inclusion Criteria:

• Male and female subjects 55-80 (inclusive)
• Subjects in good health

At screening:

• oral body temperature between 35-37.5C
• systolic blood pressure: 90-140 mm Hg
• diastolic blood pressure: 50-90 mm Hg
• pulse rate: 40-90 bpm
• Female subjects of child bearing potential must have been using a double-barrier local contraception
• Postmenopausal women must have had no menstrual bleeding
• Subjects must have weighed at least 50 kg
• All subjects must provide informed consent
• All subjects must have been able to communicate well with investigator

Specific to Major depression disorder (MDD)

Patients had to show following level of symptomatology:

• HAM-D (17-item scale) > 16
• CGI > 4 (moderately ill)

Exclusion Criteria:

Specific criteria for healthy volunteers:

- Presence and/or history of clinically significant major neurological or psychiatric disorder

Specific criteria for MDD patients:

• Presence and/or history of a clinically significant major neurological or psychiatric disorder other than MDD or generalized anxiety disorder
• Axis I co-morbidity was excluded except anxiety spectrum disorders

Criteria for both:

• Smokers
• Pregnancy
• Subjects with history of or symptoms/complaints consistent with mild cognitive impairment
• Use of any psychotropic prescription drugs
• Coffee consumers more than 6 cups/day
• Participation in any clinical investigation
• Donation or loss of 400 mL or more of blood
• Significant illness within 2 weeks prior to dosing
• A known hypersensitivity to study drug
• MRI scan that showed evidence of stroke
• Any surgical or medical condition which might have significantly altered distribution
• Clinical evidence of any abnormal lab value
• History of immunodeficiency disease
• Positive Hepatitis B surface antigen
• Presence of contraindications to PET scan investigations- Presence of contraindications to MRI investigations
• Evidence from an Allen test of incomplete communication
• History of drug or alcohol abuse
• Current use of anticonvulsant
• Significant radiation exposure

Other protocol-defined inclusion/exclusion criteria may apply.