This study is designed to evaluate the safety and tolerability of single ascending doses of UCB7665 administered by intravenous or subcutaneous infusion in healthy subjects.

Inclusion Criteria:

• Healthy male and female volunteers of non-childbearing potential who gave their consent to the written informed consent form
• Healthy volunteers in the age between 18 and 64 years old with normal weight as determined by a body mass index (BMI) between 18 and 30 kg/m^2, with a body weight of at least 50 kg for male subjects or 45 kg for female subjects
• Subjects has blood pressure (BP) and pulse within normal range in a supine position after 5 minutes rest
• Subject has clinical laboratory test results within the reference ranges of the testing laboratory or test results that are outside the specified ranges and that are deemed as clinically non-significant
• Subject's electrocardiogram (ECG) is considered "normal" or "abnormal" but clinically non-significant

Exclusion Criteria:

• Subject has had significant blood loss, or has donated blood in excess of 400 mL of blood or blood products within 90 days before Day -2, or plans to donate blood during the clinical study
• Subject has a total Immuneglobulin G <7 g/L or >16 g/L at the Screening Visit
• Subject has absolute neutrophil count <1.5x10^9/L and/or lymphocyte count <1.0x10^9/L
• Subject has known viral hepatitis, has a positive test for Hepatitis B surface antigen or is Hepatitis C virus antibody positive
• Subject tests positive to Human Immunodeficiency Virus (HIV) 1/2 antibodies
• Subject has a past medical history or family history of primary immunodeficiency
• Subject is splenectomized
• Subject has a positive TIGRA (T cell interferon γ release assay) at the Screening Visit. If TIGRA is not available, purified protein derivative (PPD) skin test can be substituted and/or chest x-ray performed within 6 months before the Screening Visit showing no evidence of latent/active Tuberculosis (TB)
• Subject has past (in the last 12 months ) or present substance abuse/dependence or concurrent medical conditions that in the opinion of the investigator could confound clinical study interpretation or affect the subject's ability to fully participate in the clinical study
• Subject has a known hypersensitivity to any components of the Investigational Medicinal Product (IMP)
• Subject has an active infection or has had a serious infection (resulting in hospitalization or requiring parenteral antibiotic treatment) within 6 weeks before the first dose of IMP
• Subject has 12-lead electrocardiogram (ECG) with changes considered to be clinically significant upon medical review
• Subject has renal impairment
• Subject has hepatic impairment
• Subject has corrected serum calcium of >11.5 mg/dL (>2.9 mmol/L) or <8.0 mg/dL (<2.0 mmol/L) at the Screening Visit
• Subject has active neoplastic disease or history of neoplastic disease within 5 years of entry in the clinical study
• Subject has any other acute or chronic illness which, in the opinion of the investigator or study physician, could pose a threat or harm to the subjects
• Subject has received a vaccination within 6 weeks before the Screening Visit or intends to have a vaccination before Day 43 of the Safety Follow-up Period
• Subject has received any IMP or experimental procedure within 90 days before the first dose of IMP
• Subject requires a treatment other than Paracetamol during the clinical study
• Subject is a vulnerable subject (e.g. subject kept in detention) (Declaration of Helsinki, 1996)