The goal of this clinical trial is to evaluate the safety, tolerability, and pharmacokinetics of the investigational monoclonal antibody TBE025 when given intravenously to healthy adult volunteers aged 18 to 55.

The study aims to determine the recommended Phase II dose of TBE025 and to assess the incidence and severity of adverse events related to its administration.

There is no comparison group, as this is a single-arm, open-label study. Participants will receive a single intravenous infusion of TBE025 at one of three escalating dose levels, will be monitored for safety with regular clinical and laboratory assessments, and will provide blood samples for pharmacokinetic, anti-drug antibody, and neutralization testing.

The tick-borne encephalitis virus (TBEV) is an infection spread by ticks that is becoming more common in Europe, including Switzerland. Currently, there is no specific treatment-care focuses on relieving symptoms. Vaccines exist, but few people get vaccinated, and sometimes the vaccine does not offer full protection. There is therefore a real need for new ways to prevent or treat this disease.

TBE025 is a fully human antibody taken from people who have recovered from TBEV. It can strongly block the virus. In lab studies with mice, TBE025 was able to protect against infection, both when given before exposure and soon after infection. It also appears to be safe, with no harmful effects on other tissues.

This first-in-human study (phase 1) will evaluate intravenous administration of TBE025 in healthy adult volunteers using a standard 3+3 dose-escalation design (200 mg, 600 mg, 2000 mg). Between 3 and 18 participants will be enrolled sequentially across three cohorts. The study is designed to establish the maximum tolerated dose and recommended Phase II dose, as well as to characterize the pharmacokinetics, immunogenicity, and neutralization capacity of TBE025.

Participants will be followed for safety, laboratory, and pharmacokinetic assessments over a three-month period. The study will provide the first clinical data on TBE025 in humans and will inform the design of subsequent efficacy trials in populations at risk of TBEV infection.

Inclusion Criteria:

• Adults aged 18 to <55 years
• Good general health (medical history, physical exam, normal labs)
• Written informed consent provided
• Ability to read and understand local language
• Contraception requirements : Women of childbearing potential: negative pregnancy test, surgically sterile, postmenopausal, or using highly effective contraception for 3 months post-infusion. Men: surgically sterile or using highly effective contraception (self or partner) and abstain from sperm donation for 3 months post-infusion.

Exclusion Criteria:

• BMI <19 or >30
• Infections: HIV, active hepatitis B (HBsAg), active hepatitis C
• Prior participation in investigational study within 30 days
• History of hypersensitivity to monoclonal antibodies
• Recent surgery or unresolved adverse events
• Active autoimmune disease requiring systemic treatment
• Recent use of immunosuppressive agents or immunoglobulins
• Immunodeficiency diagnosis
• Significant cardiovascular events within 6 months
• Live/attenuated vaccines within 28 days
• Major surgery within 28 days
• Pregnancy or breastfeeding
• Professional or private link with the research team
• Any condition judged by the investigator to interfere with safety or study results

No individual participant data (IPD) will be shared. Only de-identified subject-level data may be provided if requested, and only in accordance with participant informed consent and applicable laws.