The purpose of this study is to evaluate the safety, tolerability, pharmacokinetics (PK), and pharmacodynamics (PD) of QLH2405 injection in healthy participants and participants with mild cognitive impairment (MCI) due to Alzheimer's disease (AD) and mild Alzheimer's disease.

Inclusion Criteria for Part A:

1. Voluntarily participate and sign the informed consent.
2. Male or female, aged 18 to 65 years (inclusive).
3. Healthy status as confirmed by medical evaluation.
4. Body mass index (BMI) of 19-28 kg/m² (inclusive).
5. Agree to use effective contraception and have no plans for sperm/egg donation or pregnancy from ICF signing until 9 months post-dose.

Inclusion Criteria for Part B:

1. Voluntarily participate and sign the informed consent, able to communicate well with the investigator and complete the trial per protocol.
2. Male or female, aged 50 to 85 years (inclusive).
3. A diagnosis of mild MCI due to AD or mild AD according to the National Institute on Aging and the Alzheimer's Association (NIA-AA) criteria.
4. Has subjective cognitive and memory decline for ≥6 months.
5. Confirmation of Alzheimer's disease pathological changes by amyloid PET (Aβ-PET) scan.
6. Has one (or more) reliable study partner (s)/adult caregiver(s) can accompany the participant to all visits.
7. Agree to use effective contraception and have no plans for sperm/egg donation or pregnancy from ICF signing until 9 months post-dose.

Exclusion Criteria for Part A:

1. History of significant diseases, or any existing disease may affect the study or pose an unacceptable risk to the participant.
2. Positive for HBsAg, HIV-Ab, Treponema pallidum antibody, or HCV-Ab.
3. History of blood donation, significant blood loss (total volume ≥400 mL), or blood transfusion within 3 months prior to screening.
4. History of alcohol abuse within 1 year prior to screening; or positive alcohol breath test.
5. History of drug abuse and/or substance abuse; or positive drug screening.
6. Participation in any drug or medical device clinical trial within 3 months prior to screening.
7. Pregnant or lactating women, or women of childbearing potential with a positiveβ-hCG test.
8. Any other condition that, in the investigator's judgment, makes the participant unsuitable for participation in this study.

Exclusion Criteria for Part B:

1. Cognitive impairment or dementia caused by any disease other than AD.
2. History of stroke within 6 months prior to screening, or imaging evidence of clinically significant central nervous system diseases.
3. History of epileptic seizures or epileptiform abnormalities on EEG within 6 months prior to screening.
4. Unstable or severe diseases within 6 months prior to screening that, in the investigator's judgment, make the participant unsuitable for study participation.
5. eGFR <60 mL/min/1.73 m².
6. AST or ALT >3 × upper limit of normal (ULN), or total bilirubin >2 × ULN.
7. Current use or anticipated need during the study period for any medication prohibited by the study protocol.
8. Any other condition that, in the investigator's judgment, makes the participant unsuitable for participation in this study.