Evaluation of MTX-439 in Healthy Adults and Adults With Diabetic Kidney Disease

Inclusion Criteria: 1. All genders, ages 18 to 65 years, inclusive. 2. Able to read and understand the study and all related materials (including the ICF), and provide a signed, written informed consent. 3. Willing and able to complete all protocol-required study visits and procedures. 4. Consumption of no more than 5 cigarettes or other cotinine-containing products (including tobacco, nicotine gum, patches, nicotine pouches, and e-cigarettes) per week and agreement to abstain from all such products during inpatient stays. 5. Willing to refrain from marijuana- or cannabinol-containing products for 30 days before Screening and until the last study visit. 6. Agrees to not donate eggs (if applicable) from the time of the first infusion until 65 days after the final dose, or 125 days after the final dose for sperm (if applicable); additionally agrees to not donate blood from 56 days prior to the time of first infusion until 90 days after the last study visit or platelets/plasma from 14 days prior to the time of first infusion until 90 days after the last study visit. 7. Participants are required to follow specific contraception measures as follows: * Participants assigned male at birth must use a condom (even if vasectomised) at the time of Screening and for 125 days after the final dose. * Participants assigned female at birth must be of nonchildbearing potential (defined as either at least 6 months surgically sterilised or at least 1 year postmenopausal and confirmed by follicle-stimulating hormone \[FSH\] level \> 40 U/L) OR, if of childbearing potential (defined as not sterilised via bilateral oophorectomy or hysterectomy; still menstruating; or \< 1 year has passed since the last menses, if menopausal), must use a combination of 1 highly effective method of contraception and 1 effective method of contraception. This contraceptive practice should begin at least 28 days before the first dose of study drug, continue during the study, and persist for 65 days after the final dose of study drug, if applicable. * Male participants agree to ensure that their female partners who are of childbearing potential will use a highly effective method of contraception.. 8. Vital signs within the following ranges: * Systolic blood pressure: 90-140 mmHg * Diastolic blood pressure: \< 90 mmHg * Pulse rate: 55-100 bpm * Respiration rate: 10-16 respirations per minute DKD participants: 9. All of the above. * Well-controlled diabetes requiring minimal dose adjustments of antidiabetic medications in the past 3 months and no adjustments within 30 days of Screening. * Haemoglobin A1C \< 9.5%. * Estimated glomerular filtration rate (eGFR) between 30 and 60. Exclusion Criteria: 1. Any active medical condition determined clinically significant by the Investigator, except for DKD (for DKD participants). 2. Body mass index (BMI) \> 32 kg/m2 for healthy participants; \> 40 kg/m2 for DKD participants. 3. Use of any systemic immunosuppressant medications, medications to treat diabetes (except DKD participants), antipsychotics, anticoagulants, or other prescription medications other than contraceptives within 90 days of Screening that, as determined by the Investigator, could confound their participation in the study. 4. Received a vaccine within 30 days of Screening. 5. Cancer or a history of cancer or lymphoproliferative disorder within 5 years of Screening except for adequately treated non-melanoma cancers of the skin and cervical carcinoma in situ. 6. Current infection with hepatitis B, hepatitis C, or human immunodeficiency virus (HIV) as evidenced by a positive hepatitis B surface antigen (HbsAg) test at Screening or a positive HIV test at Screening. For hepatitis C, a positive hepatitis C antibody (antiHCV) test is exclusionary unless the participant has previously been treated for hepatitis C, in which case they would be eligible with a negative HCV ribonucleic (RNA) test. 7. Currently pregnant, lactating, or planning to conceive or contribute to pregnancy during the trial and up to 65 days (for participants of childbearing potential) or 125 days (for males) after the participant's last dose of study drug, if applicable. 8. History of severe depression, psychosis, or suicidal ideation, as determined by the Investigator, within 5 years of Screening. 9. History of anaphylaxis or other significant allergies in the opinion of the Investigator. 10. History of substance use disorder as specified in Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, within 1 year of Screening. 11. Positive screen for drugs of abuse or alcohol at Screening or admission to the CRU (Day -1). 12. Donation of blood within 28 days of Screening. 13. Any clinically significant disease or laboratory abnormality detected at Screening, including diabetes mellitus, that might interfere with a participant's ability to complete the study, on-study evaluations, or participant safety, including the following: * Haemoglobin \< 115 g/L (female) or \< 135 g/L (male); \> 160 g/L (female) or \> 180 g/L (male) * Absolute neutrophils \< 2.0 × 109/L or \> 7.5 × 109/L * White blood cells \< 4.0 × 109/L or \> 11.0 × 109/L * Platelet count \< 150 × 109/L or \> 450 × 109/L * Any clinically significant abnormality on any of the Screening ECGs * Alanine aminotransferase or aspartate aminotransferase greater than 1.2 times the upper limit of normal. 14. Any surgical procedure, including planned procedures within 12 weeks of Screening. 15. Known allergy to MTX-439 or any of its excipients. 16. Participation in another research study of an investigational agent within 30 days of Screening or 5 half-lives of the agent, whichever is longer.