• To evaluate safety and determine the maximum tolerated dose (MTD) or recommended phase 2 dose (RP2D) [ Time Frame: 21 day cycle ]
  Evaluation of safety of MT-5111 as measured by number of subjects with adverse events using Common Terminology Criteria for Adverse Events (CTCAE) v 5.0
• To evaluate tolerability and determine the maximum tolerated dose (MTD) or recommended phase 2 dose (RP2D) [ Time Frame: 21 day cycle ]
  Evaluation of tolerability of MT-5111 as measured by number of subjects with dose limiting toxicities (DLTs)

• To evaluate safety and determine the maximum tolerated dose (MTD) or recommended phase 2 dose (RP2D) [ Time Frame: 21 day cycle ]
  Evaluation of safety of MT-5111 measured by number of subjects with adverse events using Common Terminology Criteria for Adverse Events (CTCAE) v 5.0
• To evaluate tolerability and determine the maximum tolerated dose (MTD) or recommended phase 2 dose (RP2D) [ Time Frame: 21 day cycle ]
  Evaluation of tolerability of MT-5111 measured by number of subjects with dose limiting toxicities (DLTs)

• PK as measured by concentrations of free MT-5111 (Maximum Plasma Concentration [Cmax]) [ Time Frame: Day 1, Day 8 (cycle 1 only), and Day 15 (cycle 1 only) in Each 21-Day cycle ]
  Evaluation of the pharmacokinetic profile of MT-5111
• PK as measured by concentrations of free MT-5111 (Time to reach maximum concentration after drug administration [Tmax]) [ Time Frame: Day 1, Day 8 (cycle 1 only), and Day 15 (cycle 1 only) in Each 21-Day cycle ]
  Evaluation of the pharmacokinetic profile of MT-5111
• PK as measured by concentrations of free MT-5111 (Area Under the Curve [AUC]) [ Time Frame: Day 1, Day 8 (cycle 1 only), and Day 15 (cycle 1 only) in Each 21-Day cycle ]
  Evaluation of the pharmacokinetic profile of MT-5111
• To evaluate the tumor response to MT-5111 [ Time Frame: Screening, approximately every 6 weeks, at End of Treatment and 30 days after the last dose ]
  Objective response rate (ORR) defined as the proportion of subjects with either a complete response or a partial response as determined by investigator assessment
• To evaluate the immunogenicity of MT-5111 [ Time Frame: Screening (baseline), Day 1 of each 21 day cycle, at the End of Treatment and the Follow-up Visit ]
  Immunogenicity as measured by MT-5111 (anti-drug antibody [ADA] titer)
• To evaluate the immunogenicity of MT-5111 [ Time Frame: Screening (baseline), Day 1 of each 21 day cycle, at the End of Treatment and the Follow-up Visit ]
  Immunogenicity as measured by MT-5111 (neutralizing antibody [NAb])

• PK as measured by concentrations of free MT-5111 (Maximum Plasma Concentration [Cmax]) [ Time Frame: Day 1, Day 8 (cycle 1 only), and Day 15 (cycle 1 only) in Each 21-Day cycle ]
  Evaluation of the pharmacokinetic profile of MT-5111
• PK as measured by concentrations of free MT-5111 (Time to reach maximum concentration after drug administration [Tmax]) [ Time Frame: Day 1, Day 8 (cycle 1 only), and Day 15 (cycle 1 only) in Each 21-Day cycle ]
  Evaluation of the pharmacokinetic profile of MT-5111
• PK as measured by concentrations of free MT-5111 (Area Under the Curve [AUC]) [ Time Frame: Day 1, Day 8 (cycle 1 only), and Day 15 (cycle 1 only) in Each 21-Day cycle ]
  Evaluation of the pharmacokinetic profile of MT-5111
• To evaluate the tumor response to MT-5111 [ Time Frame: Screening, approximately every 6 weeks, at End of Treatment and 30 days after the last dose ]
  Objective response rate (ORR) defined as the proportion of subjects with either a complete response or a partial response as determined by investigator assessment
• To evaluate the immunogenicity of MT-5111 [ Time Frame: Sreening (baseline), Day 1 of each 21 day cycle, at the End of Treatment and the Follow-up Visit ]
  Immunogenicity as measured by MT-5111 (anti-drug antibody [ADA] titer)
• To evaluate the immunogenicity of MT-5111 [ Time Frame: Sreening (baseline), Day 1 of each 21 day cycle, at the End of Treatment and the Follow-up Visi ]
  Immunogenicity as measured by MT-5111 (neutralizing antibody [NAb)

• To correlate the pharmacodynamic markers of cancer under study (for breast cancer subjects using historic data, if available) [ Time Frame: Screening (baseline) ]
  The expression of HER2, Estrogen Receptor (ER), Progesterone Receptor (PgR) and Ki67 (exploratory) on the tumor cell analyzed by immunohistochemistry
• To correlate the pharmacodynamic markers of cancer under study relationship for MT-5111 using the PK, pharmacodynamics, safety, and tumor response variables. [ Time Frame: Screening (baseline), every 6 weeks (± 1 week) and within 7 days of the EoT Visit. ]
  Serum-HER2 (s-HER2)
• If warranted by the study results, to evaluate the exposure-response relationship for MT-5111 [ Time Frame: Screening (baseline), every 6 weeks (± 1 week) and within 7 days of the EoT Visit. ]
  Analyze data collected for all primary, secondary and exploratory endpoints using the PK, pharmacodynamics, safety, and tumor response variables

• To correlate the pharmacodynamic markers of cancer under study (for breast cancer subjects using historic data, if available) [ Time Frame: Sreening (baseline) ]
  The expression of HER2, Estrogen Receptor (ER), Progesterone Receptor (PgR) and Ki67 (exploratory) on the tumor cell analyzed by immunohistochemistry
• To correlate the pharmacodynamic markers of cancer under study relationship for MT-5111 using the PK, pharmacodynamics, safety, and tumor response variables. [ Time Frame: Sreening (baseline), every 6 weeks (± 1 week) and within 7 days of the EoT Visit. ]
  Serum-HER2 (s-HER2)
• If warranted by the study results, to evaluate the exposure-response relationship for MT-5111 [ Time Frame: Sreening (baseline), every 6 weeks (± 1 week) and within 7 days of the EoT Visit. ]
  Analyze data collected for all primary, secondary and exploratory endpoints using the PK, pharmacodynamics, safety, and tumor response variables

This study will be conducted in two sequential parts:

• Part 1 (Dose Escalation): The purpose of Part 1 is to determine the Phase 2 dose (RP2D) to be used in Part 2. Part 1 will include any type of HER2-positive solid cancer.
• Part 2 (Dose Expansion): The purpose of Part 2 is to confirm the safety and tolerability of the RP2D of MT-5111. Part 2 will include any type of HER2-positive solid cancer, including breast cancer, gastric or gastroesophageal adenocarcinomas (GEA).

Up to 140 eligible subjects will be identified and treated through competitive enrollment at multiple study centers.

In Parts 1 and 2 of the study, a subject may participate for the following four periods:

• Screening (up to 28 days before first dose of MT-5111)
• Treatment period (active period where a subject will receive doses of MT-5111 over a 21-day treatment cycle)
• Follow-up (30 days after last dose of MT-5111)
• Long-term follow-up (up to 24 months after the last dose of MT-5111)

MT-5111 will be given as an intravenous (IV) infusion over about 30 minutes on the same day every week (i.e., on day 1, day 8 and day 15 of each cycle, a cycle being defined as 21 days). A subject can continue receiving MT-5111 as long as it is well-tolerated, their disease has not worsened, or until the subject decides they no longer want to participate in the study.

• Experimental: Arm 1- Dose Escalation

  The dose escalation part of the study is aimed at determining the Recommended Phase 2 Dose (RP2D) of MT-5111.

  The assigned dose level of MT-5111 will be given as an intravenous (IV) infusion over about 30 minutes on the same day every week (i.e., on day 1, day 8 and day 15 of each cycle).

  Intervention: Drug: MT-5111 (experimental study drug)
• Experimental: Arm 1- Dose Expansion

  The dose expansion part of the study will begin after completion of the dose escalation phase to confirm the safety and tolerability of the RP2D.

  The RP2D dose level of MT-5111 will be given as an intravenous (IV) infusion over about 30 minutes on the same day every week (i.e., on day 1, day 8 and day 15 of each cycle).

  Intervention: Drug: MT-5111 (experimental study drug)

Inclusion Criteria:

1. Histologically confirmed, unresectable, locally advanced or metastatic solid cancers:

   • Part 1 (Dose-Escalation): All HER2-positive solid cancers are eligible
   • Part 2 (Dose-Expansion): Any type of HER2-positive solid cancer, including breast cancer, gastric or gastroesophageal adenocarcinomas (GEA).
2. HER2-positive in the latest tumor sample tested for HER2 (testing to be done on a metastatic lesion in cases of metastatic cancers).
3. Relapsed or refractory to or intolerant of existing therapy(ies)
4. At least 1 measurable or evaluable lesion according to RECIST 1.1
5. ECOG performance score of ≤ 1

6. Bone marrow function:

   • Absolute neutrophil count (ANC) ≥ 1,000/mm3
   • Platelet count ≥ 75,000 mm³ and
   • Hemoglobin ≥ 8.0 g/dL
   • No red blood cell transfusion within 4 weeks of study treatment start is allowed

7. Kidney function:

   • (eGFR) ≥ 50 mL/min calculated by the Cockcroft-Gault formula
   • Subjects with CLcr ≥ 50 mL/min will be eligible irrespective of the eGFR result

8. Cardiac Function:

   • Left ventricular ejection fraction (LVEF) ≥ 55% on the multigated acquisition (MUGA) scan (preferred) or echocardiogram (ECHO) assessment, and
   • QTcF ≤ 480 ms for women and QTcF ≤ 450 ms for men [average from three QTcF values on the triplicate 12-lead electrocardiogram (ECG)] at baseline

9. Hepatic function:

   1. Total bilirubin ≤ 1.5 x ULN, and

   2. AST ≤ 3 x ULN and ALT ≤ 3 x ULN

      • < 5 x ULN (if hepatic metastases)

Exclusion Criteria:

1. History or current evidence of another tumor that is histologically distinct from the tumor under study

2. Current evidence of new or growing CNS metastases during screening

   -Subjects with known CNS metastases will be eligible if they meet specified criteria

3. Evidence of CTCAE Grade >1 toxicity before the start of treatment, except for hair loss and those Grade 2 toxicities listed as permitted in other eligibility criteria
4. History or evidence of significant cardiovascular disease
5. Current evidence of active, uncontrolled hepatitis B virus, hepatitis C virus, human immunodeficiency virus (HIV) (evidenced by detectable viral load by PCR) or acquired immunodeficiency syndrome (AIDS) related illness
6. Current evidence of ≥ grade 2 underlying pulmonary disease
7. Certain exclusionary prior treatments