• Dose limiting toxicity (DLT) within 28 days of first single dose of FS-1502. [ Time Frame: At the end of Cycle 1 (each cycle is 28 days). ]
  To evaluate dose limiting toxicity (DLT) within 28 days from first dose according to DLT criteria per protocol
• Overall response rate (ORR) by measurement of target lesions [ Time Frame: Approximately 2 years. ]
  Assess ORR per 2 cycles according to RECIST 1.1 criteria

• Incidence Rate and type of treatment emergent adverse event (Safety) [ Time Frame: Approximately 3 years. ]
  Evaluate according to CTCAE version 5.0.
• Number of Participants with serious adverse events and adverse reactions that leading to treatment permanent discontinuation of FS1502. [ Time Frame: Approximately 2 years. ]
• Incidence of deaths and causes within 30 days after the last dose. [ Time Frame: Approximately 3 years. ]
• Progression free survival (PFS) [ Time Frame: Approximately 2 years. ]
• Overall survival (OS) [ Time Frame: Approximately 3 years. ]
• 1-year overall survival (OS) rate [ Time Frame: 1 years ]
• Duration of response (DoR) [ Time Frame: 2 years ]
  Length of time response continued
• Clinical benefit response (CBR) [ Time Frame: 2 years ]
  Percentage of participants achieve clinical benefit (complete response (CR), partial response (PR) or stable disease (SD) more than 6 months) from the treatment.
• Area under the plasma concentration versus time curve (AUC) [ Time Frame: Approximately 2 years. ]
  Measure the AUC of FS-1502 and it metabolites (MMAF).
• Peak Plasma Concentration (Cmax) [ Time Frame: Approximately 2 years. ]
  Measure the Cmax of FS-1502 and it metabolites (MMAF).
• Time to maximum concentration (tmax) [ Time Frame: Approximately 2 years. ]
  Measure the tmax of FS-1502 and it metabolites (MMAF).
• Elimination half life (T1/2) [ Time Frame: Approximately 2 years. ]
  Measure the T1/2 of FS-1502 and it metabolites (MMAF).
• Clearance (Cl) [ Time Frame: Approximately 2 years. ]
  Measure the Clearance of FS-1502 and it metabolites (MMAF).
• Anti-FS-1502 antibody [ Time Frame: Approximately 2 years. ]
  Measure the anti-FS-1502 antibody to demonstrade the immunogenicity of FS-1502.

• Incidence Rate and type of treatment emergent adverse event (Safety) [ Time Frame: Approximately 3 years. ]
  Evaluate according to CTCAE version 5.0.
• Number of Participants with serious adverse events and adverse reactions that leading to treatment permanent discontinuation of FS1502. [ Time Frame: Approximately 2 years. ]
• Incidence of deaths and causes within 30 days after the last dose. [ Time Frame: Approximately 3 years. ]
• Progression free survival (PFS) [ Time Frame: Approximately 2 years. ]
• Overall survival (OS) [ Time Frame: Approximately 3 years. ]
• 1-year overall survival (OS) rate [ Time Frame: 1 years ]
• Duration of response (DoR) [ Time Frame: 2 years ]
  Length of time response continued
• Clinical benefit response (CBR) [ Time Frame: 2 years ]
  Percentage of participants achieve clinical benefit (complete response (CR), partial response (PR) or stable disease (SD) more than 6 months) from the treatment.
• Area under the plasma concentration versus time curve (AUC) [ Time Frame: Approximately 2 years. ]
  Measure the AUC of FS-1502 and it metabolites (MAFF).
• Peak Plasma Concentration (Cmax) [ Time Frame: Approximately 2 years. ]
  Measure the Cmax of FS-1502 and it metabolites (MAFF).
• Time to maximum concentration (tmax) [ Time Frame: Approximately 2 years. ]
  Measure the tmax of FS-1502 and it metabolites (MAFF).
• Elimination half life (T1/2) [ Time Frame: Approximately 2 years. ]
  Measure the T1/2 of FS-1502 and it metabolites (MAFF).
• Clearance (Cl) [ Time Frame: Approximately 2 years. ]
  Measure the Clearance of FS-1502 and it metabolites (MAFF).
• Anti-FS-1502 antibody [ Time Frame: Approximately 2 years. ]
  Measure the anti-FS-1502 antibody to demonstrade the immunogenicity of FS-1502.

This study is being done for the following reasons:

• The study has two parts. The purpose of the first part (Phase 1a) of the study is to find out the highest dose of FS-1502 (Trastuzumab Monomethyl Auristatin F), an Anti-HER2 Antibody Drug Conjugate (ADC) that can be given safely.
• The purpose of the second part of the study (Phase 1b) is to observe the treatment effect of Recommended Phase 2 Dose (RP2D) of FS-1502 in patients with HER2+ breast cancer who are relapsed or refractory to the previous anti-HER2 therapy.

• Solid Tumor
• Breast Cancer

Inclusion Criteria:

1. Age ≥18 years and ≤ 70 years at the time of study registration (men and women eligible);

2. Phase Ia dose-escalation study:

   Patients with HER2 expressed advanced malignant solid tumor had failed to standard therapy (including surgery, chemotherapy, radiation therapy or biotherapy), or no standard therapy is available.

   1. HER2 overexpression: IHC3+, IHC2+/FISH+, or FISH+
   2. HER2 low expression: IHC1+, IHC2+/FISH-

   Phase Ib dose-expanded study:

   Histologically or cytologically confirmed breast cancer patients who have failed to prior trastuzumab treatment. Including patients with locally advanced or metastatic breast cancer and those who have relapsed after standard adjuvant chemotherapy (treatment for more than 3 months). Details as follows:

   1. HER2 positive (defined as IHC3+ or IHC2+/FISH+);
   2. At least one standard trastuzumab treatment or other biosimilar has been received and the treatment failed.
   3. Provide evidence of disease progression or intolerable toxicity as confirmed by the investigator or medical history recorded prior to enrollment.

   The enrollment can be based on written HER2 test report from certified local lab, but sufficient paraffin sections or fresh tumor tissue specimens must be provided to the central laboratory for confirmation.

3. The ECOG performance status must be 0 or 1.
4. Expected survival for at least 12 weeks.
5. Has adequate organ and bone marrow function: absolute neutrophil count (ANC) ≥ 1.5x109/L; hemoglobin ≥ 90g/L (without red blood cell infusion within 14 days); platelet count ≥ 100x109/L; Total bilirubin ≤ 1.5x upper limit normal (ULN), or ≤ 3x ULN if with Gilbert syndrome; aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 2.5x ULN; AST and ALT ≤ 5x ULN if liver metastasis; Serum creatinine < 1.5x ULN and creatinine clearance ≥ 60mL/min (Cockroft-Gault formula calculation); albumin ≥ 3g/dL; left ventricular ejection fraction (LVEF) >50%.
6. Has at least one measurable lesion by RECIST version 1.1.
7. Male or female patients with fertility must agree to use effective contraceptive methods during the study period and within 30 days of the last dose of study therapy, such as dual barrier contraceptive methods, condoms, oral or injectable contraceptives, and intrauterine devices.
8. Ability to understand and voluntarily sign written informed consent.

Exclusion Criteria:

1. Patients who received chemotherapy, radiotherapy, major surgery or other anti-tumor treatment within 4 weeks prior to the start of dosing.
2. Patients who have participated in other clinical trials 4 weeks before the start of study drug administration or within 5 drug half-life periods; patients who have received similar treatments.
3. Uncontrolled central nervous system metastasis or injury (Patients who have been treated with gamma knife and have a stable disease for more than 3 months are allowed to enroll).
4. Patients with uncontrolled diabetes mellitus (Patients who are receiving an insulin regimen or a hypoglycemic regimen and are evaluated as having good glycemic control by a specialist are allowed to enroll).
5. Has not recovered from previous anti-tumor treatment-related toxic reactions (> NCI-CITCAE 5.0 Grade 2), except for hair loss. The neurotoxicity of patients who have previously received chemotherapy needs to be restored to NCI-CTCAE 5.0 level 2 or below.
6. Patients who have used potent CYP3A inhibitors within 14 days prior to the start of dosing (Boprevir, cobaster, cognac, dannoprevir and ritonavir (j), ethiravir and ritonavir (j), indinavir and ritonavir ( j), posaconazole and ritonavir (j), lopinavir and ritonavir (j), saquinavir and ritonavir (j), telanavir and ritonavir (j), parylene and ritonavir and (obivir and or dasab) (j), grapefruit, itraconazole, ketoconazole, telaprevir, vinegar Orchard, voriconazole, clarithromycin, diltiazem, aldelis, nefazodone, nelfinavir) or strong inducer (carbamazepine, enzalutamide, mitoxantrone, phenytoin, rifampicin, St. John's wort, etc.); P-gp transporter inducer or inhibitor; (j) indicates inhibition of CYP3A when in combination with ritonavir.
7. Patients who are taking medications that prolong the QTc interval (mainly Ia, Ic, and III antiarrhythmic drugs) or patients with risk factors for QTc interval prolongation, such as uncorrectable hypokalemia, hereditary long QT syndrome. Drugs that potentially prolong the QTc interval can be found at https://crediblemeds.org/index.php/tools/pdfdownload?f=cql_en

8. Cardiac function and disease meeting one of the following conditions:

   1. Three 12-lead electrocardiogram (ECG) measurements at the research center during the screening period. Calculate the average of three measurements based on the QTc formula, QTc > 470 milliseconds.
   2. New York Heart Association (NYHA) graded ≥3 congestive heart failure.
   3. Clinically significant arrhythmias, including but not limited to complete left bundle branch conduction abnormality, II degree atrioventricular block.
9. Pregnant or lactating woman.
10. Allergic to any excipients of FS-1502.
11. Clinically significant active bacterial, fungal or viral infections, including hepatitis B (hepatitis B virus surface antigen positive and hepatitis B virus DNA over 1000 IU/ml) or hepatitis C (hepatitis C virus RNA positive), human immunodeficiency virus infection ( HIV positive).
12. Any other disease or condition of clinical significance (eg, active or uncontrollable infection, etc.) considered by investigator that may affect protocol compliance or affect patient signature of ICF.