Key eligibility: Adults (≥18) with progressive advanced sarcoma/prostate/breast/ovarian/pancreatic cancers (ECOG ≤2, life expectancy ≥6 months) post ≥1 prior therapy. Requires ACT Tumor Board recommendation, adequate organ function (ANC ≥1,500/uL, platelets ≥100,000/uL, bilirubin ≤1.5×ULN), and measurable disease. Major exclusions: active secondary malignancies, untreated CNS metastases, uncontrolled comorbidities (NYHA III-IV heart failure, severe infections), pregnancy/breastfeeding, or conditions jeopardizing protocol compliance.

Administered in monotherapy or in combination with other targeted agents or immunotherapies, chemotherapies, or radiation. Combination treatment plans may include a two-week monotherapy lead-in, followed by a combination treatment regimen. Each ACT study intervention must have an established RP2D determined in a prior clinical trial. Participants undergo a Pre-Treatment Biopsy, plus an On-Treatment Biopsy after two weeks on first dose of study drug (s) and prior to starting Cycle 2, regardless of regimen. Participants continue to receive study agent (s) after the On-Treatment Biopsy, according to the biopsy results and the results of ongoing safety and clinical assessments. Treatment cycles repeat every 21 to 28 days in the absence of disease progression or unacceptable toxicity. Cycles are determined based on the study agent (s). Upon disease progression, participants are given the option to undergo an additional biopsy.

Key eligibility: Adults (≥18) with HER2+ (IHC3+/FISH+) metastatic breast cancer (1-3 prior lines, ECOG 0-1). Exclusions: prior HER2-ADC use, active CNS metastases (except stable treated lesions), uncontrolled effusions/ILD, significant cardiac disease, QTc risks, active infections (HBV/HCV/HIV), or pregnancy. Requires adequate organ function (ANC ≥1.0×10⁹/L, LVEF >50%) and measurable disease (RECIST 1.1).

Key eligibility: Adults (≥18) with untreated HER2+ (IHC3+) metastatic breast cancer (ECOG 0-1, LVEF ≥50%), no prior systemic therapy except THP within 6 weeks. Exclusions: prior invasive breast cancer treatment, uncontrolled cardiac/ILD conditions, active infections (unless controlled HIV/HBV/HCV), CYP2C8/3A4 modifiers use, or pregnancy. Requires adequate organ function and stable brain metastases if present.

Secondary objectives assess median Overall Survival (OS), Objective Response Rate (ORR by RECIST 1.1), Grade 3-5 treatment-related toxicity (CTCAE v5.0), completion rates of sequential therapy parts (A: taxane/trastuzumab/pertuzumab; B: trastuzumab deruxtecan; C: T-DM1/tucatinib; D: trastuzumab/pertuzumab/tucatinib), and DFS4 by Minimal Residual Disease status over 54 months.

Eligible patients (≥18 years) must have HER2+ disease (IHC3+ or IHC2+/FISH+), 1-3 prior lines (including adjuvant trastuzumab/taxane), ECOG 0-1, LVEF>50%, and adequate organ function. Key exclusions: prior HER2-ADC use, uncontrolled CNS metastases (unless stable ≥6 months post-treatment), QTc prolongation risks, active HBV/HCV/HIV, or unresolved toxicity >CTCAE G1 (except alopecia/stable G2 neuropathy).

Eligible patients (&ge;18 years) have HER2+ advanced cancers (IHC3+ or IHC2+/FISH+), ECOG 0-1, LVEF>50%, with &ge;2 prior anti-HER2 lines. Key exclusions: CNS metastases, recent major surgery/RT (<4 weeks), QTc >470ms, active HBV/HCV/HIV, or unresolved toxicity >CTCAE G1 (except stable G2 alopecia/neuropathy). Tissue confirmation is mandatory for pivotal study participants.

Phase Ia: Patients enrolled on the 1.2 and 2.0 regimens: FS-1502 monotherapy every 4 weeks with intravenous drip, 28 days as a cycle; Patients enrolled on the 3.0 regimens: starting from the 1.0mg/kg dose group, FS-1502 monotherapy every 3 weeks with intravenous drip, 21 days as a cycle; Phase Ib: FS-1502 monotherapy, the dose and frequency of administration for Stage Ib will be obtained according to Phase Ia (RP2D).