A Clinical Study of PA5 in Patients With Advanced Solid Tumors

The goal of this clinical trial is to learn if PA5 is safe and works to treat advanced solid tumors in adults. It will also learn about the tolerability and PK/PD profile of PA5. The main questions it aims to answer are: * Is intravenous infusion of PA5 monotherapy safe and tolerable for patients with advanced/metastatic solid tumors? * What is the maximum tolerated dose (MTD) and recommended dose for Phase II clinical trials (RP2D) of PA5 monotherapy? In the escalation phase, participants will receive PA5 via intravenous infusion on Day 1 of Cycle 1 (28-day cycle). If no dose-limiting toxicity (DLT) occurs, treatment continues from Cycle 2 onward with adjusted frequency (every 2-4 weeks). In the expansion phase, participants will receive PA5 with the frequency determined based on the results of the escalation phase.