APL-501 Study for Select Advanced or Relapsed/Recurrent Solid Tumors

Major Inclusion Criteria: • Able to understand and comply with study procedures, understand the risks involved, and provide written informed consent. Dose Escalation: * Histologically and / or cytological confirmed solid tumors: colorectal, endometrial, gastric including, gastroesophageal junction adenocarcinoma (GC), head and neck (esophageal, hepatocellular (HCC), non-small cell lung cancer, mesothelioma, ovarian, and renal cell carcinoma (RCC), either refractory or relapsed to standard therapy or for which no effective standard therapy is available. * No restriction to number of prior therapies for Dose Escalation Segment except for gastric and renal cell carcinoma. Cohort Extension: * Histologically and/or cytological confirmed solid tumors with an approved labelled indication for PD-1 inhibitors. * Tumor biopsy at study entry and during therapy. Tumor sites used to satisfy this criterion must not have received any prior radiation therapy. Sites for biopsy must be distinct from target lesions used for efficacy assessment. * Measurable disease according to RECIST v1.1. Dose and Disease Expansion: * MSI-H or dMMR per local laboratory and failed at least one prior line of standard of care chemotherapy per local standards. * Carcinoma of Unknown Primary Major Exclusion Criteria: * History of severe hypersensitivity to mAbs, excipients of the drug product or other components * Prior malignancy active within the previous 2 years except for locally curable cancers that have been cured, such as basal or squamous cell skin cancer, superficial bladder cancer or carcinoma in situ of the cervix or breast * Any active autoimmune disease or a documented history of autoimmune disease, or history of syndrome that required systemic steroids or immunosuppressive medications, except for subjects with vitiligo or resolved childhood asthma/atopy * Prior therapy with an anti-PD-1, anti-PD-L1, anti-PDL-2, or anti-CTLA-4 antibody (or any other antibody targeting T cell co-stimulation pathways) (except NSCLC) * Known significant mental illness or other conditions such as active alcohol or other substance abuse that, in the opinion of the Investigator, predisposes the subject to high risk of noncompliance with the protocol treatment or assessments.