A Study to Evaluate GEH200520/GEH200521 (18F) Safety and Tolerability When Used for PET Scans in Patients With Solid Tumour Malignancies

Inclusion Criteria: * The subject is able and willing to comply with all study procedures as described in the protocol, including the imaging day pre-visit requirements, and has read, signed, and dated an informed consent form prior to any study procedures being performed. * The subject is male or female, ≥18 years of age. * Subject has a life expectancy ≥12 weeks. * Subject has Eastern Cooperative Oncology Group (ECOG) performance status 0-1. * Subject has an unresectable or metastatic solid tumour or a local and resectable head and neck squamous cell carcinoma, or an unresectable stage III-IV melanoma. * Subject is eligible for ICI treatment per Investigator judgement. * Subject has at least 1 measurable tumour lesion documented on CT/magnetic resonance imaging (MRI) RECIST v1.1 during the last 12 months. * Subject has a tumour lesion(s) of which a biopsy can safely be obtained according to standard clinical care procedures. * Subject is male or female that agrees to adhere to the protocol contraception methods. Exclusion Criteria: * Subject is unable to undergo all procedures in the study and/or is unable to remain still and tolerate the imaging procedure. * Subject has 12-lead ECG significant findings during screening, per Investigator's assessment. * Subject is not stable due to medical condition or therapy that, in the opinion of the Investigator, could compromise subject safety or protocol objectives. * Subject has active autoimmune disease or a documented history of autoimmune disease or syndrome that requires systemic steroids or immunosuppressive agents. * Subject has serious non-malignant disease or conditions that, in the opinion of the Investigator, could compromise subject safety or protocol objectives. * Subject has B or T cell lymphoma. * Subject has brain or bone-marrow metastasis that, in the opinion of the Investigator, could compromise subject safety or protocol objectives. * Subject has signs or symptoms of systemic infection within 2 weeks prior to imaging day. * Subject has history of severe allergic, anaphylactic, or other hypersensitivity reactions to chimeric or humanised antibodies or fusion proteins or known allergy to the study IMP ingredients and/or the proposed ICI therapy. * Subject has any other diseases, metabolic dysfunction, physical examination finding, or clinical laboratory finding giving reasonable suspicion of a disease or condition that contraindicates the use of the ICI treatment, or that may affect the interpretation of the results or render the subject at high risk from complications. * Subject has laboratory values out of range per protocol. * Subject has any safety laboratory test results (blood chemistry, haematology, and urinalysis) that, in the opinion of the Investigator, could compromise subject safety or protocol objectives. * Subject has had any major surgery within 4 weeks prior to enrollment. * Subject has been enrolled in another interventional clinical study within the 30 days before screening for this study, except for the study site IIS. * Subject is pregnant or planning to become pregnant or is breastfeeding. * Subject has a history of alcohol or drug abuse within the last year. * Subject has had treatment with systemic immunostimulatory agents within 6 weeks or 5 half-lives of the drug, whichever is shorter, prior to dosing with the IMP. * Subject has had treatment with systemic immunosuppressive medications within 2 weeks prior to dosing with the IMP. * Subject has received acute, low-dose, systemic immunosuppressant medications that, in the opinion of the Investigator, could compromise protocol objectives. * Subject has used systemic corticosteroids to treat inflammatory or autoimmune symptoms.