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.