After an interim analysis of the data, it became clear that the imaging results did not correlate to the primary outcome measures in this subset of patients.

T cell infiltration of tumor lesions is a known prognostic factor in several tumor types and is used as treatment mechanism in some of these tumor types. In metastatic melanoma, treatment with immune checkpoint inhibitors induces clinical benefit in about 30-50% of the patients. These immune-based therapies are however accompanied by serious immune-related adverse events and high costs.

Tumor infiltrating T cells express the high affinity interleukin-2 (IL2) receptor on their surface. These T cells could therefore be visualized by molecular imaging with a radio-labelled ligand for this receptor. For this purpose, the investigators have developed the PET tracer [18F]FB-IL2.

The study commences with a biodistribution study (phase 1) in 5 subjects. Thereafter the main study (phase 2) starts, in which 25 subjects will receive two [18F]FB-IL2 PET scans at baseline and week 6 of treatment with either ipilimumab, nivolumab, pembrolizumab or the combination of ipilimumab and nivolumab. If [18F]FB-IL2 PET is able to detect a response to treatment, it could serve as a non-invasive early indicator of T cell response to the treatment. Besides, accumulation of the PET tracer in non-target tissue could indicate infiltration of activated T cells in normal organs and thus may predict the development of an immune-related adverse event.

Inclusion Criteria:

1. Has signed informed consent.
2. ≥18 years of age.
3. Histologically confirmed cutaneous metastatic melanoma (Stage IV).
4. Evidence of at least one measurable metastatic lesion based on RECIST version 1.1.
5. At least one easy accessible metastatic melanoma lesion, of which biopsy can be performed.
6. Eligible for treatment with ipilimumab, nivolumab, pembrolizumab, or the combination of ipilimumab and nivolumab.
7. No contraindication for performing a CT scan.
8. Eastern Cooperative Oncology Group (ECOG) Performance Status of 0-1.
9. Women with child-bearing potential and men with reproductive potential must be willing to practice acceptable methods of birth control during the study.
10. Must have adequate organ function (e.g. liver, kidney) as defined

Exclusion Criteria:

1. Pre-existing auto-immune disease, which could be exacerbated by ipilimumab (e.g. Crohn, Hashimoto's Thyroiditis).

2. Presence of malignancy other than the disease under study within 5 years of study enrolment. Subjects with a history of completely resected non-melanoma skin cancer or successfully treated in situ carcinoma are eligible.

3. Brain metastases that are symptomatic or not stable for 8 weeks (must be documented by imaging).

4. The use of corticosteroids (at the start of treatment). Note: Corticosteroids are allowed during the study for immune-related toxicity of immunotherapy, as this will not interfere with activity of immunotherapy.

5. Evidence of active infection requiring antibiotic therapy at start of treatment.

6. Current use of a prohibited medication or requirement of any of these medications during treatment with immune-checkpoint inhibitors as mentioned in the summary of product characteristics (SPC) for Yervoy, Opdivo, and Keytruda.

7. Known immediate or delayed hypersensitivity reaction to ipilimumab, nivolumab or pembrolizumab or excipients.

8. Unresolved toxicity of National Cancer Institute Common Terminology Criteria for Adverse Events, version 4.0. Grade 2 or higher from previous anti-cancer therapy, except alopecia.

9. A history or evidence of cardiovascular risk including any of the following:

   1. A history or evidence of current clinically significant uncontrolled arrhythmias;
   2. A history of acute coronary syndromes (including myocardial infarction or unstable angina), coronary angioplasty, or stenting within 6 months prior to randomization.
   3. A history or evidence of current ≥Class II congestive heart failure as defined by the New York Heart Association (NYHA) guidelines.
   4. Abnormal cardiac valve morphology (≥grade 2) documented by echocardiogram (subjects with grade 1 abnormalities [i.e., mild regurgitation/stenosis] can be entered on study). Subjects with moderate valvular thickening should not be entered on study.
   5. Presence of cardiac metastases.

10. Any serious or unstable pre-existing medical conditions (i.e. diabetes mellitus, hypertension, etc), psychological, familial, sociological, or geographical conditions that do not permit compliance with the protocol; or unwillingness or inability to follow the procedures required in the protocol.

11. Altered mental status, or any psychiatric condition that would prohibit the understanding or rendering of informed consent.

12. Pregnant or nursing females.