The purpose of this study is to determine the safety, tolerability and recommended phase 2 dose of a new drug, known as IMP321, in combination with pembrolizumab when given to patients with unresectable or metastatic melanoma.

Main Inclusion Criteria

• Histologically confirmed diagnosis of locally advanced (unresectable Stage III) or metastatic (Stage IV) melanoma
• Currently receiving anti-PD-1 therapy with pembrolizumab and after 3 cycles achieved asymptomatic irPD (slowly progressive, not requiring urgent intervention, and stable performance status) or sub-optimal response (irSD, irPR) as demonstrated in imaging assessments performed within 6 weeks prior to study start
• Female or male 18 years of age or above
• ECOG performance status 0-1
• Evidence of measurable disease as defined by Response Evaluation Criteria in Solid Tumours (RECIST) version 1.1 10. Adequate Laboratory criteria

Main Exclusion Criteria

• More than four prior lines of therapies for advanced or metastatic disease.
• Prior PD-1/PDL-1 targeted therapy
• Currently receiving treatment with another investigational drug, or less than 4 weeks since ending treatment on another investigational drug
• Currently receiving systemic chemotherapy, targeted small molecule therapy, radiotherapy, or biological cancer therapy (other than pembrolizumab) or less than 4 weeks since completion of these therapies and first dose of study treatment
• History of irAEs from ipilimumab of CTCAE Grade 4 requiring steroid treatment
• Known cerebral or leptomeningeal metastases
• Serious intercurrent infection within 4 weeks prior to first dose of study treatment
• Active acute or chronic infection
• History or evidence of interstitial lung disease or active non-infectious pneumonitis
• Active auto-immune disease requiring immunosuppressive therapy
• HIV positivity, active hepatitis B or hepatitis C
• Continuous systemic treatment with either corticosteroids or other immunosuppressive medications within 4 weeks prior to first dose of study treatment