The goals of this protocol are 1) to investigate the safety and tolerability of X4P-001 in combination with Keytruda® (pembrolizumab) in patients with advanced melanoma, and 2) to assess serial biopsies of melanoma tumor lesions obtained throughout the study for inflammatory and tumor cell infiltrates. After completion of study treatment, patients with resectable disease will undergo surgery, unresectable patients may continue on pembrolizumab as standard of care.