A 28-year-old man is brought to the emergency department after a motor vehicle collision. He was the unrestrained driver. On arrival, his blood pressure is 95/62 mmHg, heart rate 118/min, respiratory rate 24/min. Abdominal examination reveals severe tenderness and distension in the left upper quadrant with guarding. FAST (Focused Assessment with Sonography for Trauma) shows free fluid in the left upper quadrant. A CT scan of the abdomen with IV contrast demonstrates a laceration of the spleen with active extravasation of contrast. The patient remains haemodynamically unstable despite 2 units of packed RBCs. What is the most appropriate next step in management?