Alteplase binds to fibrin in a thrombus and converts the entrapped plasminogen to plasmin. It also produces limited conversion of plasminogen in the absence of fibrin.
Jennings, L.K., & Saucedo, J.F. (2008). Antiplatelet and anticoagulant agents: key differences in mechanisms of action, clinical application, and therapeutic benefit in patients with non-ST-segment-elevation acute coronary syndromes. Current Opinion in Cardiology, 23(4), 302-308. PMID: 18520712
Walker, C.P., & Royston, D. (2002). Thrombin generation and its inhibition: a review of the scientific basis and mechanism of action of anticoagulant therapies. British Journal of Anaesthesia, 88(6), 848-863. PMID: 12173205