Biochemistry

Tenecteplase (TNK-tPA, Metalyse^®) is a 527 amino acid glycoprotein produced by modifying the complementary DNA (cDNA) for natural human t-PA at three key sites using an established mammalian cell line (Chinese Hamster Ovary Cells). Hepatic elimination is the main clearance mechanism. Natural human t-PA has a modular structure consisting of five domains: a "finger" domain, an epidermal growth factor region, two "kringle" structures, and a serine protease domain.

In comparison with human t-PA Tenecteplase (TNK-tPA, Metalyse^®) is modified by:

• substitution of threonine-103 by asparagine, adding a new glycosylation site on kringle-1, which decreases the rates of clearance and fibrin binding.
• substitution of asparagine-117 by glutamine, removing the existing glycosylation on kringle-1, decreasing the rate of clearance and restores fibrin binding in combination with Thr-103 by ASN.
• substitution of one lysine, one histidine and two arginines by four alanines at positions 296-299, increasing fibrin specificity and giving higher resistance to the naturally occurring inhibitor PAI-1.

These key site modifications have been shown in animal models to bring the following advantages in comparison to wild type plasminogen activator:

• a 4-8-fold slower plasma clearance in 5 animal species.
• a 14-fold higher fibrin specifity than that of alteplase.
• an 80-fold greater resistance to inhibition by plasminogen activator inhibitor 1 (PAI-1), which is secreted by activated platelets.
• clot lysis: arterial venous shunt models in rabbits indicate that bolus tenecteplase induces 50% lysis in one-third the time required by alteplase infusion.
• platelet aggregation: at the doses examined, tenecteplase produces little or no potentiation of collagen-sensitised platelet aggregation, suggesting a decreased risk of reocclusion after thrombolysis.

In vivo performance:

• in rabbits with thrombotically occluded arteries, tenecteplase produced faster and more complete recanalisation than alteplase without increasing systemic plasmin generation or peripheral bleeding.