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When tissue plasminogen activator (t-PA) was approved in 1987 as an intravenous thrombolytic agent, it appeared to be a tough competitor for streptokinase, the reigning therapy. While t-PA was, and still is, much more expensive -- a typical 100-mg dose today costs about $2200, versus about $200 for the corresponding 1.5 million units of streptokinase -- several of its properties seemed to give it an advantage.
First, because it is a human protein, t-PA was thought less likely to induce severe allergic reactions than streptokinase (a bacterial protein), particularly on repeat administration. Second, t-PA has a more specific affinity for fresh clots, suggesting that it would be less likely to produce unwanted bleeding by lysing old clots elsew…