During the last years, many works have been carried out in order to modify genetically molecules with thrombolytic activity, used in the treatment of Acute Myocardial Infarction, with the objective of obtaining proteins with better properties such as: a longer period of medium life, higher specificity for fibrin and smaller antigenicity.

The Streptoquinase (SK) obtained from the beta-hemolytic streptococcus, constitutes a molecule which has a proved effectiveness that occupies 30% of thrombolytics' world market. Nevertheless, this protein has an inconvenience, the existence of anti SK circulating antibodies in most of normal individuals immunized by the frequent infections with streptococci. After the treatment with SK, the titers of anti SK antibodies rise considerably and they cause allergic reactions until in 20% of patients. These antibodies also produce the neutralization of the SK. All this also limits SK re-administration to persons who were previously treated, for periods which could last up to 4 years.

It is described in this work the location of SK antigenic epitopes and the generation by genetic ingeneering of an SK mutant, lacking of a very antigenic region. This new thombolytic was characterized as:

• Keeping its biological activity,
• Being less recognized and less neutralized by the serums of patients treated with SK,
• Being less antigenic in green African monkeys,

The results demonstrated that through modifications it can be achieved a mutant with lower antigenicity and they allowed gathering information for the subsequent development of clinical studies with this new thrombolytic.

It has a patent presented in Cuba, Europe and USA, and 6 works published in prestigious magazines such as Biochem Biophys Res. Comun., Thromb. Haemost, and Immunology Letters.