methods for the identification , production and use of derivatives of the invention obtained by preparing a dna fragment comprising at least the part of the coding sequence of staphylokinase that provides for its biological activity ; performing in vitro site - directed mutagenesis on the dna fragment to replace one or more codons for wild - type amino acids by a codon for another amino acid ; cloning the mutated dna fragment in a suitable vector ; transforming or transfecting a suitable host cell with the vector ; and culturing the host cell under conditions suitable for expressing the dna fragment . preferably the dna fragment is a 453 bp ecori - hindiii fragment of the plasmid pmex602sakb , the in vitro site - directed mutagenesis is performed by spliced overlap extension polymerase chain reaction and the mutated dna fragment is expressed in e . coli strain tg1 or wk6 . the invention also relates to pharmaceutical compositions comprising at least one of the staphylokinase derivatives according to the invention together with a suitable excipient , for treatment of arterial thrombosis .