With the development and clinical investigation of the fibrin-specific thrombolytic agents t-PA and scu-PA, many questions relating to their optimal use remain to be answered. It is, however, becoming apparent that these agents, in addition to several advantages,suffer some shortcomings, e.g. their therapeutic dose is large and their fibrin-specificity is limited.Therefore,the quest for better thrombolytic agents remains open.We will report results of four main lines of research which we are pursuing to obtain better agents or regimens for fibrin-specific thrombolytic therapy.1. Synergism between t-PA and scu-PA. t—PA and scu—PA in molar ratios between 4:1 and 1:4 show no synergism for thrombolysis of a plasma clot immersed in plasma in vitro(Thromb. Haemost. 56, 35, 1986) but display significant in vivo synergismin a rabbit model (Circulation 74, 838, 19867and in man (Am. Heart J. 112, 1083,1986).Recently we have confirmed synergism for thrombolysis between t-PA and scu-PA in a coronary thrombosis model in the dog(Zuskind et al., unpubl.) and in the baboon (Collen et al., unpubl.). Sequential infusion of t-PA followed by scu-PA butnot of scu-PA followed by t-PA is syneristic(Collen et al., this meeting).2. Mutants of t-PA. In collaboration with Larssen et al.,deletion mutants of t-PA, obtained by in vitro mutagenesis are characterized with respect to pharmacokinetics and thrombolytic properties.Mutants lacking the finger—like domain and/or the growth factor domainand/or one or all of the glycosylation sites have a much slower clearance (Larssenet al., this meeting) but unaltered specific thrombolytic properties and fibrin-specificity (Collen et al., this meeting).3. Mutants of scu-PA. A truncated form of scu-PA, lacking the 143 NH2~terminal amino acids was shown to be pharmacologically and thrombolytically indistinguishable from intact scu—PA (Stump et al.).Mutants of scu—PA in which Lys 158 is replaced,whereby they can no longer be converted to urokinase, still haveintrinsic plasminogen activating properties and act synergistically with t-PA on thrombolysis in vivo (Nelles et al., this meeting).4. Hybrids of t-PA and scu-PA. In collaboration with Pierard et al. (this meeting) hybrids of NH2~terminal regions of t-PA and COOH-terminal regions of u-PA were constructed which, after translation in transient expression systems, showed apparent specific activities comparable to that of natural two-chain u-PA. One hybrid, composed of the finger domain of t-PA and the B-chain of u-PA, was scaled up, purified and characterized (Gheysen et al., this meeting). This hybrid had theenzymatic properties typical of single chain u-PA, but had not acquired the fibrinaffinity of t-PA.Based on the finding that the isolated A-chain of t-PA retains the intact fibrin-affinity of the native molecule (Holvoet et al.,Eur.J. Biochem. 158, 173, 1986) andthat a low Mr form of scu-PA retains the functional properties of the intact moleule (J. Biol. Chem. 261, 17120, 1986), we have constructed and expressed a hybrid consisting of the NH -terminal region of t-PA (amino acids 1 to 263) and the COOH-terminal region of scu-PA (amino acids 144to 411) (Lijnen et al., this meeting). This hybrid has both fibrin affinity of t-PA (although less pronounced) and the enzymatic properties of scu-PA. The activation of plasminogen by the hybrid is apparently stimulated by fibrin.We believe that continued research along these lines will yield thrombolytic agents or therapeutic schemes, which may be superior to t-PA and/or scu-PA in terms of specific thrombolytic activity and fibrin-specificity.
Limited proteolysis and ‘in vitro’ mutagenesis of bovine brain inositol monophosphatase identifies an N-terminal region important for activity