John W. Fenton II

Importance to Hirudotherapy

John W. Fenton II was the American thrombin biochemist whose work made it possible to translate Markwardt's hirudin into a regulated pharmaceutical product. Fenton's central contribution was the production of high-purity human thrombin — milligram quantities, electrophoretically homogeneous, with full enzymatic activity — at a time when most laboratories could only obtain partially-purified bovine thrombin contaminated with prothrombin and Factor V. Without Fenton's pure thrombin as the assay substrate, the precise kinetic characterization of hirudin-thrombin binding (Ki measurements, on-rate and off-rate constants, exosite-specific interaction surface mapping) would have been impossible. Fenton's discovery of thrombin's bipartite exosite architecture — an anion-binding exosite I (the fibrinogen recognition exosite) and exosite II (the heparin-binding region) — was the structural insight that explained why hirudin binds thrombin with such extraordinary affinity (Kd ~0.5 pM). Hirudin engages both the active site and exosite I simultaneously, using a globular N-terminal domain to occupy the catalytic cleft and a long acidic C-terminal tail to wrap into exosite I. This bipartite binding architecture is what made hirudin a 10,000-fold more potent thrombin inhibitor than any single-site small molecule, and it became the design template for the entire subsequent direct thrombin inhibitor drug class. Fenton's collaboration with Hoechst Marion Roussel between 1991 and 1998 was the regulatory keystone of recombinant hirudin development. He provided the structural rationale, the analytical methods (clotting assays, chromogenic substrate assays, and the surface plasmon resonance characterization that became the FDA's release-criteria standard), and the academic credibility that allowed lepirudin (Refludan) to be approved by FDA in June 1998 as the first direct thrombin inhibitor for heparin-induced thrombocytopenia. ASH considers him the patron of modern recombinant-hirudin pharmacology.