Antithrombotic Argatroban : crystallographic, spectroscopic and theoretical investigation on the efficiently resolved 21-(R) and 21-(S) diastereoisomers

Argatroban is a synthetic potent and direct inhibitor of thrombin, the serine protease that plays a central role in thrombotic events, and it has been approved in the USA, Europe and Japan for prophylaxis or treatment of thrombosis in patients with heparin-induced thrombocytopenia (HIT).(1) In the Argatroban structure three moieties can be well recognized: the 4-methyl-2- piperidine carboxylic acid bonded to an arginine residue bearing a methyltetrahydroquinoline sulphonyl group on its amino function. Four stereogenic centers are present in the molecule: the stereocenter on the tetrahydroquinoline is introduced via hydrogenation affording a mixture of (21R)- and (21S)-diastereoisomers, accepted as antithrombotic drug if the respective ratio is within 65/35 ± 2. Nevertheless, according to the current regulatory guidelines on active pharmaceutical ingredients, we have efficiently separated the 21R- and 21S-diastereoisomers, which are both biologically active, in particular 21S is twice potent as 21R and about five times less soluble. In this work, a deepen crystallographic analysis on the conformational profile of both diastereoisomers will be presented, and the results will be compared with the theoretical investigation, giving sound information useful for the design of new thrombin inhibitors.