By combining chemical and enzymatic synthesis, scientists have come up with a faster and cheaper way to make the smallest form of heparin.

If commercialization works out, the findings could extend the market reach of ultralow molecular weight heparin, as this form is called. And the chemoenzymatic synthesis that the researchers described in the Oct. 28, 2011, issue of Science marks progress on the way to a larger goal: synthesizing full-length heparin.

Heparin is a widely used anticoagulant drug that comes in three forms. Ultralow molecular weight (ULMW) heparin, the smallest of the three and the only one that is synthesized from the ground up rather than isolated from a natural source, currently accounts for only about 5 percent of the heparin that is used medically.

The reasons for that relatively small market share, co-corresponding author Robert Linhardt told BioWorld Today, are both economic and scientific. Linhardt is a professor at Rensselaer Polytechnic Institute.

ULMW heparin cannot be used in kidney dialysis – one large therapeutic area for heparin – because it is too small. Its half-life is also much longer than that of regular heparin, which can be good or bad depending on the particular use.

Heparin is a carbohydrate and so, unlike proteins, cannot be made recombinantly. Full-length heparin, which currently makes up about 60 percent of the total market, is derived from pigs. In fact, much of it is derived from Chinese pigs raised on small farms – that is, with a minimal level of regulatory oversight.

Production issues came to a head in 2008, when more than 80 people died and many more were sickened after receiving heparin that had been mixed with oversulfated chondroitin sulfate. (See BioWorld Today, March 3, 2008, and March 18, 2008.)

This week, lawmakers warned that the same suppliers that were the source of the 2008 contaminated heparin may still be supplying manufacturers – which is especially disturbing since there is some reason to believe that the contamination was not accidental, but motivated instead by the fact that oversulfated chondroitin sulfate is much cheaper than regular heparin starting materials, but hard to distinguish by standard tests. (See the related article, p. 1.)

Even in the absence of outright fraud, the current production method is vulnerable to contamination with everything from viruses to solvents.

Currently it is made by chemical synthesis. "When it was first synthesized, there wasn't any way to use enzymes, so you had to use brute-force chemistry – which is very complicated," Linhardt said. And that complication translates into slow going in the lab.

Chemoenzymatic synthesis, which – as its name might suggest – uses a combination of chemical and biotechnological steps, is much faster. And that, Linhardt said, means that "we can make many, many more structures . . . and that allows us to explore more structures to address some of [ULMW heparin's] limitations."

The fact that different structures are useful highlights an irony of heparin: despite the fact that medically, it is used as an anticoagulant by the pound, its natural function is unclear. This also implies, however, that there is no particular reason that the natural structure of heparin should be the best one for medical purposes. Linhardt and his team said they hope to explore a variety of different forms of ULMW heparin, and are actively seeking partners for those explorations.

In fact, first author Sayaka Masuko told BioWorld Today, with the method that her team has developed, synthesis was to a degree the easy part. "Compared to chemical synthesis, the chemoenzymatic synthesis was a lot easier," she said. Actually characterizing the exact composition of the ULMW heparins she synthesized was the more difficult part of the work.

Co-corresponding author Shaker Mousa, who is vice provost of research at the Albany College of Pharmacy and Health Sciences, told BioWorld Today that in addition to enabling the synthesis of new forms of ULMW heparin, the chemoenzymatic method simplifies the synthesis of the ULMW heparin that is currently commercially available: Arixtra (fondaparinux sodium, GlaxoSmithKline plc.) While it currently takes 17 steps to make Arixtra, the researchers were able to make it in 10 to 12 steps, and with a better yield to boot.

The ability to make Arixtra, or a generic version, more cheaply may translate into the ability to use the drug for some applications where full-length heparin's only advantage is that it is cheaper. In some instances full-length heparin can cause heparin-induced thrombocytopenia (HIT). ULMW heparin almost never causes HIT, which can lead to gangrene and, in extreme cases, kill its victims.