text: When drugs are covered under patent protection, only the pharmaceutical company that holds the patent is allowed to manufacture, market, and eventually profit from them. The lifetime of the patent varies between countries and also between drugs; in the United States, most drug patents last about twenty years. Once the patent has expired, the drug can be manufactured and sold by other companies - at which point, it is referred to as a generic drug. Its availability on the market as a generic drug removes the monopoly of the patent holder, thereby encouraging competition and causing a significant drop in drug prices, which ensures that life-saving and important drugs reach the general population at fair prices. However, the company holding the initial patent may get a new patent by forming a new version of the drug that is significantly changed compared to the original compound. Patentability of different isomers has been controversial over the past ten years and there have been a number of related legal issues. In making their determinations, courts have looked at factors including: (i) Whether the racemate was known in the prior art. (ii) The difficulty in resolving the enantiomers. (iii) The stereoselectivity of the relevant receptor. (iv) Other secondary considerations of non-obviousness such as commercial success, unexpected results, and satisfaction of long-felt needs in the art. The decisions made regarding these issues have varied and there is no clear answer to the legality of patenting stereoisomers. These issues have been resolved on a case-by-case basis. With the number of current pharmaceuticals currently being marketed as racemic mixtures, it is likely that patentability will continue to be debated in the near future.
  There are examples of common drugs, like ibuprofen, where the use of chiral switching has caused controversy. Ibuprofen is a racemic mixture where the S-enantiomer is known to play a major role in reducing inflammation as it inhibits COX-2 (cooxygenase 2) compared to the R-enantiomer; the fact that the S-enantiomer is stronger is what led to the chiral switching. But, when the racemic ibuprofen enters the body, a little over half of the R-enantiomers experience chiral inversion and transform into the favored S-enantiomer. This observation has led to a conclusion that the racemic and the S-enantiomer are potentially biologically equivalent. Because of this and the more recent evidence suggesting that the R-enantiomer may actually contribute to COX-2 inhibition, as well, but at a slower rate, there is still debate on whether or not the chiral switching seen in ibuprofen is really advantageous or if it is just to give patent protections to the manufacturers.