The UK Patents Court invalidated patent claims to an enantiomer of a known racemate mixture in Generics Ltd. V. Novartis AG  EWHC 2403 (Pat).
Generics (UK) Ltd. (Mylan) filed an action to try to invalidate a Supplementary Protection Certificate (“SPC”) for a drug for the treatment of Alzheimer’s disease called rivastigmine. An SPC extends the life of a granted “basic patent” in certain circumstances beyond the date on which the patent would otherwise come to the end of its statutory term. Novartis markets rivastigmine under the trade name Exelon.
Rivastigmine is the name for the (-)-enantiomer of N-ethyl-3-[(1- dimethylamino)ethyl]-N-methylphenyl–carbamate. Earlier, scientists at the Hebrew University of Jerusalem, made and tested the unresolved racemic compound (RA7). A racemate is an equimolar mixture of a pair of enantiomers. RA7 was one of a number of compounds proposed for the treatment of AD, but the publications made no mention of resolving it into its individual enantiomers.
The sole question in this action is whether a relevantly skilled pharmaceutical development team would find it obvious in the light of the RA7 publications to resolve the racemic mixture of RA7 into its individual enantiomers.
Stereo-isomers or enantiomers have identical physical and chemical properties in every respect, except two. They differ in their optical properties (the direction in which they rotate the plane of polarised light) and they react at different rates with other chiral compounds. Since living systems contain chiral proteins and constitute a chiral environment, the chirality of compounds administered to humans is important in therapy.
Whether to resolve?
The Court dealt witht he question of whether or not one skilled in the art would have resolved the enantiomers from the racemic mixture.
Firstly, it was common ground that the skilled team would consider the question of resolution in relation to its lead compound or compounds taken forward for development. It could scarcely have been otherwise given the fact that, of the chiral medicinal compounds introduced in 1984 and 1985 (excluding semi-synthetic compounds where nature had produced an enantiomerically pure starting point) about 50% were racemates.
Secondly, it was common ground that the actual resolution of RA7 did not involve any problematic chemistry. RA7 is easily resolved into rivastigmine using a standard stereochemical resolving agent. Novartis did not seek to make anything out of the practical chemistry involved. They contended that the skilled team would be aware that resolution could, in some cases, represent a difficult task, and that this would make the skilled team reluctant even to attempt a simple resolution. Although I accept that questions may arise as to the extent of resources which a skilled team might be prepared to devote to difficult resolution, I am not persuaded that in 1987 the skilled team would be hesitant about seeing whether a chiral compound could be easily resolved.
Novartis argued that the skilled team would see no advantage in the present case in an improvement in potency. However, the Court thought differently:
I am unable to accept that the skilled team would fail to see practical benefits in resolution. Firstly, there is the question of the metabolism of the compound. Whilst the very process of blocking the active site on the AChE results in a breakdown of the drug molecule, this is not the only metabolic process to which the drug might be subjected. Those drug molecules which do not interact with the target enzyme could be broken down by other enzymes, for example pseudo-cholinesterase, in a stereospecific way. Dr Newton was clear that metabolism was an area where there might (not would) be a stereochemical effect between enantiomers. Secondly, the skilled team would be aware that the process of penetration of the blood brain barrier could be stereo-selective. Thirdly, delivering a drug as a resolved enantiomer avoids the possibility of unknown, stereo-specific side effects emerging downstream.
Ultimately, the Court decided that the enantiomer was obvious in light of the racemic mixture:
I think the correct analysis is that a pharmaceutical composition for treatment of AD comprising rivastigmine was conceptually obvious in the light of Weinstock and would immediately occur to the skilled team. The team would consider that resolving RA7 would be a worthwhile step to take for good technical reasons. The team would find that the chemistry involved is trivial. Applying the principles outlined above I have no doubt that the inventive concept is obvious in the light of Weinstock.
I believe that conclusion to be consistent with the “problem and solution approach” employed by the Boards of Appeal. The objective technical effect demonstrated by the patent in comparison with the Weinstock prior art is simply that which one would expect from resolution of a chiral compound. The skilled person would know how to solve the problem of achieving those effects by an application of the common general knowledge about chiral compounds.