Cost-efficient synthesis of complex chiral products remains a huge challenge in the manufacture of pharmaceuticals. J W Wiffen, team leader, New Technology Products and S Taylor, a consultant, at CSS outline a collaborative approach to cutting that cost
CSS, a division of Almac Sciences, specialises in custom synthesis and service r&d and has greatly expanded its technology offering in recent years through targeted acquisitions and alliances. An example, now proving to be very successful, was the acquisition of Albachem in March 2004, giving access to specialised technology in the field of peptide and protein synthesis. This approach has been augmented by the formation of strategic alliances with other specialist technology companies, particularly in chiral technology.
The Chiral Consortium defines CSS's relationships with ChiralQuest and IEP, where the former provides asymmetric hydrogenation technology and the latter asymmetric bioreduction expertise. Combining these with the facilities and virtual manufacturing network of CSS has provided a powerful boost to customer choice in the industry.
In addition to these activities, CSS is now investing to bring new technology in-house, strengthening its position in the high-technology chemistry marketplace. The company now derives business from all stages of the pharmaceutical lifecycle - discovery, custom manufacture for clinical development and chiral building blocks at tonne scale.
Biocatalysis is a proven technology used by the fine chemical and pharmaceutical industry for the provision of stereo-pure intermediates and APIs in pharmaceutical synthesis. It complements technologies such as crystallisation and asymmetric chemocatalysis.
Biotransformation science and technology is now established within CSS, where enzyme screening and route definition for the synthesis of chiral alcohols, acids and amines is routinely practised. A key feature of the approach of the New Technology Products Group of CSS is to seek new areas where the strength of biocatalysis and separation science can be used to solve difficult problems in chemistry.
building blocks
An example is in the synthesis of building blocks for medicinal chemists. CSS formed an alliance with Biofocus plc in April 2004, providing the chemistry expertise for the synthesis of focused libraries for screening in drug discovery. Libraries based on the structure of ribose, having diverse and potentially metabolically stable nucleoside base analogues attached to a furanose or pyranose ring structure, have proved very popular.
To expand upon these, a desirable deoxyribose-derived intermediate was identified that was needed to provide different stereo-isomers of two key building blocks. The chemical synthesis of this, however, led to a mixture of two diastereomers that vary by the stereochemistry at the carboxyl group and which proved difficult to separate.
Through extensive enzyme screening and optimisation, conditions were found that allowed the bioresolution of these diastereomers into single isomer form (see figure 2). This allows subsequent production of a chirally pure library for screening and is of significance in drug discovery where chemical and optical purity are of paramount importance for the correct interpretation of screening results.
CSS is also applying biocatalysis in new ways for the synthesis of amines, in response to demands from pharmaceutical customers. There are well-established methodologies for the biocatalytic resolution of amines, such as those shown in the scheme in figure 3, where simple esters can be used as an acyl donor, either in the presence or absence of added solvent.
When these methods were tried for a range of target amines problems were encountered through low enzyme productivity or lack of sufficient enantio-selectivity. This was addressed by adapting a methodology, developed some years ago in Professor Stan Roberts's group (then at the University of Exeter) to resolve problematic tertiary alcohols, where an oxalate ester was used to overcome steric hindrance.
When an amine is reacted with an activated hemioxalate ester, an oxalamide ester derivative is formed. This is eminently suitable for bioresolution by straightforward enzyme hydrolysis of the ester, leaving a readily deprotected oxalamide.
The advantage is that a much wider range of enzymes can now be tested that stand a higher chance of success, and the system can be fine-tuned through modification of the usual reaction parameters such as solvents, ester type, temperature and the like. In direct bioresolution of amines the choice of enzyme is more limited, where most work has been focused on just a few enzymes such as the lipase B from Candida Antarctica (see figure 4).
broader options
CSS is also developing other catalytic methodologies for amines, such as the catalytic asymmetric rearrangements. This broadens the options available to CSS such that it now has internal access to asymmetric chemocatalysis, various biocatalytic resolution routes and crystallisation routes for amines. Of course, there may be synergistic advantages where these approaches are combined in a given synthetic route.
The company has further developed its competence in the area of classical resolution identification and development. Typically, when a classical resolution is developed, the resolving agents are screened against the racemic mixture. While this often provides a resolution, generally it is sub-optimal. Applying a methodology pioneered by Andre Collet, CSS develops resolution methods by examining the phase diagrams for single enantiomers and utilising this information to identify the most efficient resolving agent in terms of both cost and process efficiency. A further development of this approach is the use of sub-stoichiometric amounts of resolving agent, again significantly lowering the cost of any resolution. The combination of this methodology and CSS's expertise in bioresolution provides a powerful offering in the field of separation science that is proving popular.
The company has historically been known for its expertise in relatively small-scale custom synthesis - from mg to multi-kg of APIs - and also has a long-standing reputation in the technical transfer of innovative processes to its customers. It has now combined these attributes with access to world-leading chiral technology capability (in-house and via The Chiral Consortium). The logical next step was to develop manufacturing processes to chiral building blocks required at tonne scale. In collaboration with its Chiral Consortium and manufacturing partners, a whole new range of high quality, cost-effective chiral building blocks are becoming available. Some early examples of the output of this strategy are illustrated in figure 5.
All of this fits with the company's philosophy of developing the best possible manufacturing solutions, sited in the most appropriate assets.....anywhere.
