Technology Forum: Preparative Chromatography

Preparative chromatography has been widely used as a purification technique for many years, particularly in the pharmaceutical industry. Joining us for this forum are Frédéric Cheviron of Chiral Technologies Europe, J.T. Presley of Phenomenex, and Jean Bléhaut of Novasep.

Which factor is the most important in preparative purifications: speed, purity, or cost?

Cheviron: These three attributes are closely interrelated. Their importance is defined by specifications for a specific separation or purification project. Purity specs will define project speed and, consequently, cost.

Presley: We believe all three of these factors are important in purification projects, and try to help our customers achieve the best process economy possible. In fact, our specialized in-house services groupworks closely with our customers to identify the best media for their application based on selectivity, loading, and overall media consumption cost. This group also conducts scale-up and loading studies to streamline purification development so customers can implement their optimized method and gather their target compounds a lot more quickly than if they were to conduct a similar scaled selectivity and optimized loading study in-house.

Bléhaut: The priority in purification is very dependent on the development stage. In medicinal chemistry, the key is a quick development to have samples ready for initial activity tests. At this early stage, the synthesis is carried out on very small scale and the purification cost is not a major factor, but time is.

In contrast, in late clinical phases and in the commercialization phase, when the active pharmaceutical ingredient (API) is required at large scale, the process needs to be carefully optimized to reduce the costs of goods sold (COGS) and maximize the project’s profitability. Furthermore, high purity and a controlled impurity profile are fundamental for APIs administered to human beings as no risks can be taken. In a word, the priorities are completely reversed between early development and production.

As a result, different purification methods should be implemented to closely match these specific needs. Typically, for a chiral separation, we would recommend using supercritical fluid chromatography (SFC) to our clients in early stages because this batch technique can be rapidly developed and the purification time is dramatically reduced compare to high performance liquid chromatography (HPLC) for example. However, when it comes to production, batch techniques are generally less cost effective and continuous chromatography such as simulated moving bed or multicolumn continuous chromatography provides much-improved COGS. Furthermore, the integration of solvent recycling enables almost all solvent consumption to be eliminated (efficacy of recycling above 99.9% has been reported by UCB Pharma using commercial-scale continuous chromatography systems) and offerscost-effective production of chiral APIs and intermediates.

In recent years, flash chromatography has come on the scene as a viable alternative to high-pressure preparative chromatography. Do you see flash chromatography becoming a serious competitor to high-pressure preparative chromatography?

Cheviron: Flash chromatography is a widely used method for prepurification steps of large quantities of material to be purified. But I do not think that for the majority of applications Flash chromatography can be viewed as a competitor. They are complementary techniques.

Presley: Over the last decade, as people have moved away from combinatorial purification, we’ve seen a decrease in flash chromatography. Flash is still mainly used as a capture step and works well for that application, but due to its larger particle sizes and low-to-medium pressure systems, it will never be able to achieve the high efficiency of HPLC that is needed for the high purity level that many customers require. Most HPLC media can also be washed and reused for extended usage and even multiple campaigns compared to disposable flash cartridges.

Bléhaut: Indeed, flash chromatography is a powerful tool for relatively easy purifications (nonchiral ones in particular) because it allows for rapid and reliable purification. As mentioned earlier, this is valuable in the early stages of a molecule’s development to isolate of a few grams to a kilogram of compound. By design, flash chromatography often makes use of a solvent gradient to help the separation of the different compounds of the sample according to polarity. This affords in a single run purified samples of the desired compound and also the isolation of the impurities (for identification and quantification purposes), which helps for the later development.

For production of larger batches, however, the gradient nature of the mobile phase becomes more challenging. In particular, the eluent’s recycling involves fractional distillation, which is more complex and expensive than the simple evaporation of an isocratic solvent system. Knowing that solvent recycling is key to a viable large-scale chromatographic purification (both economically and environmentally), I cannot see flash chromatography becoming a serious competitor to preparative HPLC, at least not at large scale.

Furthermore a single-step purification (regardless of technology used —flash chromatography, SFC, or HPLC) is not necessarily the most efficient process development strategy in view of large-scale production. Quite often, the mixture to purify contains some easily separated impurities and some that are closely related to the desired product. Eliminating the first ones does not require expensive stationary phases (which can furthermore be poisoned by some impurities); simpler, cost-effective methods, such as adsorption of the impurity on cheap silica, can be implemented. After this rough purification, fewer compounds remain to be separated and the use of a more advanced HPLC process (and expensive stationary phase) is limited to the complex part of the purification, where it brings the most value. This two-step approach maximizes productivity while minimizing costs by expanding the lifetime of the costly stationary phase required in challenging purifications.

Over the last few years, where have you seen the greatest growth in demand for preparative chromatography?

Cheviron: With the current pharma environment — patent expirations, layoffs, and poor pipelines — we see growth in custom services for small-, medium-, and large-scale separations.

Presley: Since the launch of the biopharmaceutical market, we have seen tremendous growth in the purification of peptides and proteins along with the biosimilars market, which is also referred to as biogenerics. There are larger, more mature preparative chromatography markets that incorporate our media in their purification processes but this is where we have seen the largest growth of our preparative products.

Bléhaut: Preparative chromatography is now acknowledged as a standard tool by all pharmaceutical companies for the purification of drug candidates in medicinal chemistry and for preclinical stage. In addition to these gram-to kilogram-scale purifications, more and more of these companies have realized the benefits of chromatography on scales ranging from kilogram to multi-tons as well. Besides a sustained demand for chiral separation, we observe an increasing need for large-scale chromatographic purification of complex mixtures, due in great part to the ever-increasing complexity of the new APIs, which makes them harder to purify by methods like crystallization, for instance. This is particularly true for molecules such as imaging agents containing various and complex functionalities or for advanced molecules obtained by fermentation or biocatalysic processes.

How beneficial is preparative chromatography as a technique?

Cheviron: Preparative chromatography rapidly provides clients with sufficient amounts of material of desired quality to initiate preclinical and clinical trials and allows alternative commercialization routes to be explored with no loss in development time.

Presley: Preparative chromatography has been widely used among pharmaceutical and biopharmaceutical companies as a purification technique for decades. It’s the best way to economically reach the high level of purity required for active pharmaceutical ingredients (APIs). At the moment there is nothing that really compares to preparative HPLC in terms of yield, purity, cost, and purification time, so we expect it to be a viable solution for preparative chemists isolating and purifying both chiral and achiral compounds for many years to come.

Bléhaut: The ease and reliability of the scale-up process is definitely the strongest advantage of preparative chromatography: Thanks to advanced column technology, reliable stationary phases, and chromatography process design using computer simulation, preparative chromatography benefits from a linear scale-up profile. This is quite unique compared to other techniques and also very convenient. Not only does it make it the scale-up process straightforward, but also the costs of production at large scale are known from a very early stage in the development of the compound. Therefore, one knows whether chromatography is a viable production technique or not for future large-scale production.

In addition, the use of chromatography as a purification technique can separate impurities that would be quite difficult to separate with other techniques, hence preparative chromatography can sometimes allow for more straightforward or more cost-effective synthetic routes. In addition, for chiral separations, chromatography can be a rapid and cost effective technique compared to alternatives. For instance, the development of a crystallization process can be long and expensive due to a hit-and-miss approach, and potentially generates a costly process implementation due to the chiral additives required by diastereomeric crystallization and the large amounts of solvents used. In comparison, a chromatographic separation can be quickly developed, and solvent recycling minimizes the COGS. Even in cases where preparative chromatography is not necessarily the most cost-effective option, it can be used as a fast track to early supplies of product when time is of the essence, to get the product on the market, until a (possibly) more cost-effective, alternative route is found for large-scale, established production.

And last but not least, we observe a tendency for new life science active ingredients to be ever more complex. This increasing complexity is mainly driven by two phenomena. First, the development of more and more targeted therapies generates purposed-designed lead compounds with intricate formulae and numerous, complex functionalities. Second, the intellectual property minefield forces the API developers to focus on yet-unpatented molecules to protect their future markets. In this context, preparative chromatography has a key role to play because more complex molecules are more challenging to synthesize and require more advanced purification techniques, and in particular, chromatography (whether it be chiral or achiral). As a result, we anticipate a continued development of preparative and process-scale chromatography in the future.