Essential Product and Process Parameters in Summary

Transcription

1 CHAPTER 3 Essential Product and Process Parameters in Summary 3.1 General Considerations During the process development stage, a number of variables need to be considered; they include formulation details, i.e. solution composition and concentration solution fill volume type of container, method of container closure The physics and chemistry of freezing show many complex and often surprising features, but the industrial freeze-drying process is limited to very few operational degrees of freedom. Thus, for a given solution composition and a given container and closure system, only three process variables exist by which direct control over the drying cycle can be maintained and which, therefore, determine the quality of the final dried product. They are shelf temperature chamber pressure time On the other hand, the most important factor is of course the product temperature and its change throughout the duration of the process. Its measurement and control are discussed in more detail in several following chapters. 20

2 Essential Product and Process Parameters in Summary 3.2 Formulation Parameters 21 In the manufacture of a therapeutic product, the initial stage requires the purification of the biologically active component, the drug substance. Purification methods for conventional drugs are well established. They are based on synthetic organic chemistry and standard analytical techniques. The situation is more complex for biopharmaceutical products, in particular those based on proteins. The drug substance may then be of animal, plant or microbial origin; it may be obtained from the natural source materials or by recombinant DNA (rdna) methods. The extraction and purification strategy depends to some extent on the source of the protein. Possible impurities are many, as shown in Scheme 1. The downstream processing always requires several stages and, depending on the source material, may involve a combination of physical and chemical methods. A generalised flow sheet for the isolation and purification of rdna proteins (r-proteins) is shown in Scheme 2. The total yield of purified protein is related to the number of stages required and their individual yields. For instance, in an isolation and purification process consisting of n steps, each one of which can be carried out with a 90% efficiency (highly idealised!), the final percentage yield of product is given by Yield (%) ¼ 9 n 10 (2 n) Thus, for a process consisting of five stages, each of which can be performed with a 90% recovery, the yield of the final product cannot Scheme 1 Types of impurities commonly found in crude protein extracts and that have to be removed during downstream processing

3 22 Chapter 3 contained Cytoplasmic soluble inclusion body periplasmic Broth deactivated Cell harvest Centrifugation Microfiltration Secreted Perfusion Batch Eukaryote Prokaryote Speparation of cells from medium Product realease Mechanical Enzymatic Chemical Osmotic shock Inclusion body Release & cleanup Denaturation Unfolding with urea, guanidine: reduction Refolding SH/SS exchange Clarification Enrichment Precipitation Ultrafiltration Production concentration Chromatograpy Membrane concentration Purification Cation/anion exchange Affinity chromatography Hydrophobic chromatography Polishing Gel filtration Crystallisation? Scheme 2 STABILISATION Freezing Lyophilisation Active drug to formulation/dosage form Typical downstream process flow sheet for r-proteins

4 Essential Product and Process Parameters in Summary exceed 60%, which is reduced to 53% for a six-stage purification procedure. Since biopharmaceutical drug substances, whatever their origin, are usually expensive, a loss of 40% during the purification process is a serious factor. A minimisation of the number of stages is therefore aimed at, but it has to be judged against other economic considerations and an acceptable degree of purity. In the manufacture of proteins for applications other than as therapeutic products, e.g. industrial enzymes, purity criteria are not as stringent as they are for parenteral products, in which case acceptable yields may be obtained with fewer fractionation and purification steps. For biochemical and therapeutic uses, however, purity and long-term stability are the overriding requirements. Fractionation, purification and stabilisation by freeze-drying then generally account for 50% of the total production costs, but since such products command high premiums in the market place, production hardly figures in the cost equation. Examples of the typical production cost breakdown for two product/process scenarios are shown in Table 1. The data illustrate that the cost contribution allocated to freeze-drying is almost insignificant, compared to the cost of the purified raw drug substance, assumed to be a protein. Before a purified protein, usually in a dilute aqueous solution, is subjected to the final stabilisation procedure, e.g. by freeze-drying, it has to undergo a process of compounding, loosely referred to as formulation. Very few proteins can survive freeze-drying without the aid of the so-called lyoprotectant additives, which serve to ensure the recovery of full biological activity of the protein at the point of use, whether in the dry solid state or reconstituted in an aqueous medium. The science and technology of lyoprotection, as employed in freeze-drying, is only gradually being put on a quantitative basis; several unresolved mysteries remain (see Chapters 7 and 11). Apart from lyoprotectants, other substances are also commonly added to the protein solution prior to drying. Some may also have been 23 Table 1 Economics of freeze-drying: production cost breakdown (excluding labour) for two typical biopharmaceuticals Cost components Product I: % cost Small vials, 24-h cycle Pure drug substance Solution manufacture 7 10 Containers 3 3 QC assays 8 7 Source: M.J. Pikal (unpublished data). Product II: % cost Large vials, 4-day cycle

5 24 Chapter 3 carried over in the solution during downstream processing. They may include ph buffers, surfactants and salts. A common additive, phosphate-buffered saline solution (PBS) ensures the isotonicity of a reconstituted parenteral product in water, prior to injection. Although from a pharmacokinetic standpoint some of the additives in common use may be innocuous or even beneficial, their presence in the solution may increase technical problems that can be encountered during freezedrying. For instance, the presence of PBS will inevitably require a longer freeze-drying cycle than would be required for the same solution in the absence of salts. The omission of phosphates and NaCl from the initial solution is therefore helpful, since it allows for a shorter drying cycle. On the other hand, reconstitution must then be performed with PBS solution, rather than with sterile water ( water for injection ), apparently a distinct disadvantage from a marketing standpoint. 3.3 Process Parameters As already pointed out, the primary process parameters by means of which drying can be directly controlled are shelf temperature, chamber pressure and time. In principle, the condenser temperature can also be adjusted, but in practice it is usually set to the lowest possible value, so as to maximise the ice sublimation rate. Since it is the temperature of the material that determines the drying rate, the control over the process can only be indirectly maintained, usually by coupled adjustments, either continuous or ramped, of the shelf temperature and/or the chamber pressure throughout the drying cycle. Once the process cycle conditions have been decided upon and the vials have been loaded, the process is automatically controlled by a computer. The freeze-drier performance is monitored, and a sample of a typical output sheet is shown in Figure 1. It records the time course of the production cycle (48 h in this case) in terms of shelf, condenser and product temperatures and chamber pressure. As mentioned earlier, the only directly adjustable parameters are the shelf and condenser temperatures, and the chamber pressure. We shall defer a detailed discussion of the information contained in the chart to several later chapters. 3.4 Multidisciplinary Nature of Freeze-Drying As will become clearer in the following chapters, the successful application of freeze-drying is based on a complex interplay of several

6 Essential Product and Process Parameters in Summary 25 Figure 1 Freeze-drier recorder output of condenser temperature (bottom, blue), shelf temperature (black), product temperature (green) and chamber pressure (red). Ordinate scales (from top to bottom): 1 and 2 ¼ bar, 3 ¼ 1C, 4 ¼ mbar scientific principles, some of which are still very poorly explored and hardly appear in university undergraduate curricula. In the author s experience, one rarely finds managers or technologists with an overall broad view of the various stages of freeze-drying. Thus, the trained

7 26 Chapter 3 chemist s experience differs from that of the physicist, the materials scientist or the chemical engineer. And yet, an overall appreciation of how these disciplines interact is of the utmost importance. In addition, experts in the various disciplines within a company will usually be found in different divisions of a line management structure, a custom not conducive to the fostering of the necessary collaboration. This book aims to highlight the importance of a multidisciplinary approach to freeze-drying. Nonetheless it has been found necessary, although not really desirable, to separate physical, chemical, economic and engineering and other aspects into different chapters. The following paragraph is intended to provide a summary of the important disciplines, some knowledge of which is essential for the development of a suitably formulated product, coupled with a sensible process cycle. The terms printed in italics are those that are of importance and require understanding by the practitioner. Pharmacokinetics deals with all aspects of the therapeutic effect of the chosen drug substance, suitably formulated, on its target. It forms the very basis of the formulation and processing stages. Of second highest importance is the formulation stage. In the case of biopharmaceuticals based on proteins, some experience of protein biochemistry and technology is absolutely necessary. The formulation process aims to protect the bioactive substance from the severe stresses to which it will be exposed during any drying process, including those directly due to concentration increases during freezing. The freezing process itself is governed by ice nucleation and crystal growth rates in supersaturated solutions. It is accompanied by freeze concentration of the solution, a process that may result in both crystalline or amorphous solids, or in mixtures. A correct identification of the physical and chemical identities of the product during and after processing requires an understanding of the analytical techniques available for obtaining the necessary information, also for an eventual submission to the relevant regulatory body. The ice sublimation stage depends on the balance of heat and mass transfer. Secondary drying depends on the diffusion rate of water through the solid matrix. The long-term stability is governed by the materials science of crystalline and amorphous solids, and it is measured with the aid of an array of physical analytical techniques, such as thermal analysis, X-ray diffraction, spectroscopy and chromatography. For registration purposes, all process and analytical data must be submitted in the required formats, with product identity, purity, safety, stability and shelf life playing major roles in the submission for regulatory approval.

8 Essential Product and Process Parameters in Summary 3.5 Conclusions 27 In small teams where good communication between individual staff members is fostered by imaginative management, it is possible for the necessary knowledge and experience to be passed on and become common knowledge, so that high-quality freeze-drying can be performed. The complex nature of biopharmaceutical freeze-drying makes it unlikely that this can be achieved in larger institutions, because most line management structures inhibit the flow of information between groups devoted to formulation and those whose objective it is to devise economical drying cycles. This was brought home to the author when his consultancy work took him to a German company that undertook the freeze-drying of human blood products on a large scale, but with variable results. During a round-table discussion, it became apparent that those pharmaceutical chemists who were responsible for formulation development had never met or spoken to their engineering colleagues, who were responsible for the freeze-drying process. Indeed, for security reasons, the latter were not informed of the nature of the products that were to be freeze-dried. This was a scenario that our technologists also encountered in a number of other mega-pharma companies. Usually, even the mention of glass transition or its measured value met with blank stares. The complexities were not appreciated, the freeze-drying operation being regarded as a push-button affair, hardly a recipe for success.