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The current trend in the pharmaceutical industry for the manufacture of small-molecule therapeutic agents is moving toward continuous flow processes. This article is part of a special issue on APIs.
The current trend in the pharmaceutical industry for the manufacture of small-molecule therapeutic agents is moving toward continuous flow processes. In 2007, the Novartis–MIT Center for Continuous Manufacturing was established with $65 million in funding from the drug company. The center is proposing a "Blue Sky" concept where there is a continuous process from the start of a chemical synthesis through final pharmaceutical dosage form (1, 2). The Blue Sky program is an ambitious goal but is gaining ground rapidly among thought leaders in the pharmaceutical industry and US Food and Drug Administration. Consequently, the momentum for this concept is likely to have a trickle-down effect for contract manufacturers (CMOs) that design and develop early-stage manufacturing processes for clients developing innovator small-molecule drugs.
Continuous-flow technology involves the continuous introduction of a stream of chemical reactants into a flow or microreactor to yield a desired reaction product on a continuous basis. The versatility and usefulness of continuous-flow reactor (CFR) technology is expanding rapidly with an ever broadening scope of applicable chemistries and the development of new flow technologies (3). Champions of continuous-flow technology cite a wide range of potential advantages compared with traditional batch manufacturing of pharmaceuticals. In general, the greater optimization and control achievable with CFR technology can translate to significant savings in time and costs and can have a favorable safety and environmental impact. Furthermore, the small-reaction volume, broad operating pressure and temperature ranges, and mixing efficiencies of flow reactors extends the repertoire of chemistries beyond that of the safety and technical limitations of batch reactors. The capital investment for CFR technology is also substantially less as is the footprint required in the plant than a similar capacity batch-reactor system. However, even though the potential advantages of CFR technology can be significant, the technology is currently not applicable or practical in all situations.
Adapting CFR technology to early-stage development projects has significant merit, but also significant challenges. CMOs work with numerous sponsor clients, diverse chemistries, and projects in all stages of development. Many of the projects CMOs encounter are very early stage with the development candidate being licensed out of an academic laboratory or coming directly from the sponsor company's discovery laboratories. These early-stage projects more often than not require various degrees of process research and/or process development to make the discovery synthesis amenable to current good manufacturing practice (cGMP) scale-up. Also, to receive additional funding or secure a development partner, the sponsor company has a strong sense of urgency to enter the clinic and achieve proof-of-concept as soon as possible. This puts pressure on the CMO to rapidly develop a scalable process to meet the near-term active pharmaceutical ingredient (API) goals of the sponsor company and at the same time enable the process to further scale-up to meet later stage API demands.
Small-molecule drug development processes are typically in the range of six to eight synthetic steps. Given the time constraints and level of technical challenge, the design of an initial six to eight step totally continuous-flow process for an early-stage drug development project is generally not practical and is typically reserved for established commercial processes. CFR technology, however, does offer the distinct advantage of broader scaling capabilities for chemistries not suitable to batch scale-up. This scalability feature of CFRs is appealing for circumventing nonscaling problematic chemistries in a timely fashion. It is not uncommon for there to be one or more steps in an initial discovery synthesis that is not amenable to batch processing. When this occurs, significant time, effort, and money have to be invested in process research and/or development to resolve the chemistry or retool the synthesis. CFR technology, on the other hand, offers the potential to scale the existing problematic chemistry to overcome the bottleneck. For example, Johnson and Johnson (New Brunswick, NJ) demonstrated the utility of CFR technology for rapidly scaling gram to kilogram quantities of early-stage clinical trial API where batch processing was a concern (4). Several classes of reactions that presented safety or hazardous concerns for batch manufacturing were shown to scale efficiently, safely, and with shorter process research times. The reaction classes reported by the Johnson and Johnson group included exothermic reactions, reactions at elevated temperatures, reactions with unstable intermediates, and reactions involving hazardous reagents (4). Implementing CFR technology in an otherwise batch process to resolve early scalable issues provides an attractive strategy for expediting early-stage process development. Under this mixed "batch-CFR" paradigm, the problematic step(s) can be optimized to a CFR early on in the process allowing the chemistry to be readily scaled from grams to kilograms. Manufacturers of continuous CFRs such as Corning (Corning, NY) make smaller scale reactors that can be used for optimizing the continuous-flow chemistry on a small scale and employing the smaller reactor to make the desired product on a scale of grams to about a kilogram. When larger-scale production is required, the chemistry is readily transferred to an identical larger reactor simplifying the technology transfer process from laboratory scale to plant scale. Consequently, the "Batch-CFR" approach has the potential to be more expedient and cost effective as it takes advantage of CFR technology's ability to scale existing chemistry that is not suitable or safe for larger-scale batch processing. CFR technology may also allow the CMO to scale reactions beyond the capacity of their fixed reactors as an alternative to doing a technology transfer to another facility with larger fixed reactors.
The contract manufacture will still likely use fixed equipment to process the continuous-flow reaction maelstrom. Although significant gains have been made in in-process monitoring and continuous crystallization, at the present time, it is more expedient for early-stage continuous flow reactions to be worked-up using traditional methodology such as filtration, extraction, solvent removal, and crystallization in fixed equipment. If the project moves to commercialization, particularly in the hands of a large pharmaceutical company, the process is more likely to become a fully optimized continuous process from start to finish. With a "Batch-CFR" process, this transition should be facilitated since the more challenging chemistry has already been adapted to CFR technology.
The decision by a CMO to implement CFR technology to resolve a process scale-up issue is a critical risk decision requiring buy-in from the sponsor client. The technology holds significant promise for efficient and cost-effective development of early-stage cGMP processes. The "Batch-CFR" approach provides a much greater probability for scaling the initial discovery synthesis directly, thereby requiring significantly less process research and development work. CFR technology, however, requires different strategic thinking and technical expertise compared with classical batch manufacturing. Because most drug-development professionals are classically trained, there is likely to be some natural resistance to implementing CFR technology in early-drug development. This mindset has been referred to as "batch mentality (5). However, with FDA and the pharmaceutical industry encouraging the shift to CFR technology, contract manufacturers are likely to follow suit.
James Hamby, PhD, is vice-president of business development at Ash Stevens, 18655 Krause Street, Riverview, MI 48193, tel. 734. 282.3370 Ext. 1144, email@example.com.
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