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Innovations in Manufacturing Drug Substances

Production of the nation’s drug supply involves manufacture of drug substances—the active pharmaceutical ingredients (APIs)—and ultimately the drug products that are delivered to patients. In this chapter, the committee explores innovations for manufacturing bulk, purified APIs. Specifically, the committee discusses innovations in unit operations, process intensification, and process stream compositions that are associated with the upstream and downstream processing of APIs. Here, upstream refers to the portion of the process in which an API is first generated by reaction or from a host organism, and downstream refers to the portion of the process dedicated to the isolation and purification of the API. The innovations discussed here are likely to arise in filings of investigational new drugs in the next 5–10 years. Technical and regulatory challenges are also discussed with suggestions for overcoming the regulatory challenges in drug-substance manufacturing.

UNIT OPERATIONS

Unit operations refers to individual manufacturing steps and their associated equipment, such as a stirred tank reactor for synthesis of a small-molecule API from chemical precursors, a cell culture for producing monoclonal antibodies (mAbs), a harvest operation that uses a filtration unit to separate a biologic API from host cells and host-cell debris after cell culture, a crystallizer for final purification and generation of a solid form of a small-molecule API, or a polishing purification operation that uses a column chromatography unit to remove residual contaminants to yield a highly purified biologic API stream from a stream of intermediate purity. Innovations in unit operations arise when traditional, expected operations are replaced with atypical alternatives, when technologies are adopted from other industries, when new formats or operating strategies are instituted for existing unit operations, or when completely new process equipment and technologies are created. The following sections describe innovations for those situations.

Replacement of Traditional Process Technologies with Atypical Alternatives

The physicochemical or biophysical properties of new APIs and changes in the composition of process streams are likely to drive the replacement of traditional technologies. The inability to crystallize small-molecule APIs of increased molecular complexity and the production of amorphous forms of API solids that have desirable release kinetics might lead to the replacement of typical crystallization operations with chromatographic purification operations and leave the formation of the solid phase to a later drying step. Column chromatography, although long the mainstay of the downstream purification of biologics, is much less familiar in the context of small molecule APIs.

For biologics that are produced by secreting host cells, substantial increases in API titers during upstream processing have been made possible by host-cell engineering, adoption of alternative hosts, cell-growth media and feeding-strategy innovations, and bioreactor engineering. Those advances have pushed the limits of capacity and mass-transfer kinetics of traditional column chromatography. For mAbs—the largest class of biopharmaceuticals by number of approved drugs, production scale, and sales volume—fed-batch production titers (currently 5–10 g/L, Shukla et al. 2017; Xu et al. 2020) are expected to grow to 40 g/L within 10 years (BPOG 2017a). At such high protein concentrations, bulk-separation alternatives to the traditional protein A affinity column-chromatography capture step—such as precipitation, aqueous two-phase extraction, and crystallization—become attractive with respect to throughput, cost, and complexity. In fact, for high-titer proteins, the development of purification trains¹ completely devoid of column chromatography is likely.

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¹Trains are defined here as sequences of unit operations.

Adoption of Process Technologies from Other Industries

The similarities between the properties of process streams in biologic-drug production and product streams in other industries—such as the food and beverage, industrial enzyme, plasma fractionation, and wastewater-processing industries—provide opportunities for the adoption of alternative unit operations. Harvest operations for biologics have long been conducted by centrifugation or filtration operations, and cell flocculation and flotation-based harvest strategies that could be adopted from waste-water processing might provide low-fouling alternatives. The precipitation-based capture purification of mAbs is an example of a technology borrowed from long use in the plasma-fractionation industry. New continuous-processing unit operation formats, discussed further below, illustrate the diffusion of technology and processing approaches from the oil, gas, and chemical-process industries, and more recently the food industry, to the pharmaceutical industry. Here, the drivers for the adoption are decreased operational complexity and costs and increased throughput.

New Formats and Operating Strategies for Existing Process Technologies

New formats and operating strategies are being created for existing unit operations to increase efficiency and throughput, decrease the cost of goods and complexity, and address scalability concerns. The manufacture of biologics provides several innovative examples (Coffman 2020; Jagschies 2020). The need to limit lactate and ammonia accumulation can lead to batch operations that have new feeding strategies in which glucose is fed to the culture in a controlled manner to increase cell densities and product titers. Further advances are likely to link feeding strategies directly to sensed critical quality attributes. Cell-perfusion operations can greatly increase productivity provided that long-term cell growth can be maintained. Expectations are that perfusion-reactor productivities will grow from 0.05–1 g/L-day to 0.5–10 g/L-day (BPOG 2017a) within 10 years. Such increases will be facilitated by innovations in current cell-retention devices, such as the incorporation of new low-fouling membranes in tangential flow filtration (TFF) and alternating tangential flow (ATF) filtration, and in the application of new process technologies, such as scalable acoustic separators and hydrocyclones described below.

Multicolumn periodic continuous chromatography formats have been developed to address the capacity and throughput limitations of traditional column chromatography for high-titer protein products. Next-generation chromatographic formats, such as counter-current tangential chromatography that uses chromatographic media slurries in place of packed beds and rapid cycling adsorptive membranes, are under development to address the mass-transfer limitations of fixed beds. Single-pass tangential flow filtration, an alternative developed for traditional batch ultrafiltration-based concentration operations, might be used in new configurations to accomplish sequential concentration and diafiltration or in cascades to form a purification train. Examples of new formats and operating strategies that span both biologic and small-molecule drugs are microfluidic unit-operation formats for small-scale production of individualized therapies and continuous formats for many batch unit operations. The development of continuous formats is discussed further below.

New Process Technologies

Beyond the extension and elaboration of existing technologies, completely new types of unit operations that exploit physical phenomena that have not previously been harnessed in traditional manufacturing processes are emerging. In the synthesis of small-molecule drugs, new types of reactors that enable photochemical and electrochemical reactions are being developed (Tom 2020). In upstream operations for biologics, the use of membrane-based microcarriers for culturing adherent cells introduces a different process from the one used for culturing suspension cells. Methods to retain individual cells or microcarriers in perfusion cultures are likely to be the subject of substantial innovation. In general, such methods must be neutral with respect to cell viability and effective in retaining cells or microcarriers in the bioreactor. Alternatives to now-conventional TFF and ATF cell-retention devices—such as acoustic separators that work by concentrating cells at the nodes of a three-dimensional low-frequency standing wave and hydrocyclones that exploit density differences between cells and the suspending medium in a centrifugal-flow field to concentrate cells—might see application. In addition, precipitation methods that use various types of decanters and cell filtration and recycling have been used for cell retention in processes that involve perfusion cultures. Acoustic separators might also replace primary depth filtration in cell-harvest operations.

Other new technologies in the downstream processing of biologics have incorporated sequential membrane-based chromatographic operations that remove trace im-

purities while allowing high-concentration target species to flow through for the polishing purification of biologics. Such sequential membrane-based operations have arisen because of the availability of new membrane media and the increasing ability to predict target and contaminant binding behaviors as a function of media properties and solution conditions (Crowell et al. 2018). These new unit operations can have operational and performance advantages over traditional technology and might allow the rearrangement or elimination of surrounding operations in the overall manufacturing process.

Technical Challenges

Adoption of new unit operations can pose several technical challenges. First, new unit operations can have unfamiliar mechanisms and create uncertainty regarding the relationships between critical process parameters and critical quality attributes of the API. New process analytic technologies (PATs) and control strategies might be needed to operate new unit operations. Second, the introduction of a new unit operation can alter the composition or impurity profile of a process relative to a conventional process; for example, a novel, high-throughput capture step during purification might have lower selectivity than typical capture operations and transfer a greater share of the purification burden to later polishing steps. Third, the robustness of new unit operations to accommodate variations in feed stream flows while maintaining consistent output stream characteristics and to provide long-term operability at needed scales with associated failure modes needs to be demonstrated if the industry is to adopt them. Fourth, validation protocols for a new unit operation might not be well established or might need to be developed from scratch. Finally, new unit operations must integrate well within the broader process in which they are embedded with respect to processing timescales, transient time constants, equipment footprints, process-stream holdup volumes, and resource needs.

Regulatory Challenges

New and unfamiliar unit operations will lack the historical operating records and institutional experiences that instill confidence in established validation protocols and previously identified critical process parameters and performance characteristics and their connections to critical quality attributes of drug substances. In the absence of specific guidance, the first to introduce a new unit operation in an investigational new drug application, a new drug application, or a biologic license application will bear the burden of demonstrating that the new process and its mechanism of operation, performance characteristics, and critical quality attributes are well understood and that the validation protocol and results are sufficient to establish robustness. Both applicants and regulators will need to be convinced that the unknown risks have been minimized such that the product and patient-safety risks associated with deploying an innovative unit operation are commensurate with or smaller than those posed by the established unit operation that it is replacing.

PROCESS INTENSIFICATION

Process intensification can be defined as “the development of novel apparatuses and techniques that, compared to those commonly used today, are expected to bring dramatic improvements in manufacturing and processing, substantially decreasing equipment-size/production-capacity ratio, energy consumption, or waste production, and ultimately resulting in cheaper, sustainable technologies” (Stankiewicz and Moulijn 2000, p. 23). In its roadmap for biomanufacturing technologies, the BioPhorum Operations Group (BPOG) has classified process intensification according to how it is achieved—by manipulating the supporting infrastructure of an existing process without changing unit operations or operating parameters, by changing operating parameters within existing unit operations, by changing raw-material use in existing unit operations, or by substantially changing process flow with disruptive technologies (BPOG 2017b). In the context of anticipated innovations in the manufacture of APIs, the committee discusses intensification in terms of the last category, the one with the greatest effects, specifically addressing the integration or reduction of multiple traditional unit operations, the replacement of traditionally batch unit operations with continuous formats, and the incorporation of recirculation and recycle in unit operations and processes.

Integrated Unit Operations

By analogy with the chemical-process industries in which efficiency considerations have driven the integration of reactor-separator unit operations, such as reactive distillation and reactive extraction, the pharmaceutical industry is developing new combinations of unit operations that have enhanced performance and efficiency. For example, in the upstream processing of biologics, novel seed trains that use high-density cell lines with high-nutrient inoculation media and N-1 perfusion can shrink the number of discrete cell-expansion operations and sub-

stantially shorten overall culture times. Innovations are also expected in product harvest and capture operations, which are critical steps at the interface between upstream and downstream processes. Here, specific innovations include the use of precipitants in bioreactors to remove cell debris, host-cell proteins, and host DNA before supernatant harvest and the introduction of combined clarification and product-capture devices. Furthermore, viral filters that contain filter media with viral-inactivating coatings combine two orthogonal modes of viral clearance that are traditionally conducted in separate unit operations (viral filtration and viral inactivation) into a single unit operation.

An important element of integrative intensification for the manufacture of biologics that bears mentioning separately is solution preparation. This seemingly mundane aspect of bioprocessing is a substantial process-time, labor, and complexity bottleneck and a controlling factor in setting a facility or process footprint. Intensified cell-culture operations place increased demands on media-solution preparation in that fed-batch bioreactor media needs to scale with cell-number density, and a perfusion bioreactor needs to scale with perfusion rate. Buffer use in the downstream process scales with titer, and many buffer solutions are required, particularly to support chromatographic operations. Although traditional batch solution preparation is giving way to in-line dilution of concentrates, further intensification is expected. A unit for on-demand preparation of buffer solutions that consolidates all downstream process buffer preparation into a single unit operation is under development as part of a collaboration between the National Institute for Innovation in Manufacturing Biopharmaceuticals (NIIMBL) and BPOG with broad industry participation. Given the intensity of industry interest, it is likely to be deployed soon (BPOG 2019), and the concept is likely to be extended to on-demand cell-culture media preparation.

Continuous Unit Operations

Unit operations that have a long history of use in batch or semi-batch modes are being converted to continuous mode in an effort to capture all the benefits of continuous operations: smaller footprint, decreased material use, higher throughput and yield, and, ultimately, cost efficiencies. Continuous operation also provides the potential for achieving true steady-state conditions that ensure consistent attainment of critical quality attributes of the product during operation. For small-molecule APIs, flow chemistry offers many additional benefits in upstream processing given the often complex and hazardous reactions that are involved in API generation. It can decrease the volumes of hazardous reactants and solvents that are handled in a process at a given time, restrict extreme reaction conditions to short residence times, avoid the isolation of hazardous intermediates, control the formation of products and side-products by manipulating serial and parallel reactions, and enable more efficient reactor designs (Burcham et al. 2018). For biologics, there is precedence for continuous unit operations given the long-standing upstream use of perfusion cell culture to enable production of labile APIs that would otherwise be substantially degraded if batch operations were used. Another continuous-processing example can be found in the more recent introduction of periodic continuous chromatography in downstream processing operations to enable full use of target-binding capacity of expensive chromatographic resins, such as the protein A media used to capture mAbs. Similarly, for small-molecule APIs, precedence is provided by continuous drug-product processing, which has extended traditional continuous unit operations, such as tableting and capsule-filling steps, to end-to-end drug-product formulation and filling processes (Burcham et al. 2018).

Further innovations in continuous processing for small-molecule APIs are expected to include the incorporation of flow chemistry with novel reaction mechanisms and reactor formats to enable photochemical, electrochemical, and serial biochemical catalysis; the development of hybrid batch-continuous reactors or intermittent-flow stirred tank reactors to facilitate the conduct of heterogeneous reactions in upstream processes; and membrane separations to replace distillation or crystallization operations in downstream processes (Burcham et al. 2018). Biologics manufacturing will likely see the conversion of periodic continuous-chromatography formats to fully continuous formats, such as countercurrent tangential chromatography (Shinkazh et al. 2011); the introduction of continuous precipitation and extraction operations to replace column chromatography for capture steps (Sheth et al. 2014; Li et al. 2019); the introduction of continuous viral inactivation processes based on tubular contactors rather than traditional batch-stirred tanks (Orozco et al. 2017; Gillespie et al. 2019); continuous viral filtration formats (David et al. 2019); and continuous ultrafiltration–diafiltration for preformulation of drug substances (Jabra et al. 2019; Yehl et al. 2019).

Incorporation of Recirculation and Recycle

Recirculation is the retrograde flow of material within a unit operation, and recycle involves flows of process

streams from later unit operations to earlier unit operations. Both offer opportunities for API yield improvement, more efficient use of raw materials, reductions in waste generation, and improved process control by manipulating physical material feedback. There is ample precedence for accepting recirculation in a unit operation. For example, it is used in perfusion cell-culture systems with cell recirculation, batch ultrafiltration and diafiltration operations based on retentate recirculation, and mixed-suspension–mixed-product removal crystallization with mother-liquor recirculation. Innovative unit operations that use recirculation include countercurrent flows of wash buffers in continuous countercurrent tangential chromatography and in continuous precipitation operations. The recirculation of formulated, small-molecule API powder blends has also been used with additive manufacturing technology for tablet-formation operations as described in Chapter 3.

Incorporating recycle loops in a process is a bigger innovative leap than incorporating recirculation loops. An example is the recycle of heterogeneous catalysts used in flow chemistry by coupling flow reactors to continuous membrane separators (Burcham 2018). Another is the recycle of mother liquor from crystallizers to upstream reaction stages in small-molecule API production to improve yield (Patrascu and Barton 2019). In the production of biologics, the reuse of chromatography regeneration and equilibration solutions and the routing and augmenting of spent precipitants from downstream precipitation-based capture purification operations to upstream clarification operations are examples in which recycle can substantially reduce buffer use and waste-stream volumes. The rise of more fully continuous processes will provide opportunities for the recovery and reprocessing of APIs diverted after a processing fault.

Technical Challenges

The technical challenges associated with process intensification include those associated with the introduction of innovative unit operations and are perhaps magnified by the greater scope of innovation involved. However, additional challenges are associated with integration, continuous processing, and incorporation of recirculation and recycle. The integration of unit operations leads to several efficiencies: a reduction in the total number of unit operations, each of which has finite yields and opportunities for faults, errors, and contamination events; a reduction in process footprint that results in smaller manufacturing suites; and a reduction in cost of goods. The tradeoff is that the integrated unit operation is likely to be more complex mechanically or operationally because multiple mechanisms have been combined to achieve multiple process-quality goals simultaneously in a single unit operation. That complexity is typically overcome through the implementation of suitable process-control systems and strategies that admittedly might also be more complex than the process control implemented for less intensive operations and processes. The integrated operation might also be more reliant on specialized raw materials, media, or consumables than the separate unit operations that it replaces.

Continuous operations, as discussed further in Chapters 4 and 5, require the development of safe and efficient process startup and shutdown procedures and mechanisms for tracking and diverting nonconforming material that might have been generated as a result of faults that the process-control system cannot overcome. Continuous operation will likely require parallel enabling innovations in process-control technology and strategy and in the associated in-line PAT to achieve and maintain steady-state operation and to handle transients, fluctuations, faults, and restarts; these innovations will ensure that a “state of control” is maintained during process operations. Such innovations might include new types of sensing modalities. For example, sensors that use Raman spectroscopy have already made inroads in bioreactor monitoring and might see application to downstream unit operations. It should be noted that continuous unit operations typically have much shorter timescales in which process decisions must be made than do batch operations.

Recirculation and recycle provide enhanced efficiencies and the ability to control stream composition and flow characteristics directly. However, those benefits come at the expense of the potential for accumulation of process-related and product-related impurities associated with the reverse flow of streams within or between unit operations and the potential for delayed and oscillatory responses to process disturbances and control actions because of increased system time constants that result from retrograde stream flows.

Regulatory Challenges

Several regulatory challenges arise with process intensification and are compounded versions of the challenges associated with novel unit operations. The stakes are higher because a larger portion of the overall process or the increase in processing objectives is typically involved in an intensification innovation relative to a unit operation innovation. For integrated unit operations, the compounding arises from the concatenation of the uncertainties of

two or more processing objectives, such as a combined clarification and capture step for biologics. Process intensification also might reduce operational redundancies that are viewed as a process safety net. In continuous unit operations, the complexity of the integrated PAT and control systems and the short process decision-making timescales compound uncertainties. Sequential continuous unit operations that have low residence times also might eliminate the accumulation of a process intermediate and thus the intermediate quality-assurance and quality-control data that have traditionally supported drug-substance release. If a continuous downstream operation is connected directly to a continuous formulation operation, “drug substance” might cease to exist as anything other than as a transient intermediate and might lead to the elimination of drug-substance release testing. Furthermore, in continuous operations, there is a need to focus on residence-time distributions of process units rather than on batch histories. The committee notes that both recirculation and recycle have traditionally been avoided in API production, given concerns about retaining the identity of a lot as it progresses through unit operations and the potential for the backward propagation of out-of-specification APIs or contaminants.

PROCESS INNOVATIONS THAT CREATE NEW STREAM COMPOSITIONS

New stream compositions arise from upstream operations that incorporate innovations in synthetic chemistry and in host-cell selection and engineering. They also result from the production of completely new types of drug substances and from the introduction of excipients upstream of formulation and filling operations. The new stream compositions might include differences from conventional processing in the distribution of product variants, impurities, and additives; might lead to changes in how individual downstream unit operations perform; and might require wholesale reorganizations of downstream operations.

New Routes to Production of Drug Substances

For small-molecule APIs, innovations in upstream processing are being driven by improvements in synthetic efficiency, the increasing complexity of APIs (such as oligonucleotides, large macrocycles, and peptides), the desire to reduce the formation of side products and to use more environmentally friendly synthetic routes, and the need to reduce risks in handling hazardous reagents, solvents, and reactions. New synthetic routes are being based on photochemistry to form new types of bonds, access complex synthetic scaffolds, and control stereoselectivity; electrochemistry to take advantage of high chemoselectivity; and biocatalysis that uses engineered enzymes and single-pot multienzyme reaction cascades (Tom 2020). The latter case will likely extend to biologic APIs for which the engineering of post-translational modifications—such as N-glycan structure remodeling or elaboration for enhanced biologic activity—might be performed on partially purified material after cell culture.

For biologics, the drivers for innovation—increased volumetric productivity and simplification of and decreased burden on downstream purification operations—are similar to those for small-molecule APIs. As discussed earlier, cell engineering and bioreactor strategies have led to dramatically increased titers and specific cellular productivities of mAbs. The corresponding increased concentrations, viscosities, and physical-stability concerns will challenge the capacities, operating characteristics, and flow behaviors of traditional downstream unit operations, such as column chromatography. In addition, new cell-culture monitoring and control strategies that are based on spectroscopic probes and reporter species might reveal cell-stress levels during high-concentration cell culture and lead to culture media and feeding enhancements that result in improved product quality by narrowing the distribution of product variants formed.

Further improvements in production of biologics are likely to come from alternative hosts, including new mammalian cell lines (for example, human cell lines) that have shorter doubling times and increased genotypic and phenotypic stability (BPOG 2017b). The use of hosts that have increased stability might reduce the amount of product-related contaminants that are formed during product expression and are difficult to remove, such as glycosylation variants that are formed during mAb production or homodimers and half-molecules that are formed during bispecific antibody production with hosts designed for heterodimer expression. Eliminating those contaminants would help to increase product yields, reduce the number of challenging polishing purification steps that are required in the downstream process, and ultimately reduce important production barriers (NIIMBL 2017).

Advances in production of biologics are also anticipated to come from faster-growing, nonmammalian hosts that offer advantages over their mammalian host-cell counterparts (BPOG 2017b). Among such nonmammalian hosts, yeast is one of the most popular alternatives; multiple companies are developing this host for protein-drug expression because required upfront investment and cost of production are lower. Although native yeast cells

are problematic because they attach nonhuman glycan structures to proteins, engineered yeast-cell lines that can modify secreted protein products with more human-like glycans have been developed. Other nonmammalian expression hosts that have garnered attention include filamentous fungi, insect cells and larvae, microalgae, protozoa, silkworms, transgenic plants, and a plethora of bacterial hosts, such as Bacillus and Lactococcus genera, Pseudomonas fluorescens, and Ralstonia eutropha.

That nonmammalian hosts are typically free from contaminating mammalian adventitious virus eliminates the need for dedicated viral clearance operations that accompany mammalian hosts and thereby simplifies downstream processing. For products with post-translational modifications, pathway engineering is expected to provide enhancements to rapidly growing hosts that have limited native post-translational modification capabilities; this has been accomplished recently in yeast. Escherichia coli, which has a long history in biomanufacturing, has also been engineered for important post-translational modifications, including disulfide bond formation and glycosylation with human-like glycan structures; the post-translation modifications can be performed on both intracellular proteins and those secreted into the extracellular culture medium. Other innovations in host-cell engineering might be directed at eliminating problematic proteins that tend to co-purify with the target species and at identifying and mitigating inhibitory metabolites. The ready availability of a variety of gene-editing tools, coupled with nonmammalian hosts that have smaller genomes, will make host-cell engineering routine.

Another innovation expected in the production of biologics is the elimination of host cells altogether in favor of cell-free protein synthesis (CFPS) systems. In these systems, cell lysates derived from eukaryotes (such as Chinese hamster ovary [CHO] cells, wheat germ, and yeast) or bacteria (such as E. coli) are combined with vector DNA, amino acids, accessory proteins, nucleotides, and molecular energy sources to express recombinant proteins (Rao 2020). In CFPS-based manufacturing, the cell-culture and harvest steps have not been eliminated from the process; rather, they have been placed ahead of the product biosynthesis step to supply and refresh, as needed, the active biosynthetic reagents, which have finite half-lives. Processes that take days or weeks to design, prepare, and execute in vivo can potentially be implemented more rapidly in a cell-free system. In addition, CFPS systems that use E. coli can produce grams-of-protein-per-liter reaction volume; can support co-translational or post-translational modifications, such as glycosylation (Oza et al. 2015; Jaroentomeechai et al. 2018); and have reaction scales that have reached 100-L (Zawada et al. 2011). Such systems also offer the potential for less complex, better-defined process streams that are less susceptible to adventitious agents and would dramatically simplify the downstream process. They also offer the potential for producing products that would otherwise be toxic to intact host cells. Finally, CFPS systems can be freeze-dried for long-term storage at ambient temperature (Pardee et al. 2014, 2016a; Salehi et al. 2016) and then reconstituted for on-demand protein synthesis by adding water; this was recently demonstrated for protein subunit vaccines (Pardee et al. 2016b) but could be envisaged for other biologics. CFPS technology has also been adapted for portable expression of therapeutic proteins by using an integrated manufacturing platform that fits inside a suitcase (Adiga et al. 2018, 2020). In that situation, the cell-culture and biosynthetic-reagent harvest steps are operated asynchronously from the rest of the process; the cell lysates become another raw material for biosynthesis of the biologic. Accordingly, CFPS systems will give rise to new supply chains that are ideal for decentralized, cold-chain–independent production of biologics. Given the challenges of larger scale operation of this new, bifurcated approach to upstream processing, CFPS will likely debut with smaller scale production systems, perhaps even portable production systems, in which the target patient population is small, product potency is high, or remote access is required.

New Modalities with New Attributes and New Impurity Profiles

The array of new modalities is poised for rapid expansion. Antibody-related products make up one wave of expansion. An example is next-generation antibody–drug conjugates (ADCs) that are designed for site-specific warhead (cytotoxin) conjugation by incorporating one or more unnatural amino acids into the amino acid sequence of the mAb portion to enable bioorthogonal click chemistry for warhead attachment (NIIMBL 2017). That approach would necessitate an array of process innovations, including the introduction of a novel host-cell line that can carry out the incorporation during protein synthesis, the use of an unnatural amino acid in the culture media, the conduct of a new bioorthogonal conjugation reaction that uses different solvents to link the modified mAb with the cytotoxin, and the presumed simplification of the later chromatographic or filtration-based conjugate-purification operations. The physical and chemical stability of the new conjugate will also have implications for formulation operations and process safety given the extreme toxic-

ity of the warheads used. Future anticipated modalities that are within the Food and Drug Administration (FDA) Center for Drug Evaluation and Research oversight span oligonucleotides, cell-derived vesicles (such as mammalian exosomes and bacterial outer membrane vesicles), species that are purposely designed to be labile, and high-complexity small molecules. Such new modalities enable exploitation of new therapeutic routes and might rely on multiple catalytic or biocatalytic steps and new purification-unit operations.

Formulation During Downstream Processing

Formulation operations traditionally begin after the generation of an API with a primary aim of stabilizing and preserving its activity. However, it is possible to add excipients before formulation operations to boost API yields and manipulate stream properties during downstream processing. Innovations in this context include the use of stabilizing excipients during the chromatographic purification of recombinant protein-based and nucleic acid-based APIs and the addition of viscosity-reducing excipients to facilitate the downstream processing of high-concentration recombinant-protein streams, such as mAbs.

“Co-processed” small-molecule APIs in which a nonactive excipient, additive, or carrier component is added during the production of a drug substance—typically in particle formation, crystallization, or drying operations—can offer the possibility of improved stability of a desired solid state or tailored API physical properties (Schenck et al. 2020). Co-processing also might enable the tableting of an otherwise unprocessable API. For example, a highly hydrophobic, poorly soluble small-molecule API will typically be easier to dissolve and have much greater bioavailability in an amorphous, precipitated form vs a crystalline form because the crystalline solid is more thermodynamically stable than the corresponding amorphous solid. However, the more desirable, but less stable, amorphous form will be prone to crystalize because of energy inputs and random energetic fluctuations during processing to make the drug substance. To prevent the crystallization, an API in solution might be adsorbed into a porous carrier particle, and the loaded particle suspension dried to form a stabilized amorphous API phase within the pores of the particle. In that case, the API-loaded particles effectively make up the drug substance.

Technical Challenges

New stream compositions might have different distributions of product variants, impurities, and additives from those in conventional processing and might require changes in or wholesale reorganization of downstream unit operations. For novel synthetic approaches to small-molecule APIs, new reagents, reactor types, PAT, and operating and control strategies will likely be required, and these changes will have important implications for manufacturing processes. Similarly, novel cellular hosts used in the production of biologics might require novel growth media, feeding strategies, and monitoring and control strategies. For both novel cellular hosts and cell-free synthesis platforms, the achievable scale of production and nonhuman glycosylation are substantial impediments. Also challenging for the development of innovative expression systems based on living cells or cell-free extracts are the various impurities—for example, intracellular and secreted biomolecules, such as proteins, nucleic acids, and lipids or glycolipids—that each system introduces. The impurities are different from those arising during conventional CHO-based manufacturing and thus will need to be carefully characterized at all scales of production and will require appropriate analytic tools for offline and in-line monitoring. In addition, depending on the nature and quantities of the impurities, alternative hosts and expression systems will likely require customized downstream processing steps to ensure efficient removal of any system-specific contaminants. As discussed above, a variety of process innovations will likely be required for producing novel modalities, such as antibody–drug conjugates, and the stability of the new conjugate will also have implications for formulation operations and for process safety. Finally, for co-processed APIs, the unit operations required for production are more closely aligned with the equipment or capabilities of solvent-based processing operations found in a drug-substance manufacturing facility. And these operations are not compatible with most drug-product manufacturing facilities.

Regulatory Challenges

Production of APIs by using new synthetic routes or new host cells creates uncertainties in the type and distribution of contaminants and raises questions about the appropriate or tolerable levels of contaminants in setting product specifications. The same uncertainties and questions will arise with the production on new modalities.

An important regulatory issue arises in the case of co-processed APIs. If a co-processed API is defined as a drug substance, key quality attributes and the impurity profile would be determined for the co-processed API, and the stability dating period that is established for the drug product would be independent of the time of production of the

co-processed API. However, defining the co-processed API as a drug-product intermediate would require that the stability date be set at the point of manufacture of the co-processed API rather than when the co-processed API is converted to a drug product. The effect of that difference in the start of stability date could lead to the drug product entering the supply chain with an earlier expiration date and thus could create a risk to supply. In addition, if no drug substance is isolated, the drug-substance stability testing expected under ICH Q1A(R2) (FDA 2003) is not possible; this will necessitate an uncertain importation of the associated stability-testing requirements into the drug-product testing regimen.

OVERCOMING REGULATORY CHALLENGES

Perhaps the main challenge associated with innovation in the manufacture of a drug substance, and with innovation more generally, is the lack of familiarity on the part of process-development scientists and engineers and on the part of regulators. The antidote to lack of familiarity is experience. In some cases, the experience might already be in house as in the adoption of techniques traditionally associated with plasma fractionation for the purification of biologic APIs that are under the purview of the FDA Center for Biologics Evaluation and Research. In the absence of in-house expertise, FDA active participation in public-private partnerships, such as NIIMBL, to alleviate risk associated with precompetitive innovation spaces might have great utility. The committee notes that the formation of consortia requires the acknowledgment by industry that the key intellectual property is vested in APIs rather than in the manufacturing process.

As noted in Chapter 1, FDA has provided a vehicle for providing preliminary feedback on technologic innovations with the establishment of the Emerging Technology Team (ETT); the effectiveness of the ETT in increasing the pace of innovation throughout the pharmaceutical industry would be enhanced by its working with consortia vs one-off interactions with individual manufacturers. Furthermore, periodic rotation of FDA reviewers and inspectors through assignments within the ETT might empower a broader cadre of regulators to be better informed and deal efficiently with innovations in drug-substance manufacture. The compilation and availability of case studies of successful introductions of innovations and even of common themes and characteristics of unsuccessful introductions would also be an extremely useful resource if confidentiality limitations can be overcome. Finally, FDA might consider providing some extramural research funding to consortia (such as NIIMBL and the Advanced Mammalian Biomanufacturing Innovation Center), other relevant Manufacturing USA institutes (including America Makes, the Smart Manufacturing Institute, and the Rapid Advancement in Process Intensification Deployment Institute), or independent, FDA-sponsored pharmaceutical-manufacturing innovation centers specifically targeted to help drive research and development efforts to alleviate risks associated with new technologies. FDA does offer extramural funding through the Broad Agency Announcement process; this mechanism could be used to advance manufacturing innovation further with additional support. Any new targeted funding initiatives would likely require new resources, which might be provided through consortium agreements or included as part of a new Prescription Drug User Fee Amendment agreement.

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