Process Development and Control with Recent New FBRM, PVM, and

Article pubs.acs.org/OPRD

Process Development and Control with Recent New FBRM, PVM, and IR George Zhou,* Aaron Moment, James Cuff, Wes Schafer, Charles Orella, Eric Sirota, Xiaoyi Gong, and Christopher Welch Merck Sharp & Dohme Corporation, P.O. Box 2000 RY818-C306, Rahway, New Jersey 07065, United States ABSTRACT: Process analytical technologies (PATs) have played an important role in process development and optimization throughout the pharmaceutical industry. Recent new PATs, including in-process video microscopy (PVM), a new generation of focused-beam reflectance measurement (FBRM), miniature process IR spectroscopy, and a flow IR sensor, have been evaluated, demonstrated, and utilized in the process development of many drug substances. First, PVM has filled a technical gap by providing the capability to study morphology for particle engineering by visualizing particles in real time without compromising the integrity of sample. Second, the new FBRM G series has closed gaps associated with the old S series with respect to probe fouling, bearing reliability, data analysis, and software integration. Third, a miniaturized process IR analyzer has brought forth the benefits of increased robustness, enhanced performance, improved usability, and ease of use, especially at scale-up. Finally, a miniaturized flow IR sensor has provided process flow chemistry development with a smaller, faster-performing, less expensive analytical tool.

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smaller footprint for continuous flow analysis. This kind of lowvolume IR flow cell is designed to be used as a true inline analytical detector for flow chemistry, providing quantitative and qualitative data as a smaller, faster, better-performing, less expensive platform analytical tool.17 It could enable the screening and optimization of flow reactions in real time, saving valuable time and materials because optimization of flow chemistry reactions is often slowed when offline analytical techniques must be used to provide yield and purity information. For drug substances, one of the most critical areas of process development is the selection, design, optimization, and scale-up of crystallization processes. A fundamental understanding of the controlling features as well as the dynamic nature of these particulate processes is essential to support the development of formulations, to ensure a facile and robust manufacturing process, and to add to the body of knowledge needed for regulatory acceptance. PAT based on focused-beam reflectance measurement (FBRM) is the online method most commonly used for the determination of particle size distributions in the development of crystallization processes. It has been used for process development and scale-up and found wide applications in labs, pilot plants, and factories for monitoring of particulate processes.4,8−10 Real-time monitoring of the solid phase in a crystallization slurry is particularly attractive because it obviates the need for sample manipulation. Compared with offline methods such as MicroTrac, FBRM offers the distinct advantage of chord length distribution profiling in the crystallization process. However, while using the traditional FBRM S series, probe fouling (e.g., stuck particles) often occurs. Moreover, instrument nonlinearity, reliability of bearings, bias of surface features, and outdated software are among the limitations and

INTRODUCTION Process analytical technology (PAT) has been widely applied in chemical synthesis or purification as a means to monitor reactions or crystallization in the laboratory and manufacturing sites.1−4,15 It has been used as a valuable tool to provide better understanding of synthetic processes and hence ensure the quality of drug substances. It also serves as a useful tool to monitor synthetic processes under hazardous or harsh conditions by avoiding sampling of potent reaction intermediates or compounds at high temperature and/or pressure. In addition, it serves as an effective tool to gain information on transient, nonisolated, unstable intermediates via inline or online monitoring. PAT based on Fourier transform infrared (IR) spectroscopy has been used as a versatile means of monitoring chemical processes.1,2,5 With the online IR analyzer one can not only obtain kinetic information at an early stage of process development but also carry out online analysis during production.2 IR spectrometers are widely used for either online, at-line, or offline measurements because they can be used to elucidate reaction mechanisms and obtain kinetic information for the design of robust processes, achieving safer process scale-up.6 Recent advances in IR instrumentation have added more flexibility to the existing IR analyzers in the lab and at scale-up facilities. With a miniaturized interferometer, process IR can utilize reduced-size optics that bring forth high light throughput, allowing an ambient deuterated triglycine sulfate (DTGS) detector to be used and avoiding the need for a sensitive but cryogenic detector. Thus, compared with the existing IR technology at scale-up, this kind of miniaturized IR could bring forth the following benefits: (a) increased robustness, (b) enhanced performance, (c) improved usability/ease of use, and (d) increased applicability in pilot plants and at manufacturing sites. Similarly, for lab settings, this miniaturized interferometer has brought forth a small-flow-cell IR detector with a much © XXXX American Chemical Society

Special Issue: Process Analytical Technologies (PAT) 14 Received: March 21, 2014

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dx.doi.org/10.1021/op5000978 | Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development

Article

IR analyzers was used to control the operation and collect IR spectra. Lasentec FBRM (models G400 and S400 by Mettler Toledo AutoChem, Columbia, MD) was employed to measure the chord length distribution profiles or to check for the presence of crystals or solid species during the process. An in-process video microscope (model 819 by Mettler Toledo AutoChem, Columbia, MD) was employed to capture the inline morphology of crystals or solid species during the processes, especially those of crystallization.

drawbacks associated with the FBRM S series. With recent advances in instrumentation, especially the new FBRM G400, these problems associated with the older S series have been mitigated. The FBRM G series can achieve more accurate measurement, improved data quality, and more reliable performance.18 To enhance particle engineering, in-process video microscopy (PVM) can be used in conjunction with other particle characterization techniques such as FBRM to provide a suite of information on particle size, morphology, and crystal habit/ form.9−13,19−21 It enables the study of morphology by visualizing particles inline and in situ without compromising sample integrity or requiring sample preparation. This technology enables visualization of crystal growth or phase conversion and differentiation between particle agglomeration, deaggregation, crystal growth, and primary and secondary nucleation as well as providing dynamic information on polymorph transitions in crystallization processes that otherwise is not available to engineers or chemists. As early as the beginning of this century, first-generation PVM was deployed in industry sectors for particle engineering.12 However, because of the limit of resolution, it did not find wide application until the new generation appeared in recent years. Therefore, in view of the challenges and high demands of process development and scale-up, new PAT tools such as miniaturized IRs, FBRM G400, and PVM can be used to study processes with improved agility, improved data quality, and reliable performance. To do so, herein we highlight the utility and benefits of these new PAT tools in selected case studies.

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RESULTS AND DISCUSSION Process Scale-Up with Miniaturized FTIR. To highlight the impact of recent new PAT tools on process development and scale-up, a synthetic step of an intermediate for a Merck compound was chosen for online monitoring and control with ReactIR247 at scale-up. This step, shown in Scheme 1, illustrates Scheme 1. Reaction of n-butyllithium with diisopropylamine (DIPA) to form lithium diisopropylamide (LDA)

that lithium diisopropylamide (LDA) is formed from diisopropylamine (DIPA) and n-butyllithium. Subsequently, LDA is reacted with ethyl 4-(diethoxyphosphoryl)piperidine-1-carboxylate (PIP-P) to form the Li-PIP-P intermediate (Scheme 2).

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Scheme 2. Metalation of PIP-P with LDA to form Li-PIP-P

EXPERIMENTAL SECTION Materials. Several active pharmaceutical ingredients of drug candidates were used in the case studies. They are MK-A, MK-B, and MK-C for crystallization studies. Solvents such as ethanol (HPLC grade), n-heptane (ACS grade), toluene (ACS grade), THF (ACS grade), DMF (ACS grade), acetic acid and sodium acetate (both ACS grade), methylmagnesium chloride in THF, diisopropylamine (DIPA), n-butyllithium in hexanes, isoprene, maleic anhydride, 2-fluoro-4-methoxyacetophenone, and conventional chemicals (ACS grade) were all obtained from SigmaAldrich (St. Louis, MO). Ethyl 4-(diethoxyphosphoryl)piperidine-1-carboxylate was obtained from Ipca Laboratories (Maharashtra, India). Equipment. An FTIR analyzer (ReactIR247 by Mettler Toledo AutoChem, Columbia, MD) equipped with an attenuated total reflectance (ATR) probe (DiComp) was inserted in a flow-through cell of a recirculation loop to measure the concentration of species in a process stream. This reactor/ pipeline was gastight with a tightly controlled moisture level. Spectral resolution of 8 cm−1, Happ−Genzel apodization, spectral region of 2000−650 cm−1, and a DTGS detector were used. Each spectrum was composed of 64 coadded scans. The spectrum of nitrogen gas at room temperature was used as the background. Similarly, a miniaturized IR detector (FlowIR by Mettler Toledo AutoChem, Columbia, MD) with a low-volume flow cell of ATR sensor (SiComp) was used to measure the concentration of species in processes. Spectral resolution of 8 cm−1, Happ− Genzel apodization, spectral region of 4000−650 cm−1, and a DTGS detector were also used. Each spectrum had 32 coadded scans, and background spectrum of nitrogen gas at room temperature was used. The software “iC IR” version 4.2 for both

Process variations such as the n-butyllithium concentration and moisture level are inherent and would affect the total amount of LDA available. Meanwhile, when LDA is left unreacted, it readily reacts with other species in the subsequent steps, leading to high levels of impurities. On the other hand, if LDA is all consumed and PIP-P is overcharged, this is not detrimental to the impurity profile of the final product but does waste the extra amount of PIP-P. To minimize impurity formation, process operation requires real-time monitoring of the LDA and PIP-P concentrations by controlling the consumption rate of LDA. For the activation of butyllithium, when butyllithium was charged into the solution of DIPA in dry THF, LDA was formed, as indicated by its absorptions at 1157 and 1350 cm−1 (Figure 1). Peak areas of the 1157 cm−1 peak were therefore used for profiling of LDA to obtain the process information and control the process, especially the end point of LDA consumption (see Figure 2). In regard to the formation of Li-PIP-P from the metalation reaction of PIP-P with LDA, the peak at 1700 cm−1 was chosen to track the concentration of PIP-P. During the titration of LDA with PIP-P, the intermediate, Li-PIP-P, does not have a stable carbonyl absorption. Only after all of the LDA is consumed does the absorption of the carbonyl group of PIP-P at 1700 cm−1 appear. Therefore, the appearance of the peak at 1700 B



Article

Figure 1. (a) Selected IR spectra (1800−1100 cm−1) of interest during the processes of the reaction step of the Li-PIP-P intermediate and (b) 3D spectra illustrating the end point of Li-PIP-P formation at scale-up.

cm−1 can be used exclusively for calling the end point of the LDA titration. Two possible dosing strategies were examined in the lab, one with PIP-P overcharged slightly (Figure 2a) and the other with PIP-P undercharged (Figure 2b). When PIP-P is overcharged slightly, which is the case in Figure 2a, all of the LDA is consumed completely, eliminating the risks of forming impurities in the subsequent step due to the reactive nature of LDA. In addition, the slight excess of PIP-P can be removed downstream and does not impact the impurity profile. As in Figure 2a, the slight increase in PIP-P while LDA has leveled off helps to call the end point of the reaction qualitatively. That is, when PIP-P is slightly overcharged, the PIP-P profile popped up and maintained a consistent level because Li-PIP-P does not have a carbonyl absorption. In contrast, when PIP-P is undercharged, which is the case in Figure 2b, not all of the LDA is consumed, resulting in the formation of impurities when the active LDA reacts with species such as diphenyl methanone in the subsequent step. Therefore, the dosing strategy is to have all of the LDA reacted away at low temperature with PIP-P slightly overcharged. In order to scale it up, the process procedure has been developed to have an online ReactIR247 to monitor and control the LDA formation, PIP-P consumption, and subsequent diphenyl methanone addition. Figure 2c shows the results of a scale-up run. The batch with dry DIPA solution in THF is recycled through the recirculation loop of ReactIR247 prior to starting the n-butyllithium charge. The n-butyllithium charge takes ∼7 h, which is controlled by holding the batch temperature around −18 °C. As shown in Figure 2c, the LDA profile increases continuously as more nbutyllithium is added. After the reaction mixture ages over half an hour, PIP-P is added, which is illustrated by the decrease in the LDA profile since LDA is reacted away by PIP-P and there is no accumulation of PIP-P. When the LDA has been entirely consumed, its profile levels off, and at the same time the profile of PIP-P (carbonyl group around 1700 cm−1) increases, reaching the end point of the reaction. Furthermore, with the blow-out of the PIP-P addition line, the PIP-P peak increases a little bit further. Meanwhile, a detection limit of 0.95% equivalent of LDA has been estimated on the basis of the IR profile. This allows the reaction end point to be determined on the basis of the IR

profiles of LDA and PIP-P to ensure the complete consumption of LDA when PIP-P is slightly overcharged at ∼1% equivalent. Thus, it can minimize the formation of impurities in the subsequent steps when a small amount of LDA is left unreacted and avoid excessive consumption of PIP-P when it is overcharged, ensuring the correct addition of diphenyl methanone in the subsequent step. Process Development with FlowIR. Grignard reactions are widely used for carbon−carbon bond formation. Because of the potential exothermic nature of these reactions, process scaleup of Grignard reactions inherently carries potential safety hazards.6 Therefore, when a Grignard reaction runs in batch mode, especially at scale-up, it is important to ensure that the reaction proceeds at a controllable rate.2,6,16 In a continuous process mode, the risk that a Grignard reaction will run away can be substantially reduced because the size or volume involved in the reaction is much smaller than in a batch process. In such a case, online monitoring with an FTIR sensor can play an important role in developing and controlling the continuous process. As an example of Grignard reaction scale-up, the formation of the tertiary alcohol 2-(2-fluoro-4-methoxyphenyl)-2-propanol, an intermediate of a drug candidate under development by Merck Sharp & Dohme Corp., was followed using a FlowIR sensor. Scheme 3 shows the reaction, in which the two starting materials, the Grignard reagent and the acetophenone (2-fluoro4-methoxyacetophenone), form the tertiary alcohol. The FlowIR sensor is connected to a flow reactor with two syringe pumps introducing the starting materials, one for 3 M methylmagnesium chloride in THF and the other for 1 M acetophenone in THF. The reactor and IR flow cell are thermally controlled at 42−44 °C to control this exothermic reaction. In addition, both starting materials are prepared under nitrogen coverage to avoid potential moisture impact. The reaction starts with 0.8 equiv of Grignard reagent, and every 3 min the amount of Grignard reagent is increased by 0.2 equiv, up to 2.0 equiv of Grignard reagent (Figure 3b). In the end, the reaction is quenched, and the desired tertiary alcohol is isolated with further work-up. As the amount of Grignard reagent is increased, the addition rates of the two reactant pumps are varied accordingly so that the total flow rate is maintained at 1.2 C



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Figure 2. (a, b) Li-PIP-P runs with PIP-P titrating LDA in lab reactions with PIP-P (a) overcharged and (b) undercharged. (c) Scaled-up reaction with end point determination based on profiles of LDA and PIP-P.

sensitivity to accurately monitor either impurity in this study. This shows that the miniature FlowIR could be used as an online monitoring tool at the lab scale for flow reactions, allowing conditions to be optimized quickly and exothermic reactions to be run safely. Another case of application with miniaturized FlowIR is a Diels−Alder reaction in which maleic anhydride reacts with isoprene in DMF to form 1,2,3,6-tetrahydromethylphthalic anhydride7 (Scheme 4). The FlowIR sensor and three syringe pumps are connected to a column reactor with a total volume of 9.6 mL. Two syringe pumps are used to pump reagents (2 M isoprene and 2 M maleic anhydride in DMF) into the reactor. At the outlet of the flow reactor after the FlowIR sensor, a quench solution of DMF is introduced by the third syringe pump via a “T” connector. The reaction temperature is controlled with a chiller connected to the jacket of the flow reactor. This Diels−Alder reaction has been tracked with online IR monitoring. Figure 4 shows the 3D reaction spectra in the selected spectral region and profiles of maleic anhydride,

Scheme 3. Reaction of a Grignard reagent (CH3MgCl) with an acetophenone to form a tertiary alcohol in THF

mL/min to ensure a consistent reaction time. As the amount of Grignard reagent is increased from 0.8 to 2.0 molar equiv (Figure 3b), the formation and consumption of components are tracked by IR (Figure 3a), with clear steps in the reaction profiles in response to changes. In this case, the reaction does not reach completion until 2.0 equiv of Grignard agent has been added (excess Grignard agent is added to avoid impurity formation). Two main components, the starting material and product, are identified by the spectra of individual components. There are two impurities, hemiketal and aldol. However, the hemiketal impurity is typically about 1% of the final mixture as determined by LC area percentage, and the aldol is a much smaller impurity (around 0.3% of the total reaction product). Thus, IR has insufficient D



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Figure 3. Flow reaction of an acetophenone with a Grignard reagent at variable concentrations: (a) 3D reaction spectra in the selected spectral region and (b) profiles of the acetophenone and tertiary alcohol at a reaction temperature of 42−44 °C.

Process Development with PVM and FBRM G400. To ensure a facile and robust manufacturing process for drug substances, a better fundamental understanding of the controlling features of particulate processes is essential, and this can be realized by using FBRM and PVM instruments simultaneously.10,12 The new FBRM G400 probes address several problem areas associated with the older S series. Specifically, higher accuracy in interpretation has been achieved with improved linearity and sensitivity, less noise due to surface features, and software integration with other instruments such as PVM. In addition, the G series features an improved bearing design that leads to increased instrument reliability. More experiments can be interpreted and analyzed with a correction feature for fouling or stuck particles, and more flexible interpretation can be realized with features accessible in real time, including coarse/primary views and detection count. The capabilities of PVM have allowed scientists and engineers to collect meaningful microscopic images in situ. More importantly, PVM can be used in concert with other particle characterization techniques such as FBRM to provide comprehensive information on particle size, morphology, and crystal habit or form.13,14

Scheme 4. Diels−Alder reaction of maleic anhydride with isoprene in DMF to form 1,2,3,6-tetrahydromethylphthalic anhydride

isoprene, and product at a reaction temperature of 40−42 °C. Different residence times of reaction (6, 12, 24, and 48 min) are achieved as the total flow rate is varied from 1600 to 800, 400, and 200 μL/min, respectively. As the residence time varies, the reaction reaches several steady states at different degrees of completion (Figure 4b). This indicates that at 40−42 °C this reaction takes ∼48 min to reach completion. This also shows that the miniature FlowIR sensor can serve as an easy-to-use online monitoring tool to track the main components of flow reactions, allowing flow reaction conditions to be screened and optimized quickly.

Figure 4. Diels−Alder reaction in which maleic anhydride reacts with isoprene in DMF to form 1,2,3,6-tetrahydromethylphthalic anhydride: (a) 3D reaction spectra in selected spectral region and (b) profiles of maleic anhydride, isoprene, and product at a reaction temperature of 40−42 °C. E



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Figure 5. Seeded cool-down crystallization of MK-A on wet-milled seed. (a) At the seed point, new crystals nucleate on the wet-milled seed; (b) 46 min after seeding, before cool down, clusters of rod-shaped crystals have formed; (c) 61 min after seeding, before cool down, the clusters have fallen apart; (d) after the cool down, larger (>200 μm in length), wider crystals have formed; (e) in the absence of the stuck particle correction feature, PSDs by FBRM S400 for all four conditions (a−d).

To illustrate the usage of FBRM G400 and PVM for seeding, nucleation, growth, cluster formation, and breakup, a seeded

cool-down crystallization of Merck compound MK-A is discussed. A small amount (e.g., 0.5 wt % relative the to total F



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Figure 6. Seeded cool-down crystallization of MK-B on wet-milled seed. In this case, the crystal growth is primarily on the seed, leading to the formation of rod-shaped particles (>200 μm in length). The chord length distribution shifts up and to the left post-seeding, which may be misinterpreted as nucleation but is in fact growth of rod-shaped crystals.

surface on which new crystals nucleate and then grow, forming starburst clusters that subsequently break up. This is an instance in which FBRM data alone could easily be misinterpreted; more specifically, the breakup of clusters may be misinterpreted as a nucleation event. To highlight the crystal growth directly on seeds, another seeded cool-down crystallization of a Merck compound, MK-B, is discussed. In this case, about 1% wet-milled seed is added at the seeding point (Figure 6). The batch is aged until the supersaturation has been relieved and then cooled to continue the crystallization process. This is a similar cooling crystallization as discussed previously (Figure 5), but the observations concerning the seeding-point behavior are different. In contrast to the previous example, in this case there is no observed nucleation of small crystals on the seeds and no subsequent cluster formation and cluster breakup. Instead, this a more ideal case in which the crystal growth occurs on the seeds directly to form rods. In Figure 6, FBRM and PVM data together are used to unfold the story of this crystallization process. The FBRM data alone are not straightforward to interpret, as they may appear to be nucleation-dominated because the mean of the chord length distribution appears to be shifting to the left with a higher particle count. However, the PVM clearly shows the growth of rods in both the length and width directions. The reason for the increase in particle count has to do with the increasing “projected area” of the rods into the scan circle of the FBRM. There are more chord length counts, primarily of rod widths, which are more frequently counted because as the rods grow longer they more frequently encounter the scanning laser. In a similar way as in the last

weight of batch crystallized) of wet-milled seed is added at the seeding point. The batch is aged at a constant temperature until the supersaturation has been relieved at the seed point and then cooled to continue the crystallization process. The combination of FBRM and PVM was utilized to illuminate the particle behavior at the seeding point and during the subsequent crystallization operation, and the results are shown Figure 5. Figure 5a indicates that at the seeding point, small crystals nucleate on the surface of the seed particles, which are “wet milled” at ∼30 μm in size. These seeds then grow out, forming clusters that are starburst-shaped and ∼200 μm in diameter (Figure 5b). At a certain point (i.e., the 61 min mark; Figure 5c), the clusters fall apart into individual rods that then continue to grow. This breakup point is likely a function of the hydrodynamic forces in the system. At this point, the FBRM chord length distribution trace moves sharply up and to the left, consistent with an increase in the number of particles and cluster breakup. In the end, crystals grow into large rods (Figure 5d) that in some cases are twinned. These phenomena were captured with both the FBRM and PVM working in tandem. In addition, the stuck particle correction feature on the new FBRM G400 provided smooth data profiles for all of the particle size distributions (Figure 5a−d), whereas the older S400 instrument provided much noisier data under the same circumstances because of stuck particles (see Figure 5e). Under supersaturation conditions in a crystallization, crystallization of stuck particles on the surface of the probe is a common problem. With the stuck particle correction feature of this new G400 probe, a chord length distribution with much better quality can be obtained. In summary, this was a case in which the original seeds act as a G



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Figure 7. Evaluation of crystallization performance of two different lots of an intermediate using the FBRM S400 and G400 probes. In lot 21, an oil droplet phase is present, whereas in lot 9 no oil phase is present. Lot 9 crystals have sharp edges that may lead to the large peak in the FBRM S400 trace at around 5 μm. The PVM image of lot 9 shows that there are no

Process Development and Control with Recent New FBRM, PVM, and

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