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Review pubs.acs.org/OPRD

Is Avoidance of Genotoxic Intermediates/Impurities Tenable for Complex, Multistep Syntheses? David P. Elder† and Andrew Teasdale*,‡ †

GSK, Park Road, Ware, Hertfordshire SG12 ODP, U.K. AstraZeneca, Charter Way, Silk Road Business Park, Macclesfield, Cheshire SK10 2NX, U.K.

‡

S Supporting Information *

ABSTRACT: A survey of over 300 synthetic publications published in Organic Process Research & Development over a 10-year period (2001−2010) provides a top-level overview of current synthetic strategies. It reaffirms the widely held view within the pharmaceutical industry that the synthesis of complex, multistage pharmaceuticals is untenable without the use of reactive, potentially mutagenic intermediates and that calls for “avoidance” reflect a lack of awareness of the challenges inherent in modern synthetic chemistry. On the basis of this survey, we can conclude that the average number of steps required to synthesize each active pharmaceutical ingredient (API) was 6 (5.9) and that the average number of reactive intermediates per synthetic route was 4 (4.1). It was also noted that there are four major classes of reactive intermediate that are commonly utilised in the later stages of API syntheses, (i.e., the last four stages): alkyl halides, acid chlorides, aromatic amines, and Michael acceptors. There was minimal usage of highly potent compounds from the “cohort of concern”, which suggests that any additional focus on “cohort of concern” would be misplaced. Most of the cited publications gave several different alternative synthetic routes. In all cases there was no evidence to suggest that any of these alternative routes could produce the final API (of typical complexity) without the need to use reactive intermediates at some stage of the synthesis. In addition, the number of reactive intermediates remained broadly similar irrespective of which route was selected, strongly challenging the notion that avoidance was ever a viable option. This again underpins the argument that control, not avoidance or ALARP, is the most appropriate strategy in the overwhelming majority of cases.

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INTRODUCTION In the original European Medicines Agency (EMA) guidelines on genotoxic impurities (now termed mutagenic impurities (MIs)),1 a decision tree for the assessment of acceptability of MIs was provided (see Scheme 1). This decision tree indicated that for MIs presumed to be nonthresholded, the applicant needs to confirm that the presence of the impurity is unavoidable (or can be eliminated from the synthesis), to confirm that the level is as low as reasonably practicable (ALARP) (or can be reduced to ALARP levels), and finally to then apply the threshold of toxicological concern (TTC) approach, which states that if the MI (or potentially mutagenic impurity (PMI) based on structural alerts alone) is less than 1.5 μg/day, then the risk is negligible, whereas if the TTC is exceeded, then the applicant needs to either restrict or reject the affected batch of active pharmaceutical ingredient (API). Thus, the regulatory approach was originally enshrined as avoid > reduce > control (TTC). Scheme 1 shows the original EMA decision tree, the FDA guidance2 had a very similar approach. However, many industry commentators have reflected that since reactive intermediates are essential to synthesize complex, multifunctional APIs, the avoidance of risk from MIs is not tenable, as it is practically impossible to avoid entirely all reactive intermediates. It may be possible to redesign the synthesis to avoid a nominated MI or MIs, but any potentially different syntheses will in all probability have other routespecific MI(s) that would also need to be eliminated. In addition, it was strongly argued that since the TTC was acknowledged to be extremely conservative in nature, the © XXXX American Chemical Society

ALARP requirement was superfluous and the TTC was in fact a virtually safe dose (VSD). This approach was recognized in the subsequent EMA Q&A.3 The use of alternative, less reactive reagents/intermediates might be considered as a partial resolution, as any potential residues in the API would most likely have reduced DNA reactivity. However, this strategy is likely to be suboptimal, as it will often impact negatively on synthetic yields. The use of less reactive reagents may also lead to a reduction in control over other impurities, resulting in an overall increase in impurities. Other impacts include financial and environmental costs such as increased waste. Although the primacy of TTC (or staged TTC) has emerged over the last 5 years and is fully embraced in the emerging M7 guidance,4 there are still commentators (from both the pharmaceutical industry and regulatory bodies) that continue to argue that avoidance and/or reduction (i.e., ALARP) are still the prime considerations. Recent review articles5−7 have appeared to support this view. Raman et al.5 stated that “if GTIs are found then alternative synthetic routes which can control these impurities should be developed.” Similarly, a recent article7 on validation of an in silico toxicology model to predict the mutagenic potential of drug impurities indicated that ALARP was still a prime consideration: “Therefore, efforts Special Issue: Genotoxic Impurities 2015 Received: November 4, 2014

A

DOI: 10.1021/op500346q Org. Process Res. Dev. XXXX, XXX, XXX−XXX

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Organic Process Research & Development

company assessments that will have discharged the mutagenicity risk. The purpose of this publication is to provide a top-level overview of synthetic strategies and to demonstrate that complex APIs typically cannot be synthesized without resorting to the use of reactive intermediates. Typical alerting structures are shown in Scheme 2. The identities of any alerting reagents or reactive intermediates (or any resultant MIs/PMIs) have been separately tabulated, together with the appropriate synthetic stage. A reactive intermediate (or similar) flagged for more than one alerting motif within the structure (e.g., alkyl aldehyde and aromatic nitro) is represented by the alerting motifs separated by slashes (e.g., “alkyl aldehyde/aromatic nitro”), whereas alerting motifs used separately during a single synthetic stage are represented individually (e.g., “alkyl aldehyde” and “aromatic nitro”). The overall number of synthetic stages used in the manufacture is also reported along with the supporting references.

Scheme 1. EMA decision tree for assessment of acceptability of genotoxic impurities1 (now termed mutagenic impurities)

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DISCUSSION The review covered 302 publications from this journal covering a 10-year period (2001−2010). The review data are provided in Tables S-1 to S-10 in the Supporting Information. There was only one synthetic route (i.e., for netilmicin, published in 2002;10 Table S-2) that avoided the need to employ reactive intermediates during the entire synthesis. This equates to about 0.3% of the total surveyed. In this instance, the total number of synthetic stages (3) was less than the average typically employed over this time period (5.9). Netilmicin was synthesized from the corresponding naturally occurring aminoglycoside (sisomicin) using a simple protection (Zn(OAc)2/ (Ac)2O in methanol), ethylation (NaBH4/ACOH in CHCl3), and deprotection (NaOH) approach. This type of simple chemistry building from a reasonably complex starting material, often of natural origin, would appear to be readily amenable to circumventing the need for reactive intermediates, as it relates to transformation of the molecule as opposed to construction of its intrinsic framework (C−C and C−N bond formation). This reaffirms the widely held view within the pharmaceutical industry that the synthesis of complex, multistage synthetic pharmaceuticals is untenable without the use of reactive intermediates and that calls for “avoidance” immediately prior to the introduction of the guidance documents reflected a lack of awareness of the challenges inherent in modern synthetic chemistry. This point was highlighted by Delaney,11 who indicated that “the existing EMA position ignores the reality that pharmaceutical syntheses frequently require the use of intermediates that are mutagenic in order to be practical.” He also argued that “by emphasizing the need to avoid mutagenic intermediates whenever possible, the guidance sets an expectation for process designers to spend considerably longer time and effort to employ options that may be considerably less efficient, without providing significant return on investment to patients in terms of enhanced safety.” On the basis of this survey, we can conclude that the average number of steps required to synthesize each API was 5.9 and that the average number of reactive intermediates per synthetic route was 4.1. The yearly overview is compiled in Figure 1. In the first three years of the survey (2001−2003), although the average number of reaction stages remained reasonably constant at about 5, the average number of reactive intermediates per synthesis dropped from 5.8 to 4.1. The data for 2004 (Table S-4) are probably biased by several long

are made to achieve the lowest level technically feasible that would not convey significant health risks to humans.” The purpose of this review is first to assess whether it is indeed possible to synthesise complex, multifunctional pharmaceuticals while avoiding the use of reactive intermediates (or generating reactive impurities) and second, for those compounds where alternative synthetic routes have been published, to assess whether or not the risk posed by reactive intermediates is in any way reduced.

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METHODOLOGY The review was restricted to papers that were published in this journal over a 10-year period straddling the introduction of the original regulatory guidances1,2 (2001−2010) and were focused solely on pharmaceutical drug substances. Publications focused on general chemistry, key pharmaceutical intermediates, optimization of a specific stage in the synthesis, or using specialized approaches (e.g., solid-phase chemistry for peptides, oligonucleotides, etc.) were excluded. The published synthetic routes were assessed using in cerebo techniques based on Ashby−Tennant8 alerting structures, without resorting to the use of established in silico approaches (e.g., DEREK, Leadscope, CASEUltra, etc.) and without prior knowledge with respect to genotoxicity/mutagenicity/carcinogenicity, in order to mirror the process currently utilised by a significant number of regulatory chemistry, manufacturing, and control (CMC) reviewers. Obviously, some of the reactive intermediates that were flagged will have undergone additional inB


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Scheme 2. Some examples of structural alerting motifs that are known to be involved in reactions with DNA (reproduced with permission from the author9)

be added to the final API specification (i.e., option 1 in the ICH M7 guidance4). Four stages back from the API was chosen to reflect commentary by Pierson et al.,16 who indicated that a reactive intermediate can typically be purged effectively if it is introduced four or more stages prior to the final API isolation stage (i.e., option 3 in the ICH M7 guidance4). This is obviously a coarse measure, as it takes no account of the prevailing downstream chemistry or the total number of synthetic stages in the process (and indeed, some of the syntheses reported here have ≤4 stages in total). However, those caveats aside, it provides some indication of the number of syntheses (expressed as a yearly percentage) that introduce either a reactive intermediate or PMI into the “final stages” of the synthesis. The data (summarised in Figure 2) are quite variable and obviously dependent on the chemistry utilised. However, there is some evidence that there was a reduction in the number of syntheses that introduced either a reactive intermediate or a potentially genotoxic impurity (PGI) into the final stages of the synthesis over this 10-year time period (68.4% in 2001 to 42.5% in 2010), which probably reflects the increased level of focus on these impurities since the introduction of EU1 and FDA2 guidance documents in 2007 and 2008, respectively. Obviously, the natures of the reactive intermediates (and corresponding PMIs/MIs) are dictated solely by the chemistry, but an overview of the types and relative percentages of reactive intermediates is instructive and is provided in Figures 3 and 4 (split into two time periods covering 2001−2005 and 2006−

Figure 1. Yearly summary of the average numbers of synthetic stages and reactive intermediates between 2001 and 2010.

syntheses (farnesyl transferase inhibitor (11 stages),12 MX-68 (11 stages),13 fidoxosin (11 stages),14 and in particular cortisone acetate (27 stages)15). However, from 2005 to 2010 there was a trend towards longer synthetic routes (from 5.0 to 6.9 stages) and commensurately an increased number of reactive intermediates per synthesis (from 4.2 to 6.3). We have attempted to exemplify the most significant impact of reactive intermediates, which will be during the latter stages of any synthesis (i.e., API-1 (final stage), API-2 (penultimate stage), API-3 (third stage from the end, and API-4 (fourth stage from the end)). This is because these impurities are typically the most difficult to purge and as a consequence may need to C


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Figure 2. Yearly summary of the percentages of syntheses introducing either a PGI or a reactive intermediate into the final four stages of the synthesis.

Figure 4. Yearly summary of the relative percentages of different types of reactive intermediates used in synthetic reactions from 2006 to 2010.

2010, respectively, for ease of viewing). Again, the data are quite variable and obviously chemistry-dependent. However, the two most predominant classes of alerting structures over the first part of this decade (2001−2005; Figure 3) appear to be

With regards to the “cohort of concern” (aflotoxin, azoxy, and N-nitroso compounds), there was no evidence of aflotoxin or azoxy use in any of the syntheses reviewed over this 10-year time period and only one example of N-nitroso usage (in the synthesis of a 5HT-2c receptor agonist, published in 2007; Table S-7).20 Thus, the incidence of use of these “cohort of concern” reactive intermediates was 1 out of 1782 synthetic stages assessed, i.e., 0.06%. This N-nitroso was formed as a nonisolated intermediate in the preparation of a hydrazine intermediate. These findings of minimal or no use of these very toxic intermediates substantiates the view of Delaney,11 who argued that the majority of compounds containing aflotoxin, azoxy, and N-nitroso compounds that were used in the databases underpinning the TTC would never be used by synthetic chemists as reactive intermediates in the synthesis of APIs. It further suggests that any specific focus on “cohort of concern” would be misplaced because of the very low usage of these compounds as reactive intermediates. It is also worth focusing on the usage trends for one of the newer classes of reactive intermediates, i.e., boronic acids, which historically were not covered by Ashby−Tennant8 structural alerts but are seeing increased usage and have been shown to have mutagenic potential. There was no reported usage of boronic acids during 2000 and 2001. However, in 2003−2010 the average usage was about 2.5 per year. There also appeared to be no underlying trend as to where in the synthesis they were utilised, that is, they were used at various points in the syntheses, including in the final stage, which is somewhat surprising given the added focus on residual metals (e.g., Pt, Pd). Most of the cited publications gave several different alternative synthetic routes (usually developed to circumvent low yields or poor impurity profiles or as parts of green chemistry initiatives or scale-up strategies, etc.). In all cases there was no evidence to suggest that any of these alternative routes could produce the final API (of typical complexity) without the need to use reactive intermediates at some stage of the synthesis. Two case studies in support of this contention are provided below. Case Study 1: Synthesis of EP1 Antagonist GSK269984B via Four Different Approaches. During the assessment of potential commercial synthetic routes for GSK269984B (1),21 whose structure is shown in Figure 5, it was apparent that the formation of the methylene bridge was the major synthetic challenge. Two different synthetic strategies

Figure 3. Yearly summary of the relative percentages of different types of reactive intermediates used in synthetic reactions from 2001 to 2005.

acid halides and alkyl halides. For the former, there are historical data showing that their purported mutagenicity may be attributable to the reaction between the solvent used in the Ames test (DMSO) and the analyte to yield the mutagenic byproduct chlorodimethyl sulfide (CDMS).17 With respect to alkyl halides, ICH M74 describes the potential to assign a classrelated ADI (acceptable daily intake) higher than the established TTC or staged TTC.18 In the second half of the decade (2006−2010; Figure 4), whilst these two classes of reactive intermediate (acid halide and alkyl halide) remained prevalent, they were joined by aromatic amines and Michael acceptors as the predominant classes. Aromatic amines (along with the related aromatic nitro precursors) have attained popularity because of the desire to introduce late-stage coupling reactions (e.g., Suzuki, Heck, etc.) into chemical schemes as part of green chemistry initiatives. Similarly, activated alkenes (or Michael acceptors) can readily undergo addition reactions with nucleophiles, one of the most attractive methods for the formation of C−C bonds, using mild reaction conditions. These findings are somewhat at odds with an earlier survey of good manufacturing practise (GMP) bulk reactions run in a research facility between 1985 and 2002,19 which showed that Michael addition reactions remained constant at about 7% of the total over that time period. However, this may be a vagary of the chemistries studied. D


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Table 2. Comparison of the opportunities for PMIs to be formed in the different synthetic routes used to manufacture GSK269984B (1) route

no. of synthetic stages

Aa

5

Figure 5. Structure of GSK269984B (1).

stage where the reactive intermediate was introduced 2 3

alkyl halide (4) substituted arylboronic acid (6) acid halide (thionyl chloride) substituted arylboronic acid (7) alkyl halide (10) alkyl halide (4) substituted arylboronic acid (6) substituted arylboronic acid (7) alkyl halide (10) acid halide (thionyl chloride) alkyl halide (10)e alkyl halide (13) alkyl halide (4) alkyl aldehyde (15)e alkyl halide (4) alkyl aldehyde (15)e alkyl halide (4) alkyl aldehyde (19) alkyl halide (4) alkyl aldehyde (20)

3′

were obvious to the authors:21 coupling of the picolinic acid (or corresponding ethyl ester) with a chlorophenyl derivative (Table 1, routes A−C) or coupling of 2,6-dibromopyridine with chlorosalicylaldehyde derivatives (Table 1, route D). A comparative overview of the six alternative synthetic routes used to produce API 1 is provided in Table 2, and the chemistry is summarised in Schemes 3-1 to 3-6. Perhaps unsurprisingly, the initial medicinal chemistry route (route A; Scheme 3-1) has the greatest number of PMIs (five), which was decreased slightly (to four) during optimisation of this route (Scheme 3-2), but the optimized version introduces an alkyl halide (10) in the final stage. Route B also has four PMIs, including an alkyl halide (4), which is also introduced in the final stage (Scheme 3-3). In comparison, route D introduces the PMIs early in the synthesis (stages 1 and 2 in Scheme 3-5). Route C was ultimately selected as the optimal approach on the basis of processability. Both the initial route C (Scheme 3-4) and the optimized version (Scheme 3-6) have two PMIs, but perhaps surprisingly, the optimized route introduces alkyl halide 4 in the final stage. Although potentially a risk, this telescoped process showed excellent purging capability, resulting in low-ppm levels in the final API. Therefore, irrespective of which route was selected, the number of reactive intermediates remained constant (four or five PMIs), exposing the fallacy that avoidance was ever a viable option. This again underpins the argument that control, not avoidance or ALARP,

4

Ab

4

4′ 1 2 3

B

4

Cc

6

Cd

4

D

5

nature of the reactive intermediate

4 1 2 3 4 3 5 2 4 1 1 2

a Initial medicinal chemistry route. bOptimized medicinal chemistry route. cInitial phenolic aldol route. dOptimized phenolic aldol route (proposed commercial route). eNonisolated intermediate.

Table 1. Different coupling approaches for the synthesis of GSK269984B (1) (adapted from ref 21)

E


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Scheme 3-1. Medicinal chemistry route to GSK269984B (1) (route Athe conversion of the Na salt 12 to the free acid 1 is not shown)

Scheme 3-2. Optimized medicinal chemistry route to 1 (route A)

Scheme 3-3. Friedel−Crafts alkylation route to 1 (route B)

Scheme 4-1). Two principal deficiencies of this route were immediately apparent: (i) the preparation of benzylamine 11 was nonoptimal and (ii) the condensation between diketone 4 and methylhydrazine gave poor regioselectivity (a 1:2 ratio of the desired isomer to the undesired isomer), which necessitated

is the most appropriate strategy on the basis of a robust appraisal of the risks and ultimate control strategy. Case Study 2: Synthesis of PPAR-α/γ Agonist GSK183390A. The initial synthetic route for GSK183390A22 involved the coupling of pyrazole 11 and benzylamine 10 (see F


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Organic Process Research & Development Scheme 3-4. Phenolic aldol route to 1 (route C)

Scheme 3-5. 2,6-Dibromopyridine route to 1 (route D)

Scheme 3-6. Optimized phenolic aldol route to 1 (route C)

chromatographic isolation of the desired isomer, 5. The authors described two new routes to the key intermediates, pyrazole 11 and benzylamine 10. The two alternative synthetic routes are shown in Schemes 4-2 and 4-3. Alternative Synthesis of Benzylamine 10 (Scheme 4-2). Initially, α-bromoisobutyrate was used to alkylate o-cresol (30) to give the corresponding ester and ethyl methacrylate, but the

reaction was slow and sometimes did not progress to completion. The more reactive alkylating agent chloretone was assessed, which resulted in a much more rapid reaction that yielded the desired acid and methacrylic acid. However, as the reaction did not progress fully to completion, there were opportunities for this alkylating agent to react with base to produce a corresponding epoxide, which could undergo ring G


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was identified as an efficient reagent that could quantitatively alkylate 30 to give the desired acid 39, with methacrylic acid as the main byproduct. The methacrylic acid impurity could be removed using a bisulfate wash, and the acid 39 could be extracted into tert-butyl methyl ether (TBME). Unfortunately, the resultant amidomethylation reaction using 2-chloro-Nhydroxymethylacetamide in sulfuric acid was slowed in the presence of residual TBME and yielded two alerting impurities (50 and 51) (Scheme 4-4).

Scheme 4-1. Original medicinal chemistry route to GSK183390Aa

Scheme 4-4. PMIs arising as a result of side reactions occurring during the amidomethylation reaction (the reaction between 2-chloro-N-hydroxymethylacetamide and TBME solvent)

Reagents and conditions: (a) NaOEt, 0−80 °C, yield 99%; (b) CH3NHNH2, EtOH, 90 °C, yield 22% (unwanted isomer yield 45%); (c) NH2OH·HCl, NaOAc, EtOH, room temperature, yield 93%; (d) ammonium formate, Pd/C catalyst, reflux, MeOH, yield 50%; (e) aqueous HBr, yield 97%; (f) Boc2O, CH2Cl2, Et3N, yield 96%; (g) K2CO3, DMF, ethyl bromoisobutyrate, yield 69%; (h) TFA, CH2Cl2, yield 82%; (i) NaOH, room temperature, yield 98%; (j) SOCl2, toluene, 80 °C, then Et3N, 10, room temperature, yield 96%; (k) NaOH, 80 °C, yield 73%. a

Scheme 4-2. New route for key benzylamine intermediate 10 in the synthesis of GSK183390A The solvent was changed to acetic acid, and the reaction proceeded smoothly to yield amide−acid 48, which was isolated and purified. It is worth reflecting on the use of 2chloro-N-hydroxymethylacetamide rather than the corresponding acetamide as a protecting group. This was done to facilitate ease of removal of the resultant protecting group, as the former has an alcoholysis rate 5 times faster than that of the corresponding acetamide. Finally, deprotection was achieved in refluxing ethanol/sulfuric acid to yield benzylamine 10. Alternative Synthesis of Pyrazole 11 (Scheme 4-3). In order to overcome the regioselectivity issues of the initial route, the authors proposed a 1,3-dipolar cycloaddition between the nitrile (which can be generated by base-mediated removal of HX from the corresponding hydrazonoyl halide) and the alkyne to yield the desired isomer 5. The authors assessed two hydrazonoyl halides (bromo derivative 18 and chloro derivative 19), which were synthesised via condensation of methyl hydrazine with ethyl gloxate followed by halogenation of the intermediate hydrazone 17 with either NBS or NCS to yield 18 or 19, respectively, and four olefins (20−23). Unfortunately, chloro derivative 19 was generally unreactive, whilst bromo derivative 18 reacted slowly with 4-tert-butylphenylacetylene, TMS enol ether 20, or enol phosphate 21 (Scheme 4-5). In contrast, 18 reacted rapidly with morpholine enamines to yield the desired 1,3,5-pyrazole 5 (Scheme 4-6). Upon scale-up it was found that 18 reacted with all bases (not just the intended triethylamine), and the unwanted morpholino adduct 28 was formed, which was more stable and did not generate the reactive nitrile imine 13. This could be controlled by lowering the reaction temperature and then extraction into water, whilst the organic layer containing 1,3,5-pyrazole 5 could be hydrolysed with LiOH to yield pyrazole 11. Then the pyrazole could be activated by conversion to an acid chloride, which reacts with benzylamine 10 to yield the API GSK183390A. A comparison of the alternative strategies to synthesize the API is given in Schemes 4-1 to 4-3 and Table 3. Whereas the original medicinal chemistry synthesis (Scheme 4-1) generated five alerting structures, both of the alternative syntheses

Scheme 4-3. New route for key pyrazole intermediate 11 in the synthesis of GSK183390A

opening to generate the corresponding acid chloride and methacrylic acid chloride. Eventually, α-bromoisobutyric acid H


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Organic Process Research & Development Scheme 4-5. Preparation of hydrazonoyl halides and alkyne equivalentsa

Table 3. Comparison of the opportunities for PMIs to be formed in the different synthetic routes used to manufacture GSK183390A route

no. of synthetic stages

stage where the reactive intermediate was introduced

Aa

4b

B1c

5

2 4 1′ 2′ 1′ 2′

C1g

a

Reagents and conditions: (a) MeNHNH2 (16), THF; (b) NBS or NCS, THF; (c) LiHMDS, TMSCl, THF, −25 °C; (d) LDA, ClPO(OEt)2, THF, −25 °C; (e) TiCl4, morpholine, toluene, 80 °C; (f) SO2·pyridine, DMSO, CH2Cl2; (g) morpholine.

7

3′ 6 7 1 2 3 4 6 7

nature of the reactive intermediate hydrazine acid halide hydroxylamine alkyl halide alkyl halide (36, 38, or 46),d epoxide (41)e Michael acceptor byproduct (37, 40, or 42),f alkyl halide (34) alkyl halide (48) acid halide acid halide (49) hydrazine hydrazone (17), NBSh hydrazone (18) hydrazone byproduct (28) acid halide acid halide (49)

a

Initial medicinal chemistry route. bTwo convergent four-step syntheses whose steps are designated as 1−4 for pyrazole 11 and 1′−4′ for benzylamine 10. cNew route to benzylamine 10. dEthyl αbromoisobutyrate (36), chloretone (38), and α-bromoisobutyric acid (46) were assessed as alkylating agents. eChloretone (38) can rearrange in the presence of base to give the corresponding epoxide. f Ethyl methyl acrylate (37), methacrylic acid (40), or methacrylic acid chloride (42) can form depending on the nature of the alkylating agent. gNew route to pyrazole 11. hN-Bromosuccinimide.

Scheme 4-6. Formation of morpholino adduct byproduct 28

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CONCLUSION A survey of over 300 synthetic publications from within this journal over a 10-year period (2001−2010) provides a top-level overview of current synthetic strategies. It reaffirms the widely held view within the pharmaceutical industry that the synthesis of complex, multistage pharmaceuticals without the use of reactive intermediates is untenable and that calls for “avoidance” reflect a lack of awareness of the challenges inherent in modern synthetic chemistry. On the basis of this survey, we can conclude that the average number of steps required to synthesize each API was 5.9 and that the average number of reactive intermediates per synthetic route was 4.1. There is some evidence that there was a slight reduction in the number of syntheses that introduced either a reactive intermediate or generated a PMI in the final stages of the synthesis over this 10-year time period, possibly reflecting the increased level of focus on these impurities since the introduction of the EU1 and FDA2 guidance documents in 2007 and 2008, respectively. Such concerns might have been expected to predicate a drive to ensure that most of the reactive intermediates would be used during the early stages of a synthesis, which in theory would allow adequate time for downstream chemical/physical purging. However, the attractiveness of late-stage coupling reactions (e.g., Suzuki, Heck, Negishi, etc.) means that some synthetic procedures utilise reactive intermediates with the potential to form MIs/PMIs in the final stages of a synthesis (which has implications from an ICH Q3D23 perspective, i.e., control of metal ion catalysts). Four major classes of reactive intermediate are commonly utilised in the later stages (i.e., the last four stages) of API

(Schemes 4-2 and 4-3) were longer and produced more alerting substances. The alternative synthesis of benzylamine 10 gave five alerting structures (see Scheme 4-2), and the alternative synthesis of pyrazole 11 gave seven alerting structures (see Scheme 4-3). This demonstrates that overall needs of the chemistry program can often supersede concerns about residual reactive components, especially if they are readily purged from the reaction sequence. For example, the introduction of the chloroactamido protecting group into 48, the precursor to benzylamine 10, rather than the nonalerting acetamido protecting group was driven by reaction efficiencies: the former is a much better leaving group, and the deprotection reaction to give the benzylamine is 5 times faster than with the corresponding acetamido derivative. The identification of the best alkylating reagent (α-bromoisobutyric acid) for the reaction with o-cresol involved assessing the relative merits of three different alkylating agents and comparing the purging power of reactive byproducts that were formed. I



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syntheses: alkyl halides, acid chlorides, aromatic amines, and Michael acceptors. Despite this apparent enhanced risk, there is no evidence to suggest that such approaches have resulted in any significant issues in terms of ineffective process control over such impurities. This points to the innate reactivity of the reagents in question, which facilitates their effective purging irrespective of proximity to the final product. Another key observation was minimal usage of highly potent compounds from the “cohort of concern”, supporting the view of Delaney,11 who argued that the majority of compounds containing aflotoxin, azoxy, and N-nitroso compounds, which were used in the databases underpinning the TTC, would never be used by synthetic chemists as reactive intermediates in the synthesis of API. This further suggests that any additional focus on “cohort of concern” would be misplaced. We have also assessed the usage of boronic acids, which historically were not covered by Ashby−Tennant structural alerts but do appear to have some mutagenic potential. Interestingly, there appears to be no underlying trend as to where in the synthesis they were typically utilised. Boronic acids were used both early and (somewhat surprisingly given the added focus on residual metals, e.g., Pt, Pd) in the final stages of API synthesis, perhaps again reflecting the attractiveness of latestage coupling. Most of the cited publications gave several different alternative synthetic routes (usually developed to circumvent low yields or poor impurity profiles or as part of green chemistry initiatives or scale-up strategies, etc.). In all cases there was no evidence to suggest that any of these alternative routes could produce the final API (of typical complexity) without the need to use reactive intermediates at some stage of the synthesis. In addition, the number of reactive intermediates remained broadly similar irrespective of which route was selected, seriously challenging the notion that avoidance was ever a viable option. This again underpins the argument that control, not avoidance or ALARP, is the most appropriate strategy.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/op500346q.

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Review

Details of the specific publications that served as the raw data for the survey (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The authors thank and acknowledge the case study contributions from current GSK colleagues (Matthew Whiting and Peter G. Turner) and former GSK colleagues (Kathy Harwood, Frank Hossner, Mark C. Wilkinson, Giuseppe Guercio, Damiano Castoldi, Nicola Giubellina, Alessandro Lamonica, Arianna Ribecai, Paolo Stabile, Pieter Westerduin, Riet Dams, Anna Nicoletti, Sara Rossi, Claudio Bismara, Stefano Provera, and Lucilla Turco). J


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