Benzene as contaminant

June 24th, 2016 No comments »

Benzene is a highly toxic compound, and long-term exposure to benzene can cause anemia, leukemia, and other medical conditions [1].  The ICH has classified benzene as a Class 1 solvent with a limit of NMT 2 ppm [2], and people in pharma and pharma-related industries are very concerned about controlling and minimizing the levels of benzene in their products.  Benzene and 50 other solvents of the four ICH classes have been screened by headspace GC to detect residual solvents as impurities in APIs [3].

Benzene has been used to make ethyl benzene, styrene, aniline, cyclohexane, cumene, phenol, chlorobenzene, and other simple feedstocks.  So perhaps we shouldn’t be surprised when benzene is found in our APIs, reagents, and solvents.  Although benzene could accidentally contaminate a drum of material, manufacturers catering to the pharma market are probably very alert to sources of benzene contamination in their products.  It is more likely that benzene is present in solvents or reagents, or is formed as an artifact.

Benzene in solvents Toluene is commercially available with less than 0.1 wt% of benzene [4].  Benzene can be separated from toluene by distillation; a high reflux ratio is key [5].

Acetone can contain low levels of benzene.  Acetone is primarily manufactured through alkylation of benzene with propylene to afford cumene, followed by generation of cumyl hydroperoxide, and scission through the Hock reaction to generate phenol and acetone [6].  Hence unreacted benzene could contaminate acetone manufactured from cumene.  A minor route to acetone is that from oxidation of propylene; in principle benzene could be generated as a minor byproduct under these conditions.  (Possible reaction: 2 propylene + 1.5 O2 produces benzene + 3 H2O.)  Due to similar volatility of benzene and acetone, separating acetone from benzene is difficult, and azeotropes may be employed [7].

Absolute ethanol can be produced by adding benzene, distilling off the EtOH – benzene – water azeotrope, and then distilling off the benzene – EtOH azeotrope to remove benzene; of course, not all the benzene is guaranteed to be removed [8].  Ethanol can be denatured with many compounds, including benzene.  EtOH denatured with benzene can be labeled as S.D.A. 2-B, S.D.A. 2-C, or S.D.A. 12-A. [9].  Denatured fuel ethanol can contain gasoline, toluene, xylene, and benzene [10].  Hence if denatured EtOH is used in manufacturing it is prudent to confirm that supplies are not contaminated by benzene.

Benzene in reagents Benzene was found in benzenesulfonic acid used to crystallize an NCE [11].

Benzene formed as an artifact Sometimes questioning the method of sample preparation / analysis can be informative.  For instance, benzene was formed from the preservative sodium benzoate when soft drinks containing ascorbic acid were heated at 100 ºC for 30 min [12].  Some control reactions may be needed to ensure that artifacts are not generated during sample preparation and analysis.

Bottom line We are well-advised to look at our reactions to anticipate if benzene could be present or generated in our process streams.  Only a small amount of benzene could derail the development or sale of a drug.  Nature doesn’t care if we think a “good reaction” is needed to generate an impurity.

  1. http://www.atsdr.cdc.gov/PHS/PHS.asp?id=37&tid=14#bookmark05
  2. http://www.ich.org/fileadmin/Public_Web_Site/ICH_Products/Guidelines/Quality/Q3C/Step4/Q3C_R5_Step4.pdf
  3. Li Qin, L.; Chang-qin Hu, C.; Yin, L., “Establishment of a Knowledge Base for Prescreening Residual Solvents in Pharmaceuticals”, Chromatographia 2004, 59(7),
  4. http://www.citgo.com/WebOther/CITGOforYourBusiness/Petrochemicals/Toluene.pdf, dated January 2016
  5. http://seperationtechnology.com/benzene-distillation
  6. Weissermel, K.; Arpe, H.-J. Industrial Organic Chemistry; 3d ed.; VCH: Weinheim; 1997; pp 276-7, 342-3, 353-5.
  7. Berg, L., “Separation of Benzene from Acetone by Azeotropic Distillation,” US 4,931,145 (1990).
  8. http://cool.conservation-us.org/waac/wn/wn28/wn28-3/wn28-304.pdf. “…smart chemists know to stay away from the lab punch made with absolute ethanol.”
  9. https://www.law.cornell.edu/cfr/text/27/21.151
  10. http://itecref.com/pdf/MSDS_Fuel_Alcohol_ITEC_Refining.pdf
  11. Gross, T. D.; Schaab, K.; Ouellette, M.; Zook, S.; Reddy, J. P.; Shurtleff, A.; Sacaan, A. I.; Alebic-Kolbah, T.; Bozigian, H. Process Res. Dev. 2007, 11, 365.
  12. Hileman, B. Chem. Eng. News 2006, 84(17), 10.

Removing Metal Ions from Reactors

September 20th, 2014 No comments »

Residual metal ions can cause significant problems with operations. For instance, hydroxylamine has been found to react exothermically with Ti, Fe, Cu, Ni, Cr, and Mn, which may be components of metal reactors and fittings; to avoid contamination by metal ions a process involving hydroxylamine was developed using a glass microreactor (1). Contamination by 15 ppm of residual copper ions in new lines decomposed some of the performic acid charged in a process (2). MnO2 and Fe(III) each react with H2O2 (3, 4). Pt is used for hydrogenation, but if residual Pd is present hydrogenolysis may also occur. Removing residual metal ions from reactors can be essential to ensure safe operating conditions and completion of reactions.

Many ions are more soluble under acidic conditions than under basic conditions, as for FeCl3 vs. Fe2O6. Organic acids are often used to solubilize metal ions. ∝-Hydroxy acids are more acidic than the non-hydroxylated analogues (5); carboxylic acids have often been used to clean reactors. ∝-Hydroxy acids may also be good ligands because they can form stable 5-membered rings in octahedral coordination complexes (6).

Citric acid has been used to solubilize ions of Fe, Ca, and other metals. Citric acid forms soluble chelates of Fe, Cu, Mg, Mn and Zn (7). Citric acid has been used synthetically to solubilize Ti(IV)(8), Os(VIII) (9), and ruthenium (10). A citric acid (0.3 M) – oxalic acid (0.2 M) combination has been used to decontaminate nuclear reactors, and oxalic acid can be used to remove MnO2 and rust from iron (11). Malic acid was used to solubilize Al(III) (12).

Under basic conditions (or at least conditions that are not strongly acidic), chelants such as EDTA and nitrilotriacetic acid (NTA) will complex with metals. Glyphosate (Roundup) forms a 5-membered ring with ions of Mn (13). EDTA and NTA strongly complex with Ni(II) and Cu(II), and can corrode reactors and fittings (14).

Matching the metal ion, the right oxidation state, the complexing reagent, and the right pH can all be important. For instance, a solution of 2,4,6-trimercaptotriazine sodium salt was ineffective in removing Cu(I), but after the process streams were sparged with air the Cu(II) generated was effectively removed by TMT treatment. Trivalent ions such as Al(III), Cr(III), and Fe(III) were not bound to TMT (15). (Since TMT precipitates metal ions it is not appropriate for removing trace metals from reactors.) Cu(II) oxalate precipitates from solutions made strongly acidic with HCl (16).

All such reagents can be corrosive. It is wise to test for the corrosivity of solutions to glass and metal reactors by soaking representative pieces of glass or metal coupons in the solutions that could be used for cleaning.

A general procedure for cleaning new equipment has been described (17). Cleaning the surface of reactors is called pickling, and then the reactors are passivated (oxidized) to prevent corrosion of the surface. With usage sediment can accumulate in the jackets of reactors, decreasing the efficiency of heat transfer.  Some companies have provided contract services to clean the jackets of reactors (18).

1)      Vörös, A.; Baán, Z.; Mizsey, P.; Finta, Z. Org. Process Res. Dev. 2012, 16, 1717.

2)      Thanks to Paul Jass for this personal communication.

3)      Decomposition of excess hydrogen peroxide to work up an epoxidation: Vaino, A. R. J. Org. Chem.2000, 65, 4210.

4)      Swaddle, T. W. Inorganic Chemistry: An Industrial and Environmental Perspective; Academic Press; 1997, p. 252.

5)      Some pKas of selected carboxylic acids: oxalic, 1.23, 4.19; citric, 3.08, 4.74; glyoxylic, 3.2; malic, 3.46; glycolic, 3.83; acetic, 4.75; peracetic, 8.2.

6)      Swaddle, T. W. ibid, p. 245.

7)      http://www.citric-acid.us/Citric_Acid_uses.html

8)      Working up a Kulinkovich reaction: Young, I. S.; Haley, M. W.; Tam, A.; Tymonko, S. A.; Xu, Z.; Hanson, R. L.; Goswami, A. Org. Process Res. Dev. 2014, XX, XXXX; DOI: 10.1021/op500135x.

9)      For achiral dihydroxylation: Dupau, P.; Epple, R.; Thomas, A. A.; Fokin, V. V.; Sharpless, K. B. Adv. Synth. Catal. 2002, 344(3+4), 421.

10)   Cleaning nuclear reactors: Row, T. H. Nucl. Sci. Abstr. 1967, 21, 27690; Reference 7 in Couturier, M.; Andresen, B. M.; Jorgensen, J. B.; Tucker, J. L.; Busch, F. R.; Brenek, S. J.; Dubé, P.; am Ende, D. J.; Negri, J. T. Org. Process Res. Dev. 2002, 6, 42. The Pfizer researchers used acetonitrile as solvent to contain volatile and toxic RuO2 in solution.

11)   Kumar, V.; Goel, R.; Chawla, R.; Silambarasan, M.; Sharma, R. K. J. Pharm. Bioallied Sci. 2010 Jul-Sep; 2(3), 220–238; doi:  10.4103/0975-7406.68505

12)   Workup of a Friedel-Crafts alkylation: Wu, G.; Wong, Y.; Steinman, M.; Tormos, W.; Schumacher, D. P.; Love, G. M.; Shutts, B. Org. Process Res. Dev. 1997, 1, 359.

13)   http://cabecahealth.com/wp-content/uploads/2013/05/FoodPlaguePrimer.pdf

14)   Swaddle, T. W. ibid, p. 269.

15)   Malmgren, H.; Bäckström, B.; Sølver, E.; Wennerberg, J. Org.Process Res. Dev. 2008, 12, 1195.

16)   Sedergran, T. C.; Anderson, C. F. U.S. 4,675,398, 1987 (to E. R. Squibb & Sons).

17)   Mukherjee, S. “Preparations for Initial Startup of a Process,” Chemical Engineering 2005, 112(1), 36.

18)   For instance, the OptiTherm service from GE Water and Power Technologies: https://www.gewater.com/kcpguest/documents/Capability%20Profiles%20FAQ_Cust/Americas/English/cp102.pdf.

Recent review on statistical DoE

September 13th, 2014 No comments »

Statistical design of experiments (DoE; also termed factorial experimental design or FED) is a powerful tool for developing processes efficiently. DoE has been used in many phases of API development, ranging from initial process optimization to design space studies in validation to assay development. In the time spanning 2003 – 2013 there were over 100 manuscripts in Organic Process Research & Development that described using DoE for various stages of API optimization. The recent review below summarizes selected publications from that group of manuscripts to show how DoE findings have been applied for process scale-up and other aspects of API development.

Weissman, S. A.; Anderson, N. G. “Design of Experiments (DoE) and Process Optimization. A Review of Recent Publications” Org. Process Res. Dev. XXXX, XX, XXXX; DOI: 10.1021/op500169m

How Much? 2-Ethylhexanoic Acid Limits

March 14th, 2014 No comments »

Salts of 2-ethylhexanoic acid, inexpensive carriers of the sodium or potassium ion, have been used to make salts of carboxylic acids, such as fosinopril sodium and sodium clavulanate. Limits should be set for residual impurities such as 2-ethylhexanoic acid in APIs. The toxicity of 2-ethylhexanoic acid must be reviewed in order to set realistic limits, especially since developing highly sensitive assays can consume considerable effort [1].

CAUTION: 2-ETHYLHEXANOIC ACID IS A TERATOGEN AND REPRODUCTIVE TOXIN. WOMEN ESPECIALLY MUST TAKE SUITABLE PRECAUTIONS TO MINIMIZE CONTACT WITH THIS COMPOUND AND ITS SALTS. This SAFETY information should be made available to everyone, including chemists, QA, QC, and operators, who handles 2-ethylhexanoic acid and its salts.

Some people have avoided using 2-ethylhexanoic acid because of a reputation for being highly toxic. The question is: How Much? How toxic is it? BG Chemie has manufactured 2-ethylhexanoic acid, and stated in 2000 that it has “low acute oral toxicity (LD50 rat oral 2043 to 3640 mg/kg body weight)” [2]. An EPA report in 2001 [3] described toxicity to rat embryos and fetuses with high dosing, referencing a 1987 report. The latter stated that valproic acid (2-propylpentanoic acid, isomeric to 2-ethylhexanoic acid) was about twice as potent to rat embryos and fetuses as 2-ethylhexanoic acid [4]. (Valproic acid is marketed generically as an anticonvulsant and mood stabilizer. The starting dosage for sodium valproate is 600 mg/day, increasing up to 1000 – 2000 mg/day. Contraindications are listed for women taking this drug [5].) The EPA report also cited a 1993 report that mentioned that effects on reproduction and post-natal development in rats were seen with high doses [6]. 2-Ethylhexanoic acid was examined on a cellular level in 2005 and found to increase the incidence of sister chromatid exchange [7]. In a 1998 study 2-ethylhexanoic acid was shown to be a weak embryotoxic compound in male rats and mice, and the NOEL (no observable effects limit) was 60 mg/kg [8]. A 2005 study showed that 2-ethylhexanoic acid was toxic to pregnant rabbits, and the NOEL was determined to be 25 mg/kg [9]. 2-Ethylhexanoic acid has no functional groups alerting for mutagenicity, and tested negative in vitro for mutagenicity [9]. Female mice developed some liver cancers when exposed to 2-ethylhexanol, which can be oxidized in vivo to 2- ethylhexanoic acid, and the TD50 was calculated at 1650 mg/kg/day [10]. In a study with rats in which 2-ethylhexanoic acid was administered orally, 7 – 12% of 14C-labelled 2-ethylhexanoic acid was detected in the feces, indicating that about 90% of 2-ethylhexanoic acid was orally absorbed [11].

Several approaches could be taken to set permissible daily exposure (PDE) limits for residual 2-ethylhexanoic acid in drug candidates. One could propose that the PDE limit should be the same as for residual 2-ethylhexanoic acid in clavulanate, thus minimizing discussions with regulatory authorities. Another, sound approach is to calculate a limit for 2-ethylhexanoic acid based on toxicology data and applying the calculations discussed in ICH Q3C (R5) [12]. Using those calculations, the PDE limits for a 50 kg patient are calculated at 12 mg/day from the study in rats and mice (F1, F2, F3, F4, F5 = 5, 10, 5, 1, 1), and 50 mg/day from the study in rabbits (F1, F2, F3, F4, F5 = 2.5, 10, 1, 1, 1). The more conservative approach would be to propose the lower limit, a PDE of 12 mg/day for a 50 kg patient, for a drug substance administered orally. For other means of administration a limit could be proposed at 90% of the oral dosing, or 10.8 mg/day.

The toxicity of residual 2-ethylhexanoic acid and its salts should of course be acknowledged in reviews with the FDA and other regulatory authorities. Dionex has developed a procedure to quantitate residual 2-ethylhexanoic acid, which may be better than the USP assay for 2-ethylhexanoic acid in clavulanate [13].

Many thanks to Dr David Snodin for his helpful comments.

1. Snodin, D. J. Org. Process Res. Dev. 2014, 18, XXXX (doi.org/10.1021/op5000368)
2. http://www.bgrci.de/fileadmin/BGRCI/Downloads/DL_Praevention/Fachwissen/Gefahrstoffe/TOXIKOLOGISCHE_BEWERTUNGEN/Bewertungen/ToxBew275-E.pdf (2000)
3. http://www.epa.gov/hpv/pubs/summaries/metalcarb/c14172rr12.pdf
4. E. J. Ritter, E. J.; W. J. Scott, W. L., Jr., Randall, J. L.; Ritter, J. M. Teratology 35(1), 46 (February 1987).
5. http://www.medicines.org.uk/emcmobile/medicine/26115/spc
6. Pennanen, S.; Tuovinen, K.; Huuskonen, H.; Kosma, V. M.; Komulainen, H. Fundam Appl Toxicol. 1993 Aug;21(2), 204.
7. Song, J.-Y. ; Cho, Y.-H. ; Kim, Y.-J. ; Chung, H.-W. Environmental Mutagens and Carcinogens 2005, 25(3), 110.
8. Juberg, D. R., et al., 1998; see http://www.contactalimentaire.com/fileadmin/ImageFichier_Archive/contact_alimentaire/Fichiers_Documents/Avis_de_AESA/cef_ej959-963-966-968_22list_op_en.pdf
9. http://www.vkm.no/dav/c067aa526e.pdf (Norwegian Scientific Committee for Food Safety)
10. http://toxnet.nlm.nih.gov/cpdb/chempages/2-ETHYLHEXANOL.html
11. http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@DOCNO+5649
12. http://www.gmp-compliance.org/guidemgr/files/Q3C_R5_STEP4.PDF
13. http://www.dionex.com/en-us/webdocs/88443-AN262-IC-2EthylhexanoicAcid-Clavulanate-01Oct2010-LPN2608-R2.pdf

ICH Guideline M7 on DNA-reactive impurities: salient points

April 16th, 2013 No comments »

ICH Guideline M7 on DNA-reactive compounds was issued on 6 February 2013 [1]. This Draft Consensus Guideline, formulated with input from representatives from pharma, may change after regulatory bodies receive comments.

“Assessment and Control of DNA Reactive (Mutagenic) Impurities in Pharmaceuticals to Limit Potential Carcinogenic Risk”

INTRODUCTION. The ICH has set limits down to as low as micrograms / day for impurities in APIs that might react with DNA. Both controlling and assaying for impurities at such low levels can require large amounts of resources.

RELATED TERMS. Mutagens alter the genetic code, while genotoxins are defined as compounds that react with DNA and cause cancer. Carcinogens cause cancer by damaging genetic material directly or indirectly [2,3]. Clastogens are compounds that cause a change in the appearance of chromosomes, due to the breakage, addition or deletion of genetic material [4]. Teratogens produce abnormalities in developing fetuses, and are not necessarily mutagens [2]. Altered genetic code may be repaired enzymatically [5], or lead to mutations in offspring, or to programmed death of the cell (apoptosis) [4]. Strictly speaking, genotoxic impurities (GTIs) have been demonstrated to be genotoxic, while potentially genotoxic impurities (PGIs) may be genotoxic [6].

SALIENT POINTS OF ICH M7. Although it is not specifically stated, ultimately the final guidance should replace the 2008 draft Guidance, “Genotoxic and Carcinogenic Impurities in Drug Substances and Products: Recommended Approaches” [7]. In the current Guideline five categories of impurities with potential carcinogenic and mutagenic activity were proposed, with corresponding actions. Applicants need to propose limits for GTIs, based on data from toxicology studies. Two complementary in silico analyses are recommended to predict mutagenicity of PGIs. Permissible levels of PGIs are higher in many categories than in the staged TTC levels provided in the earlier Guidance; limits again depend on the number of days over which the compound will be administered to humans.

The M7 Draft Consensus Guideline is consistent with earlier guidance in many ways. For instance, a QbD approach may be accepted if an applicant understands processing parameters and chemical fate of impurities. Following the processing over the lifecycle of the drug is also stressed: knowledge from manufacturing can be used to control operations. The M7 guideline does not override ICH S9, which mentions that higher limits may be set for impurities in drugs to treat advanced stages of cancer [8].

For strategies on identifying and controlling genotoxic impurities, see the excellent volume by Teasdale [9] and other discussions [6].

1. http://www.ich.org/fileadmin/Public_Web_Site/ICH_Products/Guidelines/Multidisciplinary/M7/M7_Step_2.pdf
2. Frank, P.; Ottoboni, M. A. The Dose Makes the Poison; Wiley; 3d ed.; 2011; pp. 90, 145, 158.
3. For instance, aflatoxin B1 is enzymatically activated and the resulting epoxide reacts with DNA: Baertschi, S. W.; Raney, K. D.; Stone, M. P.; Harris, T. M. J. Am. Chem. Soc. 1988, 110, 7929.
4. Juo, P.-S. Concise Dictionary of Biomedicine and Molecular Biology; 2nd ed.; CRC Press; 2002, pp. 283 & 105.
5. For instance, photodimers of thymine can be repaired by photolyase: Kao, Y.-T.; Saxena, C.; Wang, L.; Sancar, A.; Zhong, D. PNAS 2005, 102(45), 16128.
6. Anderson, N. G. Practical Process Research & Development – A Guide for Organic Chemists; 2nd edition; Academic Press: San Diego; 2012; pp. 379 – 386.
7. http://www.fda.gov/downloads/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/ucm079235.pdf
8. http://www.fda.gov/downloads/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/UCM085389.pdf
9. Genotoxic Impurities: Strategies for Identification and Control; Teasdale, A., Ed.; Wiley Interscience; 2010.

Quenching carbodiimides

December 7th, 2012 No comments »

Carbodiimides are widely available and relatively inexpensive reagents that are often used to generate amides from the corresponding amines and carboxylic acids.  Such carbodiimides include dicyclohexylcarbodiimide (DCC), diisopropylcarbodiimide (DIC), and N-ethyl –N’-(3-dimethyaminopropyl)-carbodiimide (available as free base or HCl salt; a.k.a. EDC, EDCI, EDAC, water-soluble carbodiimide, and WSC).  EDC can couple amines and carboxylic acids in predominantly aqueous solutions,[1] and is also widely used as a crosslinking reagent in biochemistry.[2]

Carbodiimides are known to be relatively toxic.  For instance, EDC is irritating to the skin and may cause severe eye damage.[3]  DCC is a known skin sensitizer.[4]  Exposure to DIC led to temporary blindness to a worker who cleaned up a spill of 1 L while wearing a respirator and gloves.[5]  On a molecular level EDC has been shown to crosslink double-stranded DNA, which could be responsible for cell death;[6] little has been published on the genotoxicity of carbodiimides.  People should undertake precautions to avoid contact with carbodiimides, and to quench reactions containing carbodiimides as part of workups.

Treatment with carboxylic acids or aqueous acids can decompose carbodiimides, thus affording effective quenches.  Oxalic acid, acetic acid, and phosphoric acid have been used to quench DCC.[7],[8]  The decomposition of EDC in aqueous solutions has been studied quantitatively: decomposition occurred in minutes at pH 4.0, and was said to be instantaneous at pH 2.8 (1% aq. AcOH) and pH 2.2 (1% aq. HCO2H).[9]  Using the kinetic data in the latter paper it may be possible to develop a QbD argument for the decomposition of EDC.



[1] Merck used EDC to prepare initial batches of a sitagliptin intermediate: Hansen, K. B.; Balsells, J.; Dreher, S.; Hsiao, Y.; Kubryk, M.; Palucki, M.; Rivera, N.; Steinhuebel, D.; Armstrong, J. D., III; Askin, D.; Grabowski, E. J. J. Org. Process Res. Dev. 2005, 9, 634.

[2] http://www.piercenet.com/browse.cfm?fldID=F3305493-0FBC-93DA-2720-4412D198A9C9

[3] See MSDS for EDC at http://www.sigmaaldrich.com/catalog/product/sial/e7750?lang=en&region=US

[4] See MSDS for DCC at http://www.sigmaaldrich.com/catalog/product/fluka/36650?lang=en&region=US

[5] http://ntp.niehs.nih.gov/ntp/htdocs/Chem_Background/ExecSumm/Diisopropylcarbodiimide/DIC_carc.html

[6] Moshnikova, A. B., et al., Cell. Mol. Life Sci. 2006, 63, 229.

[7] Fieser, L. F.; Fieser, M. Reagents for Organic Synthesis; Vol. 1; Wiley: New York, 1967; p 231.

[8] Anderson, N. G., Practical Process Research & Development – A Guide for Organic Chemists; 2nd edition; Academic Press: San Diego; 2012; pp 291, 297-8.

[9] Lei, Q. P.; Lamb, D. H.; Heller, R. K.; Shannon, A. G.; Ryall, R.; Cash, P. Anal. Biochem. 2002, 310, 122.

Highlighting Continuous Flow Chemistry

April 6th, 2012 No comments »

Continuous flow chemistry is a hot topic these days for academia, pharma and CROs / CMOs. Continuous operations are selected for reasons of safety, quality, yield, and economics. Safety is a primary reason to develop continuous operations for making large quantities of materials; for instance, 60% of the ASAP articles on continuous operations on the Organic Process Research & Development web page state that those conditions were developed for reasons of safety.

• My review “Using Continuous Processes to Increase Production” is available on-line: Org. Process Res. Dev. 2012, 16, XXX (dx.doi.org/10.1021/op200347k).

• The 6th International Current Process Chemistry Conference (13 – 14 June, Princeton) will feature some excellent presentations on industrial process R&D, including continuous flow chemistry. A half-day short course on continuous processing is offered on June 12. Instructors are Nicholas Leadbeater (University of Connecticut), Bryan Li (Pfizer), and Timothy Braden (Lilly). http://www.healthtech.com/processchemistry/

• Flow Chemistry Congress meets 23 – 24 April in Boston, with an impressive line-up of speakers. Paul Watts (The University of Hull) will present a short course on 25 April. http://selectbiosciences.com/conferences/index.aspx?conf=FCC2012

• The April issue of Organic Process Research & Development will include a special feature section on continuous processing, with at least 20 contributions.

• The Journal of Flow Chemistry is available on-line: http://www.akademiai.com/content/122156/?p=4d774b8a62434f08a980e2cb10bc4dfc&pi=1

Controlling Residual Arylboronic Acids as Potential Genotoxic Impurities in APIs

November 5th, 2011 No comments »

Arylboronic acids, but not the corresponding deboronated arenes, recently have been found to be weakly mutagenic in microbial assays [1].  Hence arylboronic acids may be considered potentially genotoxic impurities, and controlling the levels of residual arylboronic acids in APIs could become a regulatory requirement.  The issues should be decided by toxicology studies for the specific arylboronic acids in question.

Several approaches have been successful in removing boronic acids.  Diethanolaminomethyl polystyrene (DEAM-PS) [2],[3] and immobilized catechol [4] have been used to scavenge boronic acids.  Complex formation with diethanolamine may solubilize residual boronic acids in mother liquors.  Since arylboronic acids ionize similarly to phenols, basic washes of an API solution may remove arylboronic acids.  A selective crystallization can purge an arylboronic acid from the API.

The best means to control residual aryl boronic acids in APIs at the ppm level may be to decompose them through deboronation.  Sterically hindered, electron-rich aryl boronates and heteroaromatic boronates are especially prone to deboronation [5],[6],[7].  Deboronation of a hindered difluorobenzeneboronic acid was accelerated in the presence of Pd, and increased in the presence of H2O [8].  Kuivila and co-workers found that protodeboronation of 2,6-dimethoxybenzeneboronic acid was slowest about pH 5, and rapid under more acidic or basic conditions [9].  Butters and co-workers found that protodeboronation occurred for an arylboronate anion, but not for the corresponding arylboronic acid [10].  Amgen researchers found that deboronation of an ortho-substituted arylboronic acid was fastest in the presence of K2CO3 [11].  The Snieckus group found that deboronation occurred readily when pinacol was added to 4-pyridylboronic acid [12].  Percec and co-workers found that deboronation of neopentylglycol boronates, especially an ortho-substituted arylboronic acid ester, was catalyzed by nickel species [13].  Kuivila and co-workers found that CuCl2 catalyzed the deboronation of 2,6-dimethoxybenzeneboronic acid and other arylboronic acids, with formation of the corresponding aryl chlorides [14].  Unfortunately, adding reagents to a reaction mixture increases the burdens of analysis and impurity removal, but additives such as these may accelerate deboronation in difficult cases.  Simply extending the reaction conditions, which are generally basic for efficient Suzuki coupling, or heating with some amount of aqueous hydroxide are probably the preferred treatments to decompose an arylboronic acid.  By knowing the kinetics of the decomposition of the arylboronic acid it may be possible to show by QbD that analyses for the residual arylboronic acid in an API are not necessary.


[1] O’Donovan, M. R.; Mee, C. D.; Fenner, S.; Teasdale, A.; Phillips, D. H. Mutat. Res.: Genet. Toxicol. Environ. Mutagen. 2011, 724(1-2), 1.

[2] Antonow, D.; Cooper, N.; Howard, P. W.; Thurston, D. E. J. Comb. Chem. 2007, 9, 437.

[3] Hall, D. G.; Tailor, J.; Gravel, M. Angew. Chem. Int. Ed. 1999, 38, 3064.

[4] Yang, W.; Gao, X.; Springsteen, G.; Wang, B. Tetrahedron Lett. 2002, 43, 6339.

[5] Goodson, F. E.; Wallow, T. I.; Novak, B. M. Organic Syntheses; Vol. 74; Smith, A. B., III, Ed.; Organic Syntheses, Inc.; 1997; p. 64.

[6] Baudoin, O.; Cesario, M.; Guénard, D.; Guéritte, F. J. Org. Chem. 2002, 67, 1199.

[7] Dai, Q.; Xu, D.; Lim, K.; Harvey, R. G. J. Org. Chem. 2007, 72, 4856.

[8] Cammidge, A. N.; Crépy, K. V. L. J. Org. Chem. 2003, 68, 6832.

[9] Kuivila, H. G.; Reuwer, J. F., Jr.; Mangranite, J. A. Can. J. Chem. 1963, 41, 3081.

[10] Butters, M.; Harvey, J. N.; Jover, J.; Lennox, A. J. J.; Lloyd-Jones, G. C.; Murray, P. M. Angew. Chem. Int. Ed. 2010, 49, 5156.

[11] Achmatowicz, M.; Thiel, O. R.; Wheeler, P.; Bernard, C.; Huang, J.; Larsen, R. D.; Faul, M. M. J. Org. Chem. 2009, 74, 795.

[12] Alessi, M.; Larkin, A. L.; Ogilvie, K. A.; Green, L. A.; Lai, S.; Lopez, S.; Snieckus, V. J. Org. Chem. 2007, 72, 1588.

[13] Moldoveanu, C.; Wilson, D. A.; Wilson, C. J.; Leowanawat, P.; Resmerita, A.-M.; Liu, C.; Rosen, B. M.; Percec, J. Org. Chem. 2010, 75, 5438.

[14] Kuivila, H. G.; Reuwer, J. F., Jr.; Mangravite, J. A. J. Am. Chem. Soc. 1964, 86, 2666.

Recent Process Validation Guidance and Process Implementation

February 3rd, 2011 No comments »

In January 2011 the FDA issued guidance entitled “Process Validation: General Principles and Practices,” which replaces the 1987 guidance [1].  The guidance stresses lifecycle management, with the ability to store and retrieve data being important.  The document describes three basic stages of process validation: process design, process qualification, and continued process verification.  Design of experiments (DoE) and statistical process control (SPC) are urged.  The document states that operations controlled by process analytical technology (PAT) may be preferred over non-PAT systems.  The FDA urges readers to contact them with specific questions in areas not covered by this and related guidances.

In December 2010 Organic Process Research & Development posted a manuscript I wrote with two colleagues, entitled “Current Practices of Process Validation for Drug Substances and Intermediates.” [2].  This manuscript is consistent with the current FDA guidance.  Described in the manuscript are a number of previously unpublished examples of challenges and successes of implementing processes.  More experienced process chemists and engineers may groan at some of these “war stories,” which can serve as valuable lessons.


[1] Guidance for Industry. Process Validation: General Principles and Practices” issued 1/24/2011 http://www.fda.gov/downloads/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/UCM070336.pdf

[2] Anderson, N. G.; Burdick, D. C.; Reeve, M. M. Org. Process Res. Dev. 2011, 15, 162.

A Perspective on Biocatalysis

February 1st, 2011 No comments »

The attractiveness of using biocatalysis can be assessed by considering the benefits, drawbacks, and cost of biocatalytic systems. Benefits may include a route with fewer intermediates, or intermediates with higher yield and higher quality; these changes could result in fewer unit operations and fewer intermediates to assay, hence lower cost of goods. Other benefits may include decreased emissions of volatile organic compounds (solvents), decreased cost of remediating solvent-laden waste streams, and decreased solvent recovery operations. A significant benefit may be increased flexibility in selecting equipment to prepare the API.

Drawbacks of biocatalytic systems can include a potentially significant time to develop a biocatalytic process, unless a commercially available biocatalyst can be suitably employed. Ownership of intellectual property and licensing should be clearly addressed before a contract research organization develops a biocatalyst specifically for the API. If only one vendor can supply the biocatalyst, contracts must be carefully composed to ensure reliable delivery at an attractive price. Conditions and compounds that inhibit the biotransformation must be identified and controlled, similar to other catalytic processing. If the biocatalyst must be shipped to the site of API manufacturing, stability of the biocatalyst should be considered.

Recently authors have reviewed in detail the factors for estimating the cost of a biocatalyst [Tufvesson, P.; Lima-Ramos, J.; Nordblad, M.; Woodley, J.M. Org. Process Res. Dev. 2011, 15, 266]. (These authors did not estimate the cost of developing a biocatalyst.) With high fermentation yield the cost of the biocatalyst decreases; not all enzymes may be produced in high titer. To increase the turnover number an enzyme may be purified or even immobilized. The extent of purification has a great impact, with costs increasing as the processing evolves from whole cells to crude enzymes to purified enzymes to immobilized enzymes. As production volume increases the cost of the biocatalyst decreases, as expected through economy on scale. Costs associated with equipment may be detailed for an in-house COG estimate, or may not be mentioned in a proposal by a contract manufacturing organization.

Many factors should be considered to assess the desirability of a biocatalytic process. The “greenness” of a biocatalytic process must include the operations and waste both in the process and in generating the biocatalyst. Ultimately the acceptability of the cost of a biocatalyst must be judged relative to the value of the product. The value of increased flexibility in equipment choice and improved ruggedness in quality of a product may not be quantifiable before routine manufacture of an API. Nonetheless biocatalytic processes may provide clear advantages even early in the development of API candidates.