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Manning MC, Holcomb RE, Payne RW, Stillahn JM, Connolly BD, Katayama DS, Liu H, Matsuura JE, Murphy BM, Henry CS, Crommelin DJA. Stability of Protein Pharmaceuticals: Recent Advances. Pharm Res 2024:10.1007/s11095-024-03726-x. [PMID: 38937372 DOI: 10.1007/s11095-024-03726-x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2024] [Accepted: 06/03/2024] [Indexed: 06/29/2024]
Abstract
There have been significant advances in the formulation and stabilization of proteins in the liquid state over the past years since our previous review. Our mechanistic understanding of protein-excipient interactions has increased, allowing one to develop formulations in a more rational fashion. The field has moved towards more complex and challenging formulations, such as high concentration formulations to allow for subcutaneous administration and co-formulation. While much of the published work has focused on mAbs, the principles appear to apply to any therapeutic protein, although mAbs clearly have some distinctive features. In this review, we first discuss chemical degradation reactions. This is followed by a section on physical instability issues. Then, more specific topics are addressed: instability induced by interactions with interfaces, predictive methods for physical stability and interplay between chemical and physical instability. The final parts are devoted to discussions how all the above impacts (co-)formulation strategies, in particular for high protein concentration solutions.'
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Affiliation(s)
- Mark Cornell Manning
- Legacy BioDesign LLC, Johnstown, CO, USA.
- Department of Chemistry, Colorado State University, Fort Collins, CO, USA.
| | - Ryan E Holcomb
- Legacy BioDesign LLC, Johnstown, CO, USA
- Department of Chemistry, Colorado State University, Fort Collins, CO, USA
| | - Robert W Payne
- Legacy BioDesign LLC, Johnstown, CO, USA
- Department of Chemistry, Colorado State University, Fort Collins, CO, USA
| | - Joshua M Stillahn
- Legacy BioDesign LLC, Johnstown, CO, USA
- Department of Chemistry, Colorado State University, Fort Collins, CO, USA
| | | | | | | | | | | | - Charles S Henry
- Department of Chemistry, Colorado State University, Fort Collins, CO, USA
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2
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Bramham JE, Wang Y, Moore SA, Golovanov AP. Assessing Photostability of mAb Formulations In Situ Using Light-Coupled NMR Spectroscopy. Anal Chem 2024; 96:9935-9943. [PMID: 38847283 PMCID: PMC11190875 DOI: 10.1021/acs.analchem.4c01164] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/01/2024] [Revised: 04/29/2024] [Accepted: 05/29/2024] [Indexed: 06/19/2024]
Abstract
Biopharmaceuticals, such as monoclonal antibodies (mAbs), need to maintain their chemical and physical stability in formulations throughout their lifecycle. It is known that exposure of mAbs to light, particularly UV, triggers chemical and physical degradation, which can be exacerbated by trace amounts of photosensitizers in the formulation. Although routine assessments of degradation following defined UV dosages are performed, there is a fundamental lack of understanding regarding the intermediates, transient reactive species, and radicals formed during illumination, as well as their lifetimes and immediate impact post-illumination. In this study, we used light-coupled NMR spectroscopy to monitor in situ live spectral changes in sealed samples during and after UV-A illumination of different formulations of four mAbs without added photosensitizers. We observed a complex evolution of spectra, reflecting the appearance within minutes of transient radicals during illumination and persisting for minutes to tens of minutes after the light was switched off. Both mAb and excipient signals were strongly affected by illumination, with some exhibiting fast irreversible photodegradation and others exhibiting partial recovery in the dark. These effects varied depending on the mAb and the presence of excipients, such as polysorbate 80 (PS80) and methionine. Complementary ex situ high-performance size-exclusion chromatography analysis of the same formulations post-UV exposure in the chamber revealed significant loss of purity, confirming formulation-dependent degradation. Both approaches suggested the presence of degradation processes initiated by light but continuing in the dark. Further studies on photoreaction intermediates and transient reactive species may help mitigate the impact of light on biopharmaceutical degradation.
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Affiliation(s)
- Jack E. Bramham
- Department
of Chemistry, School of Natural Sciences, Faculty of Science and Engineering, The University of Manchester, Manchester M1 7DN, U.K.
| | - Yujing Wang
- Dosage
Form Design & Development, BioPharmaceutical
Development, R&D, AstraZeneca, Cambridge CB2 0AA, U.K.
| | - Stephanie A. Moore
- Dosage
Form Design & Development, BioPharmaceutical
Development, R&D, AstraZeneca, Cambridge CB2 0AA, U.K.
| | - Alexander P. Golovanov
- Department
of Chemistry, School of Natural Sciences, Faculty of Science and Engineering, The University of Manchester, Manchester M1 7DN, U.K.
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3
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Schöneich C. Primary Processes of Free Radical Formation in Pharmaceutical Formulations of Therapeutic Proteins. Biomolecules 2023; 13:1142. [PMID: 37509177 PMCID: PMC10376966 DOI: 10.3390/biom13071142] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2023] [Revised: 07/12/2023] [Accepted: 07/13/2023] [Indexed: 07/30/2023] Open
Abstract
Oxidation represents a major pathway for the chemical degradation of pharmaceutical formulations. Few specific details are available on the mechanisms that trigger oxidation reactions in these formulations, specifically with respect to the formation of free radicals. Hence, these mechanisms must be formulated based on information on impurities and stress factors resulting from manufacturing, transportation and storage. In more detail, this article focusses on autoxidation, metal-catalyzed oxidation, photo-degradation and radicals generated from cavitation as a result of mechanical stress. Emphasis is placed on probable rather than theoretically possible pathways.
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Affiliation(s)
- Christian Schöneich
- Department of Pharmaceutical Chemistry, University of Kansas, 2093 Constant Avenue, Lawrence, KS 66047, USA
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4
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Bana AAK, Mehta P, Ramnani KAK. Physical Instabilities of Therapeutic Monoclonal Antibodies: A Critical Review. Curr Drug Discov Technol 2022; 19:e240622206367. [PMID: 35748546 DOI: 10.2174/1570163819666220624092622] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2021] [Revised: 03/26/2022] [Accepted: 03/30/2022] [Indexed: 01/27/2023]
Abstract
The proteinaceous nature of monoclonal antibodies (mAbs) makes them highly sensitive to various physical and chemical conditions, thus leading to instabilities that are classified as physical and chemical instabilities. In this review, we are discussing in detail the physical instability of mAbs because a large number of articles previously published solely focus on the chemical aspect of the instability with little coverage on the physical side. The physical instabilities of mAbs are classified into denaturation and aggregation (precipitation, visible and subvisible particles). The mechanism involved in their formation is discussed in the article, along with the pathways correlating the denaturation of mAb or the formation of aggregates to immunogenicity. Further equations like Gibbs-Helmholtz involved in detecting and quantifying denaturation are discussed, along with various factors causing the denaturation. Moreover, questions related to aggregation like the types of aggregates and the pathway involved in their formation are answered in this article. Factors influencing the physical stability of the mAbs by causing denaturation or formation of aggregates involving the structure of the protein, concentration of mAbs, pH of the protein and the formulations, excipients involved in the formulations, salts added to the formulations, storage temperature, light and UV radiation exposure and processing factors are mentioned in this article. Finally, the analytical approaches used for detecting and quantifying the physical instability of mAbs at all levels of structural conformation like far and near UV, infrared spectroscopy, capillary electrophoresis, LC-MS, microflow imagining, circular dichroism and peptide mapping are discussed.
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Affiliation(s)
- Arpit Arun K Bana
- Department of Pharmaceutical Analysis, Institute of Pharmacy, Nirma University, Ahmedabad 382481, India
| | - Priti Mehta
- Department of Pharmaceutical Analysis, Institute of Pharmacy, Nirma University, Ahmedabad 382481, India
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5
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Zhang Z, Chow SY, De Guzman R, Joh NH, Joubert MK, Richardson J, Shah B, Wikström M, Zhou ZS, Wypych J. A Mass Spectrometric Characterization of Light-Induced Modifications in Therapeutic Proteins. J Pharm Sci 2022; 111:1556-1564. [DOI: 10.1016/j.xphs.2022.02.002] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/23/2021] [Revised: 02/04/2022] [Accepted: 02/04/2022] [Indexed: 12/15/2022]
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Hipper E, Blech M, Hinderberger D, Garidel P, Kaiser W. Photo-Oxidation of Therapeutic Protein Formulations: From Radical Formation to Analytical Techniques. Pharmaceutics 2021; 14:72. [PMID: 35056968 PMCID: PMC8779573 DOI: 10.3390/pharmaceutics14010072] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/07/2021] [Revised: 12/09/2021] [Accepted: 12/14/2021] [Indexed: 12/25/2022] Open
Abstract
UV and ambient light-induced modifications and related degradation of therapeutic proteins are observed during manufacturing and storage. Therefore, to ensure product quality, protein formulations need to be analyzed with respect to photo-degradation processes and eventually protected from light exposure. This task usually demands the application and combination of various analytical methods. This review addresses analytical aspects of investigating photo-oxidation products and related mediators such as reactive oxygen species generated via UV and ambient light with well-established and novel techniques.
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Affiliation(s)
- Elena Hipper
- Institute of Chemistry, Martin-Luther-Universität Halle-Wittenberg, von-Danckelmann-Platz 4, 06120 Halle (Saale), Germany; (E.H.); (D.H.)
| | - Michaela Blech
- Boehringer Ingelheim Pharma GmbH & Co. KG, Innovation Unit, PDB, Birkendorfer Strasse 65, 88397 Biberach an der Riss, Germany;
| | - Dariush Hinderberger
- Institute of Chemistry, Martin-Luther-Universität Halle-Wittenberg, von-Danckelmann-Platz 4, 06120 Halle (Saale), Germany; (E.H.); (D.H.)
| | - Patrick Garidel
- Boehringer Ingelheim Pharma GmbH & Co. KG, Innovation Unit, PDB, Birkendorfer Strasse 65, 88397 Biberach an der Riss, Germany;
| | - Wolfgang Kaiser
- Boehringer Ingelheim Pharma GmbH & Co. KG, Innovation Unit, PDB, Birkendorfer Strasse 65, 88397 Biberach an der Riss, Germany;
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Gupta S, Jiskoot W, Schöneich C, Rathore AS. Oxidation and Deamidation of Monoclonal Antibody Products: Potential Impact on Stability, Biological Activity, and Efficacy. J Pharm Sci 2021; 111:903-918. [PMID: 34890632 DOI: 10.1016/j.xphs.2021.11.024] [Citation(s) in RCA: 41] [Impact Index Per Article: 13.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2021] [Revised: 11/24/2021] [Accepted: 11/26/2021] [Indexed: 12/25/2022]
Abstract
The role in human health of therapeutic proteins in general, and monoclonal antibodies (mAbs) in particular, has been significant and is continuously evolving. A considerable amount of time and resources are invested first in mAb product development and then in clinical examination of the product. Physical and chemical degradation can occur during manufacturing, processing, storage, handling, and administration. Therapeutic proteins may undergo various chemical degradation processes, including oxidation, deamidation, isomerization, hydrolysis, deglycosylation, racemization, disulfide bond breakage and formation, Maillard reaction, and β-elimination. Oxidation and deamidation are the most common chemical degradation processes of mAbs, which may result in changes in physical properties, such as hydrophobicity, charge, secondary or/and tertiary structure, and may lower the thermodynamic or kinetic barrier to unfold. This may predispose the product to aggregation and other chemical modifications, which can alter the binding affinity, half-life, and efficacy of the product. This review summarizes major findings from the past decade on the impact of oxidation and deamidation on the stability, biological activity, and efficacy of mAb products. Mechanisms of action, influencing factors, characterization tools, clinical impact, and risk mitigation strategies have been addressed.
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Affiliation(s)
- Surbhi Gupta
- Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi-110016, India
| | - Wim Jiskoot
- Division of BioTherapeutics, Leiden Centre for Drug Research (LACDR), Leiden University, Leiden, the Netherlands
| | | | - Anurag S Rathore
- Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi-110016, India.
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8
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Lei M, Quan C, Wang JY, Kao YH, Schöneich C. Light-Induced Histidine Adducts to an IgG1 Molecule Via Oxidized Histidine Residue and the Potential Impact of Polysorbate-20 Concentration. Pharm Res 2021; 38:491-501. [PMID: 33666838 DOI: 10.1007/s11095-021-03010-2] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/06/2020] [Accepted: 02/02/2021] [Indexed: 12/15/2022]
Abstract
PURPOSE Histidine (His) undergoes light-induced reactions such as oxidation, crosslinking and addition. These reactions are initiated by singlet oxygen (1O2) to generate His photo-oxidation products, which are subject to nucleophilic attack by a non-oxidized His residue from another protein or by nucleophilic buffer components such as Tris and His. This report aims to identify light-induced His-adducts to a monoclonal antibody (mAb-1) due to the reaction of His molecules in the buffer with the photooxidized His residues under ICH light conditions. Since polysorbate-20 (PS-20) is a commonly used excipient in biotherapeutics formulation, it is also important to study the impact of PS-20 concentration on protein photostability. RESULTS We identified and characterized light-induced His-adducts of mAb-1 by LC-MS/MS. We showed that the levels of light-induced His-adducts generally correlate with the solvent accessibility of His residues in the protein. In addition, the presence of PS-20 at concentrations commonly used in protein drug formulations can significantly increase the levels of light-induced His-adducts. CONCLUSIONS Since His residues are present in a conserved region in the Fc domain, and may be present in the complementarity-determining region (CDR), the impact on the biological functions of the His-adducts observed here should be further studied to evaluate the risk of their presence.
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Affiliation(s)
- Ming Lei
- Protein Analytical Chemistry, Genentech, Inc., 1 DNA Way, South San Francisco, California, 94080, USA.
| | - Cynthia Quan
- Protein Analytical Chemistry, Genentech, Inc., 1 DNA Way, South San Francisco, California, 94080, USA
| | - John Y Wang
- Late Stage Pharmaceutical Development, Genentech, Inc., 1 DNA Way, South San Francisco, California, 94080, USA
| | - Yung-Hsiang Kao
- Protein Analytical Chemistry, Genentech, Inc., 1 DNA Way, South San Francisco, California, 94080, USA
| | - Christian Schöneich
- Department of Pharmaceutical Chemistry, University of Kansas, 2095 Constant Avenue, Lawrence, Kansas, 66047, USA.
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9
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Mass spectrometric and kinetics characterization of modified species of Growth Hormone Releasing Hexapeptide generated under thermal stress in different pH and buffers. J Pharm Biomed Anal 2020; 194:113776. [PMID: 33272786 DOI: 10.1016/j.jpba.2020.113776] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/13/2020] [Revised: 11/12/2020] [Accepted: 11/13/2020] [Indexed: 11/22/2022]
Abstract
Growth Hormone Releasing Peptide-6 (GHRP-6) is a promising molecule (H-His1-d-Trp- Ala-Trp-d-Phe-Lys6-NH2) for the treatment of several diseases. Studies on the degradation pathways of this molecule under stressed conditions are needed to develop appropriate formulations. Degradation products (DPs) of GHRP-6, generated by heating in the dark at 60 °C with pH ranging from 3.0 to 8.0 and in presence of common buffers, were isolated by RP-HPLC and characterized by ESI-MS/MS. C-terminal deamidation of GHRP-6 was generated preferentially at pH 3.0 and 8.0. Hydrolysis and head-to-tail cyclization were favored at pH ranging from 6.0 to 7.0 in phosphate containing buffers. A DP with +12 Da molecular mass was presumably originated by the reaction with formaldehyde derived from some of the additives and/or elastomeric closures. Certain DPs derived from the acylation reaction of the tri- and di-carboxylic buffering species were favored at pH 3.0-6.0 and indicate that buffer components, including those "Generally Recognized as Safe", may potentially introduce chemical modifications and product heterogeneity. Nano LC-MS/MS analysis revealed GHRP-6 was also detected as a low-abundance species with Trp oxidized to 5-hydroxy, kynurenine, and N-formylkynurenine. The kinetics for the formation of the major degradation products was also studied by RP-HPLC.
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10
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Schöneich C. Photo-Degradation of Therapeutic Proteins: Mechanistic Aspects. Pharm Res 2020; 37:45. [DOI: 10.1007/s11095-020-2763-8] [Citation(s) in RCA: 14] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/02/2019] [Accepted: 01/15/2020] [Indexed: 12/11/2022]
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11
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Le Basle Y, Chennell P, Tokhadze N, Astier A, Sautou V. Physicochemical Stability of Monoclonal Antibodies: A Review. J Pharm Sci 2020; 109:169-190. [DOI: 10.1016/j.xphs.2019.08.009] [Citation(s) in RCA: 118] [Impact Index Per Article: 29.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/16/2019] [Revised: 08/19/2019] [Accepted: 08/19/2019] [Indexed: 01/10/2023]
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12
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Wang W, Ohtake S. Science and art of protein formulation development. Int J Pharm 2019; 568:118505. [PMID: 31306712 DOI: 10.1016/j.ijpharm.2019.118505] [Citation(s) in RCA: 55] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2019] [Revised: 07/08/2019] [Accepted: 07/08/2019] [Indexed: 02/07/2023]
Abstract
Protein pharmaceuticals have become a significant class of marketed drug products and are expected to grow steadily over the next decade. Development of a commercial protein product is, however, a rather complex process. A critical step in this process is formulation development, enabling the final product configuration. A number of challenges still exist in the formulation development process. This review is intended to discuss these challenges, to illustrate the basic formulation development processes, and to compare the options and strategies in practical formulation development.
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Affiliation(s)
- Wei Wang
- Biological Development, Bayer USA, LLC, 800 Dwight Way, Berkeley, CA 94710, United States.
| | - Satoshi Ohtake
- Pharmaceutical Research and Development, Pfizer Biotherapeutics Pharmaceutical Sciences, Chesterfield, MO 63017, United States
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13
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Krause ME, Sahin E. Chemical and physical instabilities in manufacturing and storage of therapeutic proteins. Curr Opin Biotechnol 2019; 60:159-167. [PMID: 30861476 DOI: 10.1016/j.copbio.2019.01.014] [Citation(s) in RCA: 49] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2018] [Revised: 11/02/2018] [Accepted: 01/21/2019] [Indexed: 12/13/2022]
Abstract
Development of a robust biologic drug product is accomplished by extensive formulation and process development screening studies; however, even in the most optimal formulation, a protein can undergo spontaneous degradation during manufacture, storage, and clinical use. Chemical changes to amino acid residues, such as oxidation of methionine or tryptophan, or changes in charge such as deamidation or carbonylation, can induce conformational changes in the overall protein structure, potentially leading to changes in physical - in addition to chemical - stability. Oxidation is often caused by light exposure or the presence of metal ions or peroxides. Asparagine deamidation is more likely to occur at higher pH and/or elevated temperature. Mechanical and interfacial stresses during manufacturing can lead to physical instabilities (i.e. various forms of aggregation). A well-defined manufacturing process and effective in-process controls are essential in minimizing chemical and physical instabilities, enabling robust production and distribution of a safe and efficacious drug product. In this work, the authors provide a review of developments in these areas over the past two years, with emphasis on manufacturability of therapeutically relevant proteins and protein-based drug products.
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Affiliation(s)
- Mary E Krause
- Drug Product Science and Technology, Bristol-Myers Squibb Company, One Squibb Drive, New Brunswick, NJ 08903, United States.
| | - Erinc Sahin
- Drug Product Science and Technology, Bristol-Myers Squibb Company, One Squibb Drive, New Brunswick, NJ 08903, United States
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14
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Leinisch F, Mariotti M, Hägglund P, Davies MJ. Structural and functional changes in RNAse A originating from tyrosine and histidine cross-linking and oxidation induced by singlet oxygen and peroxyl radicals. Free Radic Biol Med 2018; 126:73-86. [PMID: 30031072 DOI: 10.1016/j.freeradbiomed.2018.07.008] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/20/2018] [Revised: 07/12/2018] [Accepted: 07/16/2018] [Indexed: 12/21/2022]
Abstract
Oxidation can be induced by multiple processes in biological samples, with proteins being important targets due to their high abundance and reactivity. Oxidant reactions with proteins are not comprehensively understood, but it is known that structural and functional changes may be a cause, or a consequence, of disease. The mechanisms of oxidation of the model protein RNAse A by singlet oxygen (1O2) were examined and compared to peroxyl radical (ROO•) oxidation, both common biological oxidants. This protein is a prototypic member of the RNAse family that exhibits antiviral activity by cleaving single-stranded RNA. RNAse A lacks tryptophan and cysteine residues which are major oxidant targets, but contains multiple histidine, tyrosine and methionine residues; these were therefore hypothesized to be the major sites of damage. 1O2 and ROO• induce different patterns and extents of damage; both induce cross-links and side-chain oxidation, and 1O2 exposure modulates enzymatic activity. Multiple products have been characterized including methionine sulfoxide and sulfone, alcohols, DOPA, 2-oxohistidine, histidine-derived ring-opened species and inter- and intra-molecular cross-links (di-tyrosine, histidine-lysine, histidine-arginine, tyrosine-lysine). In addition to methionine modification, which appears not to be causative to activity loss, singlet oxygen also induces alteration to specific histidine, tyrosine and proline residues, including modification and cross-linking of the active site histidine, His12. The high homology among the RNAse family suggests that similar modifications may occur in humans, and be associated with the increased risk of viral infections in people with diabetes, as markers for 1O2 have been found in early stages of this pathology.
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Affiliation(s)
- Fabian Leinisch
- Dept. of Biomedical Sciences, Panum Institute, University of Copenhagen, Copenhagen, Denmark
| | - Michele Mariotti
- Department of Biotechnology and Biomedicine, Technical University of Denmark, Kongens Lyngby, Denmark
| | - Per Hägglund
- Dept. of Biomedical Sciences, Panum Institute, University of Copenhagen, Copenhagen, Denmark; Department of Biotechnology and Biomedicine, Technical University of Denmark, Kongens Lyngby, Denmark
| | - Michael J Davies
- Dept. of Biomedical Sciences, Panum Institute, University of Copenhagen, Copenhagen, Denmark.
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