Development and validation of a (RP)UHPLC-UV-(HESI/Orbitrap)MS method for the identification and quantification of mixtures of intact therapeutical monoclonal antibodies using a monolithic column
Introduction
Monoclonal antibodies (mAbs) are large glycoproteins with a molecular weight of around 150 kDa. They are currently the most important class of therapeutic proteins on the market and are used worldwide in the treatment of a broad spectrum of disorders, above all cancers [1,2]. Bevacimab (BVZ), cetuximab (CTX), infliximab (INF), rituximab (RTX) and trastuzumab (TTZ) are among the most prescribed therapeutic mAbs as they are indicated for the treatment of widespread disorders such as cancer and autoimmune diseases. They were the top-selling biopharmaceuticals in 2015 and 2016 [3,4]. BVZ (Avastin®) is indicated for the treatment of several kinds of cancer, i.e. metastatic carcinoma of the colon or rectum, breast cancer, lung cancer, etc [5]. CTX (Erbitux®) is approved for the treatment of colon, head and neck cancers [6]. INF (Remicade®) is used in the treatment of psoriasis, Crohn's disease, ankylosing spondylitis, psoriatic arthritis, rheumatoid arthritis and ulcerative colitis [7]. RTX (MabTera®) is indicated for use in the treatment of non-Hodgkin's lymphoma, rheumatoid polyarthritis and chronic lymphoid leukaemia [8]. TTZ (Herceptin®) is indicated for the treatment of patients with metastatic breast cancer with tumours that overexpress HER2 (25% of patients) [9]. All of the mAbs share the same IgG1 structure (Fig. 1), differing only in the part involved in specific interaction with the antigen (Fab). This structural similarity makes reversed phase (RP) chromatographic separation using packed bed-columns difficult [[10], [11], [12]]. The use of monolithic column could benefit separation of mAbs due to the expected more favoured mass transfer kinetics because of the lack of mesoporores in polymer monoliths [13,14].
Reversed-phase liquid chromatography ((RP)LC) is generally considered as more efficient and more sensitive for the analysis of intact biotherapeutic proteins than other types of chromatography (i.e. size exclusion or ionic exchange chromatography) [15,16]. However, this mode of chromatography has frequently been associated with low recovery due to the absorption of the proteins by the column stationary phase material and due to the low diffusion capacity towards the mobile phase, an issue that is even more complicated in large proteins such as mAbs [21]. To solve these drawbacks, high-performance reversed-phase liquid chromatography ((RP)HPLC) requires mobile phases composed of complex mixtures of organic solvents [10,17,18] combined with high temperatures in the column (up to 90 °C [16,19]) to increase the diffusion of the proteins into the mobile phase, so increasing the efficiency of the chromatographic process. Gradient elution mode must always be used. In conventional analytical chromatography, analysis is usually performed on a column packed with 3–5 μm wide-pore (300 Å) silica particles with a pressure of up to 400 bar. Nevertheless, the chromatographic separation of the mixtures of intact mAbs such as those described above has never been achieved using this approach. This could be due to the high degree of structural similarity (sharing the IgG structure), which promotes similar hydrophobicity features that prevent the separation of therapeutic mAbs by (RP)HPLC.
Ultra-high-performance liquid chromatography in reversed-phase mode ((RP)UHPLC) uses sub-2 μm particle sizes and therefore needs higher pressures in the system (around 1000 bar). This significantly enhances the efficiency of the separation of intact proteins using common organic solvent composition in the mobile phase, such as mixtures of water and acetonitrile and common ion pairing agents, such as trifluoroacetic acid (TFA) or formic acid (FA). Column temperatures of up to 80 °C are also required [20,21], as are gradient elution modes. These conditions have the advantage of being suitable to analyse intact protein when coupled with a mass spectrometer. Rigid polymer-based monolithic columns can also be used as an alternative to packed-bed columns [13,14]. In terms of protein recovery and carryover, monolithic columns have been shown to be clearly superior to particle based stationary phase formats for analysing mixtures of intact protein in the 5.7–150 kDa size range (including recombinant mAbs). This method is also very sensitive, detecting at femtomol level [22]. This means that, as indicated in [23], organic polymer-based monolithic columns are suitable for both high-resolution and high-speed biomolecule separations. High peak capacity peptide and protein separations have been reported which were at least as good, if not better, as those achieved using packed columns. An additional advantage is that the unique porous structure and chemistry of the organic polymer-based monolithic columns decreases the carryover [24].
Notwithstanding the above, the analysis of therapeutic mAbs by (RP)LC has tended to focus on the analysis of a single mAb. Normally this is done by analysing intact mAbs or by applying enzymatic strategies to analyse Fc and Fab fragments [14,16]. So far little research has been done into mixtures of mAbs. This could be due to the fact that they are mainly formulated and prescribed individually. Nevertheless, mAbs could theoretically work well in combination therapy because of their limited overlapping toxicity and the lack of pharmacokinetic interactions [25,26]. It has already been reported that a combination of mAbs can have a greater therapeutic potential for treating cancer than a single mAb [27,28]. By combining mAbs in the treatment of a particular cancer, several pathways may be targeted simultaneously, potentially creating additive or synergistic effects; preclinical and clinical studies of twenty-five different combinations of therapeutic mAbs have been reported [25]. In addition, mixtures of mAbs are using in on-going clinical trials to overcome acquired resistance to approve mAbs treatment [29,30] or to overcome resistance developed during clinical trials to new specific mAbs therapeutic agents [31]. For all this, mAb combination is therefore likely to become a promising cancer therapy in the near future. MAb mixtures can be produced either by individually manufacturing the constituent mAbs or by producing a single batch of two or more mAbs [32].
In this context, it is important to provide rigorous and reliable analytical methods that enable us to study mixtures of mAbs, and not only the individual mAb constituents [33]. It is evident that Size-Exclusion Chromatography (SEC) is not suitable for analysing intact mAb mixtures since all the mAbs have similar molecular weights of around 150 kDa [34] although it represents an excellent strategy for the analysis of individuals mAbs (and related formats) when coupling to native ion mobility mass spectrometry [35]. Cation Exchange Chromatography (CEX) has proved to be a fast, reliable method for the characterization of charge variants [36,37], also be used for the analysis of mixtures [33,38]. Very recently has been proposed the development of a universal method for charge variant characterization of single mAbs based on the use of pH gradient elution using volatile, low ionic strength buffers with direct coupling to high-resolution Orbitrap mass spectrometry [39]. Also, hydrophobic interaction chromatography (HIC) is a technique which allows separate mAbs on the basis of their hydrophobicity under native conditions [40,41]. Capillary zone electrophoresis (CZE) is another option to analyse mAbs mixtures also resolving charge variants simultaneously [42,43]. Nevertheless, as mentioned earlier, (RP)LC has greater resolving power and efficiency [44] and sharper peaks [10,11], which can be used for quantification purposes in single mAbs or in mixture solutions. (RP)LC has the additional advantage of straightforward coupling to several ionization methods, such as heat electrospray ionization (HESI), for mass spectrometry (MS) detection. Native mass spectrometry on an Orbitrap™ platform was recently used to characterize the composition of complex mAbs mixtures generated from single production to be used for batch to batch assessment [33]; the analysis focused on the intact mAbs without previous LC separation.
Our previous research has focused on the development and validation of (RP)HPLC/DAD methods for the rigorous analytical quantification of single therapeutic mAbs using a wide porous (300 Å) 5 μm particle size packed column [[10], [11], [12]]. We also tried to analyse mAb mixtures, but this proved impossible using this form of analytical chromatography.
In this paper, we present the development and validation of an (RP)UHPLC-UV-(HESI/Orbitrap)MS method for the quantification of mixtures of five commercially available, therapeutic mAbs (i.e. BVZ, CTX, INF, RTX and TTZ). A monolithic column based on a poly(divinylbenzene-co-ethylvinylbenzene) co-polymer was selected to perform mAb separation. The experiment was statistically designed (DoE) to optimize the liquid chromatographic conditions. Direct MS characterization of the intact isoform profile of each mAb was achieved and is described below. We also demonstrated the compatibility of the use of TFA or FA as ion pairing agents for mass spectrometric analysis and for chromatographic separation. Validation was performed according to an internal protocol based on well-known international guidelines. We also propose a new approach for obtaining a model, which we call the heteroscedasticity function, which describes the dependence of the experimental measurement standard deviation vs the mAb concentration.
Section snippets
Chemicals, reagents and mAb solutions
Reverse-osmosis-quality water was purified with a Milli-RO Milli-Q station from Merck Millipore (Darmstadt, Germany). The reagents used were LC–MS purity grade. Acetonitrile (ACN) was supplied by VWR International Eurolab, S.L. (Barcelona, Spain). Trifluoroacetic acid (TFA) was obtained from Scharlab S.L. (Barcelona, Spain). Lysozyme from chicken egg white was purchased from Sigma Aldrich (Madrid, Spain). Isotonic solution (0.9% NaCl in water for injection) was supplied by B. Braun Medical
(RP)UHPLC-UV method optimization
The experimental operational conditions of the chromatographic method were optimized by applying the methodology of statistical design of experiments (DoE). The signal from the UV detector was used for optimisation purposes as it is more sensitive than the mass detector. The latter detector was later used for isoform mAbs profile/ identification.
Before applying DoE, two ion-pairing agents (TFA and FA) were checked in order to improve the mass spectral signal. It is well known that TFA
Conclusions
This paper demonstrates that the chromatographic separation of intact commercially available therapeutic mAbs can be achieved using (RP)LC with a divinylbenzene-based monolithic column. Separation can be achieved despite the similarity in their structure –typically IgG1– which results in similar hydrophobicity features. To the best of our knowledge this is the first method to analyse mixtures of therapeutic intact mAbs focused on quantification. Up to now, (RP)LC has been used for the analysis
Acknowledgments
This work was partially funded by Projects FIS-PI10/00201 and FIS-PI17/00547 (Instituto Carlos III, Ministerio de Economía y Competitividad, Spain); therefore this project has also been partially supported by European Regional Development Funds (ERDF). The authors would like to thank the Hospital Pharmacy Unit of the University Hospital San Cecilio for kindly supplying all the medicine samples for this research.
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