Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N13/00—Investigating surface or boundary effects, e.g. wetting power; Investigating diffusion effects; Analysing materials by determining surface, boundary, or diffusion effects
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume or surface-area of porous materials
  • G01N15/06—Investigating concentration of particle suspensions
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N13/00—Investigating surface or boundary effects, e.g. wetting power; Investigating diffusion effects; Analysing materials by determining surface, boundary, or diffusion effects
  • G01N2013/003—Diffusion; diffusivity between liquids

Definitions

• the invention relates to a method of determining diffusion properties of a sample, and more particularly doing so from a Taylorgram.
• the diffusion interaction parameter ⁇ k D is one metric that describes the interactive forces between solute molecules in a given medium and a growing body of evidence has shown that it can successfully predict the aggregation propensity of formulations
• k D determination by DLS may be made more difficult by methodological problems.
• the first such difficulty relates to the biased scattering intensities exhibited by differently sized particles; whereby the scattering intensity is approximately proportional to the sixth power of the molecular radius (r 6 ). Since it is the average diffusion coefficients that are reported, the resulting values may be susceptible to skew by larger particles. At worst, results can be rendered unusable in the presence of high-order aggregates or dust. In practice, this means that samples must be relatively pure and stable over the duration of the measurement. This may require clean-up procedures, such as filtering, which may be neither desirable nor appropriate for some types of sample.
• Nuclear magnetic resonance (NMR) is a second technique that can be used to determine the k D ; however, such determinations are non-trivial.
• the use of specialised solvents or sample constructs may introduce complexities into sample preparation and the conditions of measurement may be different from those in final formulations.
• NMR measurements molecular diffusion must also happen within a specific relaxation regime and, in particular, this makes obtaining accurate diffusion coefficients for larger molecules is more challenging. Consideration must also be given to possible misinterpretation due to presence of contamination, and to overly complicated spectra that may require dedicated personnel to make meaningful analyses.
• using NMR to determine diffusion coefficients is extremely time-consuming, as well as financially costly, and thus not applicable to large scale screening.
• Sedimentation velocity ultracentrifugation (SV-AUC) and self-interaction chromatography (SIC) cannot directly measure k D but can measure analogous parameters such as the sedimentation interaction parameter (k s ) and second virial coefficient (A 2 ), respectively.
• Taylor dispersion analysis can be used to infer a mutual diffusion coefficient.
• the width of a Gaussian distribution found from a best fit to a pulse Taylorgram can be used to determine a mutual diffusion coefficient for the inj ected sample.
• the concentration corresponding with such a single value of mutual diffusion coefficient is generally assumed to be the concentration of the sample at inj ection, without taking account of any reduction in concentration due to dispersion.
• a sequence of such measurements at different sample inj ection concentrations would be required to determine a relationship between concentration and mutual diffusion coefficient.
• a method for investigating diffusion properties of a sample for example the variation in mutual diffusion coefficient with solute concentration, and the diffusion interaction parameter, that overcomes or ameliorates at least some of these problems is desirable.
• a method of determining a relationship between a mutual diffusion co-efficient D m and the concentration c of a solute within a solvent comprises obtaining a Taylorgram comprising a plurality of measurements of solute concentration c; and deriving from the Taylorgram a plurality of mutual diffusion coefficient values D m corresponding with a plurality of different concentrations c of solute in the solvent.
• Deriving a plurality of mutual diffusion coefficients corresponding with a plurality of different solute concentrations from a (single) Taylorgram avoids the need for preparing a plurality of samples at different concentrations and obtaining a Taylorgram for each of the plurality of samples.
• the preparation and separate analysis (whether by DLS or TDA) of a plurality of samples at different (initial) concentrations represents the prior art approach for determining the relationship between the mutual diffusion coefficient D m and concentration c. Since embodiments of the present disclosure enable determining the relationship between mutual diffusion coefficient D m and concentration c from a single Taylorgram (and therefore a single sample), the present disclosure represents a significant advance over the prior art.
• the method may further comprise fitting a function to the Taylorgram.
• the function may be of the form:
• the method may further comprise differentiating the Taylorgram to determine a rate dc
• the mutual diffusion coefficient values D m may be derived from the function that is fitted to the Taylorgram and from the rate of change of concentration with respect to time .
• the mutual diffusion coefficient values D m may be determined using the following expression:
• the method may further comprise performing a transform on the Taylorgram to find a
• the method may further comprise :
• the method may further comprise determining a diffusion interaction parameter k D of the solute in the solvent from the relationship D m (c) between the mutual diffusion coefficient values D m and the corresponding concentrations c.
• the method may further comprise determining the second virial coefficient A 2 from the diffusion interaction parameter k D and an estimate of the coefficient of friction kf and an estimate of the partial specific volume v 2 of the solute .
• the method may further comprise estimating a measure of aggregation of solute particles from the values of the mutual diffusion coefficient over the duration of the Taylorgram.
• an apparatus comprising a processor, configured to perform the method of the first aspect.
• the apparatus may further comprise an instrument for performing a Taylor dispersion analysis, so as to obtain a Taylorgram.
• an instrument for performing a Taylor dispersion analysis so as to obtain a Taylorgram.
• Figure 1 is a schematic of a method for investigating diffusion properties
• Figure 2 is a schematic showing the outline of three alternatives for extracting a relationship between mutual diffusion coefficient and concentration
• Figure 3 is a schematic showing the first of three alternative methods for extracting a relationship between mutual diffusion coefficient and concentration
• Figure 4 is a schematic showing the second of three alternative methods for extracting a relationship between mutual diffusion coefficient and concentration
• Figure 5 is a schematic showing the third of three alternative methods for extracting a relationship between mutual diffusion coefficient and concentration
• Figure 6 is a graph showing a concentration plotted against ⁇
• Figure 7 is schematic of an apparatus according to an embodiment
• Figure 8 is a frontal Taylorgram of BSA in iodide solution, which can be used in an embodiment to determine the relationship between mutual diffusion coefficient and concentration
• Figure 9 is a graph showing a series of DLS measurements used to obtain estimates for self-diffusion coefficient D 0 and diffusion interaction parameter k D for comparison with estimates obtained from a Taylorgram, according to an embodiment.
• Taylor dispersion is a process by which shear flow is used to enhance the effective diffusivity of a sample.
• Laminar flow in a capillary results in a variation in flow velocity with radial location. Near the walls, the flow is substantially stationary, and flow velocity is at a maximum at the centre. This results in shearing of the adjacent lamina, which acts to enhance dispersion of a sample .
• Taylor dispersion analysis can be used to analyse properties of species within a sample.
• a plug of the sample may be inj ected into a microbore capillary and subsequently disperse as it traverses along the capillary within a laminar flow regime .
• the injected plug of the sample may be narrow (having a short duration) this being referred to as a pulse of the sample, resulting in a pulse Taylorgram.
• the injected plug of the sample may be long (i.e . having a longer duration) this may be referred to as a slug of the sample, resulting in a frontal Taylorgram.
• the degree of dispersion exhibited by the plug is dependent on the diffusivity of the molecules within the plug and can be measured at one or multiple points downstream of the injection site .
• a concentration detector responsive to the species of the sample, may be positioned at one or more locations downstream of the injection location.
• the concentration detectors e .g. a UV -Visible spectrometer
• the resultant signal from the detector typically referred to as a Taylorgram, corresponds to a temporally-resolved distribution of molecular concentration whose width is related to the hydrodynamic radius of the sample species.
• a method comprises obtaining a Taylorgram at step 100, and then extracting (or determining) from the Taylorgram a relationship D m (c) between the mutual diffusion coefficient D m and the concentration c, at step 200, for example by calculating a plurality of values of mutual diffusion coefficient at a corresponding plurality of different concentration values .
• the methods disclosed more fully hereinafter can determine the relationship between mutual diffusion coefficient and concentration from a single Taylorgram.
• the Taylorgram obtained at 100 may be generated from a plug or pulse inj ection, and may comprise a single detection point or multiple detection points .
• a function may be fitted to D m (c), for example by plotting D m (c) and fitting a straight line to the relationship .
• the parameters of the function may then be extracted and used to determine the diffusion interaction parameter, k D .
• the relationship D m (c) between the mutual diffusion coefficient and concentration may be used to determine an aggregation % over the course of the Taylorgram. For example, aggregation will result in a change in the mutual diffusion coefficient, so a discontinuity in the mutual diffusion coefficient D m (c) may indicate aggregation of particles .
• the diffusion interaction parameter k D may in turn be used to determine the second virial coefficient A 2 , for instance using estimates of the molecular weight, coefficient of friction and the partial specific volume of the solute .
• Step 200 in which D m (c) is extracted, is expanded into three different methods .
• the first of these methods 210 is different from the second method 230 and third method 240.
• Both of the second and third method 230, 240 involve determining an interface ⁇ ⁇ , at step 220.
• the present invention is suitable for determining the diffusion in interaction parameter k D from a Taylorgram generated from a single measurement.
• the sample may be injected in a slug, so as to create a frontal Taylorgram, or in a pulse, to create a pulse Taylorgram.
• UV absorption is typically used to determine concentration in a Taylorgram
• the method will also work with any detection method that produces signals in which the concentration and signal from a particle can be rationally correlated, for example, based on any of: refractive index, fluorescence via an extinction coefficient).
• Each point in a Taylorgram represents a concentration, so a single Taylorgram provides a plurality of solute concentrations.
• step 21 1 a function is fitted to the Talorgram.
• t is the measurement time
• t M is a parameter to be determined from the fit. Plus or minus signs in the t ⁇ t M term are used to designate analyses for the leading and trailing edges of a frontal Taylorgram, respectively. Note, that although y is estimated, it may be redundant in this method.
• the Taylorgram (or the function fitted to the Taylorgram) is differentiated, for instance using Savitzy-Golay differentiation (or the difference dc
• a value of b! can be determined for each datapoint in the Taylorgram. Note that the determination of h! is not an essential step, but merely provides for a more elegant description of the calculation. In some embodiments the expression for h! may be substituted into Eq. 5 below, and D m (c)
• Both the second and third example methods share the steps 220 for determining a reference value, ⁇ ⁇ , as shown in Figures 2, 4 and 5.
• equation 6 can be rearranged to give :
• ⁇ 7 ⁇ defines a reference point, which must be properly determined for the mutual diffusion coefficients to be physically representative .
• the reference point is constrained by the requirement: where c L and c R are the limits of the concentrations to the left and right of the Taylorgram (i.e . corresponding with a maximum and minimum value of the concentration, depending on whether the method is performed on a leading edge or trailing edge of a frontal Taylorgram) .
• this edge is ill-defined after considerable dispersion, but can be determined from the following relation:
• This method can be performed numerically or graphically, and involves the estimation of evaluation of the integral J Q ⁇ — ⁇ m dc and the differential— in equation 9 directly from the transformed concentration profile ⁇ ( ⁇ ).
• the skilled person will be aware that a wide range of techniques exist for approximating differentials and integrals from such data, either numerically or graphically from a plot. Smoothing or filtering of the data may be performed before, or after performing the transform, for instance using a moving average, or by a Savitzky-Golay filter.
• a 2 - also known as the osmotic virial coefficient (B 22 or B 2 ).
• the second virial coefficient is linked to k D by the following expression (Eq. 15); where M w is the molecular weight of the protein, k j is the coefficient of friction and v 2 is the partial specific volume .
• k D 2M W A 2 - k r - 2v 2 (Eq. 15)
• the A 2 parameter can also be extracted using k D .
• the mutual diffusion coefficient D m (c) provides a measure of the average size of the species under analysis. As the species transition through the capillary they are spatially distributed within the plug. With this knowledge and with the measurement of the diffusion coefficient at every data point collected over a certain time period a measure of aggregation can be estimated (e .g. a proportion or % aggregation) from the change in diffusion coefficient D m (c) over the Taylorgram.
• the apparatus 40 comprises an instrument 50, processor 5 1 , output means 52 and input means 53.
• the instrument 50 is operable to perform a Taylor dispersion analysis on a sample, so as to produce Taylorgram data 71.
• the processor 5 1 may be configured to estimate parameters for fitting a model (e.g. Gaussian, error function) to the Taylorgram data 71 , in accordance with an embodiment (for instance as described above).
• the processor 51 may provide an output 72 to the output means 52, which may comprise a display or printer.
• the output 72 may comprise model parameter estimates, and/or estimates of the properties of the sample analysed by the instrument 50, based on a model fitted to the data 71 by the processor 5 1.
• the processor 5 1 may be configured to use estimated model parameters (determined according to an embodiment) as a starting point for a numerical search for a best fit to the Taylorgram data 71 (for instance via regression analysis based on least squares).
• An input means 53 may be provided for controlling the processor 5 1 and/or instrument.
• the input means 53 may comprise a keyboard, mouse or other suitable user interface device.
• the instrument 50 may comprise a capillary linking two containers . Liquid is driven (e.g. at constant pressure) from the first container to the second container.
• the first container contains a run (or carrier) solution so that the capillary is initially filled with the run solution.
• the first container is then disconnected from the capillary , and a third container is connected that contains a sample solution.
• the sample solution may be a pharmaceutical or biopharmaceutical species dissolved either in the run/carrier solution, or in a different medium .
• the different medium may differ from the run/carrier solution in having an excipient, e.g. a salt or a sugar, dissolved at a different concentration than in the run/carrier solution. This may be appropriate in formulations which are designed to stabilise active drug species.
• a first and second window are spaced apart along the length of the capillary between the first and second containers.
• the capillary may be formed in a loop so that both the first and second windows may be imaged using a single optical assembly, for instance by arranging for them to be adj acent to one another in an area imaged by the pixel array of an area imaging detector.
• a single window may be used, or the detector may comprise a single element, rather than a pixel array.
• the third container may be connected to the capillary and then disconnected after a suitable volume of the sample has been injected under pressure.
• the second container is connected the capillary when the third container is disconnected from the capillary.
• the detector captures a frame sequence comprising measures of the received light intensity at the detector as the pulse of sample solution or the flow front passes through each of the first and second windows. The detector output thereby provides data on absorbance versus time: a Taylorgram.
• Figure 8 shows an example of a frontal Taylorgram 801 obtained from a sample of BSA in an iodide buffer solution from which a self-diffusion co-efficient D 0 and diffusion interaction parameter k D can be obtained, in accordance with an embodiment.
• Figure 9 shows diffusion co-efficient values for BSA in an iodide buffer 901 and a sulfate buffer 203, obtained by performing DLS on each of a plurality of sample concentrations, according to conventional prior art methodology for determining D 0 and k D .
• a best fit 902, 904 can be used to determine D 0 and k D (from equation 1).
• Table 1 Summary of results. Determination of the self-diffusion coefficient and interaction parameter using the first, second and third example methods. Comparison with the traditional DLS method is also shown.

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• 2015
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