Aqueous pk a Determination Using the pk a Analyzer PRO

Transcription

1 - Application - Aqueous pk a Determination Using the pk a Analyzer PRO Jeremy Kenseth, Ph.D. Ho-ming Pang, Ph.D. Andrea Bastin Introduction The acid dissociation constant (pk a value) of an ionizable compound is a key physicochemical parameter and of vital importance when attempting to understand issues related to ADMET (Absorption, Distribution, Metabolism, Excretion, Toxicity) and to drug formulation development. The ionized form of a compound typically possesses increased solubility, lower lipophilicity and decreased membrane permeability relative to the neutral form. Experimental knowledge of drug compound pk a values allows one to predict the extent of ionization at different ph values of physiological significance. Experimentally determined pk a values also aid in the establishment of improved algorithms for the prediction of drug behavior in biological systems. Capillary electrophoresis (CE) has gained wide acceptance in the pharmaceutical industry as a tool for the determination of drug compound pk a values. 1-5 CE offers many advantages for pk a determination: minimal sample requirement ( µg), no knowledge of sample concentration is required for analysis, and no spectral differences need to exist between the neutral and ionized forms of a compound. A unique advantage of CE for pk a determinations is the ability to separate impurities and/or degradants present in the sample, in contrast to spectroscopic or potentiometric determinations. Data analysis is intuitive, as results are plotted as ionic mobility (directly related to compound charge state) versus ph value. CombiSep recently introduced the pk a Analyzer PRO, the first and only CE-based technology specifically designed for medium to high throughput pk a determinations. The pk a Analyzer PRO system provides fully integrated software for performing multiplexed 96-capillary CE analysis, interpolation of compound pk a values from the CE data, and reporting and/or exporting of pk a results. Predefined methods are incorporated into the software to eliminate the need for method development and to ensure system reproducibility between different analyses and laboratories. This application note will briefly demonstrate the capabilities of the pk a Analyzer PRO for performing aqueous pk a determination. Experimental All chemicals were purchased from Sigma or Fisher. Four compounds were evaluated: acyclovir, a monoacidic monobasic zwitterion; amitriptyline, a monobase of low aqueous solubility; cefadroxil, a monobasic diacidic compound; and furosemide, a diacid. Samples were prepared at 50 µg/ml (50 ppm) concentration except for amitriptyline, which was analyzed at a concentration of 10 µg/ml. All sample solutions contained 0.1% dimethyl sulfoxide (DMSO, an EOF neutral marker). Basic compounds were dissolved in 1 mm HCl and acidic compounds were dissolved in 1 mm NaOH. Samples were pipetted into 24 wells each of a 96-well sample plate (Figure 1A).

2 A 24-component buffer series bracketing a ph range from at I = 50 mm (CombiSep Product # PK ) was used for collecting the CE data. An inlet buffer plate containing a matrix of the 24 different ph buffers in replicates of 4 was prepared (Figure 1B). The buffer plate could be used for three successive analyses before replenishment. The cycle time per four compound sample plate was approximately 20 min (12 compounds/h). A H G F E D C B A Sample 4 Sample 3 Sample 2 Sample 1 B ph 11.2 ph H G F E D C B A ph 6.8 ph 1.7 Figure 1. A: Arrangement of four compounds in a 96-well sample tray (PCR plate, 50 µl/well). B: Inlet buffer arrangement in a 96-well tray for four compound parallel pk a analysis (deep well plate; 1.1 ml/well). Aqueous pk a Analysis The pk a Analyzer PRO software contains preloaded methods for capillary flushing and CE analysis. To start an experiment, the user selects the appropriate assay from the main menu (Figure 2). A screen is opened which allows the user to enter compound names, molecular weights, predicted pk a values if available and buffer ph information (Figure 3). Figure 2. Main screen of pk a Analyzer PRO software showing selection of the 24 point aqueous experimental mode.

3 Figure 3. Sample and buffer information entered into the pk a Analyzer PRO software prior to performing a 24 point aqueous experiment. The CE experiment is then automatically performed and the resulting data sent to the pk a calculation software for analysis (Figure 4). Figure 4. Software display of CE data showing cefadroxil (red) and DMSO neutral marker (black) peaks obtained for the lowest 12 ph points. The effective mobility (m eff ) is calculated at each ph value from the migration time difference between the analyte and DMSO and plotted by the software. To determine the most appropriate fitting equation to apply to the data, the positive and negative m eff limits are identified and, together with the MW, the valency of the compound is predicted (e.g., monobase or dibase). 6 A non-linear regression analysis is then performed on the data plot. The inflection points of the titration curve correspond to the best-fit apparent (I = 50 mm) pk a values for the compound.

4 A report showing the pk a result, titration curve, experimental information and any comments can be created in an Excel format for printing or exporting to an external database (Figure 5). When the final pk a result is saved, a log of the experiment is also automatically generated. This log is stored in a tabular format which can be viewed at any time to compare previously obtained results (Figure 6). Figure 5. Report generated in Excel for the pk a analysis of cefadroxil. Figure 6. Searchable data table displaying previously determined pk a values. pk a Results Table 1 summarizes the pk a screening results for the four different compounds and provides a comparison to literature values. Good agreement is obtained when one considers the variations in experimental methods employed. The low standard deviations (typically <0.05) indicate the exceptional reproducibility of this method.

5 Compound pka Analyzer PRO pk a ' Value (I = 50 mm) SD Literature pk a Values Acyclovir ; ; ; ; Amitriptyline Cefadroxil ; ; Furosemide ; ; ; Reference 1 Multiplexed CE (I = 50 mm) 6 Reference 6 CE (I = 50 mm) 7 Reference 7 CE (I = 30 mm) 8 Reference 8 UV Spectrophotometry (I = 150 mm) 9 Reference 9 Potentiometry (I = 150 mm) Table 1. Comparison of pk a Analyzer PRO and literature pk a values (n = 10 runs). The results obtained for amitriptyline are of particular interest as it possesses a rather low aqueous solubility (-log S 0 = 5.19 or 1.8 µg/ml) [9]. When analyzed at a concentration of 30 µg/ml, precipitation of amitriptyline was observed at ph However the pk a value could still be estimated as more than half of the titration curve was observed prior to precipitation. Importantly, the detection sensitivity of the pk a Analyzer PRO permitted the dilution of amitriptyline to a concentration of 10 µg/ml where precipitation was avoided. This example illustrates the ability and versatility of the pk a Analyzer PRO to analyze low solubility compounds under aqueous conditions where traditional methods would often fail or require the use of cosolvents. Summary This application note has demonstrated the use of the pk a Analyzer PRO for performing accurate, reproducible and high throughput aqueous pk a measurements. The integration of multiplexed CE hardware with instrument control and pk a calculation software provides a powerful tool for performing dedicated pk a analysis. Predefined CE methods alleviate the need for method development and serve to increase reproducibility between laboratories, while enhanced report generation allows results to be exported to an external database if desired. For compounds with extremely low aqueous solubility (< 1 µg/ml), a cosolvent method has been developed which is described in a separate application note. References (1) Zhou, C.; Jin, Y.; Kenseth, J. R.; Stella, M.; Wehmeyer, K. R.; Heineman, W. R. J. Pharm. Sci. 2005, 94, (2) Poole, S. K.; Patel, S.; Dehring, K.; Workman, H.; Poole, C. F. J. Chromatogr., A 2004, 1037, (3) Jia, Z.; Ramstad, T.; Zhong, M. Electrophoresis 2001, 22, (4) Wan, H.; Holmen, A.; Nagard, M.; Lindberg, W. J. of Chromatogr., A 2002, 979, (5) Ishihama, Y.; Nakamura, M.; Miwa, T.; Kajima, T.; Asakawa, N. J. Pharm. Sci. 2002, 91, (6) Miller, J.; Blackburn, A. C.; Shi, Y.; Melzak, A. J.; Ando, H. Y. Electrophoresis 2002, 23, (7) Mrestani, Y.; Neubert, R.; Munk, A.; Wiese, M. J. Chromatogr., A 1998, 803, (8) Avdeef, A. In Absorption and Drug Development; John Wiley & Sons, Inc.: Hoboken, NJ, (9) Box, K.; Bevan, C.; Comer, J.; Hill, A.; Allen, R.; Reynolds, D. Anal. Chem. 2003, 75.

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