Cent. Eur. J. Chem. • 7(1) • 2009 • 8-13 DOI: 10.2478/s11532-008-0099-z Central European Journal of Chemistry Re-evaluated data of dissociation constants of alendronic, pamidronic and olpadronic acids Invited Paper Alexander P. Boichenko1, Vadim V. Markov1, Hoan Le Kong1, Anna G. Matveeva2, Lidia P. Loginova1* 1Department of Chemical Metrology, Kharkov V.N. Karazin National University, 61077 Kharkov, Ukraine. 2A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow, Russian Federation Received 01 April 2008; Accepted 23 October 2008 Abstract: The dissociation constants of (4-amino-1-hydroxybutylidene)bisphosphonic (alendronic) acid, (3-(dimethylamino)-1- hydroxypropylidene)bisphosphonic (olpadronic) acid and (3-amino-1-hydroxypropylidene)bisphosphonic (pamidronic) acid were obtained in aqueous solutions (0.10 М КСl) and micellar solutions of cetylpyridinium chloride (0.10 М CPC) at 25.0°C. With the exception of the third dissociation constant of alendronic acid, the dissociation constants of alendronic, olpadronic and pamidronic acids in aqueous solutions matched literature data. The possibility of sodium alendronate determination by acid-base titration by NaOH solution was theoretically grounded on the basis of re-evaluated data of alendronic acid dissociation constants. Keywords: Alendronate • Olpadronate, pamidronate, dissociation constant • Micellar media effect © Versita Warsaw and Springer-Verlag Berlin Heidelberg. 1. Introduction spectrometric [12], refractive index [13] and Sodium salts of (4-amino-1-hydroxybutylidene) conductometric [14] detection have been proposed. bisphosphonic (alendronic) acid, (3-(dimethylamino)- The anion-exchange chromatography with refractive 1-hydroxypropylidene)bisphosphonic (olpadronic) acid index detection is recommended by British and (3-amino-1-hydroxypropylidene)bisphosphonic Pharmacopoeia for the determination of sodium (pamidronic) acid are successfully used for the medical alendronate [15]. Most chromatographic analysis of treatment of bone diseases, such as osteoporosis, aminobisphosphonates have required extensive sample Paget disease, metastatic bone diseases, etc. [1]. preparation, the use of a non-routine detector and Thus, numerous pharmaceutical formulations that a special column and/or apparatus. contain these active substances have been developed In addition to chromatographic methods, [2]. The analysis of aminobisphosphonic acids is an the spectrophotometric method after derivatization interesting problem. The use of chromatographic of alendronate with o-phthalaldehyde [7] and titrimetric methods introduced several problems: 1) UV-Vis method [16] were proposed for alendronate or fluorescent detection can not be used owing to lack determination. The acid-base titrimetric method will be of chromophores in the molecule; 2) conventional quite useful for routine analysis of sodium alendronate reversed-phase liquid chromatography (RP-HPLC) is due to its accuracy, simplicity and low cost. Therefore, not a good choice, because aminobisphosphonates most of pharmacopoeia methods of active substance are hydrophilic and ionized in the pH range used with quantification are titrimetric. However, the theoretical reversed-phase silica based columns and strong metal basis of determination of sodium alendronate by chelating ability causes adsorption problems; 3) gas acid-base titration is complicated by very close third, chromatography can not be used due to insufficient fourth and fifth dissociation constant of alendronic acid volatility of aminobisphosphonates. [17] (Table 1). Moreover, alendronic acid is very weak Several high-performance liquid chromatographic by the third dissociation step (Table 1). As a result, methods based on post- or precolumn derivatization the acid-base titration of sodium alendronate should be of bisphosphonates have been developed [3-11]. problematic (Fig. 2, curve 1). For example, Ion-pair chromatography with mass- * E-mail: [email protected] 8 A. P. Boichenko, V. V. Markov, H. Le Kong, A. G. Matveeva, L.P. Loginova The dissociation constants of alendronic acid have been obtained [17] and are widely cited by other authors and referral databases [18]. However, it is interesting that other authors [20,21] have determined different dissociation Figure 1. Zwitterionic structures of alendronic (1), olpadronic (2) and pamidronic (3) acids constants of alendronic acid and have not dissolved this discrepancy. On the other hand, the stability constants of copper complexes with alendronic acid have been determined [22] without observation of disagreement between dissociation constants obtained in the previous works [17] and [20].Thus, the determination of reliable dissociation constants of alendronic acid and its analogs are of primary importance. They will be useful for modeling and prediction of the technological, environmental and pharmaco-kinetic equilibria, waste management etc. In this work the dissociation constants of alendronic, olpadronic and pamidronic acids in aqueous solutions Figure 2. Calculated and experimental titration curves of 0.02 M were re-evaluated. This should confirm or reject sodium alendronate with 0.04 M alkali solution. the possibility of sodium alendronate determination by 1 – Calculated curve by using pKa values from ref. [17]; acid-base titration in aqueous solutions. The dissociation 2 – Experimental curve obtained in aqueous solution; constants of alendronic, olpadronic and pamidronic acid 3 – Calculated curve by using pKa values obtained in this work in aqueous solution; were also determined in micellar solutions of cationic 4 – Calculated curve by using pK values obtained in this work in micellar a surfactant cetylpyridinium chloride (CPC), because CPC solution. it is known that the strength of an acid increases in Table 1. Indexes of dissociation constants of aminobisphosphonic the presence of cationic micelles [23]. This observation acids allows the development of simple titrimetric methods of Literature data This This aminobisphosphonates determination in pharmaceutical work work formulations. Moreover, micelles of surfactants are 0.1 M KCl 0.2 M KCl 0.1 M 0.1 M KCl 0.1 M CPC [17]1 [20]2 [21]3 often considered as adequate models of biological Alendronic acid membranes and the data of dissociation constants of 4 4 aminobisphosphonates in CPC media can be useful for pKa2 2.72±0.05 2.16 2.22 2.24±0.01 2.34±0.02 modeling the behavior of drugs in biological systems [19]. pKa3 8.73±0.05 6.21 6.39 6.38±0.03 5.97±0.02 pKa4 10.5±0.1 10.77 10.96 10.68±0.06 10.25±0.03 pK 11.6±0.1 12.04 11.82 11.4±0.2 10.5±0.1 2. Experimental Procedures a5 Olpadronic acid pKa2 2.55±0.05 2.1 Reagents and standard solutions 5 5 Certified reference material (CRM) of sodium alendronate pKa3 5.83±0.05 5.97±0.03 5.39±0.03 pKa4 9.9±0.1 9.75±0.05 9.35±0.06 trihydrate tested by pharmacopoeia method [15] was pKa5 10.8±0.1 10.96±0.14 10.30±0.08 kindly donated by Join-stock company «Stoma», Pamidronic acid Kharkov, Ukraine. The olpadronic and pamidronic acids pKa2 2.35±0.05 1.93 were synthesized and purified in A.N. Nesmeyanov 5 5 pKa3 5.89±0.05 6.04 6.01±0.01 5.39±0.01 Institute of Organoelement Compounds, Russia. Tablets pKa4 9.7±0.1 10.18 9.97±0.02 9.46±0.02 “Sodium alendronate” containing 10 mg of sodium pKa5 10.8±0.1 12.14 10.74±0.05 10.40±0.04 alendronate were purchased in a Ukrainian pharmacy. 1concentration of aminobisphosphonic acids 1.0 × 10-3 M; The cetylpyridinium chloride was purchased from Merck. pH-potentiometric titration, calibrated by standard pH buffers; 25.0°C 2concentration of aminobisphosphonic acids 2.0 × 10-3 M; The 0.100 М HCl solution was prepared from analytically pH-potentiometric titration, calibrated in hydrogen ion concentration, grade concentrated HCl and standardized by titration 25.0°C 3concentration of aminobisphosphonic acids from 2.0 × 10-3 to of weighted amounts of Na2CO3. Carbonate-free 8.0 × 10-3 M; pH-potentiometric titration at 25.0°C and 31P[1H]NMR pH 0.100 M KOH solution was prepared by technique titration at 22°C, calibrated by blank titration 4 -3 described in [24] and standardized by titration of weighted 5.0 × 10 M sodium alendronate; 25.0°C; Ka2 is stoichiometric constant; Ka3 - Ka5 are mixed constants; for other conditions see Experimental amounts of adipinic acid. Carbonate-free 0.100 M NaOH Procedures 5 -3 solution was prepared from saturated NaOH solution 1.0 × 10 M olpadronic or pamidronic acid; 25.0°C; Ka3 - Ka5 are mixed constants; for other conditions see Experimental Procedures and standardized by titration of weighted amounts 9 Re-evaluated Data of Dissociation Constants of Alendronic, Pamidronic and Olpadronic Acids of adipinic acid. Working solutions of НСl, NaOH and acids consisted of approximately 40 data points and KОН were prepared by appropriate dilution of standard these curves were used to calculate the dissociation solutions before titrations. The working solution of constants. Since the aminobisphosphonates were tetraethylammonium hydroxide was prepared from 10% titrated in a basic medium for determination of рKа3-рKа5 (w/w) analytical grade solution of tetraethylammonium the influence of carbon dioxide had to be prevented by hydroxide by appropriate dilution. Analytical grade KCl using nitrogen. The nitrogen was purified according to was used for preparation of supporting electrolyte. known procedure [24] and used for excluding of carbon Acid-base indicators thymol blue and phenolphthalein dioxide from titration vessel. were used as provided from Ukrainian suppliers. 2.4 Software and processing 2.2 Apparatus The data of potentiometric titrations for dissociation The electromotive force was measured with using constants determination were treated by CLINP 2.1 a compensation scheme (P-307 potentiometer, program (http://www.bestnet.kharkov.ua/kholin/clinp.html). a pH-121 pH-meter as a null-instrument) with standard The titration curves of sodium alendronate were modeled deviation of at most 0.2 mV. The glass electrode by program EQUIL 1.1 developed by Prof. A. Bugaevsky ESL-63-07 and silver/silver chloride electrode EVL-1M3 on the basis of Cruise algorithm [26].