The dissociation constants of the cytostatic bosutinib by nonlinear least-squares regression of multiwavelength spectrophotometric and potentiometric pH-titration data

Abstract

Potentiometric and spectrophotometric pH-titration of the multiprotic cytostatics bosutinib for dissociation constants determination were compared. Bosutinib treats patients with positive chronic myeloid leukemia. Bosutinib exhibits four protonatable sites in a pH range from 2 to 11, where two pK are well separated (Delta pK>3), while the other two are near dissociation constants. In the neutral medium, bosutinib occurs in the slightly water soluble form LH that can be protonated to the soluble cation LH43+. The molecule LH can be dissociated to still difficultly soluble anion L-. The set of spectra upon pH from 2 to 11 in the 239.3-375.0 nm was divided into two absorption bands: the first one from 239.3 to 290.5 nm and the second from 312.3 to 375.0 nm, which differ in sensitivity of chromophores to a pH change. Estimates of pK of the entire set of spectra were compared with those of both absorption bands. Due to limited solubility of bosutinib the protonation in a mixed aqueous-methanolic medium was studied. In low methanol content of 3-6% three dissociation constants can be reliably determined with SPECFIT/32 and SQUAD(84) and after extrapolation to zero content of methanol they lead to pK(c1)=3.43(12), pK(c2)=4.54(10), pK(c3)=7.56(07) and pK(c4)=11.04(05) at 25 degrees C and pK(c1)=3.44(06), pK(c2)=5.03(08) pK(c3)=7.33(05) and pK(c4)=10.92(06) at 37 degrees C. With an increasing content of methanol in solvent the dissociation of bosutinib is suppressed and the percentage of LH32* decreases and LH prevails. From the potentiometric pH-titration at 25 degrees C the concentration dissociation constants were estimated with ESAB pK(c1)=3.51(02), pK(c2)=4.37(02), pK(c3)=7.97(02) and pK(c4)=11.05(03) and with HYPERQUAD: pK(c1)=3.29(12), pK(c2)=4.24(10), pK(c3)=7.95(07) and pK(c4)=11.29(05).