Remarkable differences between the species distributions of various bis(guanylhydrazones) at physiological...

The first systematic study on the acid-base properties of the antileukemic agents glyoxal bis(guanylhydrazone) (GBG) and methylglyoxal bis(guanylhydrazone) (MGBG) and of their non-antileukemic monoalkyl- and dialkylglyoxal analogs is reported. At physiological conditions (pH 7.4, 37 degrees C), the species distribution of GBG and MGBG differs remarkably from that of their inactive congeners, a noteworthy proportion of GBG (10.2%) and MGBG (4.0%) existing in the form of the free base while the corresponding proportion of their non-antiproliferative analogs is only 0.5% or less. Ethylglyoxal bis(guanylhydrazone) (EGBG), which has antiproliferative properties in vitro but is devoid of antileukemic activity in vivo, is intermediate between the two groups, 2.6% of it existing in the free base form. In contrast to what has been generally assumed, at physiological conditions, the predominant species of GBG, MGBG, and EGBG is the monocation form and not the dication. Considerable proportions of other congeners also exist in the monocation form. At slightly higher pH values that are of interest because of the known antimitochondrial effects of GBG and MGBG (and, in high concentrations, EGBG), the species distribution of GBG and MGBG differs even more remarkably from that of the dialkylglyoxal analogs. Thus, at pH 8.0 and 37 degrees C, as much as 36% of GBG and 19% of MGBG exist in the free base form, the corresponding proportion of EGBG being 14% and that of the other congeners studies only ca. 3-4%. On the basis of the results, it appears possible that the unusually strict structure-activity relationships of this class of antineoplastic agents may be based on the remarkable differences between the species distributions of the various congeners. The hypothesis is presented that the actual antiproliferative and antimitochondrial species of the compounds is the free base form. A compilation of pKa1 and pKa2 values, measured by potentiometric methods in 0.1 M NaCl (aq) at 25 degrees C and 37 degrees C, is given for six bis(guanylhydrazones). The species distribution curves of the compounds (at 37 degrees C) are given for the pH range 6-10.