Studies on solution conformations and mechanism of action of amrinone and milrinone

There are two structural differences between the positive inotropes amrinone (A) and milrinone (M): (1) M has a pyridone 2-methyl substituent and; (2) the pyridone 5-amino substituent of A is replaced with a nitrile in M. SAR studies confirmed that the 2-methyl substituent is responsible for the dramatically increased potency of M relative to A (canine i.v. ED50's=37 and 1891 μg/kg, respectively). Both M and A inhibited canine heart PDEase III (IC50's=12 and 150μM, respectively). Moreover, in a series of M/A congeners, the correlation between in vitro PDEase III IC50's and in vivo inotropic ED50's was highly significant (r = 0.98, p < 0.01). In the crystal-state, the methyl substituent of M induces a marked alteration in its 3-dimensional topology relative to A. They have now completely assigned the 13C- and 1H-NMR spectra of A and M and have used the chemical shifts of the pyridone C-4 protons as conformational reporters. These studies revealed that the C-4 protons of methyl containing congeners (M-like) experience a 0.38-0.53 ppm upfield shift relative to the corresponding proton resonance in unmethylated congeners (A-like). These data suggest there are marked, methyl-induced differences in the solution conformations of A and M