Inhibitory effects of diprafenone stereoenantiomers on cardiac Na+ channels

A subset of highly effective propafenone-type multidrug resistance modulators lacks effects on cardiac action potential and mechanical twitch parameters of rat papillary muscles

In this study, we tested a series of 12 previously identified, highly effective propafenone-type multidrug resistance (MDR) modulators for their possible undesirable effects on cardiac tissue. We used rat papillary muscle preparations and quantitatively determined the potency of these substances to block action potential (AP) upstroke velocity (Vmax) and to prolong APD50. Simultaneously, the effects on isometric twitch parameters were evaluated. Concentration-response curves were obtained for all parameters. Within a subset of the compounds, we found a significant rank correlation (r' = 0.87; p < 0.05) between potencies to block Vmax (kiVmax) and to inhibit daunomycin efflux in MDR cells (IC50). Surprisingly, the most lipophilic compounds with additional aromatic side chains completely lacked effects on AP and mechanical twitch parameters, although they are the most effective MDR modulators. Additional structural modifications such as fluoride substitution of the aromatic ring, introduction of arylpiperazine or piperidine side chains, as well as modifying the hydrogen bond acceptor strength of the carbonyl group did not reestablish cardiac side effects. In contrast, when these substances were truncated at the phenylpropiophenone moiety of the propafenone core structure, cardiac effects reoccurred. We conclude that aromatic substituents in the vicinity of the nitrogen atom prevent interaction with ion channels, likely due to steric hindrance, and are thus a prerequisite for eliminating unwanted cardiac effects.

Electrophysiological effects of diprafenone, a dimethyl congener of propafenone on guinea-pig ventricular cells

1. The effects of diprafenone and propafenone on transmembrane action potential were examined and compared in papillary muscles and single ventricular myocytes isolated from guinea-pig hearts. 2. In papillary muscles, both diprafenone and propafenone > or = 10(-6) M caused a significant and dose-dependent decrease in the maximum upstroke velocity (Vmax) of the action potential. 3. In the presence of either drug, trains of stimuli at rates > or = 0.1 Hz led to an exponential decline in Vmax. A time constant (tau R) for Vmax recovery from the use-dependent block was 15.5 s for diprafenone and 8.8 s for propafenone. 4. The use-dependent block of Vmax with diprafenone was enhanced when the resting potential was depolarized by high (8, 10 mM) [K+]o, whereas that with propafenone was virtually unchanged. tau R with diprafenone was shortened by the depolarization, while that with propafenone was rather prolonged. 5. In single myocytes perfused with diprafenone or propafenone, 10 ms conditioning clamp to 0 mV caused a significant decrease in Vmax of subsequent action potential. A prolongation of the clamp pulse duration resulted in a modest enhancement of the Vmax inhibition with diprafenone, while a large enhancement of the Vmax inhibition occurred with propafenone. 6. These findings suggest that diprafenone, like propafenone, may block the sodium channel during both the activated and inactivated states. The relative contribution of inactivation block is less important for diprafenone than for propafenone. The different voltage-dependence of use-dependent block with diprafenone from propafenone would contribute to its high antiarrhythmic potency.