Electrophysiological evaluation of novel blockers of If current.

Purpose: In the sino-atrial node (SAN), a major role in the initiation and autonomic control of rhythm generation is played by f-current, a mixed sodium-potassium inward current activated upon hyperpolarization and directly modulated by cyclic nucleotides. If current is encoded by Hyperpolarization-activated Cyclic Nucleotide-gated sodium-potassium channel family genes (HCN1-4). In physiological condition HCN expression is functionally relevant in the SAN region and in other parts of the conduction system. f- current becomes upregulated in many cardiac disease at ventricular level contributing to the increased propensity for cardiac arrhythmias. Thus, selective f-channel blockers have a potential for therapeutic use as bradycardic and antiarrhythmic agents. Zatebradine and ivabradine act as f-channel blockers but both of them blocking the neuronal HCN isoforms. From this, the necessity to develop new compounds selective for the channel isoform typical of mammal SAN, HCN4. Material and Methods: On the basis of zatebradine structure different analogues (C1-C5) were synthesized and were tested on HEK293 cells expressing mHCN1, mHCN2 and hHCN4 and on SAN cells of guinea-pig and rabbit by patch clamp tecnique. Results: The compounds used for this study were tested for the first time on HEK293 cells expressing HCN1, HCN2 and HCN4 isoforms of f-channel and their activity was compared with that of ivabradine. All compounds (C1-C5) at the concentration of 10 μM, produced a reduction of maximal f-current amplitude, elicited by a step to -120 mV, although with different potency and selectivity as shown by their EC50 values. C1 and C4 were more potent on HCN1 (2.31±0.4 and 0.60±0.07 μM respectively) and C2 displayed a major activity on HCN4 (5.19±0.6 μM). C3 was more effective on HCN1 and HCN4 and C5, the enantiomer of C4, had a low activity on all isoforms. Current-voltage curves revealed that the effect of all compounds, including ivabradine, was concentration-dependent, did not reverse upon drug removal and did not change current properties. Data obtained in SAN cells suggest that the effects on native f-current resemble those obtained on HCN4 isoform, in line with the hypothesis that HCN4 represent a major contributor in SAN cells. Conclusions: These findings indicate that the interaction with the different channel isoforms seems to have different structural requirements. Work is underway to clarify the structural requirements to improve the selectivity and to better characterize the pharmacological profile of these substances.