Abstract: There is provided immediate release pharmaceutical formulations comprising, as active ingredient, 4-([3-[7-(3,3-dimethyl-2-oxobutyl)-9-oxa-3,7-diazabicyclo[3.3.1]non-3-yl]propyl]amino)benzonitrile, tert-butyl 2-{7-[3-(4-cyanoanilino)propyl]-9-oxa-3,7-diazabicyclo[3.3.1]non-3-yl}ethyl-carbamate, tert-butyl 2-{7-[4-(4-cyanophenyl)butyl]-9-oxa-3,7-diaza-bicyclo[3.3.1]non-3-yl}ethylcarbamate or tert-butyl 2-{7-[(2S)-3-(4-cyanophenoxy)-2-hydroxypropyl]-9-oxa-3,7-diazabicyclo[3.3.1]non-3-yl}ethylcarbamate, or a pharmaceutically-acceptable salt of any of these compounds, and a pharmaceutically-aceptable diluent or carrier. There is further provided solid pharmaceutical compositions comprising an above-stated active ingredient, which compositions are suitable for the preparation of immediate release pharmaceutical formulations, and which compositions may be prepared for example by freeze-drying. The formulations and compositions are useful in the prophylaxis and/or treatment of cardiac arrhythmias.