MVVM: update property by PropertyChanged

[Igor]

Hi!
There are PropertyGrid in View and ViewModel with some binding properties.
I want to change property in ViewModel immediate, in PropertyChanged (not in LostFocus).
But the corresponding property in ViewModel updates after propery definition lost focus.
Any ideas?
Code:
<telerik:RadPropertyGrid ... Name="radPropertyGrid" AutoGeneratePropertyDefinitions="False" Item="{Binding}" EditorTemplateSelector="{StaticResource dataTemplateSelector}" >


    <telerik:RadPropertyGrid.PropertyDefinitions>
        <telerik:PropertyDefinition Binding="{Binding VMSomeProperty, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" ... />
...