[WPF]ComboBox multi column récuperer valeur item

Bonjour,
Soit la combobox L_Cre suivante:

Code:

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    <Window.Resources>
        <Style x:Key="txtStyle" TargetType="{x:Type TextBlock}">
            <Setter Property="TextAlignment" Value="Center"></Setter>
            <Setter Property="FontSize" Value="13"></Setter>
            <Setter Property="FontWeight" Value="Bold"></Setter>
            <Setter Property="TextWrapping" Value="WrapWithOverflow"></Setter>
            <EventSetter Event="ComboBoxItem.PreviewMouseDown" Handler="cb_PreviewMouseDown"></EventSetter>
        </Style>
        <DataTemplate x:Key="EmpKey">
            <Grid Height="66" Width="444" ShowGridLines="True">
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="100"></ColumnDefinition>
                    <ColumnDefinition Width="70"></ColumnDefinition>
................................................................................................................
                <Label Grid.Column="0" VerticalAlignment="Bottom" VerticalContentAlignment="Bottom">Titre</Label>
                <Label Grid.Column="1" VerticalAlignment="Bottom" VerticalContentAlignment="Bottom">Date</Label>
................................................................................................................

                <TextBlock Grid.Column="0"  Text="{Binding Titre}"
                    Style="{StaticResource txtStyle}"></TextBlock>
                <TextBlock Grid.Column="1"  Text="{Binding Date}"
                    Style="{StaticResource txtStyle}"></TextBlock>
................................................................................................................

            </Grid>
        </DataTemplate>

    </Window.Resources>
................................................................................................................
            <ComboBox Height="66"  HorizontalContentAlignment="Stretch" RenderTransformOrigin="-6.5,1.4" Width="444" Grid.Column="1"  Grid.ColumnSpan="5" SelectedValue="{Binding Titre}" 
                  x:Name="L_Cre"
                  VerticalAlignment="Top" ItemsSource="{Binding}"
                  ItemTemplate="{StaticResource EmpKey}"
                   IsSynchronizedWithCurrentItem="True"
                   HorizontalAlignment="Left" >

            </ComboBox>

Code:

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        private void cb_PreviewMouseDown(object sender,MouseButtonEventArgs e)
        {
            ComboBoxItem item = e.Source as ComboBoxItem;
            if(item != null) { MessageBox.Show("item: " + item.Content.ToString()); }      
        }

Comment récupérer,sur un clic, l'item sélectionné, puis la valeur contenue dans "Titre" ?
D'avance merci.