sanitize_text_field and apostrophe problem

I've written a custom plugin to send data via a form to a personal database, different from the WP one. Everything is working fine except the fact that when you write a name, for example "D'ALESSANDRO", it's saved as "D\'ALESSANDRO".

This is how I'm sending data to database:

$custom_field = sanitize_text_field($_POST[custom_field])
if ( count($reg_errors->get_error_messages()) < 1) {
    $userdata = array(
    ........
    'id_locality'   =>  $custom_field
    ........
    );
    $where_data = array(
      .......wherefield...........
    );
    $pldb = new wpdb('user', 'pwd', 'db', 'localhost');
    $update_table = $pldb->update('table',$userdata,$where_data);

I need to save data into the database without the "\", I can't sort this when printing it.

How can I do this?