Getting data out of Profile2 with field_get_items()
As @Clive mentioned profile2_load_by_user() takes user account object not uid, but you also need type name as second argument to retrieve field_lastname using field_get_items function.

I recommend you to use this function profile2_by_uid_load instead of profile2_load_by_user

So your code should look like the following:

$profiledata = profile2_by_uid_load($variables['comment']-&gt;uid, 'profile_type_name');

$lastname = '';

if($profiledata) {

  $lastname  = field_get_items('profile2', $profiledata, 'field_lastname');
}