// Next Post Mod
add_filter( 'get_next_post_where', function( $where, $in_same_term, $excluded_terms )
{
    global $post, $wpdb;

    // Edit this custom post type to your needs
    $cpt = 'post';

    // Current post type
    $post_type = get_post_type( $post );

    // Nothing to do
    if( $in_same_term || ! empty( $excluded_terms ) || $cpt !== $post_type )
        return $where;

    // Next CPT order by last word in title
    add_filter( 'get_next_post_sort', function( $orderby )
    {
        return " ORDER BY p.post_title ASC LIMIT 1 ";
    } );

    // Modify Next WHERE part
    return $wpdb->prepare(
        " WHERE
            p.post_title > '%s'
            AND p.post_type = '%s'
            AND p.post_status = 'publish'
        ",
        $post->post_title,
        $post_type
    );

}, 10, 3 );


// Previous Post Mod
add_filter( 'get_previous_post_where', function( $where, $in_same_term, $excluded_terms)
{
    global $post, $wpdb;

    // Edit this custom post type to your needs
    $cpt = 'post';

    // Current post type
    $post_type = get_post_type( $post );

    // Nothing to do
    if( $in_same_term || ! empty( $excluded_terms ) || $cpt !== $post_type )
        return $where;

    // Previous CPT, order by last word in post title
    add_filter( 'get_previous_post_sort', function( $orderby )
    {
        return " ORDER BY p.post_title DESC LIMIT 1 ";
    } );

    // Modify Prev WHERE part
    return $wpdb->prepare(
        " WHERE
            p.post_title < '%s'
            AND p.post_type = '%s'
            AND p.post_status = 'publish'
        ",
        $post->post_title,
        $post_type
    );

}, 10, 3 );