SELECT Table_name, constraint_name 
FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS 
WHERE constraint_type = 'PRIMARY KEY'

declare @name_pk varchar(255)

select @name_pk = name from sysindexes where id in (select id from sysobjects where name = 'table_name') and name like 'PK%'

execute ('alter table table_name drop constraint ' + @name_pk)

declare @name_pk varchar(255)

SELECT @name_pk = constraint_name 
FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS 
WHERE constraint_type = 'PRIMARY KEY' and Table_name = 'table_name'

Print 'pk ' + @name_pk  -- show the results

/* or use the execute as in the original */
-- execute ('alter table table_name drop constraint ' + @name_pk)