DECLARE @sql1 NVARCHAR(MAX);
SELECT @sql1 = c
FROM
(
    SELECT 'ALTER TABLE DOC_INVS_1 DROP CONSTRAINT ' + CONSTRAINT_NAME + '; '
    FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS WHERE TABLE_NAME = 'DOC_INVS_1'
    and CONSTRAINT_NAME LIKE 'FK__DOC_INVS___kntr%' 
) T(c);

EXEC(@sql1);