California biotech Isis Pharmaceuticals , known for its drugs for rare and cardiovascular diseases using RNA-based technology, is forming a Cambridge subsidiary to further development of its drugs to control fat and cholesterol in the blood. Akcea Therapeutics is being called the "lipid franchise" of Carlsbad, Calif.-based Isis in a statement today, to be headed by former Genzyme Corp. and Moderna Therapeutics executive Paula Soteropoulos .