Enzyme replacement biologics were the tenth largest selling class of biological products, with total sales in 2005 estimated at $1.25 billion.

Enzyme replacement therapy involves replacing an enzyme in patients in whom that particular enzyme is deficient or absent.

Enzyme replacement therapy is currently available for four lysosomal diseases. Lysosomal enzymes are responsible for breaking down complex chemicals within a cell which have expended their useful life. The breakdown products are then eliminated from the cell or reused. A deficiency of any one of these enzymes will lead to a "storage disease" usually associated with developmental regression.

Cerezyme (imiglucerase for injection) is a recombinant analogue of the human enzyme ß-glucocerebrosidase and is used for treatment of patients with Type 1 Gaucher disease. Patients with Gaucher disease do not produce enough of this enzyme, resulting in accumulation of a fatty substance in the body called glucocerebroside. Cerezyme has been shown to arrest, decrease, or normalize many of the major signs and symptoms of Gaucher disease.

Fabrazyme (agalsidase beta) is a recombinant enzyme nearly identical to the alpha-galactosidase A, or alpha-GAL, enzyme produced by the body. It is used as enzyme therapy for people with Fabry disease, who are missing or have insufficient quantities of alpha-GAL, which is important in the healthy functioning of organs.

Aldurazyme (laronidase) contains a recombinant enzyme nearly identical to a naturally occurring form of the human enzyme alpha-L-iduronidase. People with Hurler and Hurler-Scheie forms of Mucopolysaccharidosis I, also called MPS I, have a deficiency of this enzyme, which breaks down substances called glycosaminoglycans (GAG). Without adequate amounts of the enzyme, GAG builds up in many tissues and organs causing damage.

Elaprase (idursulfase) is the most recently approved enzyme replacement therapy designed to treat the underlying cause of Hunter syndrome by replacing the iduronate-2-sulfatase (I2S) enzyme, which is deficient or absent in people with Hunter syndrome.

The three leading enzyme replacement therapy biologics are: