Biogen Idec Inc. on Tuesday says it has agreed to pay Isis Pharmaceuticals Inc. an initial $29 million to develop and commercialize a drug to treat spinal muscular atrophy, with the possibility for additional payments, bringing the total to $299 million.

Isis’ antisense investigational drug is meant to treat the inherited muscle-weakening disease that affects children.

Biogen will pay Isis $29 million up front, and another $45 million in potential milestone payments to have the opportunity to license the experimental medicine after a successful efficacy trial, the companies said. Biogen may also pay another $225 million for a license fee and regulatory milestone payments.

The ISIS-SMNRx treatment is designed to compensate for the primary genetic defect that causes SMA. The U.S. Food and Drug Administration has granted the treatment orphan drug status and fast track designation.

The treatment is designed to treat childhood SMA by modifying the link of a similar gene to increase production of fully functional SMN protein.

Antisense drugs work by blocking the body’s production of disease-causing proteins and traditional drug treatments interact with disease causing problems.

SMA is a genetic disease in which afflicted children lack a gene for a protein called SMN which is required for proper nerve cell development. The drug is injected into the spinal fluid and aims to boost levels of the SMN protein.

Biogen CEO and Chairman Stanley T. Crook said in a statement the deal is consistent with the company’s strategy to develop antisense drugs to ‘proof-of concept,’ a term associated with a trial to show clinical efficacy with a small number of patients.

He said the strategy also requires “a knowledgeable partner that is committed to supporting the rapid development of the drug.”