Shares in

Allos Therapeutics

(ALTH)

more than doubled Wednesday as the biotech company cleared a major regulatory hurdle for the approval of its treatment for brain metastases.

Shares in the Westminster, Colo.-based company gained $1.98, or 102%, to $3.93, after the company said it has received an approvable letter for a new drug application from the Food and Drug Administration.

Allos said the FDA had requested additional clinical data to support approval of its drug known as RSR13, or efaproxiral, for the treatment of patients with brain metastases originating from breast cancer.

The company said no other specific deficiencies in its application process were noted in the FDA's approvable letter.

According to the company's release, "RSR13 is the first synthetic small molecule designed to sensitize hypoxic, or oxygen-deprived, areas of tumors prior to radiation therapy by facilitating the release of oxygen from hemoglobin, the oxygen-carrying protein contained within red blood cells, and increasing the level of oxygen in tumors. The presence of oxygen in tumors is an essential element for the effectiveness of radiation therapy in the treatment of cancer."