European health regulators on Friday conditionally approved Amgen Inc's rare blood cancer drug, Blincyto, which is one of the world's most expensive cancer treatments.

The US Food and Drug Administration in December approved Blincyto for acute lymphoblastic leukemia (ALL), which has few treatment options once a patient has relapsed.

Blincyto is a so-called bispecific antibody, an emerging class of drugs that could prove more potent than conventional antibodies - the mainstay treatment for many forms of cancers.

An estimated 6,250 Americans, mostly children, will be diagnosed with ALL this year and 1,450 of them will succumb to the disease, according to the American Cancer Society.

The European Medicines Agency conditionally approves a drug when it is meant for a severely debilitating disease with few or no treatment options, and when the benefits of its immediate availability outweigh the risks.

The conditional approval is granted despite the lack of complete trial data and is renewed annually until the EMA is satisfied with the complete data to fully approve the drug.

Amgen, the world's largest biotechnology company, expects final European approval for Blincyto in the coming months.

The drug, which it got through the acquisition of biotechnology company Micromet in 2012, costs about $178,000 for two courses of treatment.

Pharmaceutical companies have recently come under fire over high prices for specialty drugs and U.S. Democratic presidential candidate Hillary Clinton unveiled a plan on Wednesday to address "price gouging". (