By Bill Berkrot

Jan 14 (Reuters) - U.S. patent officials on Thursday denied petitions by Amgen to review two formulation patents on AbbVie's Humira, a potential setback in Amgen efforts to market a biosimilar version of the world's top-selling prescription medicine.

In June, Amgen, the world's biggest biotechnology company, asked the U.S. Patent and Trademark Office for the review, arguing that the patents should not have been granted in the first place for Humira, a rheumatoid arthritis drug with annual sales approaching $14 billion.

In declining to review the patents, the agency said "we determine, based on the Petition and the accompanying evidence, that Amgen has not shown a reasonable likelihood of prevailing on any of its challenges."

Amgen in November became the first company to file with U.S. health regulators seeking approval to sell a less expensive biosimilar version of the blockbuster medicine. Several companies are also developing biosimilar version of Humira.

Amgen has said it expects the first of its several biosimilars in development to reach the market between 2017 and 2019. The company did not have an immediate comment on the ruling.

AbbVie this week said it expects Humira sales to reach $18 billion by 2020. Based on its various patents, AbbVie said it does not expect competition from biosimilars until 2022.

Unlike chemical medicines, biotech drugs are made from living cells so exact copies are not possible, thus the term biosimilar rather than generic as with copies of pills.

(Additional reporting by Andrew Chung; Editing by David Gregorio)