UPDATE 1-Amgen says Prolia increases bone density over time

* Gains seen in lumbar spine, hip

* Serious adverse events in 19 pct of patients

LOS ANGELES, March 23 (Reuters) - Amgen Inc's (AMGN.O) Prolia bone drug continues to improve bone mineral density in post-menopausal women treated with the drug for up to five years, according to new data released on Wednesday.

Amgen, which presented the follow-on trial results at a medical meeting in Spain, said that around 19 percent of the 4,550 patients followed in the trial experienced serious side effects, including two cases of osteonecrosis of the jaw.

Prolia is the first in a new class of medicines that work by blocking a protein that activates bone-destroying cells called osteoclasts.

Women treated with the drug -- an injection every six months -- for five continuous years saw average bone mineral density gains of 13.7 percent for the lumbar spine and 7 percent for the total hip, the company said on Wednesday.

In years four and five respectively, women taking Prolia experienced further 1.9 percent and 1.7 percent increases in lumbar spine density, and further 0.7 percent and 0.6 percent increases in total hip bone mineral density.

Sales of Prolia, which was launched in June, totaled $33 million last year.

(Editing by Bernard Orr)