NEW YORK/MADRID (Reuters) - Eli Lilly staked its claim for a slice of sales in a new class of breast cancer drugs on Sunday as clinical data showed adding its medicine abemaciclib to standard therapy reduced the risk of disease progression by 46 percent.

Experts at the European Society for Medical Oncology (ESMO) congress in Madrid said the experimental drug’s efficacy was comparable to that of rival drugs from Pfizer and Novartis already on the market.

Abemaciclib, like Pfizer’s Ibrance and Novartis’s Kisqali, belongs to a class of oral medicines called CDK 4/6 inhibitors that block cancer cells’ ability to divide and proliferate.

Adding abemaciclib to standard endocrine therapy led to significant tumour shrinkage in 59 percent of patients compared with 44 percent in those on endocrine drugs alone in the ongoing study dubbed Monarch-3.

For the study’s main goal of progression-free survival (PFS), women who only received endocrine therapy of letrozole or anastrazole on average went 14.7 months before disease worsening.

Not enough patients on abemaciclib had seen their disease worsen after 18 months of treatment to calculate median PFS for this group.

Evandro de Azambuja, a breast cancer expert at the Jules Bordet Institute, who was not involved in the study, said the rival drugs from Lilly, Pfizer and Novartis all appeared “quite similar” in terms of efficacy.

“I don’t think there is a preferred drug at this moment. It will depend on availability in different countries and also cost,” he told reporters in Madrid.

Abemaciclib is given continuously rather than the on-and-off cycles used for its approved rivals as it does not cause as high rates of neutropenia, a suppression of white blood cells that can lead to infection. It does, however, cause more diarrhoea.

Angelo Di Leo of Italy’s Istituto Toscano Tumori, who presented the Monarch-3 findings, said the choice of which CDK 4/6 drug to use would depend on individual patients’ needs.

Pfizer’s Ibrance, which was first to market, is currently the market leader, ahead of Kisqali, but industry analysts forecast the new Lilly drug will reach annual sales of $1.8 billion by 2023, according to Thomson Reuters data.

Di Leo said there was substantial benefit from abemaciclib in patients with particularly challenging metastases, such as tumors in the liver or lungs. However, patients with bone metastases had a good prognosis with endocrine therapy alone.

Monarch-3 enrolled 493 post-menopausal women who had not yet received systemic treatment for advanced breast cancer that had spread.

Five patients, or about 2 percent, who received abemaciclib experienced complete responses, meaning no detectable cancer, a number that could rise.

“The longer you’re on (these drugs) the greater the chance that you can have complete responses,” said Matthew Goetz of the Mayo Clinic, another investigator on the study.

“This trial gives us full confirmation that CDK 4/6 inhibitors are here to stay. They’re a new standard of care for breast cancer.”