NEW YORK (AP) — Shares of Bristol-Myers Squibb slumped Thursday after the drugmaker reported disappointing clinical trial data for an experimental cancer treatment.

On Wednesday the Princeton, New Jersey, company revealed data from several early-stage clinical studies of its cancer drug nivolumab. Nivolumab hasn't yet been approved, but Bristol-Myers Squibb is getting ready to file for approval of the drug as a treatment for non-small cell lung cancer. A drug regimen that combined nivolumab with an approved drug, Yervoy, has showed promise as a treatment for melanoma.

BMO Capital Markets analyst Alex Arfaei said Thursday that nivolumab and Yervoy had only a "modest" effect on lung cancer and they came with a high rate of side effects.

"We were wrong to extrapolate the strong efficacy (of the combination) in melanoma to lung and kidney cancer," he wrote. Arfaei downgraded Bristol-Myers Squibb shares to "Market Perform" from "Outperform" and cut his price target to $55 per share from $60.

Bristol-Myers Squibb Co. shares stumbled $3.67, or 7 percent, to $48.45 in midday trading as the broader markets moved lower.

Nivolumab is part of a new class of treatments called PD-1, or programmed death, therapies. The drugs allow a patient's immune system to target hidden tumor cells. In 2013 the FDA gave nivolumab fast-track status as a treatment for types of lung cancer, kidney cancer, and melanoma. Fast-track status gives companies extra meetings and correspondence with regulators throughout the review process, and it allows the drugmaker to submit data as it compiles it.

The company expects to finish filing for approval of nivolumab as a third-line treatment for non-small cell lung cancer before the end of 2014.

Yervoy was approved in the U.S. in 2011 and in the European Union in 2012. Sales of the drug rose 18 percent to $271 million in the first quarter.