Moody's Investors Service affirmed its ratings and negative outlook for Abbott Laboratories on Wednesday and warned of a potential downgrade, citing the company's decision to buy Facet Biotech Corp. for $450 million.

On Tuesday, drug and medical products company Abbott announced the $27-per-share deal, which marked a 67 percent premium to Facet's closing price of $16.21.

Moody's affirmed Abbott's 'A1', or "upper medium grade" senior unsecured and Prime-1 short-term ratings, while also affirming a "Negative" outlook. Abbott's pursuit of additional buyouts or ongoing liquidity constraints could result in a downgrade, Moody's said.

"Moody's recognizes that Facet will offer additional pipeline products in the areas of multiple sclerosis and oncology. However, following Solvay and a series of other acquisitions, this transaction highlights the company's continued strong appetite for acquisitions," Moody's said, in a statement. "Also, based on Facet's recent stock price, this transaction appears richly priced."

In December, Facet shareholders rejected a bid worth $17.50 per share from Biogen Idec Inc., which is a partner on the potential multiple sclerosis drug daclizumab.

In its deal, Abbott will gain that potential product along with potential cancer treatments.

Moody's said the buyout will place additional stress on some of Abbott's credit metrics. Also, cash coverage of debt is much weaker following the closing of the $6.2 billion buyout of Solvay's pharmaceutical business in February.

Shares of Abbott, based in North Chicago, Ill., rose 26 cents to $55.06 in afternoon trading.

Meanwhile, shares of Facet, based in Redwood City, Calif., surged $10.83, or 67 percent, to reach $27.04, signaling that investors agree with the buyout value.