The following are the M&A deals, rumors and chatter circulating on Wall Street for Thursday May 21, 2015:

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CVS Health Corporation (NYSE:

) and Omnicare, Inc. (NYSE:

) announced Thursday that they have entered into a definitive agreement for CVS Health to acquire Omnicare for $98.00 per share in cash or $12.7 billion, including approximately $2.3 billion in debt. The deal is expected to close near the end of 2015. Omnicare shares rose $1.59 Thursday, to close at $96.22.

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Jefferies said Thursday that M&A could be a major driver for Pfizer (NYSE:

) outperformance this year and says Shire (NASDAQ:

) is a potential acquisition target and overseas cash could be used to acquire Shire. Shire shares gained $2.46 Thursday, closing at $258.20

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EXOR S.p.A. (OTC:

), the largest shareholder of PartnerRe Ltd. (NYSE:

), said Thursday, it is willing to negotiate in good faith to provide PartnerRe shareholders with improved closing certainty with regards to its binding all-cash $137.50 offer. EXOR wants the PartnerRe Board to declare that EXOR's binding offer is reasonably likely to be a superior proposal, as called for under the PartnerRe agreement with AXIS Capital Holdings Limited (NYSE:

). EXOR says its binding offer of $137.50 per share delivers a premium of 10% to the implied per share value under the AXIS agreement of $125.17 using the closing price of AXIS shares as of May 5, 2015, the last trading day prior to published reports of takeover interest in AXIS and has no substantive regulatory risks. PartnerRE closed Thursday at $133.53, a gain of $0.87

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AbbVie (NYSE:

) announced Thursday, the details of its cash and stock offer for Pharmacyclics, Inc. (NASDAQ:

). AbbVie is offering $152.25 in cash and 1.6639 shares of AbbVie common stock and, subject to proration, the all-stock consideration will consist of 3.9879 shares of AbbVie common stock for its previously announced $21 bilion takeover. Pharmacyclics shares closed Thursday at $259.55, a gain of $0.82.

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Shares of The Macerich Company (NYSE:

) spiked higher Thursday, on a

story, that the Ontario Teachers Pension Plan was considering a bid for the REIT. Simon Property Group Inc. (NYSE:

) offered $99.50 per share for Macerich on March 20. That offer was rejected by the Macerich Board. Macerich has been under pressure from activist investors to reconsider the Simon Offer. A spokesperson for the Ontario Teachers declined comment on the report. A Macerich spokesperson was not available. Macerich shares closed Thursday at $82.89, a gain of $0.75.