shares of merck  co inc sank monday to buck the gains in the health care sector and the broader stock market after citi research backed away from its bullish stance on the drug maker citing concerns over its hiv and covid19 treatments the stock mrk slumped 54 in midday trading toward a twomonth low the stock is the biggest decliner among the components of the dow jones industrial average djia  which ran up 322 points or 09 and of the spdr sp health care select sector exchangetraded fund xlv  which rallied 09 the stock has now tumbled 173 since closing at a record 9054 on nov 4 citi’s andrew baum cut his rating to neutral after being at buy for at least the past 2 12 years he cut this stock price target to 85 from 105 baum said his “longstanding” bullish thesis on merck was based on the “underappreciation” of the company’s drug pipeline especially its hiv treatment islatravir which he expected would offset the coming loss of exclusivity of its blockbuster cancer treatment ketruda however he no longer expects any revenue from islatravir “we place a high probability that merck will abandon islatravir development in the next three months given likely high regulatory concerns” baum wrote in a note to clients baum said he removed all estimates for islatravir from his financial models following the company’s announcement earlier this month that a dosedependent decrease in lymphocyte counts was observed in a phase 2 trial he said that suggests its very likely that materially higher doses required would lead to “diverse and unacceptable” adverse events on nov 18 merck said it stopped dosing in the trial and on nov 23 the company announced a “temporary pause” in enrollment in the phase 2 study “we expect the diminishing outlook for islatravir to further expedite merck’s business development efforts” baum wrote “we open a negative catalyst watch on the stock along with today’s report” but disappointment over islatravir isn’t merck’s only problem baum said the clinical profile of merck’s antiviral to treat covid19 lagevrio continues to deteriorate putting estimates for lagevrio for next year and beyond at “material risk” he said it was “obvious” to him from inception that lagevrio would have a risk evaluation and mitigation strategy rems drug safety program required by the food and drug administration because of risk of birth defects “since that time we have learnt that lagevrio’s efficacy is materially lower than that reported by either monoclonals such as regeneron’s ronapreve as well as pfizer’s paxlovid on an interim analysis” baum wrote he also believes it is likely that resistance will emerge over time to monotherapy lagevrio usage “we expect the fda to recommend monoclonal antibodies to be preferred therapy in immunocompromised patients in order to reduce the risk of resistance” baum wrote as a result he said the fda could end up limiting lagevrio approval to only nonvaccinated or immunocompromised patients” merck’s stock has lost 40 year to date while shares of regeneron pharmaceuticals inc regn have climbed 354 and of pfizer inc pfe have hiked up 463 meanwhile the spdr health care etf has advanced 174 this year and the dow has gained 178