** Shares of drug developer Summit Therapeutics SMMT.O fall 3.6% to $21.28 premarket

** Brokerage Leerink Partners starts coverage on SMMT with "underperform" rating and $12 PT, a 46% downside to the stock's last close

** Brokerage says it doesn't believe SMMT's lung cancer therapy, ivonescimab, is "the next Keytruda", Merck's MRK.N blockbuster cancer drug

** Views SMMT as "by far the worst-positioned" competitor, requiring a major partnership beyond their Pfizer PFE.N ADC collaboration to compete effectively

** Leerink believes SMMT's drug faces "unrealistic share" expectations in the cancer immunotherapy market, given the "intense and heterogeneous competitive landscape"

** Summit faces "unfavorable cost dynamics" including "low-double-digit royalties" to Chinese partner Akeso 9926.HK and reliance on contract manufacturers - brokerage

** As of last close, stock up 23.7% YTD

(Reporting by Kamal Choudhury in Bengaluru)

((Kamal.Choudhury@thomsonreuters.com;))