Moderna slips after Citi downgrades to 'sell'
** Biotech firm Moderna's MRNA.O shares fall 6.2% to $190.89 premarket
** Brokerage Citigroup downgrades MRNA to "sell" from "neutral", says optimism related to cancer drugs has already been priced in
** Brokerage says the stock's re-rating reflects potential expansion into multiple tumor types and unrealistic implied sales expectations
** Citi says stock's current valuation is unjustifiable, with a negatively skewed risk-reward profile
** Citi raises price target on MRNA to $80 from $60, representing a near 61% downside to co's last close of $203.46
** As of last close, MRNA shares up roughly seven-fold this year