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** OncoMed Pharmaceuticals down 15 pct premarketafter cancer drug developer prices its follow-on launched afterthe bell Wed at a steep discount
** If losses hold, stock set to open at lowest levels sinceearly May. On course for biggest one-day pct fall since Jan
** OncoMed offers 5.5 mln shares at $10.00, a 14.6-pctdiscount to stock's $11.71 last sale
** Deal represents about 18 pct of co's 30.84 mln sharesoutstanding as of end-Jun
** OncoMed expects to use offering proceeds along with cashon hand to fund its experimental immuno-oncology drug platform
** OMED has seven drug candidates in various stages ofdevelopment. Has collaboration agreements with top shareholderGlaxoSmithKline, as well as Bayer AG andCelgene
** YTD, OncoMed's stock has lost half its value, while theNasdaq Biotech index has shed 14 pct
