These new drugs are markedly improved analogs of {HEAD ~ previously known fluorenone compounds} that were never commercialized or developed into {TAIL ~ medically useful treatments} 
never developed into
These new drugs are markedly improved analogs of { HEAD ~ previously known fluorenone compounds } that were never commercialized or developed into { TAIL ~ medically useful treatments }
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH REL OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL