A class of po­tent can­cer treat­ments could shine brighter than pre­vi­ous­ly thought in a broad­er ar­ray of pa­tients, new re­search sug­gests.

PARP in­hibitors, in­clud­ing As­traZeneca’s $AZN pi­o­neer­ing Lyn­parza, Clo­vis’ $CLVS Rubra­ca and GSK’s $GSK Ze­ju­la — work by thwart­ing PARP pro­teins that help re­pair dam­aged DNA in cell — there­by steer­ing can­cer cells on­to a path of an­ni­hi­la­tion. So far, their use has pri­mar­i­ly been in ovar­i­an can­cers con­tain­ing BR­CA mu­ta­tions, rare ge­net­ic mu­ta­tions that dis­able a DNA re­pair path­way in can­cer cells, as well as BR­CA-mu­tat­ed breast can­cer. (Al­though last month, Ze­ju­la was grant­ed pri­or­i­ty re­view to ex­pand its use in late-stage ovar­i­an can­cer pa­tients with or with­out BR­CA mu­ta­tions).

While the as­sault on DNA re­pair is be­ing waged, these drugs are al­so at­tack­ing ri­bo­somes — the ma­chin­ery that makes pro­teins, said Lee Kraus, di­rec­tor of the Green Cen­ter for Re­pro­duc­tive Bi­ol­o­gy Sci­ences at UT South­west­ern. “These find­ings could in­crease the pa­tient pop­u­la­tion ben­e­fit­ing from these drugs by two, three, or four-fold. Up to 70 per­cent of breast can­cer pa­tients could now be good can­di­dates.”

The da­ta, pub­lished in the jour­nal Mol­e­c­u­lar Cell on Wednes­day, could ex­plain why breast can­cer pa­tients can be re­spon­sive to PARP in­hibitors, de­spite not car­ry­ing BR­CA mu­ta­tions.

Kraus and his team iden­ti­fied a po­ten­tial bio­mark­er — a pro­tein called DDX21, which is re­quired for the pro­duc­tion of ri­bo­somes in small sub­cel­lu­lar com­part­ments called nu­cle­oli. But DDX21 in the nu­cle­o­lus re­quires PARP-1, which is tar­get­ed by ex­ist­ing PARP in­hibitors. The use of these drugs, there­fore, blocks DDX21, there­by in­hibit­ing ri­bo­some pro­duc­tion. This means en­hanced DDX21 lev­els in the nu­cle­o­lus could in­di­cate can­cers that might be the most re­spon­sive to PARP in­hibitors, the re­searchers posit­ed.

“Can­cer cells are ad­dict­ed to ri­bo­somes. Can­cer cells grow fast and must make pro­teins to sup­port cell di­vi­sion and oth­er es­sen­tial process­es go­ing on in the cell. If you can slow down or in­hib­it the pro­duc­tion of ri­bo­somes, then you can slow down the growth of the can­cer cell,” Kraus said in a state­ment.

Kraus et al are now work­ing on de­sign­ing clin­i­cal tri­als with UT South­west­ern on­col­o­gists to test their the­o­ry.

Kraus is a founder and con­sul­tant of Ri­bon Ther­a­peu­tics, which ear­li­er this year raised $65 mil­lion to tar­get oth­er PARPs in the broad fam­i­ly of 17 en­zymes. The Cam­bridge, Mass­a­chu­setts-based biotech’s lead pro­gram is first go­ing af­ter PARP7, a pro­tein al­so sim­i­lar­ly ac­ti­vat­ed by stress and cel­lu­lar re­sponse mech­a­nisms.

89bio said its drug was better than placebo at lessening fibrosis without worsening nonalcoholic steatohepatitis, or NASH, in two of three dose groups.

The San Francisco biotech said it thinks the Phase IIb data pave the way for a potential Phase III, following in the footsteps of another biotech in its drug class, Akero Therapeutics. To fund a late-stage study, CEO Rohan Palekar told Endpoints News 89bio “would need to raise additional capital,” with the company having about $188 million at the end of last year.