Imtesty,

Yes, Resverlogix has emphasized that apabetalone is a BD2-selective BET inhibitor that binds to BD2 of BRD4. No argument there. However, I can't tell from your previous response if you are challenging/disagreeing that apabetaone also binds selectively to BD2 vs. BD1 of other BET proteins (BRD2, BRD3, BRDT). As shown in three examples below, apabetalone has been shown to bind BD2 of BRD4 stronger than BD1 of BRD4, and apabetalone binds BD2 of BRD4 stronger than BD1 or BD2 of BRD2, BRD3 and BRDT. However, apabetalone binding to BD2 of BRD2 and BD2 of BRD3 come in pretty close 2nd and 3rd place relative to binding to BD2 of BRD4. There is much greater BD-selectivity (BD2 over BD1) for apabetalone than BET protein selectivity.

Tian et. al. in the Cell paper tested apabetalone IC50 binding affinities for BRD4-BD1 (1142 nM), BRD4-BD2 (135 nM), BRD2-BD1 (5780 nM) and BRD2-BD2 (251 nM). IC50 is the concentration required to elicit binding/inhibition of half of the total available BET protein molecules. A lower nanomolar (nM) concentration means stronger binding affinity. This study shows apabetalone binds BD2 of BRD4 about 10 times stronger than BD1 of BRD4. Apabetalone bound BD2 of BRD2 about 20 times stronger than BD1 of BRD2. Strongest binding out of the four is to BD2 of BRD4, but binding to BD2 of BRD2 was still strong at about half that of binding to BD2 of BRD4.

Picaud et al. in the 2003 PNAS paper did a comprehensive study using a few differnt techniques. But I like best the data from Table S2 in that paper that performed isothermal titration calorimetry of human BET bromodomains BD1 and BD2 for BRD2, BRD3, BRD4 and BRDT with RVX-208 to calculate the nM dissociation contanct (Kd) as a measure of binding affinity. Again, the lower the number the stronger the binding affinity. You can clearly see the BD2 vs. BD1 selectivity for all BET proteins tested. But you can also appreciate that although apabetalone binds BD2 of BRD4 the strongest, that apabetalone also binds other BD2s with pretty strong affinity too (moreso for BRD2 and BRD3 than BRDT).

BRD2: BD1 5780; BD2 251

BRD3: BD1 4065; BD2 195

BRD4: BD1 1142; BD2 135

BRDT: BD1 5405; BD2 637

In McLure et al. 2013 in PLoS One, they performed an assay to determine the binding affinity (nM IC50) of BD1 and BD2 in BRD1, BRD2 and BRD4 (did not do BRDT, presumably because not expressed in liver). Again, lower the number is stronger affinity. As seen in the other studies above, you can clearly see the BD2 vs. BD1 selectivity for all BET proteins tested, appreciate the relative strongest BRD4/BD2 binding, but also appreciate that apabetalone binds to BD2 of BRD2 and BD2 of BRD3 with pretty strong affinity too.

BRD2: BD1 2600; BD2 90

BRD3: BD1 3100; BD2 280

BRD4: BD1 1800; BD2 40

I hope this helps further clarify things.

BearDownAZ