Back to the potential of apabetalone (RVX-208) in Friedreich's ataxia......I dug up this post of mine from May 2017 (text copied below). 

I found a better link to this patent where I can actually see the figures. Friedreich's ataxia is caused by mutations in the frataxin (FXN) gene resulting in decreased levels of frataxin protein. In this patent, they show that treatment of GM15850 Friedreich's ataxia lympoblasts, which harbor a mutation in the FXN gene, with various BET inhibitors can lead to the "increase" in frataxin protein levels. 

Figure 1 shows that treatment of GM15850 cells for 72 hours with the pan BET inhibitors IBET-762 and JQ1(+) at various compound concentrations (0.05 micromolar, 0.5 micromolar and 5 micromolar) increased levels of frataxin protein. However, in Figure 2 they show that these concentrations elicited cytotoxic effects as evidenced by significantly reduced levels of total cellular protein. So they backed off on the concentrations and tried again. In figure 3, they exposed GM15850 cells for 72 hours to JQ1(+) at 0.05, 0.025, 0.0125 and 0.00625 micromolar (these concentrations caused minimal to no cytotoxicity). The 0.05 and 0.025 micromolar concentrations increased frataxin by about 1.5-fold. That's great, right? Well, it's all relative. Relative to GM15850 cells not treated with a BET inhibitor, JQ1(+) gave a 1.5-fold "increase" (50% increase). However, relative to healthy AG14725 lymphoblast cells that have a normal FXN gene, the JQ1(+)-treated and untreated Friedreich's ataxia GM15850 cells still have frataxin levels that are 3-fold and 4-fold below the normal levels observed in healthy AG14725 cells. Therefore, at concentrations that avoid cytotoxicity the effects of JQ1(+) on frataxin levels are very modest.

In Figure 6, they repeat this experiment with a panel of other BET inhibitors of differing scaffolds (all at 1 micromolar concentration for 72 hours). Go to page 103 (page 118 of the pdf) in the link at the top of this message to see Figure 6. They test:

1) N-[4-(2,4-difluorophenoxy)-3-(6-methyl-7-oxo-6,7-dihydro-1 H-pyrrolo[2,3-c]pyridin- 4-yl)phenyl]ethanesulfonamide

2) N-{6-[methyl(propanoyl)amino]-3-oxo-4-[(1 S)-1^henylpropyl]-3,4-dihydropyrido[2,3 b]pyrazin-2-yl}-beta-alanine

3) N-(2-hydroxy-3-methylquinolin-6-yl)piperidine-1 -sulfonamide

4) l-BET-151

5) l-BET-762

6) RVX-208 

The different BET inhibitors increased frataxin to varying degrees. Compound 1 had barely any effect. Compound 6 (RVX-208) increased frataxin a little bit. Compound 2 was a little better than RVX-208. Compounds 3, 4 and 5 elicited the greatest increase (2- to 3-fold greater syntaxin levels than observed for RVX-208). I-BET-762 in Figure 1 behaved similarly to JQ1(+). Therefore, at the concentrations tested RVX-208 does not seem to elicit much of an increase in frataxin. The effect of RVX-208 on frataxin is even more modest than that of JQ1(+) and I-BET-762.

In Figures 7, they treat GM03665 Friedreich's ataxia fibroblasts (not lympoblasts) with JQ1(+) at 1 micromolar concentration or do a technique to knockdown the expression of BRD4 bromodomain proteins. Both JQ1 and BRD4 knockdown appear to increase frataxin by about 2- or 3-fold. However, the levels of frataxin in healthy fibroblasts is not shown so it is difficult to put this in context. In Figure 8, they knockdown the expression of BRD2, BRD3, BRD4 or BRDT and claim that knockdown of any one of these bromodomain proteins elicits an increase in frataxin expression. However, the results seem quite modest for Figure 8 in my opinion.

My opinion is that it is VERY premature to call BET inhibitors a hopeful therapy for Friedreich's ataxia. The data in the patent is based on human cells treated with BET inhibitors, and the effects on frataxin modulation seem quite modest. There is zero animal model data. I am assuming that all of the BET inhibitors screened in this patent were pan-BET inhibitors, with the exception of RVX-208. Pan-BET inhibitors (at least the early generation ones that have been published on for oncology indications) elicit side effects that necessitate a 2 week on, 1 week off treatements cycle. While these side effects may be acceptable for oncology indications, these may not be acceptable for long term use as may be required for treating Friedreich's ataxia. Selective BET inhibitors, such as RVX-208, didn't seem to elicit much of an effect on frataxin in the data shown in the patent. Overall, I am not convinced that BET inhibitors for Friedreich's ataxia is ready for prime time, and the very modest effect of RVX-208 relative to pan-BET inhibitors suggests that pan-BET inhbition but not selective-BET inhbition would be a better choice for treatment. 

I hope someone finds my ramble useful. It was a good exercise for me at least.

BearDownAZ