Does it matter how a drug works, if it works? PTC Therapeutics seems bent on giving everyone an answer to that question, because there sure seem to be a lot of questions about how ataluren (PTC124), their Duchenne Muscular Dystrophy (DMD) therapy, acts. This article at Nature Biotechnology does an excellent job explaining the details.
Premature “stop” codons in the DNA of DMD patients, particularly in the dystrophin gene, are widely thought to be one of the underlying problems in the disease. (The same mechanism is believed to operate in many other genetic-mutation-driven conditions as well. Ataluren is supposed to promote “read-through” of these to allow the needed protein to be produced anyway. That’s not a crazy idea at all – there’s been a lot of thought about ways to do that, and several aminoglycoside antibiotics have been shown to work through that mechanism. Of that class, gentamicin has been given several tries in the clinic, to ambiguous effect so far.
So screening for a better enhancer of stop codon read-through seems like it’s worth a shot for a disease with so few therapeutic options. PTC did this using a firefly luciferase (Fluc) reporter assay. As with any assay, there are plenty of opportunities to get false positives and false negatives. Firefly luciferase, as a readout, suffers from instability under some conditions. And if its signal is going to wink out on its own, then a compound that stabilizes it will look like a hit in your assay system. Unfortunately, there’s no particular market in humans for a compound that just stabilizes firefly luciferase.
That’s where the argument is with ataluren. Papers have appeared from a team at the NIH detailing trouble with the FLuc readout. That second paper (open access) goes into great detail about the mechanism, and it’s an interesting one. FLuc apparently catalyzes a reaction between PTC124 and ATP, to give a new mixed anhydride adduct that is a powerful inhibitor of the enzyme. The enzyme’s normal mechanism involves a reaction between luciferin and ATP, and since luciferin actually looks like something you’d get in a discount small-molecule screening collection, you have to be alert to something like this happening. The inhibitor-FLuc complex keeps the enzyme from degrading, but the new PTC124-derived inhibitor itself is degraded by Coenzyme A – which is present in the assay mixture, too. The end result is more luciferase signal that you expect versus the controls, which looks like a hit from your reporter gene system – but isn’t. PTC’s scientists have replied to some of these criticisms here.
Just to add more logs to the fire, other groups have reported that PTC124 seems to be effective in restoring read-through for similar nonsense mutations in other genes entirely. But now there’s another new paper, this one from a different group at Dundee, claiming that ataluren fails to work through its putative mechanism under a variety of conditions, which would seem to call these results into question as well. Gentamicin works for them, but not PTC124. Here’s the new paper’s take-away:
In 2007 a drug was developed called PTC124 (latterly known as Ataluren), which was reported to help the ribosome skip over the premature stop, restore production of functional protein, and thereby potentially treat these genetic diseases. In 2009, however, questions were raised about the initial discovery of this drug; PTC124 was shown to interfere with the assay used in its discovery in a way that might be mistaken for genuine activity. As doubts regarding PTC124’s efficacy remain unresolved, here we conducted a thorough and systematic investigation of the proposed mechanism of action of PTC124 in a wide array of cell-based assays. We found no evidence of such translational read-through activity for PTC124, suggesting that its development may indeed have been a consequence of the choice of assay used in the drug discovery process.
Now this is a mess, and it’s complicated still more by the not-so-impressive performance of PTC124 in the clinic. Here’s the Nature Biotechnology article’s summary:
In 2008, PTC secured an upfront payment of $100 million from Genzyme (now part of Paris-based Sanofi) in return for rights to the product outside the US and Canada. But the deal was terminated following lackluster data from a phase 2b trial in DMD. Subsequently, a phase 3 trial in cystic fibrosis also failed to reach statistical significance. Because the drug showed signs of efficacy in each indication, however, PTC pressed ahead. A phase 3 trial in DMD is now underway, and a second phase 3 trial in cystic fibrosis will commence shortly.
It should be noted that the read-through drug space has other players in it as well. Prosensa/GSK and Sarepta are in the clinic with competing antisense oligonucleotides targeting a particular exon/mutation combination, although this would probably taken them into other subpopulations of DMD patients than PTC is looking to treat.
If they were to see real efficacy, PTC could have the last laugh here. To get back to the first paragraph of this post, if a compound works, well, the big argument has just been won. The company has in vivo data to show that some gene function is being restored, as well they should (you don’t advance a compound to the clinic just on the basis of in vitro assay numbers, no matter how they look). It could be that the compound is a false positive in the original assay but manages to work through some other mechanism, although no one knows what that might be.
But as you can see, opinion is very much divided about whether PTC124 works at all in the real clinical world. If it doesn’t, then the various groups detailing trouble with the early assays will have a good case that this compound never should have gotten as far as it did.