We’ve had a lot of news in the last 6 weeks from ACR, ASH and the release of clinical trial results. Here’s what we’ve learned in, and some thoughts on what might come next:
1) Rigel and partner AstraZeneca threw craps in the RA game. Syk inhibition is less effective than Jak inhibition in Rheumatoid arthritis (RA), per ACR presentations on Jak inhibitor tofacintinib and clinical trial data released on Syk inhibitor fostamatinib. Whether the difference in efficacy is due to suboptimal dosing of the Syk inhibitor fosta’nib, or reflects distinct biological readouts is a critical question, particularly as fosta’nib has off-target activity on Jak1 and other relevant kinases. It is interesting that Pfizer’s tofacitinib, widely viewed as a pan-Jak inhibitor with potential toxicity issues, has scored approval in DMARD-IR patients, and can therefore be prescribed before biologic therapy. I’ll admit I did not see this coming and felt tofa’nib would get approval, to start, in TNF-IR patients only. AZN is clearly running the clinical trials needed to show a clean safety profile for fosta’nib, but may now lose out in the marketplace on the basis of efficacy. It remains to be seen how this will play out as Rheumatologists weigh the safety/efficacy profiles of these drugs, but clearly in a “treat to target” regimen, where the target is no clinical sign of disease and no advance in joint degradation, fosta’nib has a tough road ahead. They may yet persevere but it will take a combination of squeaky-clean safety data and a clear demonstration of protection from joint damage. The compound is also moving forward in B cell malignancies (see below).
We will not learn anytime soon if better Syk inhibitors would be better RA drugs, as Biogen Idec may have lost its bet on Portola’s Syk inhibitor. Biogen has quietly withdrawn its phase 2 clinical trial for PRT062607, aka BIIB057 (clintrials.gov). The withdrawal occurred prior to the recent data coming out on fosta’nib and seems more likely to be related to the specific compound than to the pathway. Looking more widely at drug development in this space, the only other clinical stage compound is the inhaled Syk inhibitor R343, for asthma and other lung diseases, currently in phase 2 and unlikely to provide any insight into the systemic inhibition of the pathway in RA. We may have to wait for clinical readouts in RA following inhibition of Btk – this is the next kinase downstream of Syk – before we understand the utility of targeting this pathway. More on Btk to follow.
2) Jak inhibitors: does three of a kind equal two pair? There are actually four Jak kinases: Jaks 1,2,3 and Tyk2. People used to spend a lot of time discussing the specificity of different Jak inhibitors and the toxicity/efficacy expectations of targeting Jak3 vs Jak2 vs Jak1. The reality is that all Jaks are obligate hybridizers: distinct Jaks hybridize to transduce signals from cell surface receptors. It is very likely that different Jak specificities matter less than we think and that the current crop of Jak inhibitors are more alike than they are different, off target specificity aside. That being said, the Jak inhibitors appear to be poised to reap hefty winnings, in wildly distinct diseases. Novartis’ and Incyte’s ruxolitinib (Jakafitm),first-in-class “Jak2-selective” inhibitor, was approved for use in myelofibrosis in early 2011 and in 2012 is already poised to take in $100MM in sales as patients move onto this drug earlier in the treatment paradigm. The therapeutic rationale of Jak inhibitor use was well understood from genetic analysis of myelofibrosis, and the commercial and clinical validation is now clear, prompting Gilead to recently pay ~$400MM to buy the phase 1 Jak1/2 inhibitor CYT387 along with the rest of YM Biosciences. Other Jak2-selective inhibitors in development include the Sanofi Aventis compound SAR302503, currently in phase 2, and the Cell Therapeutics compound pacritinib. While myelofibrosis is the clear POC arena for these compounds, look for companies to begin moving these into new therapeutic areas as the compounds advance.
3) Jaks are wild in RA. I’ve already mentioned the success of Pfizer’s pan-Jak inhibitor tofacitinib (brand name Xeljanztm), recently approved for use in RA patients who have an inadequate response to DMARDs such as methotrexate. Other Jak inhibitors being developed to compete in the inflammation area include the Jak1-specific inhibitor GLPG0634 from Galapagos, licensed in early 2012 by Abbott (now AbbVie), who paid $150MM upfront for rights to the Phase 1 compound. It should be noted that GLPG0634 does appear to have a different side effect profile than the other Jak inhibitors, in that no impact on lipid levels were seen, nor signs of anemia and neutropenia. These side effects have been seen in clinical trials of ruxolitinib and with tofa’nib and are generally ascribed to Jak2 inhibition. Given the promiscuous nature of Jak heterodimerization this differentiation remains somewhat surprising. Indeed, Cell Therapeutics, who is developing S*BIO’s Jak2-inhibitor pacritinib in myelofibrosis, claims not to see anemia or thrombocytopenia (another class side effect) in the clinic. Finally, Vertex has moved its Jak3-selective inhibitor VX-509 into phase 2b following decent results in a phase 2a RA trial. No matter how you look at it, the data suggest that hitting any combination of Jaks, or just one if we believe the Galapagos selectivity data, is sufficient for efficacy in RA. This is almost certainly due to the prevalence of heterodimerization, as mentioned, but also due to the sheer number of pathogenic pathways impacted by Jak inhibition, including numerous cytokine and growth factor pathways. Despite the risk of side effects from such broad blunting of the immune system, this approach so far appears to be more efficacious, and at least as safe, as inhibition of Syk. On the basis of efficacy in RA, one might be tempted to move Jak inhibitors into stubborn and complex diseases like systemic lupus erythematosis. That’s another bet altogether but I’ll be looking for those investigator trials!
4) PCI’s Btk inhibitor shows its hand. The recent American Society of Hematology (ASH) meeting was a showcase for Pharmacyclics’ (PCI) and JNJ’s Btk inhibitor ibrutinib, aka ib’nib. This Btk inhibitor produced remarkable efficacy in several B cell lymphomas, including chronic lymphocytic leukemia (CLL), acute lymphocytic leukemia (ALL) and Mantle Cell Lymphoma, the latter being generally viewed as a very intractable disease. The data have been reviewed in detail elsewhere (e.g. fiercebiotech story), but clearly ib’nib has validated Btk inhibition as a therapy with broad application in B lymphoma oncology. The field is still young, with the next closest drug in development Avila’s AVL-292, who reported phase 1b data at ASH. AVL-292 demonstrated efficacy in Non-Hodgkin’s Lymphoma (NHL), CLL, multiple myeloma (MM) and other indications. Avila was acquired by Celgene earlier this year, further highlighting the interest in Btk inhibition in oncology. Ib’nib and AVL-292 covalently bind to the kinase domain of Btk, forming an irreversible bond that prevents substrate from accessing the active site of the enzyme. Historically, covalent inhibition has been associated with off-target toxicity, as it is possible for the drug to link irreversibly to unwanted targets. Happily, this does not appear to be an issue with this new class of covalent inhibitors, and toxicity is generally considered mild. Regardless, non-covalent inhibitors are being developed by several companies, perhaps with the goal of differentiating from the leading compounds. The CGI compound GDC-0834, now held by Genentech, in a non-covalent active site inhibitor under development for inflammatory disease, as is the Roche compound RN486. Given the current status of the Syk inhibitor fosta’nib in RA, it will be interesting to see if these companies stay the course, or move their inhibitors onto new disease targets. It was the success of Syk inhibition in early RA and B cell lymphoma trials that stirred interest in Btk, and the field will respond accordingly to any new Syk inhibitor data.
This is a lot of information to absorb, but it stimulates the obvious question: What’s next? Here are just a couple of thoughts:
- What is considered safe and efficacious today will change tomorrow. As combination therapy takes hold in B cell oncology, additive toxicity may become problematic. If we consider the obvious combination of a Btk inhibitor with other kinase inhibition, or with biologic therapy, the safety/efficacy profile of these compounds will evolve. This is even more true in RA, where toxicity will drive physicians away from drugs.
- Escape is possible (or inevitable). As has already been described in myelofibrosis, patients needing chronic therapy may select for drug resistance. In myelofibrosis, such resistance is associated with the recruitment of other Jaks to take over for Jak2. Having a well-stocked armory may allow continued control over the disease, even as signaling pathways shift – in the context of B cell lymphomas this may mean Syk inhibitors, Btk inhibitors, PI3K inhibitors, mTOR inhibitors, in succession or in tolerable combination therapies.
- In RA the success of tofa’nib heralds a new wave of oral medications that can challenge biologic therapies, in particular the anti-TNF biologics. This is no surprise, and it will be of considerable interest to watch how clinical practice evolves. I’m hoping that diligence is being applied to the collection of samples for analysis of response and non-response in patients receiving the new, targeted orals as it will be fascinating to understand why some patients fail while some succeed – something not yet adequately understood in RA.
- Finally its important to remember that “success” in RA still means that most patients fail to reach ACR 50 and ACR 70 scores, suggesting that they are continuing to experience disease despite treatment. Until these numbers come up, perhaps in the context of personalized medicine, we cannot declare that the work in RA is done. We note in passing that there are a plethora of diseases that could benefit from these new kinase inhibitors, and expect to see their use tested more widely.