NEW YORK (TheStreet) -- Investors appear concerned about Pharmacyclics' (PCYC) leukemia drug Imbruvica relative to the competing drug ABT-199 under development by Abbvie (ABBV). ABT-199 is an exceptionally active drug and will have a significant role in the treatment of chronic lymphocytic leukemia, but I find data in the just-released ASCO abstracts to be more positive for Pharmacyclics and Imbruvica than its competition.
While many have focused on Imbruvica monotherapy data with a treatment that is efficacious, safe, and easy to use for both the patient and doctor, I was more intrigued by data from the combination therapy of Imbruvica and GlaxoSmithKline's (GSK) Arzerra (an anti-CD20 antibody.) While the clinical trial was small, just over 70 total patients, it tested three dosing regimens of the Imbruvica-Arzerra combination with overall response rates ranging from 71% to 100%. These results are significant because they help blunt ABT-199's efficacy advantage over Imbruvica, which in turn, makes Imbruvica's safety advantage over ABT-199 more meaningful.
ABT-199 as a monotherapy is generating overall response rates around 78% in CLL, which one could argue is marginally better than Imbruvica, although at the expense of decreased tolerability. In addition, ABT-199 also has a small trial with Rituxan (also an anti-CD20 antibody) which generated an overall response rate of 78%. The depth of responses in this combination study were impressive, with 39% of patients achieving a complete response. Five of the six complete responses were minimal residual disease (MRD) negative, which is seen as potentially curative. With these ASCO data, it's easy to see why ABT-199 is deemed a threat to Imbruvica.
But the downside of all that great ABT-199 efficacy is a significant decrease in tolerability, with tumor lysis syndrome (TLS) being the most worrisome. Abbvie has been working on different dosing regimes to increase ABT-199 tolerability, the TLS will always be a concern. If Imbruvica is able to get close to ABT-199 efficacy and maintain its tolerability advantage, the competitive threat diminishes. That's why I see the Imbruvica-Arzerra combination therapy so promising and bullish for Pharmacyclics. Based on the abstract, we only have response rate data available. At the ASCO meeting, we should get an update on the depth of response. I'll be looking specifically for the number of patients with complete responses and whether any patients were negative for minimal residual disease.
If we get complete response rates from Imbruvica-Arzerra similar to what's being reported for ABT-199, that would be positive for Pharmacyclics.
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