Catherine C. Coombs, MD, on B-Cell Malignancies and Long-Term Safety of Pirtobrutinib

Catherine C. Coombs: BTK has proven to be an invaluable target of inhibition for the treatment of a number of B-cell malignancies. However, the use of BTK inhibitors is dependent upon their continuous administration. Therefore, safety and tolerability are paramount importance to maintain maximal efficacy. In this abstract, we reviewed the long-term safety data of Pirtobrutinib from the Phase 1/2 BRUIN trial. The trial design enrolled patients with a number of B-cell malignancies. The entire safety population was over 700 patients. However, in this post-hoc analysis, we reviewed the patients on treatment for over a year, which amounted to 326 patients. As one would expect, this population was enriched for patients with the more chronic B-cell malignancies, and so the largest population was CLL and SLL though there were about 40 patients with mantle cell lymphoma and Waldenström's macroglobulinemia. The safety aspects that were reviewed included all of the common side effects from this drug, which fortunately were very uncommon, especially grade three or higher AEs. In reviewing treatment exposure adjusted AE rates, what we determined was that in comparing patients on the drug for over a year compared to the entire safety population, there does not appear to be an increased risk in toxicity for patients that are on the drug for longer periods of time. This is further supported by the low incidences of discontinuation for the drug, especially in those patients who were on the drug for over a year where only 1.2% of patients discontinued due to side effects. In addition, regarding the class effects of BTK inhibitors that we worry about, the incidences of atrial fibrillation, bleeding, and hypertension were all extremely low and did not suggest a temporal relationship to Pirtobrutinib. In conclusion, we can see that now with long-term administration of Pirtobrutinib, this drug is exquisitely safe and can inhibit its target, BTK, for maximal benefit to our patients with these malignancies.