In our original report on Principia Biopharma, we emphasized that BTK inhibitors have failed over and over in autoimmune diseases, which we attribute to a basic problem with their mechanism of action: their main effect is to impair the production of new B-cell lineages, leaving the previously established B-cell populations that cause autoimmunity largely unscathed. Alas, Sanofi – the French pharma firm that licensed Principia’s BTK inhibitor SAR442168 for use in multiple sclerosis – doesn’t seem to have gotten the memo. During Sanofi’s recent earnings call, its global head of R&D rattled off what