Identification of SQ609 as a lead compound from a library of dipiperidines

We recently reported that compounds created around a dipiperidine scaffold demonstrated activity against Mycobacterium tuberculosis (Mtb) (Bogatcheva, E.; Hanrahan, C.; Chen, P.; Gearhart, J.; Sacksteder, K.; Einck, L.; Nacy, C.; Protopopova, M. Bioorg. Med. Chem. Lett. 2010, 20, 201). To optimize the dipiperidine compound series and to select a lead compound to advance into preclinical studies, we evaluated the structure–activity relationship (SAR) of our proprietary libraries. The (piperidin-4-ylmethyl)piperidine scaffold was an essential structural element required for antibacterial activity. Based on SAR, we synthesized a focused library of 313 new dipiperidines to delineate additional structural features responsible for antitubercular activity. Thirty new active compounds with MIC 10–20 μg/ml on Mtb were identified, but none was better than the original hits of this series, SQ609, SQ614, and SQ615. In Mtb-infected macrophages in vitro, SQ609 and SQ614 inhibited more than 90% of intracellular bacterial growth at 4 μg/ml; SQ615 was toxic to these cells. In mice infected with Mtb, weight loss was completely prevented by SQ609, but not SQ614, and SQ609 had a prolonged therapeutic effect, extended by 10–15 days, after cessation of therapy. Based on in vitro and in vivo antitubercular activity, SQ609 was identified as the best-in-class dipiperidine compound in the series.