In vitro activity: Maritoclax blocks the binding of Bim BH3 α-helix to Mcl-1 but not Bcl-XL. Maritoclax markedly inhibits the viability of Mcl-1-IRES-BimEL cells (EC50=1.6 μM) with a selectivity greater than 40-fold over Bcl-2-IRES-BimEL (EC50=65.1 μM) and Bcl-XL-IRES-BimEL (EC50=70.0 μM) cells. Maritoclax induces cell death selectively in Mcl-1-dependent but not Bcl-2- or Bcl-XL-dependent leukemia cells. Maritoclax induces proteasome-mediated Mcl-1 degradation without induction of Mcl-1 phosphorylation and Noxa expression. Maritoclax inhibits Mcl-1 interaction with Bim in intact cells and triggers cytochrome c release from isolated mitochondria. Maritoclax synergistically sensitizes lymphoma/leukemia cells to ABT-737. Marinopyrrole A shows activity against all tested S. aureus strains, including glycopeptide-intermediate and vancomycin-resistant MRSA, and has potent activities against other Gram-positive organisms. In addition, marinopyrrole A is active against H. influenzae but is inactive against other tested Gram-negative strains. Marinopyrrole A displays substantial concentration-dependent killing against MRSA strain TCH1516 and is far more rapid in its antibiotic action than either vancomycin or linezolid. Marinopyrrole A exhibits a favorable therapeutic index, with 50% inhibitory concentrations (IC50) in excess of 20× above the MIC in each case: 32 to 64 μg/mL against HeLa cells and 8 to 32 μg/mL against L929 cells. Marinopyrrole A (3 μM) induced-cell death is associated with MCL1 decrease and translation inhibition. Marinopyrrole A induces a dephosphorylation of EIF4EBP1 concomitant to a decrease of EIF4E phosphorylation. Marinopyrrole A is much more effective against Bcl-2-dependent RS4;11 cells (IC50: 2 μM) when compared to Mcl-1-dependent HeLa cells (IC50: 20 μM).

Kinase Assay: Marinopyrrole A (also known as Maritoclax) is a natural product and a selective antagonist of Mcl-1, a protein of the anti-apoptotic Bcl-2 family which including Bcl-2, Bcl-X(L) and Mcl-1, and are well-validated drug targets for cancer treatment. Marinopyrrole A can overcome ABT-737 resistance by binding to and targeting Mcl-1 for proteasomal degradation, while ABT-737 and its orally active analog ABT-263 are the most potent and specific inhibitors to date that bind Bcl-2 and Bcl-X(L) with high affinity but have a much lower affinity for Mcl-1, they are not very effective as single agents in certain cancer types because of elevated levels of Mcl-1. Maritoclax disrupts the interaction between Bim and Mcl-1 with an IC50 of 10.1 μM. Marinopyrrole A induces Mcl-1 degradation via the proteasome system, which is associated with the pro-apoptotic activity of maritoclax. Marinopyrrole A selectively kills Mcl-1-dependent, but not Bcl-2- or Bcl-XL-dependent, leukemia cells and markedly enhances the efficacy of ABT-737 against hematologic malignancies, including K562, Raji, and multidrug-resistant HL60/VCR, by ~60- to 2000-fold at 1-2 μM.

Cell Assay: K562 cells expressing Mcl-1-IRES-BimEL are treated with DMSO, 2 μM maritoclax alone, or in combination with 1 μM MG132 for 12 h. Cells are lysed in 1% Chaps buffer (1% Chaps, 150 mM NaCl, 10 mM Hepes, pH7.4) containing protease inhibitors. Cell lysates containing 350 μg of protein are incubated with 4 μl of rabbit anti-Mcl-1 antiserum or control pre-immune serum in 250 μl of the same lysis buffer at 4 °C overnight on a rotator. Immunoprecipitates are collected by adding 20 μl of protein A-Sepharose beads for 3 h at 4 °C, followed by centrifugation at 6,000 rpm for 30 s. The beads are washed five times with the same lysis buffer, boiled for 5 min in Laemmli sample buffer and analyzed by Western blotting.