A New Class of Synthetic Type-B Polycyclic Polyprenylated Acylphloroglucinols (PPAP-B) Overcomes Venetoclax-Resistance in Acute Myeloid Leukemia (AML)

Venetoclax (VEN) resistance in AML therapy is a field of unmet need. VEN selectively binds in the BH3-binding groove of BCL-2 to release BIM leading to pore-forming BAX/BAK dimerization to finally release cytochrome C, as the “step of no return” in induction of programmed cell death. Compensatory overexpression of other BCL-2 family members such as MCL-1 and BCL-XL is a well described resistance mechanism to restore the ability to sequester BIM and therefore compensate for BCL-2 inhibition. Type-B PPAP are a novel class of neoplastic therapeutics. A new synthetic toolbox for the coupling of a bioactive head group (R 3) enabled the generation of novel, highly active PPAPs, which are low-toxic, water-soluble and photochemically stable - and therefore attractive compounds for clinical development. We now provide evidence that PPAP-B possess antileukemic activity in in vitro and ex vivo AML models via induction of the mitochondrial apoptosis pathways. Further, PPAP-B suppress transcription of MCL-1 and BCL-XL - leading to highly synergistic induction of apoptosis in combination with venetoclax, including a Venetoclax-resistant AML model. Employing structure-activity relationship (SAR) studies, newly generated PPAPs (#22-#144) were tested in acute leukemia HL60, Molm14 and OCI-AML-3 cell lines as well as freshly harvested native leukemia blasts from consented patients. Physiologic mononuclear cells (MC) extracted from cord blood or healthy donors were used as controls. PPAPs were modified step-by-step regarding their chemical structure and assessed in dose dilution series to determine antileukemic efficacy of the compounds. The capability to induce apoptosis and inhibit proliferation was assessed, using standard techniques. To shed light on the underlying mechanism of action, Proteome Profiler Arrays (R&D systems) were performed and confocal microscopy was employed to trace specific fluorescent PPAPs within the cell. Together, the SAR approach led to improvement of IC50 rates of PPAP-B ranging from 20 μM to now < 4 μM. Compound #122 was thereby identified as the most potent PPAP with respect to inhibition of proliferation and induction of apoptosis. Genotoxic studies, as evidenced in a thymidine kinase mouse lymphoma assay (MLA), did not point to potential genotoxicity. Confocal microscopy confirmed accumulation of PPAPs in the cytosol and outer mitochondrial layer using fluorescent PPAPs. In line, induction of mitochondrial apoptosis was seen as evidenced by BAX activation, release of cytochrome C and cleavage of caspases 8 and 3. Further, PPAP-B suppressed transcription of MCL-1 and BCL-XL, putatively via inhibition of the JAK-STAT pathway as discussed earlier. These observations suggest a role of PPAPs in overcoming VEN-resistance in AML. Indeed, combination of PPAP #122 with VEN was highly synergistic with regard to induction of apoptosis, already at doses where the single agents did not display pro-apoptotic activity (e.g. PPAP #122 2.5 μM: 10%, VEN 5nM: 5%, PPAP/VEN combination:74% apoptotis). Most interestingly, PPAP #122 displayed strong pro-apoptotic activity in VEN-resistant leukemia models (MOLM14, HL60, OCi-3 models) with a IC50s < 1μM, and in combination with VEN, re-sensitized cells towards Venetoclax in a combination approach. Together, further development of type-B PPAPs to target Venetoclax resistance in AML is a promising approach to overcome a so far unmet clinical need in acute leukemias.