Bcl-xL inhibition radiosensitizes PIK3CA/PTEN wild-type triple negative breast cancers with low Mcl-1 expression.

Patients with radioresistant breast cancers, including a large percentage of women with triple negative breast cancer (TNBC), demonstrate limited response to radiation (RT) and increased locoregional recurrence; thus, strategies to increase the efficacy of RT in TNBC are critically needed. We demonstrate that pan Bcl-2 family inhibition (ABT-263, rER: 1.52-1.56) or Bcl-xL specific inhibition (WEHI-539, A-1331852; rER: 1.31-2.00) radiosensitized wild-type PIK3CA/PTEN TNBC (MDA-MB-231, CAL-120) but failed to radiosensitize mutant PIK3CA/PTEN TNBC (rER: 0.90 - 1.07; MDA-MB-468, CAL-51, SUM-159). Specific inhibition of Bcl-2 or Mcl-1 did not induce radiosensitization, regardless of PIK3CA/PTEN status (rER: 0.95 - 1.07). In wild-type PIK3CA/PTEN TNBC, pan Bcl-2 family inhibition or Bcl-xL specific inhibition with RT led to increased levels of apoptosis (p < 0.001) and an increase in cleaved PARP and cleaved caspase 3. CRISPR-mediated PTEN knockout in wild-type PIK3CA/PTEN MDA-MB-231 and CAL-120 cells induced expression of pAKT/Akt and Mcl-1 and abolished Bcl-xL inhibitor-mediated radiosensitization (rER: 0.94 - 1.07). Similarly, Mcl-1 overexpression abolished radiosensitization in MDA-MB-231 and CAL-120 cells (rER: 1.02 - 1.04) but transient MCL1 knockdown in CAL-51 cells promoted Bcl-xL-inhibitor mediated radiosensitization (rER 2.35 ± 0.05). In vivo, ABT-263 or A-1331852 in combination with RT decreased tumor growth and increased tumor tripling time (p < 0.0001) in PIK3CA/PTEN wild-type TNBC cell line and patient-derived xenografts. Collectively, this study provides the preclinical rationale for early phase clinical trials testing the safety, tolerability, and efficacy of Bcl-xL inhibition and RT in women with wild-type PIK3CA/PTEN wild-type TNBC at high risk for recurrence.

RB expression confers sensitivity to CDK4/6 inhibitor-mediated radiosensitization across breast cancer subtypes.

Standard radiation therapy (RT) does not reliably provide locoregional control for women with multinode-positive breast cancer and triple-negative breast cancer (TNBC). We hypothesized that CDK4/6 inhibition (CDK4/6i) would increase the radiosensitivity not only of estrogen receptor-positive (ER+) cells, but also of TNBC that expresses retinoblastoma (RB) protein. We found that CDK4/6i radiosensitized RB WT TNBC (n = 4, radiation enhancement ratio [rER]: 1.49-2.22) but failed to radiosensitize RB-null TNBC (n = 3, rER: 0.84-1.00). RB expression predicted response to CDK4/6i + RT (R2 = 0.84), and radiosensitization was lost in ER+/TNBC cells (rER: 0.88-1.13) after RB1 knockdown in isogenic and nonisogenic models. CDK4/6i suppressed homologous recombination (HR) in RB WT cells but not in RB-null cells or isogenic models of RB1 loss; HR competency was rescued with RB reexpression. Radiosensitization was independent of nonhomologous end joining and the known effects of CDK4/6i on cell cycle arrest. Mechanistically, RB and RAD51 interact in vitro to promote HR repair. CDK4/6i produced RB-dependent radiosensitization in TNBC xenografts but not in isogenic RB1-null xenografts. Our data provide the preclinical rationale for a clinical trial expanding the use of CDK4/6i + RT to difficult-to-control RB-intact breast cancers (including TNBC) and nominate RB status as a predictive biomarker of therapeutic efficacy.

Structure-Guided Design and Initial Studies of a Bifunctional MEK/PI3K Inhibitor (ST-168).

The structure-based design of a new single entity, MEK/PI3K bifunctional inhibitor (7, ST-168), which displays improved MEK1 and PI3K isoform inhibition, is described. ST-168 demonstrated a 2.2-fold improvement in MEK1 inhibition and a 2.8-, 2.7-, 23-, and 2.5-fold improved inhibition toward the PI3Kα, PI3Kß, PI3Kδ, and PI3Kγ isoforms, respectively, as compared to a previous lead compound (4; ST-162) in in vitro enzymatic inhibition assays. ST-168 demonstrated superior tumoricidal efficacy over ST-162 in an A375 melanoma spheroid tumor model. ST-168 was comparatively more effective than ST-162 in promoting tumor control when administrated orally in a tumor therapy study conducted in an A375 melanoma mouse model confirming its bioavailability and efficacy toward combined in vivo MEK1/PI3K inhibition.

Discovery of Bifunctional Oncogenic Target Inhibitors against Allosteric Mitogen-Activated Protein Kinase (MEK1) and Phosphatidylinositol 3-Kinase (PI3K).

The synthesis of a series of single entity, bifunctional MEK1/PI3K inhibitors achieved by covalent linking of structural analogs of the ATP-competitive PI3K inhibitor ZSTK474 and the ATP-noncompetitive MEK inhibitor PD0325901 is described. Inhibitors displayed potent in vitro inhibition of MEK1 (0.015 < IC50 (nM) < 56.7) and PI3K (54 < IC50 (nM) < 341) in enzymatic inhibition assays. Concurrent MEK1 and PI3K inhibition was demonstrated with inhibitors 9 and 14 in two tumor cell lines (A549, D54). Inhibitors produced dose-dependent decreased cell viability similar to the combined administration of equivalent doses of ZSTK474 and PD0325901. In vivo efficacy of 14 following oral administration was demonstrated in D54 glioma and A549 lung tumor bearing mice. Compound 14 showed a 95% and 67% inhibition of tumor ERK1/2 and Akt phosphorylation, respectively, at 2 h postadministration by Western blot analysis, confirming the bioavailability and efficacy of this bifunctional inhibitor strategy toward combined MEK1/PI3K inhibition.