PARP inhibition and pharmacological ascorbate demonstrate synergy in castration-resistant prostate cancer.

Prostate cancer (PCa) is the second leading cause of cancer death for men in the United States. While organ-confined disease has reasonable expectation of cure, metastatic PCa is universally fatal upon recurrence during hormone therapy, a stage termed castration-resistant prostate cancer (CRPC). Until such time as molecularly defined subtypes can be identified and targeted using precision medicine, it is necessary to investigate new therapies that may apply to the CRPC population as a whole. The administration of ascorbate, more commonly known as ascorbic acid or Vitamin C, has proved lethal to and highly selective for a variety of cancer cell types. There are several mechanisms currently under investigation to explain how ascorbate exerts anti-cancer effects. A simplified model depicts ascorbate as a pro-drug for reactive oxygen species (ROS), which accumulate intracellularly and generate DNA damage. It was therefore hypothesized that poly(ADP-ribose) polymerase (PARP) inhibitors, by inhibiting DNA damage repair, would augment the toxicity of ascorbate. Results: Two distinct CRPC models were found to be sensitive to physiologically relevant doses of ascorbate. Moreover, additional studies indicate that ascorbate inhibits CRPC growth in vitro via multiple mechanisms including disruption of cellular energy dynamics and accumulation of DNA damage. Combination studies were performed in CRPC models with ascorbate in conjunction with escalating doses of three different PARP inhibitors (niraparib, olaparib, and talazoparib). The addition of ascorbate augmented the toxicity of all three PARP inhibitors and proved synergistic with olaparib in both CRPC models. Finally, the combination of olaparib and ascorbate was tested in vivo in both castrated and non-castrated models. In both cohorts, the combination treatment significantly delayed tumor growth compared to monotherapy or untreated control. Conclusions: These data indicate that pharmacological ascorbate is an effective monotherapy at physiological concentrations and kills CRPC cells. Ascorbate-induced tumor cell death was associated with disruption of cellular energy dynamics and accumulation of DNA damage. The addition of PARP inhibition increased the extent of DNA damage and proved effective at slowing CRPC growth both in vitro and in vivo. These findings nominate ascorbate and PARPi as a novel therapeutic regimen that has the potential to improve CRPC patient outcomes.

Mutant p53 elicits context-dependent pro-tumorigenic phenotypes.

The tumor suppressor gene TP53 is the most frequently mutated gene in numerous cancer types, including prostate cancer (PCa). Specifically, missense mutations in TP53 are selectively enriched in PCa, and cluster to particular "hot spots" in the p53 DNA binding domain with mutation at the R273 residue occurring most frequently. While this residue is similarly mutated to R273C-p53 or R273H-p53 in all cancer types examined, in PCa selective enrichment of R273C-p53 is observed. Importantly, examination of clinical datasets indicated that TP53 heterozygosity can either be maintained or loss of heterozygosity (LOH) occurs. Thus, to mimic tumor-associated mutant p53, R273C-p53 and R273H-p53 isogenic PCa models were developed in the presence or absence of wild-type p53. In the absence of wild-type p53, both R273C-p53 and R273H-p53 exhibited similar loss of DNA binding, transcriptional profiles, and loss of canonical tumor suppressor functions associated with wild-type p53. In the presence of wild-type p53 expression, both R273C-p53 and R273H-p53 supported canonical p53 target gene expression yet elicited distinct cistromic and transcriptional profiles when compared to each other. Moreover, heterozygous modeling of R273C-p53 or R273H-p53 expression resulted in distinct phenotypic outcomes in vitro and in vivo. Thus, mutant p53 acts in a context-dependent manner to elicit pro-tumorigenic transcriptional profiles, providing critical insight into mutant p53-mediated prostate cancer progression.

MAPK Reliance via Acquired CDK4/6 Inhibitor Resistance in Cancer.

Purpose: Loss of cell-cycle control is a hallmark of cancer, which can be targeted with agents, including cyclin-dependent kinase-4/6 (CDK4/6) kinase inhibitors that impinge upon the G1-S cell-cycle checkpoint via maintaining activity of the retinoblastoma tumor suppressor (RB). This class of drugs is under clinical investigation for various solid tumor types and has recently been FDA-approved for treatment of breast cancer. However, development of therapeutic resistance is not uncommon.Experimental Design: In this study, palbociclib (a CDK4/6 inhibitor) resistance was established in models of early stage, RB-positive cancer.Results: This study demonstrates that acquired palbociclib resistance renders cancer cells broadly resistant to CDK4/6 inhibitors. Acquired resistance was associated with aggressive in vitro and in vivo phenotypes, including proliferation, migration, and invasion. Integration of RNA sequencing analysis and phosphoproteomics profiling revealed rewiring of the kinome, with a strong enrichment for enhanced MAPK signaling across all resistance models, which resulted in aggressive in vitro and in vivo phenotypes and prometastatic signaling. However, CDK4/6 inhibitor-resistant models were sensitized to MEK inhibitors, revealing reliance on active MAPK signaling to promote tumor cell growth and invasion.Conclusions: In sum, these studies identify MAPK reliance in acquired CDK4/6 inhibitor resistance that promotes aggressive disease, while nominating MEK inhibition as putative novel therapeutic strategy to treat or prevent CDK4/6 inhibitor resistance in cancer. Clin Cancer Res; 24(17); 4201-14. ©2018 AACR.

Synergy of radiotherapy and PD-1 blockade in Kras-mutant lung cancer.

Radiation therapy (RT), a critical modality in the treatment of lung cancer, induces direct tumor cell death and augments tumor-specific immunity. However, despite initial tumor control, most patients suffer from locoregional relapse and/or metastatic disease following RT. The use of immunotherapy in non-small-cell lung cancer (NSCLC) could potentially change this outcome by enhancing the effects of RT. Here, we report significant (up to 70% volume reduction of the target lesion) and durable (up to 12 weeks) tumor regressions in conditional Kras-driven genetically engineered mouse models (GEMMs) of NSCLC treated with radiotherapy and a programmed cell death 1 antibody (αPD-1). However, while αPD-1 therapy was beneficial when combined with RT in radiation-naive tumors, αPD-1 therapy had no antineoplastic efficacy in RT-relapsed tumors and further induced T cell inhibitory markers in this setting. Furthermore, there was differential efficacy of αPD-1 plus RT among Kras-driven GEMMs, with additional loss of the tumor suppressor serine/threonine kinase 11/liver kinase B1 (Stk11/Lkb1) resulting in no synergistic efficacy. Taken together, our data provide evidence for a close interaction among RT, T cells, and the PD-1/PD-L1 axis and underscore the rationale for clinical combinatorial therapy with immune modulators and radiotherapy.