Ovarian cancer and its treatment limitations:

Despite recent advances in understanding the biology of high-grade serous ovarian cancer (HGSOC), surgical debulking and platinum-based chemotherapy remain the standard of care. Over 80% of cases experience recurrence, with approximately 12,000 patients succumbing to ovarian cancer annually in the United States [1]. Targeted therapies for common genetic alterations in HGSOC, such as poly (ADP-ribose) polymerase inhibitors (PARPis), have gained FDA approval for both frontline and recurrent use [2]. However, while PARPi therapy extends median progression-free survival for ovarian cancer patients with homologous recombination repair (HRR) deficiency, numerous mechanisms of PARPi resistance have been reported. Therefore, PARPi resistance is an emerging therapeutic challenge, necessitating the development of strategies to sensitize cancer cells to PARPis.

Background of the study:

 PG545 (Pixatimod), a highly sulfated small molecule compound, was designed with a core heparan sulfate mimetic structure to target heparanase and heparin binding growth factor (HB-GF) signaling [3]. Recently studied in a Phase 1a monotherapy trial for patients with advanced solid tumors (NCT02042781), PG545 is known for its anti-cancer activity in various pre-clinical cancer models, including ovarian, endometrial, pancreatic, colon, breast, and lung cancers [4-7]. Previous research indicated that PG545 inhibits growth factor-mediated signaling in ovarian cancer to enhance chemotherapy response [4]. Heparan sulfate proteoglycans (HSPGs) in addition to its role in signaling, are also involved in endocytosis of molecules like the oncoprotein DEK [8]. Upregulated in several cancer types, DEK, functions as a DNA repair protein in the nucleus, regulating the HRR pathway [9-10]. Secreted DEK acts as a chemoattractant for inflammatory cells and regulates inflammation in the tumor microenvironment [11]. PG545 inhibits HSPG-mediated endocytosis of DEK, impacting its repair function [12]. Overexpression of DEK in ovarian cancer is linked to cell growth, DNA damage, and increased chemotherapy sensitivity [13].

Key findings:

In our study, we discovered that PG545 induces DNA damage in ovarian cancer cells, a previously uncharacterized effect. We aimed to investigate the mechanisms and outcomes of this effect. PG545 causes DNA single- and double-strand breaks, reduces the expression of RAD51 (a crucial DNA repair protein), and inhibits the HRR pathway in cancer cells through autophagy. Additionally, PG545 prevents internalization of the DEK oncoprotein by cancer cells. Since DEK relies on heparan sulfate binding for function, PG545 disrupts HSPG-mediated endocytosis, leading to loss of nuclear DEK and inhibition of the DNA repair pathway. PG545 synergizes with PARPis in ovarian cancer cell lines in vitro, including those resistant to PARPi monotherapy. This synergy was observed in 55% of primary cultures of patient-derived ascites samples in an ex vivo model. Furthermore, the combination of PG545 and the PARPi rucaparib induced increased DNA damage and reduced tumor burden in mice with HRR-proficient OVCAR5 cell line xenografts, compared to monotherapy, in an in vivo setting. The combination also demonstrated synergistic anti-tumor activity in an immunocompetent syngeneic ID8F3 ascites model in vivo. These findings suggest that targeting the tumor microenvironment's signaling, specifically the DEK-HSPG interaction, using PG545, might inhibit DNA repair and sensitize cells to PARPi treatment.

Take-home message and future directions:

This study presents the first report on PG545's role in inducing the DNA damage pathway in cancer cells. It introduces a novel therapeutic strategy—using PG545 to sensitize ovarian cancer cells to PARPi treatment. This strategy is supported by results from patient-derived ascites samples, ex vivo and in vivo preclinical models of ovarian cancer, and in vitro ovarian cancer cell lines. Notably, HR deficiency isn't essential for the observed synergy, as PG545/PARPi synergy was seen in both HR-deficient and PARPi-resistant lines. The ability of PG545 to downregulate RAD51 via autophagy in HR-proficient cell lines adds to PARPi sensitization, warranting further preclinical investigation. Mechanistically, PG545 inhibits DEK internalization and its function as a DNA repair protein, inducing DNA damage and sensitizing cells to PARPis. To progress, additional ex vivo studies with patient-derived ascites samples are needed to correlate treatment responses with clinical characteristics or mutational statuses. Future work utilizing patient-derived xenograft models will further evaluate the combination treatment's efficacy, especially in preclinical models resistant to current therapies, with hopes of identifying additional treatment-selection biomarkers.

References:

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