Regulation of Olaparib Resistance in Ovarian Cancer via Serotonin Receptor Modulators: Insights from High-Throughput FDA-Approved Compound Screening

Study Background and Research Question

Ovarian cancer (OvCa) remains the deadliest gynecologic malignancy, largely due to late detection and the emergence of resistance to standard therapies. Maintenance therapy with olaparib, a poly(ADP-ribose) polymerase (PARP) inhibitor, has shown efficacy in delaying disease recurrence. Nevertheless, most OvCa patients ultimately develop resistance to olaparib, underscoring a critical need to identify modulators of chemoresistance and enable effective drug repositioning screening strategies. The central research question addressed by Yao et al. (2025) is whether any FDA-approved agents—including those not traditionally associated with oncology—can enhance the efficacy of olaparib or overcome resistance mechanisms in OvCa cells.

Key Innovation from the Reference Study

The reference study's key innovation lies in its unbiased, high-throughput approach to drug repositioning using an FDA-approved bioactive compound library. Rather than limiting investigation to established chemotherapy adjuvants or canonical signaling pathways, the authors systematically screened 1,578 regulatory-approved compounds against OvCa cell lines in combination with olaparib. This methodology enables discovery of unanticipated drug interactions and new pharmacological target identification, particularly among compounds with diverse mechanisms of action. The most unexpected and meaningful finding was the identification of several serotonin receptor (5-HTR) modulators as candidates with potential to alter olaparib sensitivity in OvCa cells.

Methods and Experimental Design Insights

A multi-stage experimental workflow was implemented to interrogate olaparib resistance:

• High-Throughput Screening (HTS): The study deployed a large-scale MTT viability assay, co-treating OvCa cells with olaparib at its IC₅₀ and each of 1,578 FDA-approved compounds from the DiscoveryProbe™ FDA-approved Drug Library (SKU L1021, APExBIO). This HTS format is optimized for pharmacological synergy detection and drug repositioning screening (Yao et al.).
• Validation of Serotonin Receptor Modulators: Four 5-HTR modulators (almotriptan, ketanserin, prucalopride, and tropisetron) identified in the initial screen were selected for secondary validation in multiple OvCa cell lines (TYKnu, OVCAR8, CAOV3, SKOV3).
• Secondary Viability Assay: A trypan blue exclusion assay was used to cross-validate MTT findings, focusing on proliferative capacity and cell death.
• Mechanistic Analysis: Western blot assays assessed DNA damage (gamma-H2AX phosphorylation) and cell cycle checkpoint activation (phospho-Chk1) to elucidate downstream effects of drug interactions.

Protocol Parameters

• Compound screening: 1,578 FDA-approved compounds; each tested at a concentration appropriate for cell-based HTS (typically 10 μM), co-administered with olaparib at its IC₅₀ in OvCa cell lines.
• Primary viability readout: MTT assay, 48–72 hours post-treatment, to assess metabolic activity and cytotoxicity.
• Secondary validation: Trypan blue exclusion assay to quantify live versus dead cells following compound-olaparib co-treatment.
• Mechanistic endpoints: Western blot for gamma-H2AX (DNA damage marker) and phospho-Chk1 (cell cycle checkpoint activation) after specified drug exposure durations.

Core Findings and Why They Matter

The HTS identified 183 compounds that, in combination with olaparib, reduced OvCa cell viability by at least 80% compared to olaparib alone. Among these, serotonin receptor modulators emerged as a novel class influencing chemosensitivity:

• Ketanserin and prucalopride significantly decreased olaparib’s IC₅₀ in MTT assays, suggesting potent synergy and enhanced cytotoxicity.
• Almotriptan increased olaparib’s IC₅₀, indicating antagonistic effects.
• Further investigation of prucalopride in additional OvCa cell lines confirmed its capacity to enhance olaparib efficacy in MTT-based screens.
• However, trypan blue assays contradicted the MTT findings for prucalopride, indicating no significant reduction in viable cell numbers or even a trend toward restoration of cell viability.
• Mechanistic analysis revealed that prucalopride decreased phosphorylation of gamma-H2AX and increased Chk1 activation, consistent with cell cycle arrest and potential protection from olaparib-induced cytotoxicity.

These data highlight the complexity of drug-drug interactions in cancer research drug screening and emphasize the need for multi-assay validation when repositioning FDA-approved agents for new indications.

Comparison with Existing Internal Articles

The workflow adopted by Yao et al. aligns with best practices detailed in several internal resources. The DiscoveryProbe™ FDA-approved Drug Library is recognized for supporting robust, reproducible high-throughput and high-content screening, making it suitable for translational oncology research. Mechanistic evaluations, as described in mechanistic innovation articles, are essential to elucidate the downstream effects of hit compounds, particularly when functional readouts (e.g., MTT vs. trypan blue) diverge. The current study’s combination of viability assays and pathway markers exemplifies a rigorous approach compatible with the library’s intended use in pharmacological target identification and drug repositioning. Internal analyses also stress the necessity of ready-to-screen compound collections to ensure reproducible data acquisition in both oncology and neurodegenerative disease drug discovery models.

Limitations and Transferability

While the study provides compelling evidence for serotonin receptor modulators as regulators of olaparib resistance, several limitations affect the transferability of these findings:

• Assay dependence: The apparent synergy between prucalopride and olaparib observed in the MTT assay was not corroborated by trypan blue exclusion, suggesting underlying effects on mitochondrial function or cellular metabolism unrelated to cell death.
• Mechanistic ambiguity: Decreased DNA damage signaling and increased Chk1 activation with prucalopride imply cell cycle arrest may protect, rather than sensitize, cells to olaparib.
• Cell line specificity: Although several OvCa cell lines were tested, in vivo validation is lacking.
• Further studies needed: The role of oxidative stress and mitochondrial bioenergetics in these drug interactions remains speculative and requires additional mechanistic work.

Thus, while high-throughput screening drug library approaches are powerful for hypothesis generation and early-stage pharmacological target identification, multi-level validation is critical before clinical translation.

Research Support Resources

For researchers seeking to explore similar drug repositioning screening workflows, the DiscoveryProbe™ FDA-approved Drug Library (SKU: L1021) offers a curated, ready-to-screen collection of 2,320 clinically approved compounds. Its design supports high-throughput and high-content screening in oncology and other disease models, facilitating robust pharmacological target identification and mechanistic follow-up studies. This resource can accelerate the translation of HTS findings into actionable insights for overcoming chemoresistance in cancer or other therapeutic areas.