Onvansertib and Emerging Cancer Treatment Strategies

Onvansertib is the first oral, highly selective inhibitor of polo-like kinase 1 (PLK1) to advance through mid-stage clinical trials, and its development sits at the center of a broader push to exploit cancer cells’ dependence on their own growth machinery. The drug has generated the most clinical data in KRAS-mutant colorectal cancer, where a phase II trial reported a confirmed overall response rate of about 26% in a difficult-to-treat population, but active investigations span leukemia, pancreatic cancer, prostate cancer, breast cancer, head and neck cancer, and several pediatric tumors. What makes onvansertib interesting is not just the drug itself but what it reveals about how targeting a single enzyme can ripple through cancer biology in ways that create openings for combination therapies.

Why PLK1 Is a Vulnerable Target in Cancer

PLK1 is a kinase that acts as a traffic controller during cell division, shepherding cells through the stages of mitosis. Healthy cells need PLK1, but cancer cells tend to become addicted to it. Across many tumor types, cancer cells overexpress PLK1, and that overexpression has been linked to more aggressive disease and worse outcomes. In colon cancer specifically, high PLK1 expression independently predicted poorer survival, with a relative risk of about 3.3 in patients with localized disease.¹ Similar patterns show up across many cancer types, making PLK1 a broadly relevant target.²

Block PLK1, and cancer cells stall in the G2/M phase of the cell cycle, the transition point where a cell commits to dividing.³ Stuck there, the cells accumulate DNA damage, trigger stress responses, and eventually die through programmed cell death. Onvansertib accomplishes this at very low concentrations and is more than 5,000-fold more selective for PLK1 over its close relatives PLK2 and PLK3, which matters because hitting those cousins would cause unnecessary side effects in healthy tissue.⁴

The role of PLK1 in cancer turns out to extend beyond just cell division. Emerging research suggests it also helps tumors evade the immune system and shapes the surrounding tumor environment in ways that promote growth.⁵ That broader involvement is one reason oncologists are exploring onvansertib in so many different disease settings.

KRAS-Mutant Colorectal Cancer, Where the Evidence Is Furthest Along

KRAS mutations drive roughly 40 to 50% of metastatic colorectal cancers, and for decades they have been essentially undruggable. Drugs that target other growth signals in colorectal cancer, like EGFR inhibitors, flat-out do not work when KRAS is mutated. Onvansertib’s clinical program zeroed in on this unmet need, testing the drug alongside FOLFIRI chemotherapy and bevacizumab as a second-line treatment.

A phase Ib dose-escalation study established the recommended dose and offered the first signs of clinical activity. Among 16 evaluable patients, the confirmed response rate was about 38%, the clinical benefit rate (responses plus stable disease) reached 88%, and the median time before the cancer progressed was 12.6 months. Responses were seen across all dose levels and across different KRAS mutation subtypes.⁶

The subsequent phase II trial treated 53 patients and confirmed an overall response rate of about 26%, with responses lasting a median of nearly 12 months. But the most striking finding came from a subgroup analysis. Patients who had never previously received bevacizumab responded at a dramatically higher rate, around 77%, compared with only 10% in those who had prior bevacizumab exposure. Their median progression-free survival was 14.9 months versus 6.6 months.⁷ Translational work from the same study suggests that prior bevacizumab exposure contributes to onvansertib resistance, and that blocking PLK1 interferes with a hypoxia pathway that bevacizumab also acts on, meaning the two drugs may be partly redundant if the tumor has already adapted to bevacizumab pressure.

A randomized trial comparing FOLFIRI/bevacizumab with and without onvansertib is now underway, which should answer whether the drug genuinely adds benefit or whether the single-arm results are inflated by patient selection.⁶

Using Circulating Tumor DNA as an Early Signal

One of the more practical angles in the colorectal cancer trials is the use of circulating tumor DNA, specifically the KRAS mutant allele frequency in the blood, as a biomarker that might predict response before imaging scans catch up. In the phase Ib study, patients who went on to have confirmed tumor shrinkage showed a mean decline of about 96% in circulating KRAS mutant allele frequency after just one cycle of treatment. Patients with stable disease saw a 74% drop, and those whose cancer ultimately progressed had a smaller, statistically nonsignificant decline.⁸ If this pattern holds up in larger datasets, a simple blood test after the first treatment cycle could tell doctors and patients whether the drug is working weeks before a CT scan would show it.

What Side Effects Look Like

PLK1 is involved in the division of all rapidly dividing cells, not just cancer cells, so it is no surprise that the main side effects involve blood cell counts. In the colorectal cancer dose-escalation study, neutropenia (low white blood cells) was the most common serious side effect, affecting about 56% of patients at any grade. Severe side effects overall accounted for about 15% of all adverse events, and all dose-limiting toxicities were hematologic. Every patient who experienced a dose-limiting toxicity recovered, typically within one to 17 days.⁶

In the leukemia trial, where doses climbed higher and patients were sicker at baseline, the pattern was similar but the rates were higher. The most common serious side effects were anemia (about 35% of patients), febrile neutropenia (30%), regular neutropenia (25%), and low platelets (25%). The maximum tolerated dose in combination with the chemotherapy drug decitabine was established at 60 mg/m², after two patients at the 90 mg/m² dose level experienced dose-limiting rash and mucositis.⁹

A practical advantage of onvansertib is its pharmacokinetic profile. The drug is taken by mouth, reaches peak blood levels within a few hours, and has a half-life that allows once-daily dosing. Its blood levels increase predictably with dose and do not appear to be affected by what other drug it is combined with.⁹ For patients, the oral formulation matters. Many cancer drugs in this space require intravenous infusion, which means hospital visits and chair time. An oral drug that can be taken at home is a meaningful quality-of-life difference.

Acute Myeloid Leukemia

The first-in-human combination study of onvansertib was conducted in patients with relapsed or refractory acute myeloid leukemia (AML), a blood cancer with poor outcomes after initial treatment fails. In that phase Ib trial, onvansertib was combined with either low-dose cytarabine or decitabine. The study’s primary purpose was to find a safe dose and characterize the drug’s behavior, so the efficacy data is limited. But the trial established that onvansertib can be given alongside standard leukemia drugs without unexpected toxicity at most dose levels.⁹ The rationale for AML is particularly clean: leukemia cells divide rapidly and depend heavily on PLK1, making them inherently sensitive to its inhibition.

Pancreatic Cancer and Overcoming Chemotherapy Resistance

Pancreatic ductal adenocarcinoma is one of the deadliest cancers, in part because it quickly develops resistance to gemcitabine, the backbone chemotherapy drug. Laboratory and animal studies show that adding onvansertib to gemcitabine produces meaningful anti-tumor effects where gemcitabine alone falls short. The combination works by shutting down two survival pathways that pancreatic cancer cells use to dodge gemcitabine’s effects, while simultaneously ramping up cell death signals.¹⁰

A separate line of research in pancreatic cancer explores whether blocking PLK1 can make tumors vulnerable to PARP inhibitors, a drug class that typically works only in cancers with specific DNA repair defects. Most pancreatic cancers do not have those defects, but blocking PLK1 appears to create a similar vulnerability artificially, causing DNA damage to pile up in cells that can no longer fix it. In laboratory experiments, onvansertib combined with the PARP inhibitor olaparib reduced the survival of pancreatic cancer cells that would ordinarily shrug off PARP inhibition.¹¹ This concept of manufactured vulnerability, sometimes called synthetic lethality, is being explored in colorectal cancer as well.¹²

Prostate Cancer and Hormone-Therapy Resistance

In metastatic castration-resistant prostate cancer, the disease has stopped responding to drugs that suppress or block male hormones. One of the standard treatments at that stage is abiraterone, which cuts off residual hormone production. The problem is that abiraterone resistance eventually develops. Preclinical work shows that PLK1 is actually upregulated in prostate cancer cells after androgen-deprivation therapy, and that this upregulation might be one of the escape routes the cancer uses.¹³

When researchers combined onvansertib with abiraterone in cell and animal models, the two drugs worked synergistically through a mechanism that does not rely on the androgen receptor, meaning it could potentially sidestep the resistance pathway entirely. Gene expression studies identified a signature of mitosis-related genes that correlated with which tumors responded to the combination, pointing toward a potential way to select patients most likely to benefit.¹⁴ A phase II clinical trial has been designed around this concept.

Breast Cancer and CDK4/6 Inhibitor Resistance

A similar resistance story is playing out in hormone receptor-positive breast cancer. CDK4/6 inhibitors like palbociclib have become a pillar of treatment, but cancers eventually progress through them. In patient-derived tumor models that had become resistant to palbociclib, combining onvansertib with paclitaxel chemotherapy extended the duration of response and overcame the resistance. The combination killed cancer cells more effectively than either drug alone across several breast cancer cell lines.¹⁵ The recurring theme here is worth flagging: across cancer types, onvansertib keeps showing up as a way to restore sensitivity to drugs that tumors have learned to resist.

Making Radiation Therapy Hit Harder

Radiation therapy works by damaging DNA, and cancer cells that can repair that damage survive. PLK1 plays a role in DNA repair during cell division, so blocking it with onvansertib means radiation damage accumulates rather than getting fixed. Two preclinical programs illustrate this especially well.

In HPV-negative head and neck cancer, which tends to respond poorly to standard radiation, combining PLK1 inhibition with radiation arrested cancer cells at a vulnerable point in the cell cycle and reduced tumor growth in animal models. The combination also reversed a radiation-triggered increase in a protein called MMP10 that promotes cancer cell invasion, addressing a concern that radiation can sometimes make surviving tumor cells more aggressive.¹⁶

In MYC-driven medulloblastoma, an aggressive pediatric brain tumor, onvansertib alone increased DNA damage markers, and adding radiation amplified this effect well beyond what either treatment achieved on its own. Colony formation, a measure of cancer cells’ ability to regrow after treatment, dropped sharply with the combination compared to radiation alone.¹⁷ Because medulloblastoma treatment currently relies heavily on radiation that causes long-term cognitive side effects in children, a drug that could make lower radiation doses equally effective would have enormous practical value.

Pediatric Cancers Beyond Medulloblastoma

Neuroblastoma, the most common solid tumor in young children outside the brain, is another disease where onvansertib is being explored. High-risk neuroblastoma remains deadly despite aggressive multimodal treatment. Laboratory studies show that onvansertib compromises neuroblastoma cell survival through multiple mechanisms at once: it disrupts mitosis, causes DNA damage to pile up, and triggers programmed cell death. Researchers have described the findings as providing a strong rationale for moving onvansertib into preclinical animal models and eventually clinical evaluation in this disease.¹⁸

Pediatric cancers are an area where drug development moves slowly because trials are small, the diseases are rare, and regulators rightly demand strong preclinical evidence before testing drugs in children. The medulloblastoma and neuroblastoma data collectively suggest that PLK1 inhibition has real biological relevance in at least some pediatric tumors, but clinical testing in children is still in the future.

Gynecologic Cancers

Uterine serous carcinoma, an aggressive subtype of endometrial cancer, is another disease where onvansertib has shown preclinical promise. At very low (nanomolar) concentrations, the drug inhibited the growth of uterine serous cancer cells, arrested them in the G2 phase of the cell cycle, induced DNA damage, and reduced the cells’ ability to invade surrounding tissue. It also increased the cells’ sensitivity to paclitaxel, suggesting a potential combination strategy.¹⁹ Uterine serous carcinoma accounts for a disproportionate share of endometrial cancer deaths despite being relatively uncommon, so new treatment options are genuinely needed.

How PLK1 Expression Might Guide Treatment Decisions

Not all tumors overexpress PLK1 equally, and this variation is likely to matter for patient selection. In colon cancer, PLK1 overexpression correlates with more advanced disease stage and lymph node involvement, and independently predicts worse survival.¹ The paradox is that the patients with the worst prognoses, those whose tumors are most dependent on PLK1, may be the ones most likely to benefit from blocking it.

In the prostate cancer program, researchers identified a gene expression signature related to mitosis that predicted which tumors responded to the onvansertib-abiraterone combination.¹⁴ And in the colorectal cancer trials, changes in circulating tumor DNA after just one treatment cycle correlated with eventual radiographic response, as described earlier. These biomarker efforts are still being validated, but they point toward a future where PLK1 inhibitor therapy is matched to the patients most likely to benefit, rather than offered broadly and hoped for the best.

The bevacizumab finding from the phase II colorectal cancer trial is perhaps the most immediately actionable biomarker insight. If prior bevacizumab exposure genuinely predicts poor response to onvansertib, that could reshape how clinical trials are designed and, eventually, how treatment is sequenced in the clinic.⁷

Combination Strategies and the Synthetic Lethality Concept

The thread running through nearly all of the onvansertib research is combination therapy. The drug is rarely envisioned as a solo act. Instead, the idea is to pair PLK1 inhibition with another treatment whose effectiveness is limited by a resistance mechanism that PLK1 helps sustain. Gemcitabine resistance in pancreatic cancer, abiraterone resistance in prostate cancer, palbociclib resistance in breast cancer, radiation resistance in head and neck cancer: in each case, PLK1 inhibition disrupts the tumor’s escape route.

The synthetic lethality angle deserves a closer look because it could significantly expand who benefits from PARP inhibitors. Currently, PARP inhibitors work best in cancers with BRCA mutations or other defects in a specific DNA repair pathway. Most cancers do not have those defects. But because PLK1 is involved in an alternative DNA repair process that cells rely on during division, blocking PLK1 can artificially create the same kind of repair vulnerability that BRCA-mutant cancers have naturally. Researchers have proposed combining PLK1 inhibitors with PARP inhibitors in both pancreatic and colorectal cancers where this repair pathway is intact, effectively widening the eligible patient population for PARP-based therapies.¹² This remains preclinical, but the biological rationale is compelling and distinct from the chemotherapy-boosting combinations that are further along in trials.

The immune evasion connection is the newest frontier. Evidence is accumulating that PLK1 plays a role in helping tumors dodge the immune system, which raises the question of whether combining onvansertib with immunotherapy drugs like checkpoint inhibitors could produce synergistic effects.⁵ No clinical data exists yet on this combination, but given the dominance of immunotherapy in modern oncology, it is a logical next step and likely to be tested.

1
PubMed Central. Polo-like kinase 1 expression is a prognostic factor in human colon cancer
2
PubMed Central. PLK1, A Potential Target for Cancer Therapy
3
PubMed. Single-cell transcriptomic and pharmacological studies of onvansertib for small cell lung cancer treatment
4
PubMed Central. Polo-like Kinase 1 Inhibitors Demonstrate In Vitro and In Vivo Efficacy in Preclinical Models of Small Cell Lung Cancer
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PubMed. All screens lead to polo-like kinase 1: A central node in cancer therapeutics and resistance
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PubMed Central. Onvansertib in Combination with FOLFIRI and Bevacizumab in Second-Line Treatment of KRAS-Mutant Metastatic Colorectal Cancer: A Phase Ib Clinical Study
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PubMed Central. Onvansertib in Combination With Chemotherapy and Bevacizumab in Second-Line Treatment of KRAS-Mutant Metastatic Colorectal Cancer: A Single-Arm, Phase II Trial
8
Clinical Cancer Research. Onvansertib in Combination with FOLFIRI and Bevacizumab in Second-Line Treatment of KRAS-Mutant Metastatic Colorectal Cancer: A Phase Ib Clinical Study
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Clinical Cancer Research. A Phase Ib Study of Onvansertib, a Novel Oral PLK1 Inhibitor, in Combination Therapy for Patients with Relapsed or Refractory Acute Myeloid Leukemia
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PubMed Central. PLK1 inhibition enhances gemcitabine-induced apoptosis through PLK1-dependent ERK1/2-Bim and AKT1/Noxa signals in pancreatic cancer cells
11
Cancer Research. Inhibition of BARD1-PLK1 axis enhances PARP inhibitor/platinum sensitivity in homologous repair proficient pancreatic ductal adenocarcinoma
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Cancer Research. Biomarkers of response to abiraterone and the polo-like kinase 1 (PLK1) inhibitor onvansertib in metastatic castration resistant prostate cancer (mCRPC) patients
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Journal of Clinical Oncology. A phase II study of onvansertib in combination with abiraterone and prednisone in patients with metastatic castration-resistant prostate cancer (mCRPC)
15
Cancer Research. PLK1 Inhibitor Onvansertib Extends the Response and Overcomes Resistance to Paclitaxel in Palbociclib-resistant HR+ Breast Cancer Patient-derived Xenografts
16
Clinical Cancer Research. Targeting PLK1 Reduces MMP10 to Enhance Radiosensitivity in HPV— Head and Neck Cancer
17
PubMed Central. A novel PLK1 inhibitor onvansertib effectively sensitizes MYC-driven medulloblastoma to radiotherapy
18
Cancer Research. Onvansertib-mediated PLK1 inhibition reduces cell viability in neuroblastoma cells
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PubMed Central. Onvansertib inhibits cell proliferation and increases sensitivity to paclitaxel in uterine serous cancer cells